Information

Why insects are so small compared to mammals?

Why insects are so small compared to mammals?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

I was wondering what biological limitations make the dimension of the insects small compared to the dimension of the mammals. I know in other eras insect were bigger!


The standard answer to this question is - it's down to the delivery of oxygen to respiring tissues. Insects depend upon a process that is essentially diffusion-driven (via spiracles and the tracheal system) whereas vertebrates have a circulatory system that allows oxygen absorbed in the lungs to be carried via the blood to peripheral tissues. And of course the blood contains specialised carrier cells (red cells) which in turn are packed with haemoglobin which acts as a molecular carrier for oxygen.

The giant dragonflies of the Carboniferous (Meganeura) existed at a time when oxygen levels in the atmosphere were much higher than now (32.5% vs 20.9%), so the diffusion-driven gas exchange system could support a larger body size.


Summary

The majority of conservation efforts and public attention are focused on large, charismatic mammals and birds such as tigers, pandas and penguins, yet the bulk of animal life, whether measured by biomass, numerical abundance or numbers of species, consists of invertebrates such as insects. Arguably, these innumerable little creatures are far more important for the functioning of ecosystems than their furry or feathered brethren, but until recently we had few long-term data on their population trends. Recent studies from Germany and Puerto Rico suggest that insects may be in a state of catastrophic population collapse: the German data describe a 76% decline in biomass over 26 years, while the Puerto Rican study estimates a decline of between 75% and 98% over 35 years. Corroborative evidence, for example from butterflies in Europe and California (which both show slightly less dramatic reductions in abundance), suggest that these declines are not isolated. The causes are much debated, but almost certainly include habitat loss, chronic exposure to pesticides, and climate change. The consequences are clear insects are integral to every terrestrial food web, being food for numerous birds, bats, reptiles, amphibians and fish, and performing vital roles such as pollination, pest control and nutrient recycling. Terrestrial and freshwater ecosystems will collapse without insects. These studies are a warning that we may have failed to appreciate the full scale and pace of environmental degradation caused by human activities in the Anthropocene.


A Sizeable Advantage: Why Insects Are So Small

Insects are a highly successful clade of organisms, as evidenced by the myriad forms one can daily observe. This success is due to the layering and interaction of many different advantageous evolutionary and ecological factors over the eons, allowing for such massive speciation. One such advantage may be that of size. Insects are reasonably small in comparison with the wide variation of scales other clades of living organisms have achieved. Why? The correct answer is a combination of several hypotheses, but what combination?

Among the most prevalent hypotheses are those pointing the finger of size-limitation blame towards tracheal respiratory system limitations, an unfavorable surface area to volume ratio causing biomechanical exoskeleton non-scalability, and the evolution of more potent flying life forms such as birds and bats posing potential biotic size-limiting factors including predation and competition. At the opposite end of the scale, evidence shows that insect miniaturization is limited more simply by egg size large enough to produce viable larvae (for females), brain size, and the inability to fly, a highly advantageous evolved behavior (Grebennikov 2008, Niven & Farris 2012, Polilov 2012).

Insects’ breathing system consists of pairs of holes called spiracles along their bodies connecting to tubes called tracheae which transport oxygen to cells and remove carbon dioxide. As insects increase in size, tracheae take up a proportionally greater amount of space in their bodies because they need to increase in length and width to deliver oxygen as a larger body size increases oxygen needs. This inhibits growth because at a certain point the tracheae become impractical and crowd other organs (Kirkton 2007).

Unlike an endoskeleton, which allows continuous growth, an exoskeleton limits an organism to discrete or incremental growth through molting. During molting, an organism is highly vulnerable to predation and the forces of gravity. A very large insect would take too long to molt, put too much energy into the production of a new exoskeleton, and collapse under its own weight. Also, an exoskeleton must be scaled to the dimensions of the organism to prevent it from collapsing. For example, if an ant’s exoskeleton must become thicker as body size increases (strength is proportional to length squared whereas mass is proportional to length cubed), at a certain point this thickness would cause the ant’s legs to crumple under the strain of holding up its own body weight.

One hypothesis points to biotic factors constraining size. If flying insects were large, they would be easier to prey upon because they would be more conspicuous and less acrobatically maneuverable. Also, insects would be outcompeted by other flying animals like birds and bats. This hypothesis is outlined and explored further below.

One way to logically determine the possible evolutionary pressures limiting insect size is to trace the different evolutionary paths a primitive insect may have taken on its journey of terrestrial invasion and ultimate colossal diversification. Imagine a primitive insectoid form crawling out of the ocean. It pioneered the terrestrial space for its slimy kin. This creature does fairly well for a while, but then its squishy body becomes a hindrance to its fitness to reproduce-it is too physically vulnerable and too costly in terms of moisture conservation. Therefore, the organism evolves an exoskeleton to help conserve moisture and protect itself from predators. However, this primitive exoskeleton reduces the insect’s diffusion, and it must evolve a respiratory system-either tracheal or some intermediary. If the insect possesses an exoskeleton upon colonization, but has a diffusive respiratory system causing it to lose too much moisture, it may also evolve a tracheal system.

In either scenario, the primitive land-bound insect is limited by size because of its exoskeleton and tracheal system. If for some reason the insect first has no reason to evolve an exoskeleton-perhaps it is small enough or evasive enough to avoid prying predators – then it would be limited by its tracheal system. The opposite is true if the insect has a different respiratory system than a tracheal one – it would be limited by its exoskeleton. Interestingly enough, a lack of exoskeleton could also have limited the pioneering insect’s size. If it got too large but remained squishy and vulnerable, it would have been much easier to see and capture as well as to rip apart and ingest. Fast forward to the Carboniferous period. During this time period, oxygen levels were higher, meaning insects needed smaller quantities of air to meet their oxygen demands and could thus grow much larger (Clapham & Karr 2012). In fact, one study suggests insects may have needed to grow larger to avoid hyperoxia. Aquatic larvae which respire through diffusion are more sensitive to oxygen fluctuations and primitive forms which may have been unable to regulate their oxygen intake could solve this issue by growing bigger (Verberk & Bilton 2011). Only those organisms unconstrained by the exoskeleton could have grown large during the Carboniferous period, such as odonates, which overcome the surface area to volume problem by changing their shape. Bloated boxy beetles would have been size-limited, but organisms with long skinny body parts could overcome this obstacle.

