Category: Adaptations

How the tapir got his spots

August 4, 2009 • 7:11 am

by Greg Mayer

A while back Jerry posted a video of lion cubs at the Tulsa Zoo, and noted that they have spots, remarking

Many species of cats show this pattern in the cubs, even if the pattern disappears with growth.  It almost certainly reflects (as discussed in WEIT), an atavistic trait: the persistence in a descendant of traits that were adaptive only in an ancestor.  I suspect that the ancestor of lions had spots as adults, and that’s why they show up, briefly, in lion cubs.

I posted a comment to the effect that Hugh Cott, the great student of adaptive coloration, agreed with Jerry, although I wasn’t so sure:

Hugh Cott, in his classic “Adaptive Coloration in Animals” (Methuen, 1940) agreed with Jerry on this: “Among mammals and birds, first liveries acquired by the young– whether this happens before or after birth– often differ widely from the full dress of their parents. But it must not be assumed that such differences are necessarily adaptive. Lion cubs have spotted coats, and their tails are ringed…[Cott gives some more cat examples]… Since the kittens of all these animals…are born in sheltered dens or holes, carefully hidden or guarded by the mother, the spotted pattern can hardly be explained as protective.”(p. 21). I’m not so sure, though. Lions are not sheltered in dens or burrows, but rather are kept in thickets and kopjes, and may be on their own for a day at a time (George Schaller, “The Serengeti Lion” [Chicago, 1972], so the spots might be protective coloration for keeping the young hidden before they become formidable individually. (Protective coloration in the young is well known in mammals– whitetailed and mule deer, and tapirs, being good examples: their young bear dots and vermiculations that blend with sun- or moon-dappled forest floors.)

Since then, Jerry and I have conducted an off-blog discussion on this, and he has particularly challenged me with regard to tapirs.  While tapirs (and lions!) present many interesting aspects of natural history, the general question is one one of fundamental conceptual importance for evolutionary biology: how do you tell if a feature of an organism is an adaptation? So I’m going to pursue this question over a few posts.  To set the scene, let’s introduce tapirs. The best web source of info on them is the IUCN‘s Tapir Specialist Group.

Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)
Baird's Tapir at Franklin Park Zoo, Boston (from Wikipedia)

Tapirs, along with horses and rhinoceroses, are odd-toed ungulates, members of the mammalian order Perissodactyla, which is the less species-rich of the two great extant orders of hoofed mammals. (Most hoofed mammals, such as deer, antelope, cattle, sheep, pigs, etc., are even-toed, members of the  Artiodactyla.) There are four species, all of which have short trunks.  Three are in the Neotropics (Tapirus bairdii, T. pinchaque, and T. terrestris), found from southern Mexico to northern Argentina. As adults, they are all more or less uniformly colored, brown to gray to black. The Malay tapir (T. indicus) of southeast Asia, however, is strikingly particolored.

MalayTapir_Wikipedia
Malay Tapir at Regents Park Zoo, London (from Wikipedia)

I’ll show some baby tapirs in the next post.

New stuff in Science

July 18, 2009 • 12:22 pm

Science isn’t really known for publishing a lot of work on organismal or evolutionary biology (Nature is much better at that), but there are two nice papers in the latest issue:

“Tiger Moth Jams Bat Sonar”. This is the punchiest title of a scientific paper I’ve seen in years: not a word longer than five letters.

The tiger moth (Bertholdia trigona) makes a clicking sound when it’s being chased by bats.  There are three possible reasons for this.  First, it could be an “aposematic” call, warning the bats that the moth is distasteful, so that they’ll avoid it after one or more noxious encounters (this would be the equivalent of a rattlesnake’s rattle, or the bright warning coloration of the distasteful ladybug).  This doesn’t seem likely because tiger moths seem to be palatable to bats. Second, the clicks could startle a naive bat and cause it to miss.  Finally, the click could be a “jamming” call, messing up the bat’s sonar so that it can’t locate the moth.  These three hypotheses (which of course are not mutually exclusive) make different predictions about how a bat will behave upon initial and subsequent encounters with a moth. Under the “startle” hypothesis, for example, bats should initially miss the moths but then start catching them as they grow accustomed to the clicks.

The authors  evaluated these hypotheses by presenting tiger moths to naive big brown bats (Eptesicus fuscus). They found that the bat’s behavior best conforms to #3: bats can’t seem to find the moths from the very outset. And the sonic profile of the moth calls is also that expected if they have a “jamming” function.

