209 APS Animal Behaviour
APS209: Lecture 11. The Evolution of Reproductive Behaviour (Alcock Chapter 10)
Aims 1. Present a simple model for the evolution of male (small gamete) and female (large gamete) roles. 2. How sex differences in parental investment select for typical sex roles (e.g., choosy females) & exceptions. Objectives 1. Learn examples 2. Understand the underlying logic in the evolution gamete size differences and typical sex roles. More than any other aspect of an animals life, reproduction affects the passing on of its genes. It is, literally, passing on your genes. As a result, natural selection has a very major effect on reproductive behaviour and we can find many remarkable adaptations in reproductive behaviour. 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 matings 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. Females are often choosier. Males, such as the bean weevil Callosobruchus (previous lecture and special reading by Crudginton & Siva-Jothy 2000) may harm females to disincline them from mating with another male. Both males and females reproduce, but their roles and hence their strategies vary. Females are defined as the sex that produces larger gametes (eggs). A females reproductive output is typically 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. Female coho salmon lay many eggs, c. 3500. But this is tiny compared to the billions of sperm made by a male. It is probable that the typical situation with two sexes producing different-sized gametes evolved from an ancestral state in which gametes were all the same size. Gamete specialisation then evolved, with some individuals (females) producing large gametes (eggs) with generous resources for the developing embryo, and other individuals (males) specialising in small, mobile, gametes (sperm) that are good at finding the larger gametes, but which provide few or no resources. This is a case of disruptive selection with selection favouring the extremes. Because males produce many cheap gametes, sufficient for many matings, they can normally afford to be less choosy. But there are cases where the mating is costly to males and males are choosy. Although sperm are almost always cheap, males sometimes provide additional expensive resources to the female or offspring, including paternal care and nuptial gifts. In extreme cases this can lead to sex role reversal. Sex role reversal is important because it is an exception that proves the rule (i.e., tests the hypothesis). 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 their inflate abdomens and hold their legs beside the abdomen. Males presumably are attracted to larger females because they are more fecund, on average. Darwin defined sexual selection as the advantage that certain individuals have over others of the same sex and species, in exclusive relation to reproduction. Sexual selection is the same as natural selection in that it selects for traits that increase the transmission of copies of an individuals genes. The difference lies in how this can come about. Sexual selection 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. Sexual selection has repeatedly resulted in two things: male-male competition and choosy females. (As described above, the reverse can be selected for.) Males often have adaptations for fighting and are larger than females. Large males are selected for because they are better at male-male competition. In seals, 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. Males often have interesting tactics for obtaining mates. The marine isopod Paracerceis lives inside sponges and has three male morphs. One is large and is physically
APS 209 Animal Behaviour
dominant. The second is a female mimic. The third is small and sneaky. By mimicking a female or being sneaky the smaller morphs compete in ways other than using physical force. All three male morphs are equally successful, the small males are not making the best of a bad job. However, in scorpion flies (not true flies, Diptera, but Mecoptera) small males use mating tactics that are generally less successful when competing with large males. (In the isopods, different genes cause the different male morphs. If one morph was consistently less successful, then the genes coding for it would have been selected out.) Competition among males also occurs via sperm competition. Males are strongly selected to increase the fertilisation success of their sperm in species where females mate with multiple males. In some damselflies, a males penis is not only used to introduce his own sperm but also to r...