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Re: Built Like a Race Horse, Slow as an Elephant?

Whether or not herd animals are "competing" with each other or are hunted individually, the arms race must be intraspecies. What is at issue is the average fitness of fleet versus nonfleet herd animals relative to each other, in both cases. The same will be true of the predators - slower (or less cooperative, and so forth for whatever adaptation makes them successful hunters) predators will be selected out in virtue of not being fit enough to capture food. The competition here is between variants in each species, although the actual chase is between members of different species. Selection drives to fixation or equilibrium variants in a population. If the prey were not social, the competition between them would still occur. It's the frequency of that "allele" or variant in the local population that changes due to predation. Arms races are between two interacting species. Competition is between two members of the same population of a species.

On 17/01/2007, at 8:49 AM, Sim Koning wrote:


I understand what you are saying Dann, but I think you are both partly correct. Just because an animal is in a herd, does not mean it won’t occasionally find itself alone or singled out; I think in that case, it would be an arms race, because it is not trying to outrun nearby members of its herd, but the predator itself. For example, cheetahs usually single out a gazelle before it actually starts chasing it, and if you notice, most of the other gazelles that are nearby aren’t even running that fast because once the cheetahs begins chasing its prey, it won’t have enough energy to switch targets and begin a chase with a slower gazelle that it sees out of the corner of its eye. So in the case of cheetahs, it is very much an arms race.

Now lions are different, lions will scatter a herd and will rapidly switch targets, eventually singling out the slowest animal. So, like you said, Zebra are competing with each other, not the lion. Cape hunting dogs and hyenas will also switch targets and single out the slowest animal as well.

There are some predators that are actually slower than their prey, but they still catch them because they have greater endurance, cape hunting dogs are an example of this. In fact I believe sub adult springbok only pronk in front of cape hunting dogs, though this may just be a sign of excitement rather than a demonstration of their fitness.

I’m sorry if some of you already brought up these points, I haven’t really been keeping track of this discussion until now.

Simeon Koning



From: Dann Pigdon <dannj@alphalink.com.au>
Reply-To: dannj@alphalink.com.au
To: DML <dinosaur@usc.edu>
Subject: Re: Built Like a Race Horse, Slow as an Elephant?
Date: Wed, 17 Jan 2007 09:05:40 +1100

Andrew Simpson writes:

--- Dann Pigdon <dannj@alphalink.com.au> wrote:
Actually, it was almost certainly a
pronghorn/pronghorn arms race. There would always have been far more pronghorns than
cheetahs, so cheetah predation on a population of slow coaches would
still not have been enough to threaten their long-term survival.

I doubt pronghorns would have evolved to run as fast
as a cheetah (or as close as their biology would have allowed) just to
avoid the occasional bit of cheetah predation. I think it's more likely that
pronghorns evolved to run fast to compete with each other.

It's like the old saying goes: if you and a friend
are chased by a tiger, you don't have to outrun the tiger. You just have to
outrun your friend... 

I'm not sure I'm following you Dann. How are the
Pronghorns evolving speed? Are you saying that the
North American Cheetah are or are not responsible?
Because the Proghorns can't make themselves faster and
have no reason too unless something is chasing them or
if there is an advantage to getting somewhere quicker.
(A quickly dwindling food source perhaps).

Cheetahs (and other less speedy predators) are the indirect cause, however it was almost certainly not an 'arms race' between predator and prey. Where herd animals are concerned, they only have to out run the slowest members of the herd. Hence the competition for speediness is between animals of their own species, not directly between predator and prey.

The evolution of horns seems to have followed a similar pathway. Antelopes use their horns against each other (or as sexual displays) far more often than they do defending themselves against predators with them. Therefore the development of steadily bigger or sharper horns may not have been an 'arms race' against predators, but rather due to intraspecies interactions.

The only antelope predatory defense that I can think of that seems a direct response to predation is the practice of pronking. Rather than demonstrate speed and strength to a predator directly by evading them (a huge waste of energy), pronking allows an antelope to demonstrate it's fittness to a predator without excessive energy loss. When an antelope pronks, it is communicating directly to the predator, rather than competing with herd members.

___________________________________________________________________

Dann Pigdon
GIS / Archaeologist         http://www.geocities.com/dannsdinosaurs
Melbourne, Australia        http://heretichides.soffiles.com
___________________________________________________________________

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--
John S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project
University of Queensland - Blog: scienceblogs.com/evolvingthoughts
"Darwin's theory has no more to do with philosophy than any other
hypothesis in natural science." Tractatus 4.1122