When insect size did decrease, this event coincided first with the evolution of birds, and then again with that of bats. With predatory birds in the air, the need for maneuverability and evasiveness trumped the evolutionary drive to be big, and insects decreased in size. Additionally, large insects were outcompeted by birds and bats for air space. At this point in time, biotic interactions superseded oxygen as the most important constraint on insect maximum body size (Clapham & Karr 2012).

The scenarios outlined above and their various implications on insect size-limitation are simple and consider few variables. In reality, the issue is much more complex, including many different factors (genetic, environmental, abiotic, biotic, etc.) that complicate the issue of picking apart the importance of various factors on insect size limitation and evolution in general. Currently, insects are limited by all of the above. In order of priority, basic math-related issues (exoskeleton and tracheae) are the most crucially size limiting because without any other constraints they would still limit body size. As outlined earlier, competition and predation with birds and bats supersedes oxygen availability constraints. Perhaps one could make the claim that insects are ultimately constrained by gravity – in a low-gravity world exoskeleton and body weight would be irrelevant and they could just swim through the air gulping down oxygen like fish rather than bothering with tracheae! Only by further experimentation and research can scientists piece together the highly complex puzzle depicting the size limitations on and evolutionary path of insect forms.


Why insects are so small compared to mammals? - Biology

Importance of Insects

Insects are everywhere. They are, by far, the most common animals on our planet. More than 1.5 million species of insects have been named. This is three times the number of all other animals combined. Even so, some say that the insects that have been given names are only a small fraction of the insects in nature. Many are yet to be discovered.

We can find insects in almost every conceivable habitat. Their size, shape, color, biology, and life history are so diverse that it makes the study of insects absolutely fascinating.

Without insects, our lives would be vastly different. Insects pollinate many of our fruits, flowers, and vegetables. We would not have much of the produce that we enjoy and rely on without the pollinating services of insects, not to mention honey, beeswax, silk, and other useful products that insects provide.

Insects feed on a seemingly endless array of foods. Many insects are omnivorous, meaning that they can eat a variety of foods including plants, fungi, dead animals, decaying organic matter, and nearly anything they encounter in their environment. Still others are specialists in their diet, which means they may rely only on one particular plant or even one specific part of one particular plant to survive.

Many insects are predatory or parasitic, either on plants or on other insects or animals, including people. Such insects are important in nature to help keep pest populations (insects or weeds) at a tolerable level. We call this the balance of nature. Predatory and parasitic insects are very valuable when they attack other animals or plants that we consider to be pests.

Insects are very important as primary or secondary decomposers. Without insects to help break down and dispose of wastes, dead animals and plants would accumulate in our environment and it would be messy indeed.

Insects are underappreciated for their role in the food web. They are the sole food source for many amphibians, reptiles, birds, and mammals. Insects themselves are harvested and eaten by people in some cultures. They are a rich source of protein, vitamins, and minerals, and are prized as delicacies in many third-world countries. In fact, it is difficult to find an insect that is not eaten in one form or another by people. Among the most popular are cicadas, locusts, mantises, grubs, caterpillars, crickets, ants, and wasps.

And insects make our world much more interesting. Naturalists derive a great deal of satisfaction in watching ants work, bees pollinate, or dragonflies patrol. Can you imagine how dull life would become without having butterflies or lightning beetles to add interest to a landscape? People benefit in so many ways by sharing their world with insects.

In spite of all their positive attributes, some insects can cause problems. Unfortunately, most people are more aware of the few insects that cause problems than they are of the many beneficial insects. Uninformed people think that all insects are bad and all are in need of control. We must always keep in mind that the good done by the many beneficial insects far outweighs any bad caused by a few pest species. In spite of this, texts such as this are written about the relatively few insect pests that cause us harm.


3 Answers 3

Zasso pointed it already out:

Scaling up a ant to human size means volume (weight) increasing by length proportional $l^<3>$, but the force of muscles is determined by cross section (not muscle weight), so muscle force goes proportioal to $l^<2>$.

Smaller factors are likely:

  • stiffness (or strentgh of the skeleton)
  • balance point (center of mass)
  • leverage (human skeleton is "sub-optimal" for this, we are afaik best optimized by evolution for long runs, more than any other animal)

i did some quick further search on "robot insects" on this interesting topic. This article is quite worth reading and relating biological to technological limits as well as current state of the art in nanobionics:

Interestingly, the force generated from a wide variety of actuator materials and devices has been found to be surprisingly invariant when compared with the actuator mass. A few years back, a comparison of the force-to-weight ratio of various organisms and machines found a striking similarity, with the force scaling linearly with mass over 20 orders of magnitude – from individual protein molecules to rocket engines ("Molecules, muscles, and machines: Universal performance characteristics of motors"). Remarkably, this finding indicates that most of the motors used by humans and animals for transportation have a common upper limit of mass-specific net force output that is independent of materials and mechanisms. Therefore any actuating device produces the same force per mass regardless of the material from which it is constructed and the mechanism by which it operates. This study also makes clear that biological systems dominate at the small mass, small force, range. In contrast, human-made machines dominate at the large mass range.

short example as Sonny asked for in comment:

ant with 10 mm length & 10 mg mass

$Rightarrow$ lets scale up to human size (2m) $Rightarrow$ means a factor of 200. So the mass scales with 200x200x200=8000000 (Volume $propto$ $l^<3>$ ) $Rightarrow$ human sized ant=80 kg. But muscle forces scales only by factor 200x200=40000. The small ant can carry 100x10mg of her own mass=1g, the human sized ant should be able to carry 1g x 40000=40 kg.

Conclusion: pretty comparable to a avg. 80 kg human man able to carry 40 kg!


Why Aren't There Mammals in Super Vivid Colors Like There Are Birds and Bugs?

Plumage. An incredible world, for an incredible phenomenon. Say it with me now: plumage. Picture the colors, their variety and richness. Picture, while you’re at it, some other stuff relevant to this week’s Giz Asks , such as bugs that look shaped from stained glass and sea creatures that look like they’ve been doused in neon paint.

Now picture a person. Any person will do. Pathetic, right? We—and, really, mammals generally—just do not stack up. Below, read along as a panel of experts explain why.

