In this article we will discuss about the role of males and females in parental investment.
Due to differences in gamete size (anisogamy), an egg (being larger) requires a greater investment of energy than does a sperm and are likely to be limited in number. This would lead to the males competing for the opportunity to fertilize them and will thus be subjected to sexual selection.
Due to the females already large investment, her ability to invest further may be limited and, thus, she would be choosy as to which male fertilizes her precious eggs. On the other hand, although only one small sperm fertilizes an egg, millions are generally required in each ejaculation to ensure fertilization even of a single egg.
Thus, a male’s investment in sperm is not necessarily of lesser worth. Moreover, in some species of insects, the male contributes nutritive substances in addition to sperm. Thus, males, too, can be expected to be choosy about his mate.
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Many other factors also seem to affect the contribution of each sex to parental care. Animals of a particular group seem to be predisposed to a particular pattern. Most species of fish do not care for their young; in those few species that do, it is generally only the males.
Birds tend to be monogamous (a mating type in which a male and female bond for some period of time) and both parents care for the young. Mammals, on the other hand, are generally polygynous (a mating system in which some males mate with more than one female), and the male contributes little to raising the offspring.
The explanation behind such differences has been accounted through two theories:
1. Trivers (1972), reasoned that the percentage of confidence might be the factor responsible. In species with internal fertilization, where sperm competition could take place, the male might be inclined to desert and seek additional mates. On the other hand, the female is confident of her genetic relationship to the offspring and so she invests further in it.
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2. Williams (1975) suggested that parental care possibly evolved in the sex that is most closely associated with the embryos. In eggs that are fertilized internally, evolution of embryo retention and live birth is more likely, followed by further care of the young by the mother. On the other hand, eggs that are layed and fertilized externally in a male’s territory, the male becomes most closely associated with them and additional paternal care is likely to develop.
Thus, both the above theories predict that internal fertilization favours female care, while external fertilization, male care.
Among other factors that may predispose the role of male and female in parental care are:
1. Males of birds can also incubate eggs and feed young ones. In the case of pigeons, males can even provide crop milk.
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2. In the case of mammals, gestation and milk production are restricted to the female. Males have relatively little that they can do to provide direct care for the young. In precocial (young are relatively advanced at birth) mammals, the opportunity for male investment are very low and males compete much more for multiple mates. On the other hand, in altricial (young are immature at birth) mammals, males and females generally invest equally.