PATERNAL CARE

When we finished last lecture we discussed the astouding ability of a Mexican free-tailed bat mother to direct her care only to her own pup, even though pups move about while the moms are away and other pups and moms try to steal a mother's milk as she searches through creches for her own pup. Such results show that parents will not care indiscriminantly for young.

Generally, males who help provision young show many behaviors that help to insure that they are the fathers of the young they are aiding (e.g., mate guarding -- see Alcock). In the swallow, the degree of paternal care is positively related to a maleís number of copulations with his mate and negatively related to the number of his mate's extrapair copulations. Males held captive briefly during their mateís fertile period showed much less care of offspring than control males (Moller, A.P. 1988. Anim. Behav. 36:996-1005). These results suggest that paternal care is provided only if the certainty of paternity is high.

OFFSPRING RECOGNITION

Alcock discusses interspecific variation in offspring recognition, e.g., bank vs. rough-winged swallows (Pp. 489-491). When bank swallow nestlings fledge, their parents have already learned their individually distinctive calls. In contrast, solitary nesting rough-winged swallow parents show no ability to recognize their own young when they are ready to fledge.

Another example is the black-legged kittiwake, a small cliff-nesting gull. Kittiwakes have no ability to recognize their chicks before fledging. Only rarely do chicks transfer between nests; however, some chicks forced out of their natal nest do become adopted and eventually fledge successfully from foster nests. If a chick transfers to a nest with an egg or a smaller chick, the foster parents will feed it rather than incubate their own egg or feed their own chick. Such adoption is maladaptive but perhaps occurs because (1) chick transfers are generally rare (mistakes are unlikely), and the consequences of making the mistake of evicting one's own offspring in the early stages of the evolution of chick recognition would probably be more costly to parental fitness. Kittiwakes learn to recognize their own young at the time of fledging and are very aggressive to fledglings from other nests who happen to mistakenly land at their nest. A student of mine found this out when studying kittiwakes on a hillside on Middleton Island. When she went to measure chicks, they started running from nest to nest, so she stopped but noted some of the transfers. When parents returned, they cared for whoever had ended up in their nest. One chick that transferred later flew into its natal nest as it was learning to fly; its true parents attacked and killed it as if it were an unrelated intruder!

Usually ground-nesting adult gulls learn to recognize their own chicks as the chicks become highly mobile (when the chicks are about a week old), but adoptions do occur at least occasionally. Chicks do sometimes transfer between territories and thus parents do, at least occasionally, misdirect their parental care. Ray Pierotti and I have suggested that recognition may not evolve because the opposite mistake -- rejection one's own chick, would be very costly to one's fitness if recognition isn't fool proof. Also, the tendency to feed the largest, most vigorous young may override any tendency to discriminate against foreign young (as in hosts of brood parasitic birds).

PARENT-OFFSPRING CONFLICT

Each individual behaves to maximize its own inclusive fitness even when it reduces the inclusive fitness of related individuals. Parents and offspring have the same general objective, i.e., enhancing offspring survival. Nevertheless, parents and offspring are likely to disagree about how important survival of that one offspring is relative to alternate options available to the parents. The fitness gain achieved by an offspring staying alive is greater for that offspring than for the parents, because that offspring is related to itself by one, while the parents are related to it by 1/2. Since parents are equally related to a particular offspring and other offspring, continued parental investment in a particular offspring becomes disadvantageous when the costs (measured in terms of reduced reproductive output via all other offspring) are greater than the benefits, measured in terms of the fitness accrued by investing more in that offspring. In contrast, offspring continue to benefit until the parents' cost is twice as much as the parents' benefit.

Siblicide sometimes be an example of parent-offspring conflict. In kittiwakes, parents can raise a brood of 2 chicks easily if food is abundant but at most one chick if availability is poor. At moderate levels of food abundance, the larger (older) chick in the brood may kill its smaller sibling if it isn't receiving enough food to grow rapidly. Suppose it would have a 30% chance of surviving to breed and its smaller sibling would have a 20% chance of surviving to breed if siblicide didn't occur. For siblicide to be advantageous to the parents, it would have to have a probability of survival of 50% (30+20), but it would only have to be 40% [30+(0.5 x 20)] to be advantageous from the chick's genetic point of view. Thus there is a zone of conflict (0.4 <p(S) < 0.5) in the genetic interests of parents and that one offspring. Actually, it appears that siblicidal kittiwake chicks aren't weighing genetic costs and benefits very precisely; siblicide occurs if the larger chick is hungry and aggressive and without respect to condition of the smaller chick. Weaning conflict in mammals also could be an example of parent-offspring conflict.

