poisonous bugs

Cecil's Columns/Staff Reports
21

Good job, gonz. I certainly do admire someone who is willing to go out and do a little bit of research rather than just relying on some half-remembered factoid from a nature show. (I would like to reiterate that the ingenuous/ingenious comparison was not directed at you personally, but at the theory. And it was Kingdon’s word, not mine.)

The reason you have not found more on the group cohesion hypothesis is that you are not looking in the right place. Kingdon is the author of the monumental multi-volume work, East African Mammals: An Atlas of Evolution in Africa. If you have never seen this work you should try to get hold of it. Profusely illustrated, it is a true work of art. (By the way, the social cohesion hypothesis is the one accepted as most valid by Walker’s Mammals of the World, the most authoritive reference on mammals in general; they do not even bother to discuss alternatives, I assume because they consider Kingdon’s explanation for stripes to be the only one that is well supported by available evidence.)

In Vol. III, Part B (Large Mammals) of the above work, (pp 131-140) Kingdon provides a very detailed discussion of the social cohesion hypothesis, including research on sociality on equids in general and his own research on the subject. I would say you should read this before coming to any conclusion on the matter. I cannot even begin to summarize it here, as it is quite extensive.

Re your remark about “Zebra herds are essentially the same as all other Equidae,” one of Kingdon’s points is they are not. Stripes are primarily favored in rich tropical habitats, where dense populations may occur, and of less utility in areas with low density populations such as the deserts and temperate grasslands where ancestral horses and wild asses are/were found. The Quagga, a recently-extinct form of zebra formerly found in temperate South Africa, had reduced striping confined to the head and neck, tending to support Kingdon’s explanation.

Re the two articles you cite on hunting behavior, I do not see how they address the specific subject of stripes and their possible benefit.

Re the article on tsetse fly behavior, while this effect may well occur, it would be a great stretch to say this is the ultimate cause of the striping pattern, although it could provide some additional benefit in limited circumstances. Once again, if this is such a great benefit, why don’t any other African grazers show such a pattern? And as Kingdon says, “insects are no hazard over most of the zebras’ very wide range of habitats;” tsetse flies are confined to a relatively small part of the range of the 3 species of zebra.

Yes, such hypotheses are almost impossible to test; but after reading Kingdon’s detailed analysis on the question, I find him by far the most convincing and the only one who tries to address the evidence in its totality.

22

I would just like to point out that your argument against stripes as ‘fly camouflage’ i.e why haven’t other animals adopted it…is also able to be turned to Kingdon’s recognition theory. Other exist in MUCH larger numbers and manage similar social dynamics without such striking markings. It may be that unlike other ungulates stripes developed early in Equidae and were maintained because they fulfilled other needs.

I also agree that the predator behavior is ambiguous, but it is curious that only zebra are consistently hunted by GROUPS of their top predators. In the previous articles is evidence of the same predators often hunting larger and ‘more dangerous’ prey items solo.

I would also hasten to point out that the Kingdon’s theories about flies were published over fifteen years ago, an eternity in scientific literature. Although Gibson focused on tse flies in general, conclusions are able to be directly drawn to other biting flies. This feeds directly into a report by Carl Zimmer in his book Parasite Rex (currently available): scientists have recently begun to rethink the ecology of large grazers in africa. Although large predators have always been thought to be the main regulator of grazer populations, recent evidence suggests that rinderpest (a vicious parasitic disease borne by various biting flies throughout africa) has had the primary influence browsers and grazers as evidenced by the current epidemic. This epidemic was spawned within the past several decades by the dramatic increase in the less-well adapted domestic cattle population.

Of course you realize that the ‘why didn’t other species do it’ argument we have been throwing back and forth it moot, evolutionarily speaking. Each species evolved a strategy to deal with current situations using tools at hand. Primitive Equidae may have had stripes as camouflage- these were maintained as they changed environments because they performed other functions as well. Other species sans stripes developed other means of individual recognition and parasite resistance. The world may never know.

