The below set of notes is from APS209: Lecture 7: The Evolution of Sex, Sperm Competition and Alternative Mating Strategies.
The Evolution of Sex
It is probable that anisogamy, the typical situation with two sexes producing different-sized gametes, evolved from an ancestral state in which gametes were all the same size. Gamete specialisation then evolved, with some individuals (females) producing large gametes (eggs) with generous resources for the developing embryo, and other individuals (males) specialising in small, mobile, gametes (sperm) that are good at finding the larger gametes, but which provide few or no resources. Natural selection could favour a mutant individual (invading a population for the first time) that produces twice as many half-size gametes. However this advantage will only persist if the small gametes fuse only with large gametes and not with themselves, which is exactly what happens in sexually reproducing systems.
Sperm Competition and Cryptic Female Choice
In species where females mate with multiple males, males are strongly selected to increase not just their mating success but the fertilisation success of their sperm. So males engage in sperm competition, the competition between the sperm of two or more males inside the female reproductive tract.
In some damselflies, a male’s penis is not only used to introduce his own sperm but also to remove sperm from the female’s previous mates and polygynous male insects produce a higher proportion of viable sperm than monogamous male insects. Male Adélie penguins withhold sperm from their mates, with whom they copulate frequently, and allocate it instead to extra-pair females, with whom they may have only one chance to mate. (Special Reading: Hunter et al. 2000).
Monogamous male birds often guard their mates to prevent them from copulating with extra-pair males. Male Seychelles warblers reduce the number of successful extra-pair copulations that their partners have, by mate guarding during their female’s fertile period (Komdeur et al. 1999). Males that can’t guard their mates for ecological or social reasons, may protect their paternity by copulating frequently with their partners.
Females are not passive players in the competition for matings. Chickens are more likely to expel ejaculates of low ranking cockerels. Female blue tits solicit extra-pair copulations from large males that have high survival. These males father more chicks and the chicks are more likely to be recruited into the breeding population. Female domestic fowl are able to manipulate after it has entered their reproductive tract. They eject the sperm of subordinate males and accept the sperm of dominant males (Pizzari and Birkhead 2000).
Alternative Mating Strategies
What about males that are unsuccessful in the competition for mates, are there other ways of achieving reproductive success? In scorpion flies small males use mating tactics that have low success when competing with large males. However a low level of reproduction is better than no reproduction at all. These males are essentially making the best of a bad job.
The marine isopod Paracerceis lives inside sponges and has three male morphs. One is large and physically dominant, the second is a female mimic and the third is small and sneaky. By mimicking females or being sneaky the small morphs compete in ways other than using physical force. All three male morphs are equally successful, the small males are not making the best of a bad job (Shuster and Wade 1991). In Paracerceis, the different male morphs are genetically determined. If one morph was consistently less successful, the genes coding for it would have been selected out.
Hunter F.M., Harcourt R., Wright M. and Davis L.S. (2000) Strategic allocation of ejaculates by male Adélie penguins. Proc. R. Soc. (Lond) B 267:1541-1545.
Komdeur J., Kraaijeveld-Smit K, Kraaijeveld K. and Edelaar P. 1999. Explicit experimental evidence for the role of mate guarding in minimising loss of paternity in the Sechelles warbler. Proc. R. Soc. (Lond) B. 266: 2075-2081.
Pizzari T. and Birkhead T. (2000) Female feral fowl eject sperm of subdominant males. Nature 405: 787-789.
Shuster S.M. and Wade MJ 1991. Equal mating success among male reproductive strategies in a marine isopod. Nature 350: 608-610.
Lecture 7: The Evolution of Sex, Sperm Competition and Alternative Mating Strategies