Bats evolved sonar; tiger moths evolved a jamming call. This is an example of an “arms race”: the perpetual evolution and co-evolution of predators and prey.  According to Richard Dawkins and others, such arms races were a major evolutionary cause of increased biological complexity.

Lizard swims through sand (also reported in The New York Times). The desert-dwelling sandfish lizard (Scincus scincus, found in Africa and the Middle East) not only walks on the sand, but swims through it at speeds of up to 6 inches per second. The ability to dive into the sand and “swim” is useful for escaping the desert heat and evading predators.

Using X-ray cinematography, scientists found that, when swimming undersand, the sandfish retracts its limbs, pressing them against its body, and undulates like an eel, “swimming” through the sand.  This is an efficient way of moving because sand has both liquid- and solid-like properties.

From the “Supplementary material,” three movies of the sandfish:

Burying itself in the sand

Swimming in the sand (an X-ray video, really cool)

Swimming in the sand with opaque markers on the body (shows that its limbs are pressed against the body during the undulatory swimming). The “I-L distance” is the inter-limb distance measured between markers on the limbs; it’s an index of whether the legs are splayed or appressed to the body.

_________________

Tiger Moth Jams Bat Sonar. Aaron J. Corcoran, Jesse R. Barber, and William E. Conner. Science 325:325-327.

Undulatory Swimming in Sand: Subsurface Locomotion of the Sandfish Lizard. Ryan D. Maladen, Yang Ding, Chen Li, Daniel I. Goldman. Science 325:3124-318.

New  stuff

Are Komodo dragons venomous?

May 23, 2009 • 11:23 pm

by Greg Mayer

Komodo dragons (Varanus komodoensis) are the world’s largest living lizards, reaching 10 feet in length, and are restricted to the islands of Komodo, Rintja, Padar, and Flores in Indonesia. In a soon to be published paper in the Proceedings of the National Academy of Sciences (abstract), Brian Fry of the University of Melbourne and a cast of thousands ( well not quite– there are only 27 coauthors) report evidence that dragons are venomous. This is not as shocking as it sounds. First, the only previously known venomous lizards, the Gila monster (Heloderma suspectum) and Mexican beaded lizard (Heloderma horridum) are members of the family Helodermatidae, which is part of the superfamily Varanoidea, which, as you can tell by the name also includes the Varanidae, to which the Komodo dragon belongs. Fry and colleagues have argued before that venom is more widespread in lizards (and snakes, too) than traditionally thought. Second, dragon bites have long been known to be nasty things, with death often ensuing, but this was thought to be due to sepsis.

Carl Zimmer has an interesting piece on this in the NY Times, in which he notes that Fry et al. have not convinced everyone about dragons being venomous. He talked to my old friend and colleague Kurt Schwenk (and check out UConn’s E&E dept.’s great website) of the University of Connecticut. Kurt was unimpressed by the evidence presented (I can only get the abstract yet, so can’t really comment myself), and had a wonderful way of putting the argument that venom is not necessary to explain why dragon prey die after being bitten:

I guarantee that if you had a 10-foot lizard jump out of the bushes and rip your guts out, you’d be somewhat still and quiet for a bit, at least until you keeled over from shock and blood loss owing to the fact that your intestines were spread out on the ground in front of you.

For further info on dragons, the late Walter Auffenberg‘s classic monograph on dragons, The Behavioral Ecology of the Komodo Monitor (University Press of Florida, Gainesville, 1981) is a must read.

How the giraffe got its long neck II

May 21, 2009 • 11:58 pm

by Greg Mayer

In a previous post on how giraffes got their long necks, I noted that this was a venerable question in biology. The contrast between a Lamarckian and Darwinian explanation of how giraffes got their long necks has long been a popular example in introductory textbooks. My high school textbook (Biological Sciences Curriculum Study ‘Yellow’ edition, 1968) had it, and W.T. Keeton, in the college text Biological Science (2nd ed., 1972) used it, though calling it “now rather hackneyed”. I did a little checking and found some interesting historical tidbits.

I did not consult Lamarck, but found that Wallace, not Darwin first broached the issue from the natural selection side, in his part of the famed Linnean Society paper of 1858 (all the Darwin/Wallace materials quoted here were quickly located using the marvelous The Complete Works of Charles Darwin Online). Notice that in the following extract Wallace argues that it is a process of selection among variants, with access to food being the selective factor, that drives the evolution of the neck.

