Is Being a Eunuch Adaptive?
A crucial adaptive problem for males in various species is the chance that after copulation, another male’s sperm will gain entry to the female’s reproductive tract, reducing the probability of the initial male’s successfully fertilizing the female’s gametes. In species in which this is a problem, we might expect to find adaptations designed to minimize this risk. (Baker and Bellis explored this idea in the context of humans.)
Enter a new paper by Kralj-Fišer et al (2011) in Animal Behavior, studying a spider, Nephilengys malabarensis. (Hat tip to Leo Tiokhin, who called my attention to this interesting work.)
The authors, with their fancy diacritical marks, were interested in the observation that genital amputation is common among males of this (and other) spider species. Superficially, this seems like a curious design because, by and large, losing one’s genitals would seem to run counter to an individual’s fitness interests.
Maybe not, suggest Kralj-Fišer et al. Suppose that this is actually a strategy having to do with the adaptive problem of male-male competition.
In these spiders, males use their genital organs (known as “palps”) as mating plugs, blocking the entrance to the female spider’s reproductive tract. The authors entertain two hypotheses about this phenomenon:
(1) the broken genital parts …plug the female’s copulatory opening and thus prevent rival males’ access … and (2) eunuchs are better fighters than their intact rivals, perhaps because they are more agile (they do not have to carry around the relatively large pedipalps)…
The thing to note about eunuch spiders is that after mating they have, as it were, all their sperm in one basket. (Indeed, the authors suggest recent data shows that palps can be “charged” with sperm only once.) A male in such a condition cannot increase his reproductive success by any pathway except through this female spider; he gains nothing by living to fight another day. So, one would expect that a eunuch spider should fight to the death when guarding this female. He has, in the sense of fitness, nothing else to live for. Or, to put it another way, a mutation that caused the male to take fewer risks, increasing longevity but decreasing the chance of winning the fight, would be selected against. The general principle – those with nothing to lose fight harder – shows itself in human transactions as well, as illustrated by films such as Nothing to Lose, with Tim Robbins. In turn, getting back to the spiders, rival males’ adaptations should reflect the tradeoffs faced by eunuchs, and these rivals should be more likely to retreat from eunuch males, given that these males should tend to fight to the death more readily than intact males.
The authors allowed males and females to mate and observed their behavior. Most of the time, males did, in fact, leave their palps in the female reproductive tract, which prevented subsequent copulation most, but not all, of the time. (And just to complete the somewhat gruesome life history, in three quarters of the cases, in the end, “the female attacked and devoured her mate” (p. 936). The authors similarly found that eunuchs behaved more aggressively and won more contests with rival males, who, as expected, tended to stay away from eunuchs.
Kralj-Fišer et al (2011) conclude – and this is the source for the title – “being a eunuch is adaptive for spiders” (p. 938).
(Should I point out, yet again, that this is a case in which biologists are using purely behavioral data – as opposed to direct measures of fitness and reproductive success – to make inferences about adaptation, just like evolutionary psychologists do (but get criticized for), or is that just getting old?)
But is it, really, the case that “being a eunuch is adaptive?” It seems to me that it’s the plugging of the genital tract that should be thought of as the adaptation. Becoming a eunuch is a side-effect of this particular strategy. This holds even if it turns out that the advantage of amputating one’s genitals lies in the fighting advantages because here, too, the function has to do with fighting, not with being a eunuch.
Anyway, spiders are really cool, and work like this illustrates that there are times when, set against other parts of the animal kingdom, the human mating system, as complex and frequently frustrating as it is, doesn’t seem all that bad…
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