Friday, May 18, 2007

What -ogamy are we?   posted by Razib @ 5/18/2007 01:51:00 PM

Martin mulls over the question, Are Humans Polygamous? There is lots of interesting discussion, with a FinnXPer & reindeer lover in the fray. I think part of the confusion here is simply semantical. Cultural anthropologists often tend to define an -ogamy based on the preferred ideal within a society. So you have circumstances where the social ideal is polygyny, but for various reasons most males (and even females) aren't in polygynous relationships. In contrast, behavioral ecologists tend to look at it a different way, the extent of polygyny can be thought of as the ratio of the reproductive skew of males to females. In other words, if males exhibit high skew (so that only a few males reproduce in a generation) and females exhibit low skew (so that most females reproduce) then you have a species which is highly polygynous. Traditionally one would have to use the tricks of the ecologist's trade and make observations and count broods/offspring from pairings to do this, but with modern genomics this is less critical. You can test for paternity directly within the context of a group of individuals being studied, or, you can examine population level genomic trends. For example, this paper, Reduced Y-chromosome, but not mitochondrial DNA, diversity in human populations from West New Guinea, allows us to infer a long history of polygyny within a given society. Males have a few ancestral lineages while females have many more across a given time period because so many more females reproduce per generation. Also, in many organisms the "ideal" is not necessarily the reality. "Monogamous" birds for example were found to be not so monogamous when genetic tests were performed to ascertain paternity.

In regards to our own species there are two primary issues which I think need to highlighted. First, it isn't a distribution along a simple polygyny-monogamy axis. If you use primate analogs this would be the gorilla-gibbon range. Male gorillas have harems when they are at their reproductive peak. In contrast, gibbons enter into male-female pair bonds. The size differential between males & females of the species correspond to what you would expect, since gorillas engage in a great deal of male-male competition, males are far larger than females. In contrast, gibbons are more balanced in size because there isn't a large differential between sexes in competition (females always have to "represent"). Additionally, in both species individuals seem to "cheat," so the breeding systems are ideals which are not always fully realized (both sexes are "jealous" and try to guard territory among gibbons). But there is another group of great apes which we have to look at, the chimpanzees. They don't fit the gorilla-gibbon range because females in both chimp species exhibit levels of polyandry, that is, they mate with more than one male over a short period of time . This explains the relatively large ball sacks of males chimps as well as their gooey sperm. If gorillas are at one end of this dimension and chimps at the other (gorilla females have been observed to attempt to cheat, but they don't seem to succeed much), humans seem to be in the middle (check the reliability of paternity confidence across cultures here). I think the question would be better framed as one of polygyny vs. polyandry vs. monogamy, as opposed to a one dimensional spectrum it is a multi-dimensional behavior space. Second, generalizing about the whole of humanity might not really be informative. Variation in behavior and phenotype is not an uncommon phenomenon. Humans may be characterized by alternative behavioral morphs which are extant at different proportions within various populations contingent upon the environment and society. Orangutans are a clearer case of this, there seems to be a "small male" type which females reject when offered the opportunity to male with "large males." But, the small male may remain within the population because its mobility and speed allows it to pursue a raping "strategy." That is, while large males need to be attractive to females because of their lack of agility, the smaller males can simply catch and inseminate the females. One can imagine various higher organisms being characterized by a flux of strategies within the population with might be evolutionarily stable, especially those subject to negative frequency dependence. For example, imagine that among orangs the small males are at a low frequency within the population. In this scenario it seems plausible that most females would be caught unawares because they would only rarely deal with the "threat" of approaching small males. Once the frequency of this morph rises to an appreciable level then socially (or genetically) conditioned defensive responses might arise which prevent it from increasing in frequency. In the case of gorillas, gibbons or elephant seals the modal (most common) strategy is almost exclusively practiced. But in species like orangs, gorillas or even chimpanzees, where more variation over time and tribe is the norm one must keep in mind both the central tendency (the mean) as well as the variance, and, possible multi-modalities (which would map onto the common morphs).

Finally, there is the issue of culture, the environmental and non-genetic aspect of behavioral causality. Obviously a trait must be expressed within a particular environment, and the expression of a genetic "switch" must then be properly contextualized. Consider the relationship between MAOA and abuse. The correlation between one allele and abuse on the population level needs to be understood in the context of early childhood environment at the level of the individual. The "abuse" morph will not express without particular environmental inputs. One can characterize this as a norm of reaction, or simply treat the environment as another component of the variance. The point is that a genetic component toward a behavioral bias does not imply that social & environmental forces can not play critical roles in how the phenomenon is realized (or not!). An emphasis on alternative morphs and behavioral strategies, and facultative responses, also implies that a range of behaviors might be realized dependent on the range of environments which a population is subject to over time. Ultimately I think a focus on instrumental attitude toward explanations are essential in the case of our own species because the biosocial toolkit is so multifaceted and flexible. Models need to be "thick" and microscale, nested within a set of contingent facts and amenable toward accommodating the reality of variation, heritable and non-heritable. No need for determinism, genetic or cultural, here.