Last week I wrote about female animals making choices of mates on the basis of the male’s property, nuptial gifts, songs and dances, or physical appearance. Those choices occur before copulation (in animals with internal fertilization: sperm meets egg inside a female) or spawning (in those with external fertilization: sperm meets egg outside the female). However, there are also some much subtler forms of female mate choice that have been revealed by recent research. These choices occur during or after copulation or spawning.
Here are a few examples: In some fruit flies, sperm size varies and females discriminate against small sperm—and thus against males that make small sperm. In ducks and fruit flies, preliminary studies suggest that females may be able to kill sperm that come from copulation with unwanted males. Females can eject sperm after mating in certain damselflies, flour beetles, flies, and songbirds, ridding themselves of undesired sperm and leaving themselves then available for a preferred mate.
One of the best-studied examples comes from a free-ranging population of chickens, of a breed similar to the red jungle fowl that is the ancestor of domestic chickens. Roosters maintain a strong dominance hierarchy among themselves, and hens prefer to mate with high-ranking roosters. These males provide hens with better courtship feeding, protection from predators, and protection from sexual harassment by subordinate males. Dominant males leave more descendents than subordinate males, because they mate more often. Dominance is heritable, so their sons are likely to be preferred partners also.
Subordinate males, however, obtain some copulations by forcing the hens to mate, even when the hens resist. When accosted by a subordinate male, hens make a distress call, which induces dominant males to disrupt the copulation attempt. In addition, even if the subordinate male achieves copulation, hens can eject his sperm immediately, even before he dismounts from her back. Then she is free to mate with a preferred, dominant male and the dominant male’s sperm can fertilize her eggs.
When female salmonid fishes release their eggs during spawning activity, the eggs are accompanied by ovarian fluids that can affect sperm swimming speed, longevity, and motility. Arctic charr females can change the composition and effect of their ovarian fluid, depending on just what male is trying to fertilize her eggs. Different females of rainbow trout may produce ovarian fluid with differing effects on sperm motility, and thus some females may be able to be choosier than others.
It is even possible that plants may be able to mate selectively with certain individuals. It is well known that many plants are self-incompatible—that is, they reject pollen from themselves or from genetic relatives (which share their genes). But, in addition to this common form of selectivity, experiments have indicated that some plants discriminate among the pollen grains of different unrelated pollen donors (males), thus allowing only certain males to fertilizes the ovules.
These kinds of subtle interactions that occur during or after mating have only been discovered rather recently. Many more possibilities in this hot research area are now being studied in a wide variety of organisms. The take-home message from these studies is that females are not necessarily passive receptacles for sperm, but at least in some cases they are able to exert what is called cryptic female choice.