Biology 441. Animal Behavior

Lecture 20. Wednesday, 13 November 1996

POPULATION EFFECTS OF DOMINANCE AND TERRITORIALITY

If some individuals are forced into smaller territories, suboptimal habitats or excluded from breeding altogether by the dominance or territorial behavior of others, dominance and territoriality may have population-level effects. Territoriality has been implicated as a factor affecting population dynamics of some populations of microtine rodents and of red grouse in Scotland, for example.

Dominance interactions and status in dominance hierarchies may influence survival. In winter, many birds form stable flocks; dominance hierarchies, typically quantified at concentrated food sources such as feeders, are well developed. Individuals with higher dominance status typically have higher survival. Fro example, Steve Fretwell found that most juncos in the top half of dominance hierarchies survived the winter and most in the bottom half disappeared and presumably died. In winter territorial red grouse in Scotland have a high probability of survival but floaters have a very low probability of survival.

Would different numbers of individuals survive if there were not dominance hierarchies or territoriality in juncos or red grouse, respectively? Contest competition (also called interference competition or encounter competition occurs when access to resources (e.g., food, space) is determined by the outcome of dominance interactions among individuals. Scramble competition (also called exploitation competition or consumptive competition) occurs when resources are limited and individuals reduce or deplete them without interacting behaviorally. Let's consider a species that feeds on seeds that have been produced the previous growing season but not in sufficient numbers to feed all individuals of the species for the whole winter. If there are no dominance interactions everyone will likely find sufficient resources for much of the winter but will be adversely affected, more or less equally, as resources become depleted in late winter. Thus, population numbers would stay high but then drop precipitously in late winter. In contrast, winter dominance hierarchies and territoriality put some individuals at a competitive disadvantage from the beginning and those individuals probably die well before resource depletion approaches. Thus, the winners of contest competition will likely have enough food for the whole winter because there will be less and less competition as the winter proceeds. Note that the result, not the function, of contest competition is the damping of population fluctuations--functions must be interpreted in terms of net advantages to individuals.

SEX DIFFERENCES IN DISPERSAL

In Chapter 10, Alcock discusses sex differences in dispersal differences between natal and breeding areas. In birds females typically move farther, while in mammals, males typically move farther. In both cases inbreeding is minimized but why should the sex roles be reversed? Territoriality may play a role. In birds, it is typically the males that defend the territory and males seem to be more successful if they defend territories in or near areas they have previously lived. The female's efforts in territorial defense generally are much less pronounced or nonexistent. Most male mammals do not defend territories (e.g., most ungulates). However, in those mammals in which males are highly territorial, e.g., chimpanzees and African wild dogs, males tend to stay in their natal area and females disperse. In most species it seems likely that one sex will benefit more by staying on or near the natal area, and dispersal distances seem to be shorter for that sex.

INBREEDING DEPRESSION AND OPTIMAL OUTBREEDING

The argument that sex differences in dispersal distances have evolved to avoid or minimize inbreeding presupposes that inbreeding is deleterious. However, surprisingly few studies have actually documented that inbreeding does reduce fitness. Katherine Ralls et al. (1988. Conserv. Biol. 2:188-193) tabulated extensive data on effects of inbreeding from studies of zoo populations. Matings between full siblings (r=0.5) or between parents and offspring (r=0.5) resulted in increased mortality rates of offspring in 36 of 40 species (see handout).

On average, the cost of such extreme inbreeding was 33%, but there was considerable variability and a few species showed little or no adverse effect. Extremes were brown lemurs, with a 90% increase in mortality rate and Sumatran tigers, which showed only a 0.3% increase in mortality rate. Perhaps the evolutionary history of the species affects the results. If a population occurs at low numbers and experiences inbreeding depression, some deleterious alleles may be lost, both due to selection and chance. Subsequently, inbreeding would have less deleterious effects. This may be happening with cheetahs: recent studies have found no genetic variability (see handout). However, this is a major anomaly: most natural populations have considerable genetic variability. It appears that cheetahs were reduced to low numbers and genetic variability was lost in recent geological time. Although cheetahs have not yet gone extinct, the loss of genetic variability may be the reason cheetahs seem particularly susceptible to infectious diseases. As we'll see, genetic variability seems to be a key to individual fitness and population persistence.

There have been few field studies of inbreeding levels or inbreeding depression. In shis studies of prairie dogs, Hoogland (1992. Am. Nat. 139:591-602) documented levels of inbreeding and found that most mating was between somewhat related individuals (see handout) and he could find no evidence of inbreeding depreesion.

Some authors have suggested that extreme outcrossing may be detrimental to fitness because it precludes adaptation to local conditions. Patrick Bateson showed experimentally that Japanese quail not only avoid breeding with very close relatives but also prefer distant relatives over unrelated individuals (see handout, Fig. 2). Bateson proposed that social and mating preferences are balanced to preclude either close inbreeding or extreme outcrossing (see handout, Fig. 1).

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