Risky mate search and male self-sacrifice in redback spiders

Male redback spiders twist their abdomens onto the fangs oftheir mates during copulation and, if cannibalized (65% of matings),increase their paternity relative to males that are not cannibalized.The adaptive male sacrifice hypothesis proposes that this increasedreproductive payoff from a single mating outweighs the residualreproductive value of a cannibalized male, because high mortalityduring mate searching restricts alternative mating opportunities.It has been reported that redback male residual reproductivevalue is low because males are functionally sterile after onemating—a putative intrinsic constraint that could arguablyfavor self-sacrifice in the absence of ecological restrictionson multiple mating. However, sterility and self-sacrifice mayboth arise as aspects of a terminal investment strategy if theprobability of multiple mating is sufficiently low. Here I reportfield data that support the adaptive male sacrifice hypothesis.More than 80% of redback males die without finding a potentialmate in nature. Data from two observational field studies andone release experiment suggest that in the absence of cannibalism,male redbacks would expect fewer than one mating opportunityin a lifetime. This expectation was not significantly higherfor a large male or one in good condition. A simple quantitativeanalysis confirms that even if males are assumed to be fertilethroughout life, the measured mortality rate during mate searchin combination with previously documented paternity benefitsof cannibalism is sufficient to ensure that self-sacrifice isadaptive for male redback spiders.