MATING SYSTEMS

Mating systems can be defined primarily according to the duration of the pair bond and the number of mates acquired by each sex. Most authors do not consider duration of the pair bond so that the exact meaning of particular terms, e.g., promiscuity, is variable and depends on a particular author's definition, e.g., Alcock doesn't use the term at all. We'll use the following definitions. Monogamy is mating one male and one female for one or more breeding attempts; note that y this definition a species is considered monogamous if males and females have only one mate one breeding attempt but switch mates from one breeding attempt to the next. Polygyny is mating between one male and 2 or more females; polyandry is mating between one female and two or more males. Finally, polygynandry is the simultaneous pairbonding and mating of two or more individuals of both sexes. Extra pair copulations are matings between individuals in instances when one or both of those individuals is already behaviorally pair-bonded with another individual.

Monogamy

Monogamy will tend to occur if females are uniformly (broadly) dispersed in space or if they are synchronous in their receptivity, (recall facultative monogamy in prosimian primates and explosive breeding in some toads--see below) even if there is no paternal care. Monogamy is also likely if both maternal and paternal care are extensive. Monogamy also can arise if females benefit by monogamy and control the breeding options for males (see Alcock).

If females are dispersed more or less uniformly in space, e.g., ice-breeding ringed seals, males simply can't range far enough to mate with more than one female. Small forest-dwelling, insectivorous primates are generally territorial, defending small territories that meet their energetic requirements. A male typically defends a territory that overlaps that of a single female. Thus the mating system is typically "facultative" monogamy but sometimes polygyny, depending on how many female territories are contained within a male's territory. Many small forest-dwelling primates are nocturnal, avoiding predation by cryptic behavior, which also would favor solitary behavior.

Monogamy may also be the most prevalent breeding system in "explosive breeding assemblages." Although Alcock discusses these in the context of polygyny, a high degree of synchrony among females precludes multiple mating by males, i.e., high sychrony results in monogamy.

Monogamy appears rare in mammals but is prevalent in birds. Ninety-one percent of all bird species are monogamous. Monogamy in birds is closely correlated with nearly equal parental investment by males and females. Monogamy will be advantageous if reproductive success is significantly increased by cooperation of the two mating adults. This seems the case for birds, but why not mammals? Since mammalian males do not produce milk, they have a limited ability for direct investment in offspring. Also, most mammals feed on energy-poor foods, and it would be difficult for most male mammals to effectively provision their offspring. The general contrast between birds and mammals is that male mammals emphasize mating strategies and male birds emphasize progeny rearing strategies.

Polygyny

Emlen and Oring (1977. Science 197:215-223) show how ecological factors affect mating systems. In resource defense polygyny, or territorial polygyny, males gain access to females by controlling certain vital resources that the females require. Mating systems of this type could result in strong intrasexual selection pressure on the males. If females select breeding sites on the basis of habitat quality rather than on the basis of male characteristics, epigamic selection would not be a significant factor in the male display, but there would be strong selection on traits that result in successful competition with other males.

Because monogamy is widespread and certainly the ancestral mating system in birds, considerable attention has been given to the evolution of territorial polygyny in birds. Jared Verner and Wilson developed the polygyny threshold hypothesis (see handout). If variability in habitat quality is high, unmated females should pair with already mated males in high quality territories if they can expect higher success there than by mating with an unmated male on a poor quality territory.

An alternative hypothesis of the evolution of territorial polygyny is that relatively few males may hold all the territories such that the number of females seeking to breed is much greater than the number of available territories. Consequently, a female can have some success mating with an already mated male but would have no success if attempting to mate monogamously and all males are already paired. If the operational sex ratio (number of reproductively active males:number of reproductively active females) is skewed with an excess of females, polygyny may be the only option of breeding for many females. Of course, in such situations we should also ask why the sex ratio would become so skewed in the first place. Generally, male survivorship is depressed in polygynous species because of the major emphasis on male-male competition.

Red-winged Blackbirds appear to fit the polygyny threshold model because reproductive success of females in large harems is typically equal or higher (recall our earlier discussions of the ideal-free and ideal-despotic distributions) than that of females in small harems or in monogamous breeding units (see Alcock). In most redwing populations the male provides no parental care, and success of all females on a territory is therefore similar.

The bobolink is another species in the blackbird subfamily, but bobolink males, unlike redwing males, aid in rearing the young of their first mate (but generally not the second mate). Second mates even on good quality territories typically have lower success than monogamous females. Is this a contradiction of the polygyny threshold model? Jim Wittenberger studied bobolinks and accumulated evidence that the quality of territories of unmated males is relatively poor compared to that of territories with females. Thus apparently a female is better off as a second mate on a good quality territory than as the only mate on a poor quality territory.

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