SEXUAL SELECTION (CONCLUDED)

Numerous recent studies have demonstrated a much greater role of female choice in mating patterns than had formerly been evident. Female three-spined sticklebacks preferentially mate with males with the most intense red coloration. More brightly colored males have lower parasite loads than less brightly colored males. Males in better condition can probably better maintain their nests and care for their eggs (see Milinski and Bakker. 1990. Nature 344:330-332).

Active female choice also has been demonstrated in an Australian frog (Robertson, J. G. M. 1990. Anim. Behav. 39:639-645). Females select males that are about 70% of their own weight, basing their choice on the frequency (pitch) of the maleís calls (the pitch of the call is closely related to the maleís body size). Fertilization success is highest if the maleís weight is about 70% of the femaleís weight (smaller males apparently donít release enough sperm to fertilize each egg [females lay their eggs individually], and larger males are too heavy to support throughout the oviposition process -- in the study 2 females paired with relatively heavy males were drowned).

ALTERNATIVE STRATEGIES

Whenever there is a high variance in reproductive success within one sex, some individuals of that sex that normally would be less competitive in intrasexual competition for mates might develop alternative mating strategies and consequently have some success. Such alternative strategies may simply be "making the best of a bad job" (e.g., young red deer males) or may be in an evolutionary equilibrium within a species.

Alternative mating tactics, like many other aspects of social behavior, have been evaluated in the context of evolutionarily stable strategies (ESS's). An ESS is a strategy that cannot be replaced by an alternative by the action of natural selection, i.e., fitness of individuals adopting that strategy is equal to or higher than fitness of individuals adopting other strategies. There are three general classes of ESS's: pure, mixed, and conditional. A pure ESS is a strategy that is consistently exhibited by individuals throughout their lifetimes. A mixed ESS is a complex of two or more strategies varying either within individuals through time or among individuals. The most interesting class of ESS's is the conditional ESS; in this case an individual's strategy varies under different conditions in the social or physical environment. If the evolutionary pay-off for strategy depends on what other individuals are doing and decreases as a greater proportion of the population adopts that strategy, other strategies may also be ESS's (frequency-dependent selection).

John Hogg (1984. Science 225:526-528) studied rutting behavior in bighorn sheep and found three male mating strategies, "tending, coursing, and blocking." Tending rams consort with estrus ewes within the rutting groups and prevent approaches by other (coursing) males; only large-horned rams tend. Coursing rams are usually young and sometimes are successful in disrupting a tending pair and may mount and copulate with ewes on the run before being displaced by the tending ram. Blocking rams cause ewes to move away from the ewe bands and keep individuals or small groups separated from the main rutting aggregation by aggressive threats and displays. Tending appears to be the primary strategy as it is the strategy of the largest-horned rams; coursing and blocking appear to be conditional strategies that subdominant rams adopt to make the best of a bad lot.

In ruffs the satellite and resident strategies are inherited (mixed ESS's). Residents defend territories on leks and don't tolerate one another but sometimes tolerate satellites. When a female arrives on a territory, she is courted by both and then the resident may chase the satellite away. Yet, if the resident is challenged by a neighboring resident, the satellite may copulate with the female. Satellites are tolerated better on smaller leks. The combined displays of residents and satellites on small leks may be important in attracting females there instead of to large leks with only resident morphs.

In southern populations of Coho (silver) salmon, (Oncorhynchus kisutch), there are two forms of males: "jacks" that mature and return to spawn as two-year-olds and typical "hooknoses" that return to spawn as three-year-olds. Jacks are relatively cryptic and gain access to females by sneaking around on the gravel beds used by spawning females. Large males are most successful getting close to females if they fight, and jacks are most successful if they sneak. The males that are closest to females when they spawn are most likely to fertilize the eggs. The probability that a jack will be successful in sneaking close to a female seems to be higher when jacks are relatively rare. Gross (1985. Nature 313:47-48) calculated survivorship of jacks and hooknoses between the time they left fresh water until they returned to be 0.13 and 0.06, respectively, but the jacks have a corresponding shorter life expectancy on the breeding grounds (8.4 days vs. 12.7 days, on average). You might consider the scenario for changes in the relative frequencies of jacks if fishing concentrates on hooknoses and fishing intensity increases, assuming either frequency-dependence or no frequency-dependence. The perplexing part of coho salmon male mating strategies is that the largest juveniles mature early and come back as jacks rather than maturing later and coming back as the largest hooknoses.

PARENTAL INVESTMENT

Parental investment is defined in terms of fitness in the behavioral literature and is an investment in particular offspring that reduces the parent's investments in other offspring during its lifetime. Therefore, the term is used in a very specific way in behavioral ecology.

PARENTAL CARE

Alcock suggests that high reliability of paternity is not critical for paternal care. On page 482, Alcock uses a numerical example to support his claim that indiscriminate paternal care can evolve. His example follows: if each male fertilizes 40 percent of a female's eggs when he copulates with her and a paternal male mates with two females who each lay 10 eggs and those eggs have a 50 percent change of survival, his reproductive success is 0.40 x 20 x 0.5 = 4 offspring. The nonpaternal male mating with 5 females and producing offspring with a 10 percent probability of survival without paternal care would have 0.40 x 50 x 0.20 = 2 offspring, but would also benefit greatly by the indiscriminate care by other males of eggs that he fertilized. If all 5 of those females were to associate with a male providing care indiscriminately, the noncaring male's success would be 0.40 x 50 x 0.5 = 10 offspring. Indiscriminate paternal care is not evolutionary stable -- in a population of males which provide paternal care indiscriminately a male that cheats and provides no care would be able to mate with more females and his offspring would have the same chance of survival as those of males that do provide care. Thus, it is not surprising that males behave in ways to assure their paternity when they do care for offspring.

DISCUSSION: maternal care in Mexican free-tailed bats.

Return to lecture notes index.

Back home.