Matthew Toomey

Assistant Professor, Biological Science, University of Tulsa

To understand why brilliantly colored mammals are uncommon, we must consider the world in which mammals first evolved. This was a world dominated by dinosaurs that were most likely active during the day and had excellent color vision, much like modern birds. Many of these dinosaurs were predators and a major threat to the relatively small and defenseless early mammals. In response, it appears that early mammals evolved inconspicuous coloration, and nocturnal and underground ways of life. In these dark environments, their senses of smell and hearing took priority. In the dark, color vision is of little use and early mammals lost much of their color vision.

This “nocturnal bottleneck” is apparent in the visual capabilities of mammals today. Most living mammal species perceive color with just two types of photoreceptors tuned to sense either the blue or the red portion of the light spectrum. Therefore, most mammals have limited ability to discriminate color, and many hyper-vivid hues would not be discerned by them. In contrast, bird color vision involves four different photoreceptor types sensitive to ultraviolet, blue, green, or red portions of the spectrum allowing them much finer discrimination of the colors they use to attract and intimidate one another. Interestingly, some primates, like humans, have regained more complex color vision capabilities and among these species vividly colored skin patches have evolved, such as the colorful faces of mandrills.

The “nocturnal bottleneck” may have also limited the palette of pigments available to mammals. Whereas the bodies of birds, insects, and reptiles are colored with a range of pigments, mammal hair and fur is colored with just one class of pigment molecules, melanins, which produce a limited variety of colors ranging from black to brown and auburn. While other animals employ melanin in complex optical nanostructures to create iridescence in their feathers and scales, it is exceedingly rare in mammals. Like complex color vision, it is likely that these mechanisms of vivid-color production were lost as our distant ancestors lived their secretive nocturnal lives in the shadow of the dinosaurs.

Richard Prum

Professor, Ornithology, Ecology, and Evolutionary Biology, Yale University

This relates to what we call sensory ecology: how the sensory system is used in the lives of different organisms.

Color is really about communication—all these beautiful, brilliant colors are really communication devices. It’s about what animals prefer, what they like, what’s memorable. A prerequisite for this sense, which I’d argue is aesthetic, is cognitive ability, and the ability to choose—to make social choices, or sexual choices. Who you mate with, who you hang out with, etc. All these things are in play when you’re talking about why a certain group has the colors it does.

These groups will use what they have available to make color, which means pigments, i.e. molecules that differentially absorb visible light, like paint or dye or structural colors, which are made by optical interactions of light with the material of an object (so: blue skies, rainbows, etc.). So you’ve got, on the one side, the biology of cognition and social behavior, and on the other you’ve got what a given group has to work with materially to make color.

Birds are tetrachromatic—they can just blow us away in terms of color vision. They can see colors we can’t even imagine, like ultraviolet yellow or ultraviolet green. Plus, they’ve got rich social lives—they hang out, they fly around, and most of them are engaged in mate choice. They’ve also got a certain freedom of choice. So what do birds do with that freedom? They choose beautiful things—things that are subjectively desirable. And that evolves, and leads to what I call aesthetic radiation (as distinct from adaptive radiation, which is just variations in phenotype that function in a material way to do something new).

So that’s half the question: it explains why birds are so colorful to begin with. But what’s going on in mammals?

Well, they’ve got the cognitive capacity, but what they don’t have are the other things—color-vision and, frankly, opportunity for choice. The ancestor of placental mammals was walking around in the dark for like 100 million years trying to keep from being eaten by dinosaurs. During that time, they lost the capacity for complex color-vision. And if you can’t see a color, you’re not going to evolve social signals that are colorful, period. Also, most mammals are still going around at night: bats and rodents comprise the majority of mammals, and they’re both nocturnal.

That said, one lineage of placental mammals did evolve color-vision: old-world primates. They don’t match up to your average lizard or bird or even goldfish, but they do have a lot of color: the face of a mandrill, for instance, is blue and African macaques have vividly blue scrotums.

Geoff Hill

Professor and Curator of Birds at Auburn University

Humans are atypical mammals. Most mammals are nocturnal—most mammals are rodents, who live in burrows. When you deal with your dog, you kind of get the impression that they perceive the world differently than we do—they want to smell you first, which is typical of most mammals, whose world is less vision-based and more smell-based. Most mammals don’t even have full color vision like humans do. We have three-dimensional vision (which is why you put three colors in a printer). Dogs, cows, and most other mammals only have two-dimensional vision—they don’t see colors like we do. What all of this means is that they don’t display very often with color. Where we do see some colors are in primates apes like mandrills are among the most colorful animals. So that’s basically it: mammals are more prone to smell, and humans and birds are more prone to look.

Innes Cuthill

Professor, Behavioral Ecology, University of Bristol

One factor—in reference to things like iridescence (peacocks, morpho butterflies, jewel beetles)—is the nature of their pelage. Hair is pretty unstructured (tubes of keratin with melanin pigment), whereas bird feathers, butterfly wings and insect cuticles in general, are complex multi-layer structures—which is just what you need to generate structural colors created by interference of light rays.

Kevin McGraw

Director for Undergraduate Research in Life Sciences and Co-Director of Animal Behavior PhD Program at Arizona State University

There are in fact some very colorful mammals! Golden lion tamarins and red pandas have bold red-orange pelage, Honduran white bats have vivid yellow facial skin, and several primates, such as mandrills, have quite ornate red and blue skin colors on their face. Moreover, a handful of mammals like skunks and zebras have bold coat patterns (i.e. contrasting black and white). That said, the vast majority of mammals do in fact have a drab appearance, consisting mostly of shades and earth tones. This is largely thought to be a product of their use in camouflage and disruptive coloration, and also linked to the history of mammals having poor color vision (i.e. constraining their use of diverse color signals).

Mark E. Hauber

Professor, Evolution, Ecology, and Behavior, University of Illinois

There ARE at least a few brightly colored and distinctly patterned mammals. These vary from the blue-red-yellow facial and anal regions of male mandrills to the best known black-and-white stripes of zebras, but also the deep orange pelage of the golden lion tamarins and the algal-green fur of long-lived sloths.

Body parts of mammals can also be bright focally, especially testicles (blue in vervet monkeys, white in howler monkeys, red in Japanese macaques), and, of course, the eyes, varying from light blue to deep brown in humans and huskies, and orange-reds in slender loris.

