Exchange of parental roles and sexual selection
It follows from theoretical models and empirical data that more intense sexual selection acts on the members of the sex for which the dependence of the number of fertilized offspring on the number of sexual contacts has the greatest slope, i.e. on the sex exhibiting the steepest Bateman gradient (Andersson & Iwasa 1996) (see Fig. XV.1). The reasons for the steepness of the Bateman gradient can differ for the two sexes. The commonest factor consists in the uneven costs of the production of microgametes and macrogametes (see above). However, there are a number of species of animals for which the two sexes exchange roles and the male invests a greater amount of energy into the production of progeny. In these cases, the male and female frequently also exchange roles in sexual selection; the males are more choosy and more obvious secondary sexual traits appear in the females. The reasons for exchange of parental roles are known in only a few cases.
Exchange of parental roles occurs relatively frequently in fish. In some species, the male carries the eggs and fry in a special cavity in his body (sea horses); in other cases the male builds a sort of nest in which he cares for the eggs and fry. At the present time, the opinion predominates that exchange of roles occurs in connection with territoriality in males. If a male must fight for and defend a territory, then he invests more in reproduction than the female. In addition, spawning occurs in this territory so the eggs and fry remain in the care of the male.
Some authors think that exchange of roles in fish could be related to a greater rate of diffusion of milt compared to the diffusion of eggs (Dawkins 1976). This difference means that the male must wait to release the milt into the water until the female releases her eggs. He thus finds himself at a certain disadvantage; at the moment when he fertilizes the eggs, the female can have already left. Consequently, the male must make the choice of whether to care for the embryos and ensure they survive, or to also leave and waste all the expended energy. While it may be advantageous to leave first (the partner must care for the progeny), it is clearly disadvantageous to leave second (there will be no offspring). It is quite possible that the short time that the male must remain at the spawning site after the female could play a decisive role, i.e. can decide who will finally care for the fry. It is good to recall that, in sessile aquatic organisms, the greater rate of diffusion of microgametes means that it is necessary to release microgametes first, where the release of macrogametes is frequently triggered by the presence of microgametes. However, in immobile organisms, the investments into the two types of gametes are the same, the male and female investment into care for progeny is zero and consequently ecological and ethological differentiation of males and females need not occur (Williams 1975).
Exchanged roles have been observed in insects, for example in some species of water bugs of the Belostomatinae family (Smith 1979). The female lays her eggs on the back of the male and he then carries them, defends them and ensures that they get enough oxygen for about three weeks. The total number of eggs corresponds to twice the weight of the male and care for them is a substantial burden for him. Females can copulate with a number of males, but a male decides whether he will accept eggs from a female or not. As the area of the backs of males is a factor limiting reproduction, fierce competition occurs amongst females for males willing to accept batches of eggs.