Thursday, 25 February 2010

White-Tailed deer and asymmetry

In general, we expect fluctuating asymmetry to be a sexually selected trait. How's that for an opening line? But it's generally true: Females prefer males that are more symmetric. We find that in humans, mice, rats, and about a dozen other things, so the finding is fairly robust. In fact, you can experimentally alter faces' symmetry using computers, increasing it or decreasing it artificially, and ask people to rank them. People are pretty consistent in their rankings, across cultures. 

Why? Why do animals prefer more symmetric animals? Well, there's a slew of hypotheses. Most of them revolve around the fact that the random deviations from symmetry (that's where the 'fluctuating asymmetry' term comes from) tend to be linked with past stress. It could be it was a sick child. Or a poorly fed child. Or in development, they got into problems with predators, or other sources of injury. Or it could have been environmental toxic exposure. Whatever the cause, the response tends to be pretty straight forward: do not want.

If a mouse was very sick as a pup, that means that a choosy mouse doesn't want him. After all, it might have STDs. Or it could have bad genes, that predispose it to sickness. Or it lacked behaviour that kept it away from infection. Either way, a choosy mouse doesn't want any diseases itself, and it definitely doesn't want bad genes for its offspring. No thank you! If it wasn't fed much as a pup, well, its mother gave its food finding skills/knowledge/genes, and you don't want those either. That, is, in a nutshell, fluctuating asymmetry. Or, as the cool kids call it, "Flucing A." Serious. Scientists say that with a straight face.

Weidros.

Most fluctuating asymmetry comes from early/juvinile development, because that's when most of the development occurs. But traits like antlers pose an interesting extension, because they're produced fresh each year. In theory, each new set of antlers is a demonstration of how the bull's life has gone up to that point, not just up until it reached adulthood. That's multiple points to demonstrate fluctuating asymmetry. If a male had a bad year, that information will be integrated into its antlers the next season. And antlers are some of the fastest growing tissues, so they easily react to stress to induce fluctuating asymmetry. They're also sexually selected traits, in that antlers mostly exist to woo the ladies, or intimidate other males.

In general, we expect the following pattern: Older males, less fluctuating asymmetry. That's because if you've lived a while a) you're good at what you're doing (and the worse ones have died off a bit) and b) you're more socially dominant. Also, we generally expect larger antlers to have lower fluctuating asymmetry, because only good males can make big antlers. Previous studies have found positive relationship between symmetry and low of parasite load, while others have found vague support for fluctuating asymmetry and antler size.

But here's the thing. Those were with 2-d measurements. Highly asymmetric antlers could numerically be recorded as symmetric because the data can't distinguish symmetry.

Well, Stephen Ditchkoff and Rachel L DeFreese decided to correct that. In their paper, "Assessing fluctuating asymmetry of white-tailed deer antlers in a three-dimensional context," (DOI: 10.1644/09-MAMM-A-134R.1.), they used 3-dimentional modelling to find out if a) Antler Size is negatively correlated with fluctuating asymmetry and b) age is negatively correlated with fluctuating asymmetry.

What was their sample size? What was the nutritional status of the deer? What was the sample site? All these I can't answer in any detail, because frankly, the descriptions in the paper were wanting. They mention two counties in Alabama  in 2002-2003 hunting season... well heck, that means nothing to me. In such an environmentally induced trait, population location, and position with respect to carrying capacity (K) matters. I'm going to assume that these were good years, in apex populations. But how many samples? I don't see it spelt out clearly. I think it's 121, because that's the largest n they have on any of their measurements. Still, I'm unhappy with that. I normally enjoy Ditchkoff papers, so this was bum.

But, when all is said and done, they tested their two hypotheses, and reject both of them. Age is at best weakly correlated (r≤0.33) without be statistically significant (P≥0.083). Further, there wasn't any consistent link between age and decreased fluctuating asymmetry. The stats on this are messy, and the tables numerous. But the take home message I think is sound.

So, WTF? I tell you all about fluctuating asymmetry, and then I tell you it's bunk? Well, I wouldn't say that. I don't think age is a big role in fluctuating asymmetry, because of Bowyer, one of our ex-faculty, who did some work in moose.  I've attached some graphs that show that males increase over age, before petering out at the end. Really old bulls then enter decline, when their antlers go to heck. I wanted a picture of clubbed antlers, but I don't have one handy - shame. So that would erode the relationship between age and fluctuating asymmetry. Antler size is a bit more troublesome... and I think the authors were on to something when they offered the hypothesis that maybe things weren't severe enough to produce patters.

Given antlers tend to hit a max with age, most bull moose (or buck deer, in this case) will have roughly the same size, within a normal distribution. When you really see a lopsided distribution (gamma) is when times are tough, and there's one shining winner, and a whole lot of losers who aren't looking so pretty any more. This is why ecological information is important in fluctuating asymmetry papers. I need to know how much the animals are getting pushed, because it's when the things get bad you know who's worth his salt.

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