I never thought I would get so much guff from a hummingbird…

23

Um, on the bees, bumblebees are black and yellow striped, that’s probably why all bees are drawn that way - bumblebees also have non-barbed stingers, so they can sting repeatedly, though they are pretty non-aggressive (I have been stung by honeybees, never by bumblebees).

24

Sigh. Gonz, before you go gassing on, just read Kingdon’s article. :rolleyes:

I do not have the time to go into every one of his arguments in detail. Kingdon addresses these questions quite thouroughly. He draws on certain behaviors of equids, notably specific grooming patterns, that do not occur in other grazers to account for these differences. Once you have actually read the article, come back and we can debate it. Otherwise this is pointless.

Hey, if you think this is guff you ain’t seen nothing yet. I am trying to be excruciatingly polite here. If you like we can take this to the BBQ pit. :wink:

25

No need to get testy…sounds like someone needs a nap.

Colibri…I had already read Kingdon’s article but found nothing with which to argue. As your posts declare, Zebra’s do currently use stripes for social cohesion. The point I was trying to make was that several possible factors could have acted synergistically to develop the modern zebra; I was also trying to bring in other ‘expert’ opinions.

With that in mind

THe original ancestor of all Equus species was Hyracotherium-- a small solitary forest browser. Aha! perhaps the original source for stipes, but wait it get better. As its descendendts moved to the plains it became bigger and more horse like…blah, blah.

Anyway, Kingdon notes in several places that the proto-zebra probably had localized striped markings. These commonly arise in many animals around specific targets for grooming/fighting: mainly the neck and legs. He theorizes that the patterns were selected b/c more active grooming helped the zebra overcome the natural intolerance present in most Equidae- essential for modern zebra society. Then he presents a theory as to why the striped pattern began to enlarge away from the target areas. Even he states that this argument is speculative: the increased striping could lead to better appeasement/increased social interaction or vice versa. (Sort of like the bigger brain/hand use argument in homonids). But essentailly it is unclear why the pattern became so enlarged.

Enter other benifits from stipes- predator and fly protection. These could have helped push the zebra coats into the modern zebra must-have. I will state all of the evidence that I reported was presented at least a decade after the last copyrighted zebra monograph from Kingdon I could find. We may never know what he thought about these new findings. However it seems clear that proto-zebra were stripped and that several possible factors encouraged the retention and emphasis of the trait.

As for testing and innateness, I’ll quote Kingdon,“There is no way of knowing how barring affects zebras, but” …gives examples of how it affects humans. And, “Because the zebra foal begins grooming at a very early age and tends to reduce this activity as it grows up, it is possible that the stripes exert an influence on the zebra’s behaviour through a process of conditioning, in which case the response to the stripes would not be innate.”

See, Colibri, we can share.

26

Following Dex’s comments, Colibri wrote:

“One critical aspect that the two previous posts overlook is that many animals, particularly insects, pick up the toxic principle from their food. Plants (evolutionarily) develop toxic “secondary compounds” in order to prevent insects and other herbivores from eating them. Some insects eventually (through selection) develop the ability to deal with these toxins and happily chow down on plants that are deadly poisonous to all other animals. As an added benefit, they may sequester the toxins in their tissues and so become toxic to their own predators. Under these conditions, development of aposematic coloration is a breeze.”

This is considered to be the simplest and most general explanation: toxicity evolves first, then comes warning coloration to reinforce it. I’ve never heard of anything with bright, flashy coloration IN BOTH SEXES that is not toxic/dangerous or a mimic and is commonly preyed upon by something that hunts by sight. Evidently, it’s not a good evolutionary strategy to evolve bright colors unless it’s either a warning, a good bluff, or if it’s only in males.

As for bee coloration, there are many experiments that show animals instinctively avoid yellow-black patterns, and also learn to avoid them (when associated with negative stimuli) faster than other color combos. There’s definitely an innate component there. Myself, I find it remarkable that the vast majority of dangerous bee, wasp, and ant species are NOT aposematically colored, especially honeybees. In fact, aside from yellowjackets, anything with colony sizes in the thousands or higher is NOT aposematically colored; this suggests that since only sterile workers are being eaten by predators, the selective survival advantage is not conferred at the level of the colony, which is definitely contrary to group selection theory.