Why Are There So Many Girl Spiders?
Sexually reproducing species tend to have a roughly even number of males and females. The reason is that, as Fisher and others have argued, a 50/50 mix of males and females is evolutionary stable. To see why, suppose the mix were uneven; in such a case, there is an advantage to favoring the rarer sex.
Many species exhibit the 50/50 split, and deviations from this are considered phenomena in need of explanation. Some spiders, for instance, show a highly biased sex ratio, favoring females. My first introduction to this was through a paper about a spider, Anelosimus eximius, in which Aviles (1986) suggested that the skew toward females might be due to group selection. If there is competition between groups, then having more females – which leads to greater rates of population growth – can lead to a between-colony advantage. As Aviles remarks: “A faster-growing colony would more quickly reach the threshold size for reproduction and therefore be more successful in leaving daughter colonies” (pp. 8-9).
A recent paper in BMC Evolutionary Biology addresses the reason for a biased sex ratio in a different spider (Oedothorax gibbosus) with a very different explanation. In this species, in which the ratio of female to male spiders can be as high as two to one, Bram Vanthournout and colleagues investigated the possibility that the distorted sex ratio is the rather the result of manipulation by bacteria.
Suppose you’re a bacterium – Wolbachia – and you are transmitted from one host to another through eggs. If you wind up in a male spider, you’re an evolutionary dead end. This generates a selection pressure for causing your host to bias her clutches toward daughter spiders.
Indeed, spiders infected with Wolbachia do produce a female-biased clutch, and, further, in the coolest study of the ones reported in the paper, spiders were treated with an antibiotic (tetracycline). Spiders who previously had clutches biased toward females now showed the usual, Fisherian 50/50 sex ratio among offspring.
The authors conclude their results show that Wolbachia manipulates sex ratios in these spiders and that there results are “consistent with the hypothesis that sex ratio variation in the solitary dwarf spider Oedothorax gibbosusis caused by reproductive manipulations by endosymbiont bacteria.
Add this to the list of extremely cool ways that parasites manipulate their hosts.
Reference
Avilès, L. (1986). Sex-ratio bias and possible group selection in the social spider Anelosimus eximius. American Naturalist, 128, 1-12.
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