Maybe it’s just me, but impressing women is difficult. Nowadays it takes all kinds of tricks—I mean charms—to show a woman that you’re worth even a moment of her time. Wasn’t always like this. Time was, back in the days of Olde, a knight would joust with some other guy, and the victor won the hand of the lady—maybe a princess, or a fishwife or someone. I mean they didn’t chop off her hand and give it to the winner. “Hand” is just an expression. He won her heart. No, not her actual physical heart, he wasn’t trying to acquire her courage. “Heart” is an old-fashioned term used by Shakespeare and others to mean “love,” or something. Point is, men used to physically fight other guys to determine who would end up with the princess or fishwife and sire a lot of kids to work on the farm or swab the deck or something. But what did hominins do to determine who won the hearts of the ladies?
Some animals (let’s focus on primates) don’t fight with lances like in Olden Days. Some fight dirty—they leave the competition up to their sperm. Why? Sometimes it pays not to have to fight off other suitors like Odysseus had to (hey, another Classics reference), and sometimes it can even be beneficial to have other males around. Take chimpanzees and bonobos, for instance. The live in multi-male, multi-female groups, where the males tend to be related and stay in their natal group for life, while females disperse and join a new group once they’re old enough to have their own offspring. Male chimpanzees often cooperate in patrolling their territory’s borders, hunting, and other macho activities. And females mate promiscuously—no one's wearing white at these weddings. So how do males maintain lots of buddies (who are mating competition) while still trying to father lots of offspring—sperm competition.
One thing you might notice superficially about a chimpanzees is that males’ testes are huge relative to body size. This is because part of their mating strategy is to (when mating, mind you) release copious amounts of sperm so that the chance of fertilization is higher. Also, their ejaculate forms a ‘sperm plug,' (theoretically) so that the next male that mates with that female will be less likely to achieve fertilization. Sperm competition like this characterizes species where females are promiscuous. Other species, like the gorilla, which are incredibly sexually dimorphic (males are much, much larger than females) and in which males overtly compete with one another for females do not exhibit this sperm competition. In fact, gorillas have a completely different social structure from chimpanzees see (my earlier post on the robust australopiths): (usually) a single dominant male associates and mates with a few females, and gorilla testes are tiny for their body size. Like it's always cold outside.
A recent study looked at hominoid seminal protein evolution to determine what type of mating systems humans may have evolved in (Carnahan and Jensen-Seaman 2008). The authors looked at coding sequences for three seminal proteins in extant humans, chimpanzees, bonobos, gorillas, orangutans, and gibbons/siamangs to compare the rates of non-synonymous vs. synonymous substitutions. A higher ratio—more non-synonymous substitutions—is usually evidence for some kind of selection: according to Neutral Theory, most genetic mutations should be neutral, that is, they shouldn’t alter fitness. Some substitutions, 'synonymous,' might not change what amino acid is coded; however if the substitution changes the amino acid, there is a high chance that it will be deleterious (i.e. negatively affecting fitness). So if we see a larger-than-expected amount of non-synonymous mutations, then they were likely beneficial, and so were under positive selection, i.e. were selected for.
So what did the authors find? For chimps and bonobos the non-synonymous/synonymous ratio was ‘greater than one,’ (I don’t think the paper gave an actual number other than “>1”). Also there was little variation in the sequences. This indicated that the seminal proteins were under positive selection, consistent with the aforementioned symptoms of sperm competition. Conversely, for gorillas and gibbons, characterized by no sperm competition, and little/no female promiscuity, these seminal proteins were basically non-functional.
But for humans, the ratio was 1.02, with a great amount of protein variation. Also, humans appear to be fixed for two amino acid substitutions where most other mammals are conserved. So what does this mean—is human evolution characterized by a mating system similar to that of gorillas, with overt male-male competition, and sexual body size dimorphism (this is supported by most fossil evidence)? Or is it better characterized by a chimpanzee-like mating system, with promiscuous females and sperm competition? Difficult to say. Here’s the authors’ take (Carnahan and Jensen-Seaman, in press, p. 8):
This could be interpreted in several ways. . . . these genes may have been evolving nearly neutrally with little functional constraint and therefore on their way to becoming pseudogenes, as seen in gorillas who have little or no sperm competition. . . . An alternative interpretation is that since these [non-synonymous/synonymous] ratios are estimated along the entire human lineage since our divergence from the common ancestor of humans and chimpanzees, they may be reflecting an average across fluctuating episodes of positive selection and purifying selection, in turn because of fluctuating mating systems.
I guess there are three things to take away from this study. First, it provided a novel way to integrate genetic and behavioral data to try to infer aspects of hominin behavior when there is little direct evidence, other than sexual dimorphism (or not, depending on who you ask…) in the fossil record. Second, this is another instance where molecular data are really interesting, but equivocal as to what they mean exactly (see one of my earlier posts). Just as there are gaps in the fossil record, molecular data, with all their summary statistics, models and assumptions, have their shortcomings as well. Finally, this provides another caution for using Pan as an analog for the chimp-human common ancestor (haven’t discussed this one lately). Said common ancestor, and basal hominins for that matter, may have been behaviorally similar to extant chimpanzees in some ways, but this study points out one way in which extant Pan may well be derived relative to other hominids/-oids. Oh, and who could resist linking the study’s subject (semen) with half of one of the author’s last names? I could. Grow up, seriously.
Carnahan SJ, Jensen-Seaman MI (2008) Hominoid seminal protein evolution and ancestral mating behavior. American Journal of Primatology 9999(9999):n/a