Female White-Tailed Deer (Odocoileus virginianus), taken in Hopewell, NJ
July 18th Update: LiveScience has caught onto the Nature paper and has a short article here.
A new Nature paper entitled “Sexually antagonistic genetic variation for fitness
in red deer” (Foerster, et al., 2007) has some very interesting implications for fitness selection; the most fit males may not produce especially fit female offspring, and the most fit females may not produce especially fit male offspring. While it seems to be “common sense” that the most powerful, impressive male will father the strongest offspring, male and female Red Deer (Cervus elaphus) behave differently, and what makes a fit male does not make a fit female. The authors put it this way in their introduction;
Males compete intensely for matings during the short annual rut but do not invest in offspring care, whereas female maternal investment extends over a long period during each reproductive event. Consequently, male and female life histories are likely to be under divergent selective pressures, and a particular genotype may have very different effects on fitness in males than in females.
Males have to carry around a good deal of weight on their heads and be physically strong (thus the most “masculine” males being favored when it comes to breeding), while females have to raise the young all by themselves, better mothers leaving more offspring. This is not a revelation, but I have to admit that I never really considered the differing selective pressures between males and females in a population.
The primary trend pointed out in the paper is that females who mate with robust males typically produce daughters who do not leave as many offspring; we can’t simply say that the father has “good genes” offspring of both sexes will benefit equally from them. Likewise, it is not apparent if robust fathers leave especially productive sons; the deer are polyandrous, a few males mating with most of the females, and so many (if not most) of the males never successfully mate or leave any offspring.
An itchy Elk (Cervus canadensis) at New Jersey’s Turtle Back Zoo
Of further interest to us in this topic is another new paper by Jakob Bro-Jørgensen in the journal Evolution, “The Intensity of Sexual Selection Predicts Weapon Size In Male Bovids.” While Bovids are distinct from Cervids (deer) at the Family level, I think it’s important to compare the armaments of deer with those of bovids such as antelope, gazelle, buffalo, etc. The important distinction to make here (at least for our purposes) has to deal with the weapons the males conspicuously carry about; in deer, the males grow a new set of antlers every year, the antlers being made of bone and growing from an attachment to the skull called a pedicle. Horns, on the other hand, are typically hollow, have a covering of keratin, and do not fall off annually. All male deer have antlers, and many male bovids have horns, whether females have horns/antlers or not varying from species to species (i.e. female reindeer and caribou have antlers, but not as impressive at those of males. Even among species where antlers are not a typical female characteristic, some are commonly found to have antlers). All these details, of course, make generalization difficult, but there do seem to be some larger trends at work.
A Mhorr Gazelle (subspecies of Gazella damaAntler Size in Red Deer: Heritability and Selection but No Evolution” appeared in the journal Evolution, and the authors (Kruuk, et. al) found that directional selection for more impressive antlers is not happening because the success of the competing males depends on more than just antler size. Horns and antlers are dangerous things, and at least some weapon-wielding mammalian herbivores make at least some attempt at avoiding violent conflict. A male antelope that is obviously larger and has obviously bigger/longer horns than a rival will likely be able to drive off or otherwise force into submission a rival just because the rival knows he can’t possibly compete with such an impressive specimen, fights occurring more frequently when males are close to be equally matched (although things do not always proceed as orderly as this, some inadequate combatants giving it a go despite their less-impressive characteristics).
If a fight does occur, it’s not all about the antlers or horns; the weapons do not hold any sort of magic power that make their owner necessarily any more adept at fighting or stronger than any given opponent. Imagine, if you will, two men facing off with nearly equal length swords; how well they do while fighting does not so much depend on the blade but how they use it, strength, agility, health, and experience being more important than the weapon itself. Such appears to be the case with the antlers of red deer. While Kruuk’s study found that horns were heritable and did play an important role in sexual selection, how the competing males were doing in terms of health and nutrition mattered just as much (if not more) than antler size, a male will smaller/shorter/less impressive antlers able to beat a male with more robust antlers if he was physically stronger and it better health. Thus, nutrition and health mitigates runaway sexual selection for a dimorphic trait, essentially halting evolution. I especially liked the following quote from the conclusion;
Associations between phenotype and fitness, however appealing, will give a misleading impression of the potential for evolution in a trait if the true target of selection is unmeasured or immeasurable.
Indeed, sexual selection may have played the major role in the development of impressive antlers, but such evolution is not ongoing; there eventually comes a point where the antlers (and even horns or tusks) do not get bigger because the strength of competing individuals makes more of a difference than differences in antler size or shape. This study brought to mind the famous Irish Elk (Megaloceros giganteus), an extinct deer with perhaps the most impressive set of antlers known. There are hefty prices to pay when you have to not only carry such a huge rack about, but also to grow it. While their antlers were the size one would expect for their body size, they were still huge growths of bone, and these features may very well have contributed to its demise. While there does not appear to be any conclusive finding as yet, the size of the antlers may have prohibited the elk from moving through thick vegetation as climate and local ecology changed, or even suffered from various disorders/diseases as the nutrients in the soil needed to grow so much bone began to disappear. For now all we can do is look for more evidence to confirm or refute these hypotheses, but if either were true then natural selection would push against body form generated by sexual selection, showing us that we should be careful not to be become so enthralled with just one feature of an animal that we overlook all else.
If nothing else, the studies I’ve mentioned show us that there is certainly more to consider when it comes to sexual selection, reproduction, and evolution than the “most fit” males getting together with the “most fit” females and continuing the species; a more integrated approach is needed if we’re going to truly understand what’s going on and why the evolution of certain structures (especially those influenced by sexual selection) can come to a halt.