The Evolution of Parental Care in the Context of Sexual Selection: A Critical Reassessment of Parental Investment Theory (2002) Wade MJ and Shuster SM
As we arrive at the fifth paper on the topic of mating systems, most of the themes highlighted in the discussion of Trivers’ 1972 paper have made an appearance: timing of investment, risks and rewards of mating strategies, and the influence of female choice. Again, these themes are common throughout Wade and Shuster’s paper, which emphasizes early gamete investment and its latent effects on the behaviour of males and females. Gametes are the first form of investment in offspring, and it is almost always females that invest in gametes the most heavily. Because males invest comparatively little in gametes, they can benefit from mating with multiple females, whereas females have little to gain from multiple sexual encounters, other than possibly an increased chance of successful fertilisation.
Wade and Shuster examine a classic paper by Maynard Smith (1977), which describes an Evolutionarily Stable Strategy (ESS) model. Through this model, Maynard Smith tried to formalize the ideas first articulated by Bateman (linked above); his model involves a nondescript species with typical sex roles, and assumes that re-mating within this species has both costs and benefits.
Maynard Smith’s ESS model shows that under some conditions it is possible to find both males that care for their offspring and females that desert theirs, despite the prevalence of indiscriminate mating in males and choosy behaviour in females. Past ESS models have shown that polymorphic traits, including polymorphisms in mating behaviour, can only evolve in a system if they offer equal fitness to one another. In the context of Maynard Smith’s model, this means that if atypically raised offspring (e.g. deserted by the female, cared for by the male) and those raised only by females (with deserting males) are exactly as viable as one another, the alternative mating behaviours will both be evolutionarily stable and remain.
Wade and Shuster take the ESS model by Maynard Smith and attempt to modify it in light of the state of evolutionary theory in the early 2000s. In their paper, they argue that Maynard Smith’s model made some critical invalid assumptions. For example, the original model did not take into account how male re-mating could affect female fitness (such as through withdrawal of care for the purpose of remating). To illustrate the problem with such an assumption, they demonstrate that under Maynard-Smith’s model, when the sex ratio was equal, deserting males would be unable to breed with multiple females – either that or deserting males could not exist when the sex ratio was 1:1.
Wade and Shuster added several components to the model to overcome these problems. They add a ‘payoff matrix’ into the model, in which deserters may have more offspring, but these offspring are less likely to reach adulthood, and sacrificing future reproduction to care for current offspring is an opportunity cost. Their model also takes into account the reduced number of available females when deserting males mate multiple times. It therefore considers females a limited resource, meaning deserting males shift the operational sex ratio. Wade and Shuster were able to simplify the model to a single equation.
The deserting strategy would become more common than the caring strategy when:
s/2 < p
(where s is offspring survivorship, and p is the probability of male mating.)
Because each offspring only has half of a parent’s DNA, strategic allocation to care (which increases offspring survivorship, s) must be scaled by half when assessing its value relative to male re-mating opportunities (p). When s/2 < p, desertion should be more prevalent (because fitness returns from re-mating outweigh those of caring), while care should be more prevalent when s/2 > p.
Wade and Shuster note that this solution satisfies the requirement of population genetic theory that male and female fitness should be equal, an important improvement over Maynard-Smith’s original ESS model.
In closing, they note that the presence of polygamous males and caring females under some conditions shows that sex roles can be the cause of differences in parental investment, rather than a consequence of initial differences in investment that are reflected in anisogamy, and that this inversion of the classic understanding “stands parental investment theory on its head”. While this may be a fair synthesis with respect to the conventional shorthand view, it is worth noting Trivers’ acknowledgement of the complex, reinforcing relationship between parental investment and sexual selection. Is this complex, bidirectional, causal relationship ultimately responsible for uncertainty about the causes of diversity in sexual differences among taxa? Maybe subsequent posts can help clarify this…