How does the mammalian brain effect changes in phenotype?

Question

I saw an interesting documentary ("Hogzilla") about feral hogs on National Geographic Channel. It was mentioned that it took thousands of years to domesticate hogs, but that within a few weeks a domestic hog that has gone to live in the wild will start showing bodily features only seen in wild hogs (hairy fur and large tusks). Is this an example of phenotypic plasticity? Blog concerning the show at http://blogs.nationalgeographic.com/channel/blog/2005/03/explorer_hogzilla.html
Thanks.

Answer 1

Okay, I don't know much about hogs... But I have my fair share of experience with studying plasticity, so let me give you my take on this - jsut so I can partake in a (FINALLY) really interesting discussion on YA.

I think that the two parts of your question are not necessarily related to one another. One, you ask how the brain can affect the phenotype of an individual. Two, you ask about how the *environment* can affect the phenotype. Granted, you would think that in the case of the hog these two aspects are somewhat connected; how else than through the brain would a change in the environment be perceived, you might argue. Well, let me give you a few examples.

Before I engage in this review, however, let me say that the dualism of phenotype and genotype has increasingly lost currency among biologists. While heuristically valuable, it is a distinction that does not really make much sense... But that just as a caveat lector for what is to follow.

We know from studies on various animals that environmental conditions can affect the developmental trajectory of an organism and thereby affect its phenotype irreversibly. Such organizational effect are well known from steroid hormones and their effects during embryonic development. In mammals the intrauterine position effect would be a case in point. Embryos developing between two male siblings receive more testosterone and have hypermasculinized brains. This leads to behaviroal difference, but also to morphological differences. However, steroids can of course also have activational effects, where presence or absence of the hormone causes morphological changes (testosterone and growth of muscles or facial hair). Now, in both case the brain is somewhat involved, but it is not necessarily the cause.

Now, in the hogs you are describing, the environmental conditions could drive the release of hormones, as well as the dietary and physical changes could. Different food, different body shape; more physical activity, different body shape etc. The furry hair for example could be a response to denser foliage against which the hair could act protective. The husks on the other hand could be driven by social interactions with other animals (in order to either determine rank or who gets to mate etc.). In the former case, the brain would have little to do with it, in the latter it would have all to do with it. Now, the growth of both hair and tusks is to a certain degree under hormonal control, and the control of the endocrine system lies in the brain. You could therefore argue that the brain is involved, although not necessarily as the direct cause.

So, I guess my answer to you would be that yes, what is being observed in the hog is indeed an incident of phenotyipic plasticity, but that the brain is not involved directly as cause in all of them.

Interesting question! Thanks.

Answer 2

It could be phenotypic plasticity. It could also simply be a lack of human interference. It was a fairly common practice when raising hogs to clip off the teeth in very young hogs that would normally become tusks. Left unmolested it makes sense that the teeth would eventually grow back. As for the fur. Hogs already have fur. Allow a hog to live in a more natural environment that does not involve a confinement pen and building it is very likely that the fur on these animals would grow out naturally. The very definition of phenotypic plasticity is there is wide range of "normal" for any given condition. Once again, without human interference there is every liklyhood that the range of normal would be readily expressed in a feral animal. There is no indication of an environmental extreme that would cause a marked phenotypic change. But the lack of human interfernce would allow "normal" development of certain phenotypes in feral hogs.

Answer 3

agree. furthermore, in a couple of generations there could be some selection - elimination of those animals that cant show the wild type features. Definitely it is not about brain causing changes