Alternative Mating Strategies
In many species, sexual selection has driven the male population toward alternative phenotypes usually correlated with a unique mating strategy—called alternative mating strategies. A typical case involves males that differ in body size, as in the Pacific salmon, Oncorhynchus kisutch. Reproductive success is directly related to male body size, but some small males manage to obtain access to spawning females by sneaking into the nest (Gross, 1985). These differences in male body size are the result of different life histories. Large males (as well as females, which are also large in size) spend up to 20 months in the open sea before returning to freshwater streams to reproduce. By contrast, small males return from the sea after periods ranging between 5 and 8 months. Because these two mating tactics require different phenotypes, disruptive selection is probably operating in this case. Disruptive selection refers to situations in which extreme phenotypes have greater reproductive success than intermediate phenotypes (see Figure 1.15).
In salmon, alternative males follow one of two mutually exclusive tactics, and once they commit, switching to the other strategy is not possible. More commonly, alternative
Figure 11.6 A group of juvenile southern sea lions raid the breeding colony. (A) Raiders are approaching from the sea, in the center of the picture. (B) A raider male managed to isolate a female
Source: Photos courtesy of Claudio Campagna.
reproductive strategies are conditional; that is, animals can switch from one to the other depending on various conditions, including age and the social environment.
Young elephant seal males (Miroimga angnstirostris) provide an example of an age-dependent alternative strategy. Small males cannot successfully monopolize females. Instead of engaging in cosdy aggressive contests with large resident males, they stay in the periphery of a reproductive colony where they occasionally keep a female long enough to copulate (Le Boeuf, 1974). Juvenile southern sea lion males (Otaria byronia) take a different approach (Campagna, Le Boeuf, & Cappozzo, 1988). They gather in peripheral groups of about 10 males and raid the breeding colony, provoking a great confusion. As a result, a young male can occasionally gain access to an isolated female and defend her from other males (Figure 11.6).
The social environment can also affect switching among strategies. In the green tree frog (I lyla cinerea) some males do the calling that attracts females into their vicinity, whereas other males, called satellite males, remain quiet and approach a pair to ejaculate when the opportunity for fertilization arises. Satellite males take advantage of the fact that fertilization is external in this species. Moreover, whether a male follows a “caller” or a “satellite” strategy depends on the social context (Perrill, Gerhardt, & Daniel, 1982). When calling males were removed, a large proportion of satellite males began calling within 5 min of the manipulation. By contrast, when calls were played back through speakers, a significant proportion of calling males became silent.
Sequential hermaphroditism, the capacity of females in some species to turn into males under certain conditions, illustrates a remarkable instance of an alternative mating strategy. In many species of fish that reproduce by external fertilization (no specialized genitalia or internal embryo growth), adults exhibit sex changes, such as from female to male, male to female, and bidirectional. For example, blue-head wrasse, Thalassoma bifasciatum, develop as male or female and reproduce in groups. As they grow in body size, however, males change in coloration and settle in spawning territories; but so do females, which soon begin to produce sperm. The male-to-male and female-to-male developmental trajectories can be viewed as alternative life histories. If reproductive success of one sex is higher when small (young), but that of the other sex is higher when large (older), then a sex change when body size increases above some value becomes adaptive (Todd, Liu, Muncaster, & Gemmell, 2016). Population density, competition, and predatory pressures are additional factors associated to sex change. In coral gobies, Gobiodon histrio, bidirectional sex change minimizes predatory risk by allowing any two individuals to form a breeding pair without relocating to a different area of the coral colony. Sequential hermaphroditism involves changes at multiple levels, from behavior to anatomy, neural and endocrine mechanisms, and gene expression.