Finally, we shouldn’t forget the aquatic mammals: the conspicuously white-and-black patterned orcas and the bright pink river dolphins of the Amazon.

Nina G. Jablonski

Professor, Anthropology, Penn State

And George Chaplin

Senior Research Associate, Anthropology, Penn State

First of all, there are some mammals with vivid coat colors, specifically vivid yellow and orange coats. Think of the golden lion tamarins of Brazil and the golden langurs of Bangladesh, for instance.

Second, there are mammals who have vivid-colored skin. Male mandrills and grivets (both African monkeys) and golden monkeys from China have bright blue skin on their noses and backsides. These blue colors are optical colors, not produced by pigment, but by highly oriented bundles of collagen in the skin which scatter visible light. This process, called Rayleigh scattering, is the same process which makes the sky appear blue. The same animals also have bright red skin on their backsides (and mandrills on their noses, too) produced by hemoglobin in the superficial blood vessels. Hemoglobin also makes the estrous swellings of many female primates appear bright red.

Back to coat colors: Basically, mammalian hair gets its color from two melanin pigments, the yellow-red pheomelanin and the intensely dark brown eumelanin. The melanin producing cells in mammalian hair follicles produce different mixtures of these pigments at different times to produce coats of many different colors. This makes it possible for mammals to produce anything from bright yellow to near black, as well as the absence of pigment—white, and almost every conceivable shade of grey, brown, reddish-brown, and greeny brown, in between. In contrast to birds, mammals lack the ability to produce colored hairs containing pigments called carotenoids. This is why you don’t see mammals colored like cardinals or scarlet tanagers. But, interestingly, the blue colors of bird feathers are also produced by optical effects, produced by light impinging on oriented collagen bundles.

Greg Grether

Professor, Ecology and Evolutionary Biology, UCLA

I think that can largely be explained in terms of the opportunity that animals have to use color signals in communication. Most colorful vertebrates are diurnal (active during the day) and have excellent color vision, while most mammals are nocturnal or crepuscular, lack color vision, and rely more on scent and sound for communication. Primates are an exception that supports the hypothesis—most primates are diurnal and have color vision, and some are quite colorful. Male mandrills are probably the best example.

Not all colorful invertebrates have color vision, but the ones that do not are probably signaling to potential predators that do have color vision, rather than to each other (e.g., warning coloration of bees and wasps). However, the conventional wisdom that insects do not see in color is wrong. Many do, including butterflies and dragonflies.

Tim Caro

Professor Emeritus, Wildlife, Fish and Conservation Biology, UC Davis, whose research focuses on animal coloration, among other things

Compared to birds or insects, mammals show a paucity of sexually selected coloration. In most species of mammal, female and male coloration does not differ (called sexual dichromatism) neither sex changes color at time of breeding and neither sex is particularly flamboyant or gaudy in appearance.

Sexual selection consists of two processes: intersexual selection, most commonly between males, over access to females or resources on which females depend, and intrasexual selection, usually by females choosing males with whom to mate. Both processes can drive flamboyant coloration in (usually) males.

There are factors relating to sexual selection predisposing mammals to be drab. In contrast to birds, most species of mammals are polygynous, with males employing coercion to corral females, which limits female choice of gaudy males. Most female mammals are philopatric (they don’t disperse before breeding), limiting females’ ability to choose mates. Mammals are limited in their movement compared to birds, restricting females’ ability to sample males for extra-pair copulations. In addition, male mammals rarely care for offspring, excluding one of the factors on which female birds base their mate choice. So, a priori, we might expect rather little flamboyant ornamentation in male mammals.

Some solutions: One possibility is little opportunity for sexual selection as presented above. An alternative argument is that the visual system of most mammals is dichromatic, whereas birds are tri- or tetrachromatic, so there is less necessity to rely on visual as opposed to olfactory or auditory sex differences. Another related explanation, again untested, is that most mammals are nocturnal but most birds are diurnal. At present these possibilities are difficult to tease apart moreover more than one may be relevant.

Laszlo Talas

EPSRC Innovation Fellow & Proleptic Lecturer in Animal Sensing and Biometrics at the University of Bristol

The colors of mammalian hair are primarily determined by the presence of two natural pigments: eumelanin and pheomelanin. Eumelanin is responsible for black and brown colors, while pheomelanin can create yellow and reddish hair. This means that, for mammals, it is biochemically impossible to manufacture vivid colors like bright green or deep blue using hair. The orange coat of a tiger or an infant langur is as far as it goes.

Vivid colors often evolved through sexual signaling in order to impress the opposite sex the bright feathers of a peacock would be one of the most famous examples. However, most mammals are actually red-green color blind, meaning they cannot tell apart red and green. This is true to almost all mammals, except us, Old World monkeys and some New World monkeys. Therefore bright vivid colors would not be much use to find mates.

Another strong selective force is camouflage, however brown and grey coats seem to work at most places. Many mammals can turn white during winter and remain hidden in the snow. Often prey is color blind, too: While an orange-brown tiger might be easy to spot against green vegetation by humans, a deer (the animal a tiger is really trying to hide from, not us) will struggle to tell apart the orange coat from green leaves for the reasons outlined above.

Almut Kelber

Professor, Functional Zoology, Lund University

There are a few exceptions, like an African monkey, of which Darwin already stated, “no other member in the whole class of mammals is colored in so extraordinary a manner as the adult male mandrill’s”. With their red and blue faces, this exception brings me to the first two answers to the question.

Brilliant colors are often used for communication with conspecifics, and most mammals use chemical and acoustic signals for purposes such as finding the best mate or marking a territory rather than visual signals. So there is less need of bright colors as signals among mammals. So why is the mandrill different?

A main reason is that old-world monkeys (and howler monkeys) differ in their visual system from other mammals in a way that makes them more similar to birds. Birds (as lizards, crocodiles and many fish) have highly sophisticated color vision based on four visual pigments allowing them to see more shades of color than us humans. Most mammals only have two pigments, and that reduces their color world dramatically, so even if they were colored, they would not appreciate it. The old world monkeys (the branch including us humans) had a duplication of one of the pigment genes and mutations, which together gave us three pigments, and a much more colorful world—even though not as colorful as that of birds, as we are missing all shades of ultraviolet which many birds can see.

