Thursday, October 06, 2005

Where the boys aren't

Valley Forge Park (yes, that one, with all the history).

Hmm, perhaps that answer needs a little unpacking to be at all understandable.

First of all, horns (mammalian, not lizard--that's totally something else) and antlers have some surface similarities, as well as fundamental differences. They each have bony cores, and are located in similar places on the front of the animal's skull. However, their compositions are different in important ways. What I always find surprising is that--despite the hardness and permanence of bone, as compared to hair, skin, and nails in the way we think of them--the permanent structure (horn) is covered with keratin (in most horned animals), or, in rhinos, is made totally of compressed keratin (basically, hair) with no bony core. On the other hand, the temporary structure, which is shed and regrown annually in deer, moose, and other cervids, is made totally of bone. Seems like it should be the other way around, given the ephemeralness of keratin in hair and claws, compared to bone, but there you are.

Male deer regrow their antlers after each mating season. When the antler is regrowing, before it is mature, it is covered with "velvet", which is a layer of skin full of blood vessels to promote the antler's growth, and which is shed when the antler is done growing. Cervid phenomenology being in its infancy, it's impossible to know for sure, but I wonder if the sensation of an antler growing is anything like I remember for permanent teeth coming in--not pain, not exactly, but exquisite sensitivity, and a desire to constantly rub my gums to make the feeling go away. If so, then the male deer have another reason (besides the mating season rise in testosterone levels) to be cranky.

Pennsylvania seems to be chock-full of deer anyway, and riding the bus through Valley Forge National Park frequently, I often see them standing near the road calmly eating grass, and paying no attention to the cars driving right by them. I hadn't thought about it until the bus driver (F.) asked me, but then I noticed he was right--we were never seeing any antlered deer.

So what proportion of antlered deer would we expect to see? We know that only sexually-mature males have antlers (in this species; in reindeer, by contrast, both males and females have them). In the absence of any specific knowledge, we can assume that the population is split roughly equally between males and females, so out of every four deer, we would expect two of them to be male.

Assuming that deer reproduce at a rate that roughly replaces every adult who dies with a younger one, let's say that the population is about equally distributed between mature deer and immature ones. So out of our two male deer, let's say that one should be mature, and the other immature. Out of our four deer, then, we have one antlered one, and three without antlers (as long as our initial assumptions about gender and age distribution are accurate).

So we should have been seeing roughly one deer with antlers for every four deers, or maybe fewer if our assumptions were wrong--but we were seeing zero antlered deer, way fewer than we could explain, even if we had been wildly off. So since our data were at such variance with our hypothesis, we started to look at possible systematic explanations for the discrepancy.

Possibly they were being deantlered, as I have heard is done in some Japanese deer parks, to protect the human visitors (turns out, no). Possibly their antlers hadn't fully come in yet (but mating season is earlier in the year, so that explanation doesn't work, either). We tried a couple of other possibilities, but nothing really made sense. Later, LL was able to supply a missing piece of the puzzle that hadn't occurred to us.

Hunters prefer deer with antlers, the bigger the better. Therefore, there is a selection pressure against antlers, and the population is skewed toward the non-antlered deer. Additionally, if the antlered deer have the least bit of sense, hiding from humans is a good thing when they want to kill you. So in addition to the antlered deer dying at a higher rate, probably the ones that aren't dying have learned to stay out of sight of people driving by on the nearby road.

Obviously, we haven't proven this in any rigorous sense, but it is highly plausible on the face of it. F. and I were very satisfied with our approach to this question and its resolution.

And, amusingly enough, now for three days in a row, we have seen a new antlered male who comes close to the road to eat grass. He must be very brave, or else very oblivious--but it is nice to see him; he is splendid with those antlers.


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