Sperm storage

What is sperm storage? Sperm storage is most commonly defined as the maintenance of sperm inside a female’s reproductive tract for an extended period of time. Because the fate of sperm post-copulation is important for understanding sexual selection, sperm storage is a particularly interesting characteristic that may have drastic impacts on the operation of post-copulatory sexual selection. Sperm storage aside, there is enormous variation among animals in the fate of sperm, including sperm that are viable years after mating and others that have very short periods over which sperm can fertilize eggs. For example, spawning organisms like sea urchins or salmon don’t store sperm at all, but honeybee and termite queens can keep sperm for years. Thus, sperm storage has clear implications for the evolution of mating systems and sexual conflict. A wide array of animals store sperm, including: earthworms; arthropods such as insects and spiders; birds, such as falcons, quail, finches and geese; reptiles, including turtles and tortoises, lizards, crocodiles and snakes; and a few mammals, for example bats, hares, horses and dogs (Figure A–D). In these taxa, sperm may be stored for a few hours to several years. In cases where sperm are only ‘stored’ for a few hours it may be more likely the trait observed is actually sperm longevity rather than female sperm storage. Sperm longevity is the period of time spermatozoa are able to survive independent of female maintenance in an environment with the correct conditions for sperm survival but differing from the natural site of sperm storage. Sperm are frequently observed within a female’s reproductive tract long after mating. This characteristic is often dubbed ‘sperm storage’, but for true female sperm storage the female must provide a reproductive tract environment conducive to sperm survival (with the requisite pH, immunological activity and so on). Quick guide Thus, sperm storage results from selection operating on both male (the ability of sperm to live a long time) and female (the ability to maintain sperm) traits. It is interesting that neither the simple definition of sperm storage nor the dichotomy of male versus female roles in sperm storage have received much attention. While both sperm storage and longevity are necessary for sperm maintenance in a female’s reproductive tract, careful use of terminology is necessary for researchers interested in understanding the biology (particularly co-evolution) of either trait. This problem extends to how researchers determine if a species exhibits sperm storage. The usual criteria for determining that a species stores sperm range from simply finding sperm within a female’s reproductive tract long after she has mated to more thorough evaluations of both the female’s physiology and the sperm’s viability. Indeed, implied in the definition of sperm storage above, though not always tested, is that the sperm must be viable (able to successfully fertilize an egg) at the termination of sperm storage. Thus, there are varying degrees of certainty that different taxa actually store sperm. Studies aimed at understanding the evolutionary origins or consequences of sperm storage need to be certain that species are only said to store sperm after rigorous inquiry. We suggest sperm storage requires that the female plays a role in maintaining sperm, otherwise it would be a male process (sperm longevity) occurring independent of selection on the female. For sperm storage to evolve, sperm must gain an ability to survive for long periods of time and remain viable, but the female must provide an environment conducive to sperm survival. Thus, sperm storage results from selection operating on both male and female traits. What is the use of sperm evolving long lives when survival in the environment of the female’s reproductive tract is low? Likewise, females are unlikely to evolve reproductive tracts able to maintain sperm if the sperm are unable to live longer and thus correlated evolution is a must for sperm storage to evolve. As a result, both sperm storage and longevity exist on a continuum ranging from a few hours to years.