The hypothesis of Lamarck—that progressive changes in species have been produced by the attempts of animals to increase the development of their own organs, and thus modify their structure and habits….  Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them.

A year later, in the first edition of the Origin, Darwin refers not to giraffe’s necks, but to their tails and their neck vertebrae.

The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as driving away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. (p. 195)

And later:

The framework of bones being the same in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse,—the same number of vertebræ forming the neck of the giraffe and of the elephant,—and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. (p. 479)

The giraffe’s tail is used to illustrate adaptation, or suitability to the conditions of existence, which comes about through the modifying effects of natural selection; the giraffe’s neck, with its seemingly incongruously low number of vertebrae, illustrates the unity of type (the same structures or organs in organisms of varying conditions of existence) which reveals common ancestry.  Thus the giraffe is used to illustrate the two great unexplained phenomena faced by biology prior to Darwin– adaptation and unity of type– and Darwin’s unified explanation for thes phenomena: descent with modification by means of natural selection.  These examples continue throughout all editions of the Origin. But there is no mention of the long neck as a feeding adaptation.

In the third edition of the Origin (1861), in the Historical Sketch, Darwin first mentions the neck as an adaptation for feeding.  He is here explicating Lamarck, correctly noting Lamarck’s two evolutionary mechanisms, the effects of use and disuse (the inheritance of acquired characters, which Darwin, and essentially all his contemporaries, accepted; the later naming of this as ‘Lamarckian’ inheritance is unhistorical), and the innate drive for progressive development.

With respect to the means of modification, he attributed something to the direct action of the physical conditions of life, something to the crossing of already existing forms, and much to use and disuse, that is, to the effects of habit. To this latter agency he seems to attribute all the beautiful adaptations in nature;—such as the long neck of the giraffe for browsing on the branches of trees. But he likewise believed in a law of progressive development; and as all the forms of life thus tended to progress, in order to account for the existence at the present day of very simple productions, he maintained that such forms were now spontaneously generated. (p. xiii)

It is not until the sixth edition of the Origin (1872) that Darwin gives extended consideration to the long neck as an adaptation for feeding, but in response to criticisms by Mivart, not to make a contrast with Lamarck. The problems addressed are how incipient stages of adaptive structures can be selected for, and why, if a trait is adaptive, why don’t all species (or at least all similar species) evolve the trait. Here’s the full discussion (it’s a bit long for a blog, but what the heck).

All Mr. Mivart’s objections will be, or have been, considered in the present volume. The one new point which appears to have struck many readers is, “that natural selection is incompetent to account for the incipient stages of useful structures.” This subject is intimately connected with that of the gradation of characters, often accompanied by a change of function,—for instance, the conversion of a swim-bladder into lungs,—points which were discussed in the last chapter under two headings. Nevertheless, I will here consider in some detail several of the cases advanced by Mr. Mivart, selecting those which are the most illustrative, as want of space prevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore-legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths. The Niata cattle in S. America show us how small a difference in structure may make, during such periods, a great difference in preserving an animal’s life. These cattle can browse as well as others on grass, but from the projection of the lower jaw they cannot, during the often recurrent droughts, browse on the twigs of trees, reeds, &c., to which food the common cattle and horses are then driven; so that at these times the Niatas perish, if not fed by their owners. Before coming to Mr. Mivart’s objections, it may be well to explain once again how natural selection will act in all ordinary cases. Man has modified some of his animals, without necessarily having attended to special points of structure, by simply preserving and breeding from the fleetest individuals, as with the race-horse and greyhound, or as with the game-cock, by breeding from the victorious birds. So under nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food. That the individuals of the same species often differ slightly in the relative lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; whilst the individuals, less favoured in the same respects, will have been the most liable to perish.

