Anisogamy
Anisogamy (or heterogamy) refers to the fact that, in animals, the gametes produced by males and females differ in size. The spermatozoa produced by males are much smaller than are the ova produced by females. And it’s not just the gametes; the nutrients and tissues that surround the ovum and make up an egg further add to the amount of nutrients and energy a female invests in production of offspring. What’s more, in many species (notably mammals), females retain the developing embryos inside their bodies for some period of time. Females may even continue to nourish the young after they’re born (for instance, through production of milk).
What this means is that, in many species, females invest much more in the production of each offspring than do males. In these species, what limits the reproductive capacity of a female more than anything else is the quality of her mate(s). Since the number of offspring she can produce is sharply limited, it’s in her best genetic interest to choose the best mate that’s available, that she will have the healthiest offspring possible.
By contrast, since sperm are “cheap” to produce, what typically limits a male’s reproductive capacity is the quantity of his mates. Since a healthy male is typically more than capable of fertilizing all the eggs of several different females, it’s in his best genetic interest to mate with as many different females as possible.
Of course, this is a generalization. There are plenty of species in which males and females invest roughly equally in reproduction. There are even some species in which males invest more in the production of offspring than do females.
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