So why do mammals only have two visual pigments? This gets us to the third answer: Unlike birds, early mammals survived the age of dinosaurs as mostly nocturnal and often subterranean animals. Their eyes evolved to see well at night, on the cost of seeing the brilliant colors in bright light, which they rarely saw. If these early mammals ever had brilliant colors, they must have lost them back then, as they had no need for them if they would not be not seen. And most often, production of these colors is costly, so they are easily lost.

Many bugs don’t have brilliant colors to attract mates, but to warn predators who want to eat them. Many bugs are eaten by birds so they are for birds to see and avoid them, either because they taste badly, are poisonous, or mimic other bugs that are.

Back to mammals and why they have their “dull” colors. The most mechanistic answer then is that we mammals mostly use brown, black and red pigments (melanins) and lack both colorful pigments and sophisticated mechanisms to produce the structural colors that underlie many green, blue and violet shades in butterflies, beetles and birds. However, the mandrill shows that this can evolve again when an animal evolved a better sense of color vision, within less than 30 million years.

Matthew Shawkey

Associate Professor, Biology, Ghent University

A few mammals have bright colors, for example the bright red and blue faces and rumps of mandrills. However, such bright skin color is certainly rare, and I only know of a few examples of somewhat brightly colored hair, for example the iridescent colors of some golden moles. So why indeed are bright colors so rare in mammals? From a proximate (mechanistic) perspective, they may have fewer ways to produce color. The only pigment that mammals use is melanin, which can produce a variety of blacks, browns and greys. Thus, they lack the pigments such as carotenoids needed to produce bright reds and yellows. Outside of pure pigmentary colors, birds and insects can produce so-called structural colors, produced by the nanoscale arrangement of materials within their feathers or cuticle. These make some of the brightest colors in nature, such as the flashing feathers of hummingbirds, but are virtually non-existent in mammals. Perhaps the chemistry, morphology, or developmental patterns of hairs preclude them from making these nanostructure. From an evolutionary perspective, mammals evolved as nocturnal animals, and in the shadows of dinosaurs, so perhaps camouflage has been more significant than mate choice in the evolution of their colors.


A Little About Scarab Beetles

A scarab beetle in flight. Click to enlarge.

Currently, more species of beetles are known than for any other animal group, with over 350,000 named species. About 30,000 beetle species are scarabs, representing about 10% of all beetles. Scarabs, like other beetles, are holometabolous, meaning they develop from an egg, to a “grub-like” larvae, to a pupae, and then emerge as adults. This process is called complete metamorphosis.

Scarabs are a broad group of beetles that have many different behaviors and diets, depending on the species. They may eat live plants, decaying plants, fungi, fruits, dung (poop), or other insects. Most adult scarab beetles are powerful fliers. Scarab beetles are often brightly colored to attract mates or warn competitors and predators. These are favorites of insect collectors because of their colors, horns, behaviors, and sizes.

The largest scarab beetles, including the African species Goliathus goliatus, are the most massive insects alive today, and may be the heaviest insects to have ever lived. They have always been noticed by humans. A scarab beetle was even worshipped by ancient Egyptians as the embodiment of a sun god.

Additional images via Wikimedia Commons. Thick-legged flower beetle image by Bernard Dupont.


The climate experienced by small desert mammals

Perhaps the most familiar generalization about desert climates is that heat stress is ameliorated by low atmospheric humidity “it's a dry heat” (e.g., Schmidt-Nielsen 1964, Cloudsley-Thompson 1965, Louw and Seely 1982). In addition, two well-known conclusions have followed from the observation that, as is typical for small mammals in all environments, desert species usually occupy burrows and are active on the surface only at night. In the Sonoran Desert, for example, 93% of rodent species are nocturnal. (The exceptions are four species of ground squirrel.) Because environmental temperatures decline dramatically after sunset, this finding led to the generalization that restricting activity to the night allows animals to evade the very high temperatures that are characteristic of daylight hours (e.g., Schmidt-Nielsen 1964, 1990, Louw and Seely 1982). An associated conclusion was that subterranean burrows provide cool, humid retreats for desert animals during daylight hours (e.g., Schmidt-Nielsen 1964, 1990, Louw and Seely 1982). Consequently, small mammals are usually portrayed as alternating between a cool, humid burrow during the day and the warm, dry atmosphere of the desert soil surface at night.

Environmental temperatures experienced by small desert mammals

The climates that are actually experienced by desert rodents can differ sharply from these traditional generalizations and may be much more challenging. For example, although nocturnal temperatures do typically decline 15–20 °C below diurnal maxima in many desert regions ( Sellers and Hill 1974, Pearce 1990), the consequences of this drop differ greatly among desert regions. Within hot subtropical deserts, average daily maxima during summer months are commonly so high (42–46 °C) that such drops only reduce nocturnal minima to 26–30 °C ( Sellers and Hill 1974, Pearce 1990)—much warmer than the nocturnal minima of deserts located at higher altitudes or latitudes.

In both a subtropical Sonoran Desert site (the San Cristobal Valley in Arizona) and one in the northern Great Basin Desert of the western United States (Malheur National Wildlife Refuge in Oregon), for example, air temperature typically varies approximately 16–17 °C over the daily cycle ( Figure 1). Temperatures in the Great Basin Desert site, however, average 16 °C below those in the Sonoran Desert site ( Figure 1). As a result, afternoon maximum temperatures on a typical summer day in the northern site are similar to the lowest temperatures experienced at night in the subtropical desert. Indeed, July temperatures in the northern site are similar to those of March or November in the southern desert. Clearly, the conditions experienced by mammals differ substantially between the deserts. Therefore, the consequences of traits such as nocturnality also differ. In the subtropical desert habitat of the San Cristobal Valley, for example, nocturnality allows animals to avoid very high afternoon temperatures that peak in July, on average, at 48 °C at a height of 1.5 m above the ground ( Figure 2). Nocturnality does not simply equate, however, to avoidance of high temperatures. Nighttime temperatures during July average 36.4 °C, and the air temperature typically drops below 30 °C for only 1 hour per night.

The annual cycle of air temperature in two desert sites. Values represent the daily range between the average minimum temperature and the average maximum temperature. The San Cristobal Valley (represented by data from Dateland Ranch in Yuma County Sellers and Hill 1974) lies in the Sonoran Desert of southwestern Arizona. Malheur National Wildlife Refuge is in the Great Basin Desert of eastern Oregon. Data are from NOAA (1985).