We here see that there is no need to separate single pairs, as man does, when he methodically improves a breed: natural selection will preserve and thus separate all the superior individuals, allowing them freely to intercross, and will destroy all the inferior individuals. By this process long-continued, which exactly corresponds with what I have called unconscious selection by man, combined no doubt in a most important manner with the inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is that the increased size of the body would obviously require an increased supply of food, and he considers it as “very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages.” But as the giraffe does actually exist in large numbers in S. Africa, and as some of the largest antelopes in the world, taller than an ox, abound there, why should we doubt that, as far as size is concerned, intermediate gradations could formerly have existed there, subjected as now to severe dearths. Assuredly the being able to reach, at each stage of increased size, to a supply of food, left untouched by the other hoofed quadrupeds of the country, would have been of some advantage to the nascent giraffe. Nor must we overlook the fact, that increased bulk would act as a protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck,—and the taller the better,—would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. It is from this cause, as Sir S. Baker remarks, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a means of offence or defence, by violently swinging its head armed with stump-like horns. The preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural selection be so potent, and if high browsing be so great an advantage, why has not any other hoofed quadruped acquired a long neck and lofty stature, besides the giraffe, and, in a lesser degree, the camel, guanaco, and macrauchenia? Or, again, why has not anymember of the group acquired a long proboscis? With respect to S. Africa, which was formerly inhabited by numerous herds of the giraffe, the answer is not difficult, and can best be given by an illustration. In every meadow in England in which trees grow, we see the lower branches trimmed or planed to an exact level by the browsing of the horses or cattle; and what advantage would it be, for instance, to sheep, if kept there, to acquire slightly longer necks? In every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have its neck elongated for this purpose, through natural selection and the effects of increased use. In S. Africa the competition for browsing on the higher branches of the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate animals.

Why, in other quarters of the world, various animals belonging to this same order have not acquired either an elongated neck or a proboscis, cannot be distinctly answered; but it is as unreasonable to expect a distinct answer to such a question, as why some event in the history of mankind did not occur in one country, whilst it did in another. We are ignorant with respect to the conditions which determine the numbers and range of each species; and we cannot even conjecture what changes of structure would be favourable to its increase in some new country. We can, however, see in a general manner that various causes might have interfered with the development of a long neck or proboscis. To reach the foliage at a considerable height (without climbing, for which hoofed animals are singularly ill-constructed) implies greatly increased bulk of body; and we know that some areas support singularly few large quadrupeds, for instance S. America, though it is so luxuriant; whilst S. Africa abounds with them to an unparalleled degree. Why this should be so, we do not know; nor why the later tertiary periods should have been much more favourable for their existence than the present time. Whatever the causes may have been, we can see that certain districts and times would have been much more favourable than others for the development of so large a quadruped as the giraffe. (pp. 177-179)

So, it is Wallace rather than Darwin, who has most clearly contrasted the variational mechanism of natural selection (some individuals reproduce more than others, changing the average composition of the next generation) with Lamarck’s transformational mechanism (each individual changes).  And one last point: an alert reader recommends the following as a current argument for the hypothesis of long necks as a feeding adaptation: CAMERON, E. & TOIT, J. T. du. 2007. Winning by a neck: tall giraffes avoid competing with shorter browsers. American Naturalist 169: 130-5 (abstract).

How the giraffe got its long neck

May 17, 2009 • 9:32 pm

by Greg Mayer

Matthew Cobb has kindly called my attention to this piece from the BBC about this soon to be published paper (pdf) in the Journal of Zoology by Graham Mitchell, S.J. van Sittert, and John Skinner on the neck of the giraffe. How giraffes got their long necks is a venerable question in biology, having been discussed by, among others, Lamarck , Wallace, and Darwin. The paper by Mitchell and colleagues deals with an hypothesis proposed in 1996 by Robert E. Simmons and Lue Scheepers: that the long neck of giraffes results from sexual selection by male-male competition (see chap. 6 of WEIT for a general discussion of sexual selection).  At first it may seem obvious that long necks are for reaching leaves and shoots way up in the trees, and, indeed, this is the most popular idea.  But before dismissing sexual selection, consider the behavior called ‘necking’, displayed by two male giraffes in the following clip.

It is evident that serious, and violent, combat occurs between males, and Simmons and Scheepers record several accounts of serious injury and death resulting from necking. Mitchell and colleagues, nonetheless, conclude that sexual selection is not the cause of long necks:

Support for this theory [the sexual selection hypothesis] would be that males invest more in neck and head growth than do females. We have investigated this hypothesis in 17 male and 21 female giraffes with body masses ranging from juvenile to mature animals, by measuring head mass, neck mass, neck and leg length and the neck length to leg length ratio. We found no significant differences in any of these dimensions between males and females of the same mass, although mature males, whose body mass is significantly (50%) greater than that of mature females, do have significantly heavier (but not longer) necks and heavier heads than mature females. We conclude that morphological differences between males and females are minimal, that differences that do exist can be accounted for by the larger final mass of males and that sexual selection is not the origin of a long neck in giraffes.

