This entry was written by Chloe Tan as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2
Introduction
It has been long known that consistent differences between the sexes exist. These sex differences have been documented widely among numerous taxa that the concept of sex roles have been introduced. Across many taxa, typical sex roles are as such: Males compete for mates, while females are choosier and provide more parental care (Janicke et al., 2016). It is easy to formulate proximate reasons for this trend. Females are constrained biologically by egg-laying or gestation to provide more parental care, freeing males to invest more in mating. Ultimate reasons are more subtle. To gain an evolutionary understanding of sex roles and why females tend to care more, it is essential to return to the fundamental sex difference - gamete size. In this essay, I outline how a small initial difference between the sexes, anisogamy (unequally sized gametes), can lead to large behavioural disparities in competition, choosiness, and care.
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Anisogamy
Contrary to popular belief, what defines a male and female is not dependent on who bears the babies, or outward phenotypic appearances. Anisogamy refers to a form of sexual reproduction where there are two types of gametes (sex cells): Large and small. Males are the sex with smaller gametes, while females have larger gametes. The different gamete sizes are the first sex difference in parental investment. Egg cells (female gametes) are larger and costlier to make in terms of time and resources. Females provide more resources for the offspring (thus providing more parental care), translating to higher chances of offspring survival.
Trivers’s explanation
It is impossible to discuss parental investment without reference to Trivers (1972). Trivers suggested two reasons for why females care more, which are still cited today despite their shortcomings.
(1) Because females have a greater past investment in offspring (from having large gametes), females stand to have “more to lose” by not continuing to provide ample care for their offspring.
(2) Because it takes a shorter time for males to produce sperm again after mating, males have a higher potential reproductive rate than females and thus have more to gain from investing more in mating effort instead of parenting effort.
The problem with these explanations is that (1) is flawed due to a common logical error termed the Concorde fallacy (Liker et al., 2015). Rational decisions do not consider past investments, but rather focus on future losses and gains. Logical parents should not consider their initial investment in gametes when making present and future parental care decisions. (2) does not work because although the potential reproductive rate of males is indeed higher than that of females, the actual reproductive rate of males must be equal to that of females. The Fisher condition states that every offspring in a sexually reproducing species must have a father and mother. Hence for every male that produces an offspring, there is also a female that reproduces. With a sex ratio of 1:1, the actual reproductive rate for males must be equal to that of females (Fromhage & Jennions, 2016). The main problem with this explanation is that it is implied that anisogamy alone will inevitably result in the typical female-biased care pattern.
Not anisogamy alone
How else might a small initial asymmetry in parental investment be explained to lead to widely contrasting sex roles? The answer as proposed by Fromhage & Jennions (2016) is that anisogamy alone is insufficient to lead to stable sex differences. Anisogamy can be viewed as the prerequisite to develop such stable sex differences, however, its relationship with divergent sex roles is mediated by multiple mating and sexual selection. A mathematical model yielded the result that anisogamy alone does not result in the typical pattern of females caring more. Instead, it is equally likely to result in either females or males providing more parental care up to the point of uniparental care, depending on the sex-specific costs and benefits of caring.
When the model factors in paternity uncertainty and unequal strengths of sexual selection between the sexes, the solution equilibrates at a bias towards female care. Males evolve to care less when they experience greater sexual selection pressures and sperm competition (Kokko & Jennions, 2008). Empirical support for this explanation shows that sexual selection and social environment are strong predictors of parental care patterns, while gametic investment is not (Liker et al., 2015).
The refined link between anisogamy and typical sex roles is as such: Due to anisogamy, females are the rarer sex in the ‘mating pool’ as it takes a little more time to produce a resource-rich, large egg than a sperm. The operational sex ratio is biased towards the sex initially investing less (usually males). Members of that sex are subsequently selected to become more competitive. The more competitive sex may also then experience greater mortality which further selects against caring, as caring becomes more costly. This might be the case with sexual ornaments, which hinder survival while not reaping the benefits of attracting mates when the individual is busy providing parental care. The initially more caring sex must now care even more to compensate for the loss in parental care (Fromhage & Jennions, 2016). When females mate multiply, males experience sperm competition and therefore paternity uncertainty. The rate of returns for caring is lesser for males than it is for females because males may end up caring for unrelated offspring. Sexual selection further selects for males that care less and females that care more (Kahn et al., 2013). Sexual selection and paternity uncertainty kick-start a positive feedback loop that drives anisogamy towards non-egalitarian sex roles.
Conclusion
Parental care seems to be the cornerstone trait that explains other sex-role traits such as choosiness and competitiveness. Once a sex provides more care, it affects their availability in the mating pool, altering the costs and benefits of being choosy and competitive. Typically, females care more, are less available than males, and can afford to be choosier with who they mate with. As such, males compete more to be chosen as desired mates.
If anisogamy itself does inevitably lead to female-biased care as Trivers implied, it would be impossible to explain phenomena such as sex role reversal. Females are not always more caring and choosier, and males are not always more competitive. Birds famously demonstrate a wide variety of parental sex roles with the majority exhibiting biparental care, although female birds lay eggs exceedingly larger than male birds’ sperm (Liker et al., 2015). Sexual selection, paternity certainty and possibly other environmental factors allows for the flexibility to explain for atypical sex roles occurring in nature (Kahn et al., 2013).
Literature Cited
Fromhage, L., & Jennions, M.D. (2016). Coevolution of parental investment and sexually selected traits drives sex-role divergence. Nature Communications, 7, 12517.
Janicke, T., Häderer, I.K., Lajeunesse, M.J., & Anthes, N. (2016). Evolutionary Biology: Darwinian sex roles confirmed across the animal kingdom. Science Advances, 2(2), e1500983.
Kahn, A.T., Schwanz, L.E. & Kokko, H. (2013). Paternity Protection Can Provide A Kick-Start For The Evolution Of Male-Only Parental Care. Evolution, 67(8), 2207-2217.
Kokko, H., & Jennions, M.D. (2008). Parental investment, sexual selection and sex ratios. Journal of Evolutionary Biology, 21(4), 919-948.
Liker, A., Freckleton, R.P., Remeš, V., & Székely, T. (2015). Sex differences in parental care: Gametic investment, sexual selection, and social environment. Evolution, 69(11), 2862-2875.
Trivers, R.L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man (pp. 136-179). 1871–1971. Aldine Transaction, Chicago, IL.