The annual cycle of air temperature in two desert sites. Values represent the daily range between the average minimum temperature and the average maximum temperature. The San Cristobal Valley (represented by data from Dateland Ranch in Yuma County Sellers and Hill 1974) lies in the Sonoran Desert of southwestern Arizona. Malheur National Wildlife Refuge is in the Great Basin Desert of eastern Oregon. Data are from NOAA (1985).

Average surface and air temperatures in July in the central Sonoran Desert. Data are averages for 10–27 July 1996 for sand dunes in the San Cristobal Valley, Yuma County, Arizona. Data for 1.5 m and 3 cm above the soil surface were collected using thermocouples shielded from solar radiation and thermal radiation from the soil surface. Surface temperature was measured within a 1 m 2 area using six thermocouples connected in parallel with each other to average local variations. Surface thermocouples were coated with a thin layer of local dust so that they possessed radiative properties similar to the soil with which they were in contact.

Average surface and air temperatures in July in the central Sonoran Desert. Data are averages for 10–27 July 1996 for sand dunes in the San Cristobal Valley, Yuma County, Arizona. Data for 1.5 m and 3 cm above the soil surface were collected using thermocouples shielded from solar radiation and thermal radiation from the soil surface. Surface temperature was measured within a 1 m 2 area using six thermocouples connected in parallel with each other to average local variations. Surface thermocouples were coated with a thin layer of local dust so that they possessed radiative properties similar to the soil with which they were in contact.

Moreover, all of these temperatures represent unrealistically cool indexes of the thermal environments experienced by typical desert animals because they reflect measurements made within meteorological enclosures placed approximately 1.5 m above the ground. Most mammals weigh less than 50 g and live only a few centimeters above the ground, where the air is usually much hotter due to solar heating of the soil surface ( Cloudsley-Thompson 1965, 1991, Louw and Seely 1982, Nobel 1983). The effects of solar heating occur not only during daylight hours but also at night, when heat stored in desert soils substantially elevates air temperatures ( Figure 2). During July in the San Cristobal Valley, for example, the average nocturnal temperature experienced 3 cm above the ground is 38.8 °C—more than 2 °C higher than the temperature 1.5 m above the ground.

Finally, it is important to recognize that data such as those in Figure 2 represent mean values around which considerable variation occurs. Of course, organisms do not merely have to cope with average conditions they must also survive the extremes. The hotter portions of the Sonoran and Mojave Deserts normally experience several 7–10 day episodes each summer in which air temperatures remain substantially above average. Even at heights of approximately 1.5 m, air temperatures in such extreme periods reach 45–50 °C during the day and drop to only 28–32 °C at night. Therefore, even solely nocturnal animals may routinely experience high temperatures while above ground.

In addition to facing high temperatures on the soil surface at night, desert rodents also experience much warmer temperatures within their subterranean retreats than is generally appreciated. Data quantifying burrow temperatures at locations occupied by desert rodents are sparse, but measurements supporting the view that burrows provide cool retreats were supplied by Vorhies (1945), Schmidt-Nielsen and Schmidt-Nielsen (1950, 1951, 1952), Lee (1963), Hayward (1965), Kenagy (1973), and Kay and Whitford (1978). These studies, however, were conducted outside the hottest deserts ( Figure 3). In these more moderate regions, rodents such as kangaroo rats (Heteromyidae: Dipodomys spp.) place their nest chambers at depths of 30–60 cm below ground ( Vorhies 1945, Schmidt-Nielsen and Schmidt-Nielsen 1950). Burrow depths in the hottest desert regions apparently are similar. We have excavated six burrows of Merriam's kangaroo rat (Dipodomys merriami) in the San Cristobal Valley, in the core of the Sonoran Desert, and have found that nest chambers, which are the deepest portion of the burrow, are 40–60 cm below the surface (Randall L. Tracy and Glenn E. Walsberg, unpublished data). At these depths, soil temperatures are high (e.g., 38–41 °C Figures 4 and 5). Even if other species dig more deeply, burrows more than 2.5 m deep would be required to reach soil temperatures below 30 °C during summer months in the central Sonoran Desert. Given that burrowing is energetically demanding and time consuming (e.g., Vleck 1979), the necessity to dig very deep burrows could substantially alter the animal's time and energy budget as well as constrain its thermoregulatory options.

The hottest desert regions in the United States. Within the shaded area, maximum daily air temperature in July exceeds, on average, 38 °C. Data are from Sellers and Hill (1974) and NOAA (1985). Numbers identify locations of field studies of the subterranean environment of desert rodents: 1, Vorhies (1945), Schmidt-Nielsen et al. (1948), Schmidt-Nielson and Schmidt-Nielsen (1952) 2, Lee (1963) 3, Hayward (1965) 4, Kenagy (1973) 5, Kay and Whitford (1978) 6, this article ( Figure 2) 7, this article ( Figure 4).

The hottest desert regions in the United States. Within the shaded area, maximum daily air temperature in July exceeds, on average, 38 °C. Data are from Sellers and Hill (1974) and NOAA (1985). Numbers identify locations of field studies of the subterranean environment of desert rodents: 1, Vorhies (1945), Schmidt-Nielsen et al. (1948), Schmidt-Nielson and Schmidt-Nielsen (1952) 2, Lee (1963) 3, Hayward (1965) 4, Kenagy (1973) 5, Kay and Whitford (1978) 6, this article ( Figure 2) 7, this article ( Figure 4).

Daily maximum and minimum soil temperatures during summer in the Sonoran Desert. Data are average values collected during 10–27 July 1996 in the San Cristobal Valley, Yuma County, Arizona, using 1 mm diameter thermocouples placed at 10 cm intervals at depths of 0–1 m, and at 20 cm intervals at depths of 2–3 m.

Daily maximum and minimum soil temperatures during summer in the Sonoran Desert. Data are average values collected during 10–27 July 1996 in the San Cristobal Valley, Yuma County, Arizona, using 1 mm diameter thermocouples placed at 10 cm intervals at depths of 0–1 m, and at 20 cm intervals at depths of 2–3 m.

