Trivers 1972: Parental investment and sexual selection
Continuing the series of classic papers on mating systems, Hazel and I lead some discussion on Bob Trivers’ book chapter on how parental investment relates to the sex roles. Bateman (1948) had already suggested that the difference between the sexes in the returns on investment might be related to anisogamy (the difference in gamete size across the sexes), but Trivers examined (using formal theory rather than empirical experiments) the possible effects of investment in all stages of offspring (not just the gametes themselves). He reasoned that since all kids have two parents, the sex that invests more becomes limiting to the other one. The relative investment of the sexes in their young is therefore the key variable controlling sexual selection.
Trivers was careful to point out that although “investment” can be energetic or metabolic, it doesn’t need to be for his theory to work. For example, risky behaviour like hunting or singing (which could lead to predation or parasitism) is also a form of investment and needs to be part of any calculations comparing the sexes.
Trivers also was careful to notes the circular nature of the relationship between investment and mating system, a theme that will undoubtedly resurface as we continue our tour through classic papers on mating systems: parental investment affects sexual selection (e.g., by controlling which sex is in short supply), but sexual selection also affects parental investment (e.g., by determining how much energy is left for care, for example).
The paper also pointed out a few features of the natural history of mating in animals that were likely to be important (observations whose importance we might be able to confirm with the benefit of hindsight). Here are a few haphazardly selected points that will probably feature in future discussions:
- The timing of investment (females usually invest substantially in gametes before mating) creates asymmetry between mating partners in the risk of desertion: males may have invested comparatively little in a clutch after mating and so they risk little by deserting, whereas a female may be trapped into caring for the young or losing all of her investment. Conversely, the risk of cuckoldry is asymmetrical in the opposite direction, because males can rarely be completely certain about paternity in the same way that females can trust their relationship to eggs.
- The differential risks and returns on parental investment have important implications for sexual differences in mortality. Trivers spent some time arguing that differences in mortality are not simply a consequence of chromosomal differences (i.e., the fact that in most mammals males are heterogametic), but rather imply adaptive differences in investment in longevity. Whether sexual differences in lifespan are generally adaptive is a continuing focus of quite a lot of research, including by our own Tom H.
- Female choice is probably related to some important aspects of paternal investment. Research over the past twenty years on the relative importance of direct and indirect benefits owes much to this initial analysis.
- In his closing sentence, Trivers reaffirms one of the fundamental insights that has shaped sexual selection research since this paper:
“Throughout, I emphasize that sexual selection favors different male and female reproductive strategies and that even when ostensibly cooperating in a joint task male and female interests are rarely identical.”
This recognition of sexual conflict would have to wait some time to be fully appreciated, in part because the empirical literature had plenty of work to do in testing Trivers’ theory on how parental investment affects sexual selection. In future posts, perhaps we can assess how much of that work remains to be done.