Mitchell et al. also dispute the importance of predation on giraffes, which Simmons and Scheepers thought was higher on males as a consequence of the physiological and anatomical costs of such an outsized structure.  Many people might think, What could catch and kill a giraffe? The answer: lions. In the following clip, an adult giraffe is killed by a group of lions.

Happy Easter — Aussie style

April 15, 2009 • 12:57 pm

I’m back, with lots to say, but lots of catching up to do on the day job. Let me first thank Matthew Cobb for a terrific job of filling in. His students get the benefit of his omnivorous readings in the form of a Z (zoology)-letter he sends out weekly, detailing all sorts of interesting animal stuff.

For today, until I shovel myself out from under, I post something for a belated Happy Easter. In WEIT I describe the convergences between marsupial and placental mammals, resemblances that imply that some niches antedate the animals who have evolved to fill them. Although the Australian bilby looks like a rabbit, it isn’t really herbivorous but omnivorous, although it does burrow. There used to be two species, the greater and the lesser bilby (the word “bilby” is aboriginal), but the lesser appears to be extinct. The greater bilby, Macrotis lagotis, is highly endangered due to habitat loss and predation by, among other species, feral cats; you can read about its precarious status here. Only a few hundred remain in the wild. To save the animal, extensive efforts are underway; these include widespread annual sale of chocolate Easter bilbies, which provide revenues for conservation. (In WEIT I mistakenly say “Each spring, chocolate bilbies fill the shelves of Australian supermakets. . .”, and was roundly taken to task by Aussies who pointed out, rightly, that the Australian Easter occurs in the fall.)

So, belatedly, here are some baby bilbies from down under, and the chocolate replicas that are helping save them:

Chocolate bilbies (buy them here):

easter_bilby_dl_3

NB: Goofed again. I am informed that in Australia the penultimate season is called “autumn,” not fall.

Happy Birthday, Lungless Frog!

April 9, 2009 • 2:47 am

by Matthew Cobb

A year ago, a brief paper appeared in Current Biology, written by researchers from the National University of Singapore (NUS) and from Indonesia, describing the Bornean flat-headed frog, Barbourula kalimantanensis. The frog was first described in 1978, and on the International Union for the Conservation of Nature (IUCN) Red List it is described as “endangered”.

2397503833_778e831ff3jpg

This frog, of which only a few specimens are known, is noteworthy because it has no lungs, but instead breathes through its skin. This is a trick ordinary frogs can do, too – in nearctic regions frogs may spend the winter hunkered down in the bottom of ponds, slowly respiring through their skin.

But losing your lungs is an amazingly rare event. Among all the thousands of tetrapod species – mammals, birds, reptiles, amphibians – lunglessness has evolved only in amphibians. Indeed, it has only been observed three times previously – in two families of salamander and a single species of caecilian. (Caecilians are legless amphibians that are not snakes; most of them live underground.)

Why has this species of frog lost its lungs? The authors of the paper – David Bickford, Djoko Iskandar (who first described the frog, but did not realize its lunglessness) and Anggraini Barlian – point to how the frog manages without lungs (it lives in oxygen-rich fast-flowing water, presumably has a low metabolic rate (the water is very cold), its flat shape increases the surface area for oxygen absorption), but beyond the only positive reason they can come up with is that being lungless would decrease the frog’s buoyancy . Furthermore, the closely related species Barbourula busuangensis has lungs, just like all other known frogs.

What progress has been made over the last year? Had to say – a quick scouring of the scientific literature brings up no more publications, and there’s no indication on David Bickford’s NUS website that there’s been any further discoveries. The evolution of lunglessness in this odd frog may end up in the list of adaptations we don’t fully understand, such as the zebra’s stripes (discussed here) or the stegosaur’s plates (we’ll deal with that one tomorrow, so don’t chip in until you’ve read the blog!).

In a blog discussion last year over at nusbiodiversity, one of the readers made an astute comment. “JD” wrote “Why would a lungless frog have nostrils? Where do they lead to?” Presumably it retains the nostrils for its sense of smell – which will also work under the water. Ordinary frogs breathe by swallowing (they have no diaphragm), so my bet would be that this lungless variant still gulps to get air flowing over its olfactory receptors.

The photo is taken from the Current Biology article, which can be found here.