Ensign Steve

One of the things I had a lot of fun doing in my physiological psychology class was to try to "reverse engineer" some of the traits that living things have, from an evolutionary standpoint. Taking almost any part of the anatomy of any living thing, and figuring out why it was an advantage, and why it stuck.

Well I was watching some animals on TV, and there were these deer type animals bounding around with these bright white butts under their tails. That seems like a disadvantage to me. It seems like something for a predator to easily focus on when pursuing them. Can anybody think of a good reason for the bright white butts? One possibility is the mate-attracting thing, like the baboons big red butt, but I wonder if there's a better explanation.

Thanks!

Jen

sakrilege

Warning, I am not a biologist.

If there are a bunch of them running from a predator, the many flashing white butts can be confusing. Also it can be a signal to others in a herd to run.

Just remember, my last biology class was more than 30 years ago in high school!

Mageth

The white rumps on some deer and antelope are generally considered to be used primarily as "warning flags" for others of their species, I believe. Whitetail deer, which I know the most about, get their name from this trait; the underside of their tail is white, and is raised as a warning flag to others when alarmed. The confusion part may play a role as well.

One possibility is the mate-attracting thing

They're generally found on both males and females, I believe. Further, many if not most deer and antelope males fight for control of "harems" of females, rather than attracting individual females, I believe. Thus the strongest mature males are typically the successful ones, not the best-looking.

MyKell

Jennifer,
This same issue arises not only in antelopes, but also in species throught the whole animal kingdom. It is the conflict between sexual selection and utilitarian selection.
For instance in some kinds of birds, a long tail will be a very attractive trait for a male. Males with a longer tail will have better chances to spread their genes if they survive . The problem arises when the tail reaches a length that would interfere with the birds flying ability. Attractiveness now becomes irrelevant.
Both sexual and utilitarian selection operate on opposite ends so that they result in the species we observe.
I hope that clarifies the point!
Michael

Ensign Steve

Wow, thank you to everyone for your replies. I knew this was the place to come for educated answers (or at least educated-sounding answers ).

I never even thought about the using-it-as-a-warning-for-others-in-the-herd angle. I guess because I'm always looking out for number 1!

Thanks again.

Jen

simian

A little late on the response, but I think you may be talking specifically about the White Tailed-Deer. It is an excellent flag to let other deer know you are getting the hell out of there for a reason. When the tail is down, little white is left showing. When it is up, there is a good contrast between the brown/tan fur on most of the body and the white flag of the tail/butt.

Simian

Mageth

Some other deer species and some antelope species also have a white rump. For example, here's a pronghorn antelope from the plains of N. America:



I don't think the white rumps (or other colorings on the body) are due to sexual selection, at least in most of these species. Male and female of these species tend to share similar markings (you can't tell male and female whitetail deer, or pronghorn antelope, apart by their coats alone). Rather, I think the "warning sign" (Bambi, you see something flashy and white running away, haul ass!) and perhaps also the confusion of predators are likely reasons for development of such colorations.

The prevalence of the white rump in herd animals (specifically deer and antelope) suggests that this is a "good idea" in survival terms for the herd, and thus has evolved more than once.

Look at the coloring on the antelope above. Fleeing a predator, aherd of these antelope, with their striking markings, might well present quite a confusing sight.

Ensign Steve

I see what you mean, but what was striking to me about it was, look how bright that white tail is compared to the deer's tan fur and the landscape. It's like a hunter wearing a camoflage coat and a bright orange hat! It seems a little self-defeating. I would think it would save a predator a bit of mental energy to focus on one white spec instead of a while animal. Also, the fact that it's on the rump means it's pointed toward whatever you are running away from. Maybe it does confuse the predator, though, what do I know? That warning flag idea does make a ton of sense, too. From an evolutionary standpoint, that which helps the herd survive is more important than that which helps the individual survive.

Mageth

I see what you mean, but what was striking to me about it was, look how bright that white tail is compared to the deer's tan fur and the landscape. It's like a hunter wearing a camoflage coat and a bright orange hat! It seems a little self-defeating. I would think it would save a predator a bit of mental energy to focus on one white spec instead of a while animal.

Well, whitetail dear conceal their posterior whiteness with their tail under normal conditions. They are very difficult to spot in brush or woods, particularly when motionless. Also note that, if a whitetail senses potential danger, it will typically face the source of the danger to try to figure out what's going on and "raise the flag" if it thinks the threat is real. Whitetail deer will even stomp their feet and snort to warn each other. I've also seen (or heard) whitetail deer snort and/or stomp their feet in an attempt to elicit a response from something they've sensed. Specifically, when sitting concealed in the brush, I've had deer stomp and snort to see if that odd thing they've sensed (either through smell or sight) will move.

As an aside, and as an example of interesting deer behavior, one late evening, dusk really, I was walking down a path through the woods. Suddenly, I realized there were several whitetail deer very close to me (within 10 to 20 feet). I stopped and stood perfectly still. The deer were doing much the same, trying to figure out what the hell I was before wasting energy running. The impasse lasted for half a minute or so; finally I moved or made some noise, and the deer disappeared into the brush. I've seen this behavior on several other occasions; figure out what it is before committing oneself to fleeing. For one, fleeing wastes envegy and risks injury. Second, if you're real still, there's a chance it won't see you. If you run, it's sure to. Try the easier course first and only run if it fails.

Pronghorns, on the other hand, are primarily open plains grazers. Their primary concern is not camouflage, but detection - they have amazingly powerful eyes, as well as very good ears and nose, to detect predators at a distance. Then, they can amble away, dropping over the next ridge, possibly. Run only if the danger gets close or manages to take you by surprise (difficult for any predator to do to pronghorns on the open plains). Once running, no NA predator is capable of keeping up with them, if they're healthy. Note that no amount of camouflage is likely to help you on an open plain.

Whitetail and mule deer, especially in open areas exhibit similar behavior. Spot danger at a distance, then keep an eye on it. Drop over the top of a ridge or hide behind a bush if you can. Peek over to keep an eye on whatever it is. Only run if you have to. And only run as far as you need to.

I don't know if pronghorn antelope exhibit this behavior, but some African antelopes have been observed following predators such as lions on the move. It seems they've evolved a survival skill along the lines of "the visible lion is much less dangerous than the invisible lion."

Also, the fact that it's on the rump means it's pointed toward whatever you are running away from. Maybe it does confuse the predator, though, what do I know? That warning flag idea does make a ton of sense, too. From an evolutionary standpoint, that which helps the herd survive is more important than that which helps the individual survive.

I personally think the warning flag is the bigger factor in why the white rump evolved than the confusion factor. About herd vs. individual survival, yes, it may be that herd survival or "group altruism" (i.e. behaviors/characteristics that aid survival of a group of related genes) is more important in some cases than individual survival. But note, in this case at least, the two are closely intertwined. An individual must recognize that the white rump displayed signifies potential danger, and/or that the white rump spotted running away means you should high-tail it too. Individuals that ignored these warnings didn't survive.

An additional point I thought of which may be of significance: the white rump would be much easier to see in low-light conditions. This would make the "red flag" easier to spot, and easier to follow, in such conditions.

sakrilege

My understanding is that on the open plains, the herds are larger and I think there is more up & down motion with the rump, therefore a pursuing predator will have a hard time focusing on a single individual.

Otherwise I agree entirely with the white tail deer explanation as it is also my experience. The deer near me know how far my dogs will chase them, they run a few hundred yards further and then stop to look back at the dogs.