Annual cycle of average daily soil temperature in a Sonoran Desert site. Daily minima and maxima vary greatly from average values in the upper levels of the soil (e.g., Figure 4). Data were collected by Kenneth M. Wooden from creosote bush scrub 30 km north of Gila Bend, Maricopa County, Arizona. Data were collected at 0, 10, 20, 30, 40, 60, 80, 100, 125, 150, and 200 cm below the soil surface using thermocouples as described for Figure 4. Values were collected over 24 hours at 4-week intervals throughout 1998 and then interpolated to form best-fit isotherms.

Annual cycle of average daily soil temperature in a Sonoran Desert site. Daily minima and maxima vary greatly from average values in the upper levels of the soil (e.g., Figure 4). Data were collected by Kenneth M. Wooden from creosote bush scrub 30 km north of Gila Bend, Maricopa County, Arizona. Data were collected at 0, 10, 20, 30, 40, 60, 80, 100, 125, 150, and 200 cm below the soil surface using thermocouples as described for Figure 4. Values were collected over 24 hours at 4-week intervals throughout 1998 and then interpolated to form best-fit isotherms.

Humidities experienced by small desert mammals

The portrayal of desert animals as experiencing high humidity while in their burrows and low humidity above ground may not be correct. The expectation of low humidity above ground can be deceptive for two reasons. First, humidities are often reported as relative humidity, which is the fraction of water vapor contained in ambient air compared to the maximum that the air could hold at its prevailing temperature. This index therefore subsumes both air temperature and water content. However, saturation vapor density is an exponential function of air temperature, and high temperatures in themselves produce lowered values. For example, 26% relative humidity at 45 °C represents the same absolute humidity (in terms of g/m 3 of H2O in air) as 100% relative humidity at 20 °C. Absolute humidy is a better index for evaluating effects on organisms because it, not relative humidity, sets the gradient for evaporative cooling.

The generalization that atmospheric humidities are low in desert regions is deceptive for an additional reason: many subtropical deserts receive seasonal influxes of humid tropical air. Such influxes are a notable feature of the Chihuahuan Desert, the Sonoran Desert, and portions of the Sahara, Arabian, and Middle Eastern Deserts ( Figure 6). In these regions, absolute humidity levels can be quite high (e.g., 10–15 g/m 3 ), similar to those of deciduous forest regions notorious for humid summers ( Figure 6).

Average humidity values for six different desert and temperate sites, (a) Desert regions with persistent low humidity. (b) Desert regions that receive a seasonal influx of humid air masses, (c) Temperate regions with high humidity. Data were computed from those in Sellers and Hill (1974) for Phoenix, Arizona from those in NOAA (1985) for Winnemuca, Nevada Cincinnati, Ohio and Chicago, Illinois and from Pearce (1990) for Alice Springs, Australia, and Kuwait City, Kuwait.

Average humidity values for six different desert and temperate sites, (a) Desert regions with persistent low humidity. (b) Desert regions that receive a seasonal influx of humid air masses, (c) Temperate regions with high humidity. Data were computed from those in Sellers and Hill (1974) for Phoenix, Arizona from those in NOAA (1985) for Winnemuca, Nevada Cincinnati, Ohio and Chicago, Illinois and from Pearce (1990) for Alice Springs, Australia, and Kuwait City, Kuwait.

Another common view is that burrows occupied by desert mammals provide humid environments that significantly reduce evaporative water loss. However, this generalization is based on studies by Vorhies (1945) and Schmidt-Nielsen and Schmidt-Nielsen (1950, 1952) in the Santa Rita Experimental Range in southern Arizona, an area that is not truly arid. Annual rainfall in this area averages 50 cm, far more than the 8–25 cm that is typical of the Sonoran Desert ( Sellers and Hill 1974). In addition, the Schmidt-Nielsens' data were collected during a period (May–June 1948) in which soils were probably unusually moist 10 cm of rain fell in the 2 months preceding their field work ( Sellers and Hill 1974). Indeed, basic soil physics dictates that the 100% relative humidities reported for cavities in the ground can be produced only if the surrounding soil is saturated with water ( Jury et al. 1991), a highly atypical condition for deserts. Similarly, Vorhies (1945) measured humidity within kangaroo rat burrows during a 21-month period in 1930 and 1931, two extraordinarily wet years in which total precipitation was 71 cm and 97 cm, respectively ( Sellers and Hill 1974).

Several factors are likely to oppose high humidity levels within burrows. The most obvious is that desert soils are normally dry. The humidity within a soil-confined air space can be calculated from measurements of soil water potential. Examples of such data for the Sonoran Desert include those of Szarek and Woodhouse (1977) and Monson and Smith (1982). Except for periods immediately following rainfall, humidity in soil air spaces is calculated to be extremely low ( Figure 7). In addition, desert soils are normally dominated by mixtures of gravel, sand, and clay. The relative porosity of sandy soils ( Jury et al. 1991) allows vapor to diffuse from an occupied burrow, and the strongly hydroscopic nature of clay particles also tends to reduce humidity in soil-confined air spaces ( Jury et al. 1991). Finally, desert animals generally minimize evaporative water loss, which in turn minimizes animal-induced elevations of burrow humidity. For example, evaporative water loss for kangaroo rats typically totals only approximately 70 mg/hour ( MacMillen and Hinds 1983).

Humidity within soil air spaces in the Sonoran Desert. Values were calculated from the equation for the relation between soil water potential and relative humidity in Campbell (1977), using data describing soil water potential at either 30 cm (top Szarek and Woodhouse 1977) or 40 cm (bottom Monson and Smith 1982) below the soil surface.

Humidity within soil air spaces in the Sonoran Desert. Values were calculated from the equation for the relation between soil water potential and relative humidity in Campbell (1977), using data describing soil water potential at either 30 cm (top Szarek and Woodhouse 1977) or 40 cm (bottom Monson and Smith 1982) below the soil surface.

