Multiple matings and sperm selection by females

Photo of male sand lizard
Male sand lizard (photo: T. Madsen)

Thomas Madsen
Molecular Population Biology, Department of Ecology, Lund University, Sweden.

Males of most animal species will enhance their reproductive success if they mate often and with many different partners, whereas promiscuous mating is unlikely to increase female's reproductive success. Why then is multiple copulations by females so common?

During the last 10 years we have been investigating the potential female benefits of multiple matings in adders (Vipera berus) and sand lizards (Lacerta agilis). Like in virtually all other reptiles male adders and sand lizards do not exhibit any form of parental care and thus the only contribution by the male to the female is the transfer of his genes.

In a small, inbred and isolated adder population we found that females mating with many different partners exhibited an increased offspring viability1. We suggested the increased viability could be a result of more intense sperm competition within the females's reproductive tract, resulting in a higher proportion of her ova being fertilized by genetically superior males.

In our studies of sand lizards, where females like adders accept copulations from virtually all males, did we obtain a similar result i.e. the number of sexual partners, but not number of matings, was positively correlated with hatching success and negatively correlated with the proportion malformed young2,3. However, we also found that the subsequent survival of offspring was positively correlated with number of different partners. Thus offspring from broods with high hatching success had a better survival than offspring from broods with low hatching success.

In order to test whether sperm competition or female choice of sperm could explain our results, we scored the genetic similarity (DNA fingerprinting) of male sand lizards in relation to the female. We found that males with higher genetic similarity to the female sired a lower proportion of her offspring than did more distantly related males4. This result might also arise if males that produce more sperm fertilize higher number of offspring per copulation. However, proportional paternity was not correlated with either male's body size (and hence testis size), nor the duration of sperm replenishment since last copulation. Hence, we conclude that female sand lizards and most likely also female adders select sperm from the more distantly related males. Our data suggest that females are capable of subtle discrimination among potential mates, even after copulation, and may control microevolutionary phenomena to a greater degree than has heretofore been suspected.

References

  1. Madsen, T., Shine, R., Loman, J., and Håkansson, T. 1992. Why do female adders copulate so frequently? Nature 355: 440-441.
  2. Olsson, M., Gullberg, A., Tegelström, H., Madsen, T. and Shine, R. 1994. Promiscuous lizards: Females have more viable young. Nature 369: 528.
  3. Olsson, M., Madsen, T., Shine, R., Gullberg, A. and Tegelström, H. 1994. Rewards of "promiscuity". Nature 372: 230.
  4. Olsson, M., Shine, R., Madsen, T., Gullberg, A. and Tegelström, H. 1996. Sperm selection by females. Nature 383: 585.
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Last edited on 10 March 1997