Question

1. (4m) While mating, the male redback spider does a somersault so that the back of his abdomen dangles directly over the jaws of the much larger female. This entices the female redback spider to begin to eat the male’s abdomen but does not affect the continuing transfer of sperm.

Propose two possible explanations for the male’s behavior. One explanation should deal with ultimate causation; the other explanation should deal with proximate causation.

Answer 1

Ans: Male spiders possess paired copulatory organs (palps) and female spiders have two independent sperm storage organs, each with its own insemination opening. Male redback spiders twist their abdomens onto the fangs of their mates during copulation and, if cannibalized, increase their paternity relative to males that are not cannibalized. The adaptive male sacrifice hypothesis proposes that this increased reproductive payoff from a single mating outweighs the residual reproductive value of a cannibalized male, because high mortality during mate searching restricts alternative mating opportunities.

Sacrifice during mating is thought to confer two advantages to the species. The first is the eating process allows for a longer period of copulation and thus fertilisation of more eggs.

The second is females which have eaten a male are more likely to reject subsequent males. Although this prohibits future mating for the males, this is not a serious disadvantage, because the spiders are sufficiently sparse that less than 20% of males ever find a potential mate during their lifetimes, and in any case, the male is functionally sterile if he has used the contents of both of his palps in the first mating.