from acquiring resources accumulated by the previous generation but also it places them at substantial risk while searching for new resources. In fact, when large amounts of resources are accumulated during one generation, parents will probably allow their offspring access to these resources well into adult life, unless their own life expectancy is still substantial. On the other hand, when life expectancy is still substantial, one might expect parental eviction of offspring to avoid direct competition for those resources. Indeed there is an encyclopedic literature (summarized in Baker, 1978) on the timing and extent of forced dispersal, but elderly individuals are by definition those with low life expectancy. Therefore, I expect that eviction of offspring will be rare among elderly animals. Systematic information on this sort of question has not been collected, mainly because field biologists have assumed that senescence is irrelevant to animals in the wild.

Bequeathing Resources to Young

Some mammals, at least occasionally, share access to valuable material resources with their offspring. For instance, bannertail kangaroo rat females frequently allow offspring, potentially of either sex, to share their home mounds (Jones, 1987). Thus a 3- or 4-year-old female who has a low life expectancy and shares a mound with her offspring is ultimately making a decision to bequeath them that mound. In a population in southeastern Arizona, 37 percent of males remained in their natal mound into adulthood, and 70 percent of these remained throughout their lives. Twenty-three percent of females remained in their natal mounds into adulthood, and 72 percent of these remained for their entire lives. Because mound improvement and renewing of cached seeds occur throughout life, resources acquired during the lifetime of females are passed along to their offspring. No information exists on whether males bequeath mounds to their offspring (Jones, 1987): however, it is improbable that males of this polygynous species can even recognize their own offspring.

In many species, this sort of resource bequeathing is sex-biased. As a gross generality, males are more apt to be bequeathed valuable resources such as nests, dens, or territories in birds, whereas females are more apt to inherit such resources in mammals (Greenwood, 1980). Thus in many communally breeding birds that defend valuable territories, territory ownership is passed down male lineages. For instance, among stripe-backed wrens (Campylorhynchus nuchalis), 60 percent of breeding males gained breeding status on their natal territory compared with 3 percent of breeding females (Rabenold, 1990). Conversely, among black-tailed prairie dogs (Cynomys ludovicianus) in Kansas, 98 percent of females bred in their natal coterie compared with 0 percent of males. The advantages of male-biased versus female-biased dispersal are still unresolved, but such dispersal is assumed to be related to aspects of mating system (Greenwood, 1980; Waser and Jones, 1983). It is generally assumed that dispersal of one sex is

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