Parental care is one form of parental investment and investing in one young detracts from the ability of a parent to invest in others (Trivers, 1974). Parents are thus expected to allocate investment to one young until the cost of giving that care exceeds the benefit in terms of survival of the infant or, more correctly, in terms of survival of the half set of genes a diploid organism passes on to its sexually produced offspring. Each parent should attempt to maximize its production of viable offspring, that is by judicious allocation of resources it should attempt to maximize its genetic contribution to the next generation. Investment in unrelated young, however, will detract from the ability of an animal to produce its own offspring and hence will lead to a reduction in fitness. Mechanisms are, therefore, expected to evolve to ensure that investment is not wasted. It is the purpose of this chapter to examine some of these mechanisms by which parents reduce the possibility of investing in non-kin.

A parent could also suffer a loss of fitness if it were to harm its own offspring. This is a possibility because males and females of many species commonly utilize conspecific infants as food. For example, this occurs in fish (reviewed by Dominey & Blumer, 1984), gulls (reviewed by Mock, 1984), and rodents (Elwood, 1977; Sherman, 1981). It is important that parents should avoid cannibalizing their own offspring except in extreme circumstances (Labov et al, 1985).