Lecture 6. Sexual Selection

The below set of notes is from APS209: Lecture 6. Sexual Selection.

Sexual Selection

Darwin defined sexual selection as “a struggle between the individuals of one sex, usually the males, for possession of the other sex”. Sexual selection is different to natural selection in that it selects for successful reproduction alone. Hence it frequently causes animals to do things that are good for the transmission of their genes but bad for their survival, with the red backed spider being an extreme case (Special Reading: Andrade, 1996). Sexual selection arises as a result of anisogamy. Females produce few large costly eggs while males produce many small inexpensive sperm. A female’s reproductive output is limited by the number of eggs she can produce, or the number of young she can rear. Mating with many males will not normally result in more offspring. But by being choosy she may have better quality offspring or a more helpful partner. Things are different for males. A male who mates with more females generally has more offspring because he has sufficient sperm to fertilise many females. Effectively males have a higher potential reproductive rate than females, so males compete for matings and females choose the best available male with whom to mate.

Male-male Competition and Female Choice

Male-male competition often leads to sexual dimorphism. Males often have adaptations for fighting and are larger than females. Large males are selected for because they are better at male-male competition. Large male elephant seals win more fights and become dominant. These dominant beachmasters have larger harems of females and as a result obtain more matings and more offspring. In seals as a whole, males are larger than females in species where one male can monopolize many females. But where monogamy is the rule males and females are the same size. One of the most complex areas in reproductive behaviour is female choice because it is often difficult to distinguish between male-male competition and female choice. It is necessary to carry out experimental manipulations to show female choice with any degree of certainty. Anderson (1982) manipulated the tails of male long-tailed widow birds and showed that females preferred to mate with long tailed males.

Mutual Sexual Selection and Sex Role Reversal

Because males produce lots of cheap gametes, sufficient for many matings, they can normally afford not to be choosy. But there are cases where mating is costly to males. Although sperm are almost always cheap, males sometimes provide additional expensive resources to the female or offspring, including paternal care and nuptial gifts. This can lead to mutual sexual selection and in extreme cases to sex role reversal. In crested aukets both males and females invest heavily (and equally) in raising the single chick and both display an ornamental crest. In this species mutual sexual selection occurs with both sexes preferring long crested partners and long crested individuals being dominant in both sexes.

In some species males invest more in reproduction than females and these males are choosy. Male seahorses get ‘pregnant’ and do all the parental care. They prefer to mate with large females that produce more eggs than small ones. In an Australian katydid (a type of cricket: Orthoptera) the male provides a large spermatophore (nutritional material transferred with sperm at mating). When food (pollen) is scarce males produce spermatophores slowly and are choosy in mating. But when food is abundant, males are not choosy. In the dance fly males obtain a nuptial gift of a dead insect then fly to a mating swarm of females where females display and males choose. The females inflate their abdomens and hold their legs beside the abdomen. Males presumably are attracted to larger females because they are more fecund, on average.

Cooperation and Conflict

Reproduction is an area where there is both conflict and cooperation. Males typically compete with each other for females, but occasionally a group of males will cooperate, as when low ranking baboon males may work together to obtain copulations with females guarded by dominant males. Males and females cooperate because both need to mate to pass on their genes. For example, the female insect that releases a sex pheromone and the males who are attracted to her are cooperating. But there is also conflict between the sexes. This is because their interests do not coincide fully. Male bean weevils Callosobruchus, for example, may harm females to disincline them from mating with another male. (Special Reading: Crudgington & Siva-Jothy 2000)

Andersson M. 1982. Female choice selects for extreme tail length in a widowbird. Nature 299: 818-820.

Andrade, M. C. B. 1996. Sexual selection for male sacrifice in the Australian redback spider. Science 271: 70-72.

Crudgington, H. S., Siva-Jothy, M. J. 2000. Genital damage, kicking and early death. Nature 407: 855-856.