Predicting the overall consequences of these factors on burrow humidity is challenging because they depend on the geometry and ventilation of the burrow system, the rate at which the animal supplies water vapor to the air space, and a suite of soil properties affecting vapor diffusion. Although empirical measurements of the humidity of occupied burrows have yet to be completed, I offer two types of crude estimates. One is derived by calculating vapor efflux from a nest chamber using Campbell's (1977) equation for diffusion from a spherical surface and making five assumptions: the nest is in a 16 cm spherical cavity ( Vorhies 1945, Kenagy 1973) with a plugged entrance (if the entrance were open, vapor would be lost and humidity reduced) the animal releases water vapor at 70 mg/hour ( MacMillen and Hinds 1983) vapor leaves the chamber only by diffusion through the soil air space (i.e., air is not convected out of the chamber, nor do soil particles adsorb water molecules, both of which would decrease humidity) the diffusivity of water vapor through the soil air space is 30 mm 2 /s, a reasonable value for dry, sandy soils ( Jury et al. 1991) and the burrow is deep in the soil (more than 1 m being closer to the soil surface would facilitate vapor loss and decrease humidity). Such calculations indicate that, even within a completely sealed nest chamber, humidity would rise to no more than 4 g/m 3 over a 10-hour period ( Figure 8).

Estimated humidity within a burrow. The dashed line shows an estimate based on calculated rates of water vapor diffusion through the soil the solid line shows an estimate based on empirical measurements of humidity in soil cavities supplied with water vapor at rates approximating that released by a kangaroo rat. See text for details. Vertical bars represent 95% confidence intervals. The time scale indicates hours after the animal's entry into a sealed nest chamber.

Estimated humidity within a burrow. The dashed line shows an estimate based on calculated rates of water vapor diffusion through the soil the solid line shows an estimate based on empirical measurements of humidity in soil cavities supplied with water vapor at rates approximating that released by a kangaroo rat. See text for details. Vertical bars represent 95% confidence intervals. The time scale indicates hours after the animal's entry into a sealed nest chamber.

A second method of estimation is to supply water vapor to a nest-sized cavity within desert soil and to measure the resulting humidity. In June, I constructed six 16 cm wide cavities, 1 m deep and at least 30 m apart, in silty desert soil 45 km east of Gila Bend, Maricopa County, Arizona. The cavities had a metal ceiling to prevent collapse, and measurements were made in mid-June, 3 months after excavation, to allow the soil to settle. Water was added to the chamber at a rate expected for evaporative loss from a kangaroo rat by injecting liquid water at a rate of 0.07 ml/hr through 1 mm diameter polyethylene tubing extending to the chamber interior. The water dripped onto filter paper placed on the chamber floor and evaporated. Humidity within the chamber was measured using the methods of Walsberg and Wolf (1995). Similar to the results of the calculations above, these experiments suggest that water vapor accumulates slowly in an occupied burrow ( Figure 8). Even within a chamber sealed for 10 hours, vapor densities averaged only 3 g/m 3 (i.e., 7% relative humidity at 35 °C). It thus appears improbable that burrows in dry desert soils reach high humidities, although resolution of this issue will clearly require empirical measurement of humidity within occupied burrows.

In summary, although desert mammals are often portrayed as alternating between a cool, humid burrow during daylight hours and a warm, dry atmosphere on the desert soil surface at night, the reality may be very different, and much more challenging, for species occupying subtropical deserts. Rather than being cool, burrows may be quite hot. Temperatures below 30 °C may be available only if the animal is able to excavate to great depths. Humidities within burrows are probably low most of the time. On summer nights, both soil and air temperatures commonly remain high. Finally, heat stress on surface-active animals may be exacerbated by seasons of high humidity in some desert regions.


Natural history

Insectivores make up almost 10 percent of all mammal species, and most are the size of mice or small rats. The white-toothed pygmy shrew (Suncus etruscus), however, weighs less than 2.5 grams (0.09 ounce) and is perhaps the smallest living mammal. Other insectivores, such as the moonrat (Echinosorex gymnura) and the tailless tenrec (Tenrec ecaudatus), attain the size of a small rabbit. Most insectivores are either ground dwellers or burrowers, but several are amphibious, and a few have adapted to life in the trees or forest understory. They prey almost entirely on invertebrates and small vertebrates. The olfactory lobes of the brain are highly developed, which indicates an acute sense of smell. The cerebral hemispheres, however, are small compared with those of most other placental mammals, which reflects less-developed intelligence and manipulative skills. Most have a long, flexible snout (proboscis) adorned with sensory whiskers (vibrissae) that is used to probe leaf litter, soil, mud, or water and locate prey by touch and smell. Prey may be pinned by the front feet, but it is typically grasped by the teeth and manipulated solely by mouth and proboscis until swallowed. Vision is poor eyes are small, degenerate, or covered with skin in solenodons, shrews, moles, and golden moles. Although the eyes are larger in hedgehogs, the moonrat, gymnures, and tenrecs, they are still smaller than in other orders of living mammals. Hearing is acute. Insectivores vocalize by hisses and snarls or with a range of other sounds, including ultrasonics some use specialized spines to produce sounds, and a few can echolocate.


Hibernation

Food can be scarce in winter, so micro-bats survive by either migrating to warmer regions or they hibernate. Hibernation is a state of torpor (inactivity). Micro-bats have the ability to slow their heart rate and lower their body temperature, sometimes to as low as 2°C.

Because micro-bats are so small, they cannot hibernate for very long. They can lose from a quarter to half of their body weight in a few months. To help them hibernate for up to six months they build up their body fat in summer and will roost, often in groups to increase combined body warmth, in caves, buildings, old mines or hollow trees.

Bats don’t sleep entirely through winter as they may wake up to go and feed, drink, find new roosts or in some species, mate. The energy they expend waking up mid-hibernation can reduces their chance of lasting a cold long winter. This is more of a problem where the climate drops below 0°C on a regular basis.


Acknowledgments

We would like to acknowledge Dr. Gene Fridman in the Johns Hopkins School of Medicine for lending the use of the Fridman lab’s Metrohm Autolab potentiostat for use in the cyclic voltammetry experiments. We would like to thank the Johns Hopkins Malaria Research Institute and the Department of Molecular Microbiology and Immunology Insectary and Parasite Core facility, and Dr. Joel Tang from the Johns Hopkins NMR Core facility for his help with the NMR experiments. We would also like to thank the entire Casadevall Lab for their suggestions and inputs during conversations and their feedback during lab meetings and presentations. Figures in S9 Fig were made using BioRender.



Comments:

  1. Vudozil

    cool padborka

  2. Timmy

    I apologize for interfering ... I was here recently. But this topic is very close to me. Write in PM.

  3. Dasida

    I'm sorry, but, in my opinion, they were wrong. I am able to prove it.

  4. JoJogore

    I think you will allow the mistake. Enter we'll discuss it. Write to me in PM.



Write a message