Redback Spiders and the Meaning of Life

After copulation, male redback spiders (Latrodectushasselti; relatives of the “black widow” spider), often somersault into the female’s mouthparts and are eaten (Figure 11.3A). This suicidal behavior might be adaptive, because males seldom have the opportunity to mate more than once, and it is possible that a cannibalized male fathers more offspring. Maydianne Andrade (1996)¹ tested this hypothesis by presenting females with two males in succession, recording the duration of copulation, and using genetic markers to determine the paternity of the females’ offspring. She found that females that ate the first male with whom they copulated were less likely to mate a second time, so these cannibalized males fertilized all the eggs. Furthermore, among females that did mate with both males, the percentage of offspring that were fathered by the second male was greater if he was eaten than if he survived. (Figure 11.3B). Both outcomes support the hypothesis that sexual suicide enhances reproductive success. This example suggests that prolonged survival is not necessarily advantageous, and illustrates how hypotheses of adaptation may be formulated and tested. Figure 11.3 (A) The small male redback spider somersaults into the large female’s mouthparts after copulation. (B) The proportion of eggs fertilized by the second male that copulated with a female was correlated with the duration of his copulation. On average, copulation by cannibalized males lasted longer than that by noncannibalized males. (A after Forster 1992²; B after Andrade 1996.)

(From Futuyma, Douglas J. 2009. Evolution, 2nd edition. Sinauer Associates, Sunderland MA, pp. 281–282).