This entry was written by Luke Rudloff as part of a project done in BIAN 2133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
In species which reproduce sexually, traditional sex-roles (ie those seen most frequently in
nature) involve male-male competition, vying for access to females for sex, with females
investing most into their offspring. This often involves sexual selection in males for more
extravagant features or ornaments to aid in courting females, such as the train of a peacock or the antlers of an elk. Conventional sex-roles have been attributed to numerous factors, including differences in the gametes of the different sexes, and the associated energy costs of reproduction, as well as other factors such as paternity certainty and individual fitness benefits of mating frequency related to Bateman’s principle. However, while less common, there exist species where these sex-roles are reversed, with males investing most into parental care and females competing for access to males. This reversal of sex-roles has puzzled biologists since it was recognised, with the most recent and prominent explanations focusing on the role of adult sex-ratio (ASR) and operational sex-ratio (OSR). I will discuss the evidence for the link between both OSR and ASR and sex-role reversal, as well as discuss examples in humans where sex-role may be reversed.
Main Text
The role of sex ratios
Species which exhibit sex-role reversal are those in which males invest most into parental care and are choosy when choosing a mate, while females court males and compete for mates. Studies have investigated the role of OSR (ratio of individuals of each sex ready and available to mate) in influencing sex-role reversal in insect and pipefish which exhibit sex-role reversal. Berglund (1994) experimented with a sex-role revered species of pipefish (Syngnathus typhle), manipulating the OSR from female-biased (which is usual for the species) to male-biased. He observed that when the OSR was female-biased, males exhibit preference over a female during mate choice, choosing to spend more time with larger females rather than small ones, which Berglund (1994) identifies as a predictor for mating preference. On the contrary, when OSR was manipulated to be male-biased, males spent equal time in close proximity to females without showing preference to female size. Males in male-biased sex ratio scenarios also had higher
levels of activity, representative of frequent movement between potential mates rather than maintenance of proximity to the largest female. Berglund (1994) concludes that male
choosiness increased as the OSR becomes more female-biased, decreasing as the sex ratio becomes more male-biased.
Similar to Berglund’s (1994) paper, a study of honey locust beetles (Megabruchidius dorsalis) also found that a more female-biased OSR was positively correlated with sex-role reversal (Fritzsche, Booksmythe & Arnqvist, 2016). Fritzsche, Booksmythe & Arnvqist (2016) compared beetles which had evolved under both female- and male-biased sex ratios, finding that females who had evolved in female-biased sex ratios were significantly more successful in mating and courtship compared to those from male-biased evolution lineages. This increase in mating success was associated with sex-role reversal, and thus a product of sexual selection resulting from the female-biased sex ratio, supporting the conclusion of Berglund (1994).
Contrary to the aforementioned studies, a paper by Liker, Freckleton and Székely (2013), which investigated ASR rather than OSR, found that sex-role reversal occurred as the population became more male-biased. This result matched their prediction, that the rarer sex would be under selection to provide less care than the opposite sex, with their proposed explanation that it may be more beneficial for males to invest in parent care in their offspring rather than trying to acquire a new mate when the ASR is male-biased. Although this result seems to contradict studies which found female-biased OSR to be associated with sex-role reversal, as ASR and OSR are used in similar contexts, a female-biased OSR does not necessitate a female-biased ASR and vice versa. If, for example, males in a population had a significantly longer period of “time out” from the breeding pool compared to females, then a population with a male-biased ASR could have a female-biased OSR, as females would enter the breeding pool more quickly after reproduction. In Liker, Freckleton and Székely’s (2013) study, it may have been the case that when the adult sex ratio was male-biased, the reversal of sex role and associated greater investment into parental care by males caused males to be significantly under-represented in the breeding pool, causing the OSR to be female-biased.
Sex-role reversal in humans
In humans (Homo sapiens) anthropologists have noted, cross-culturally, a greater emphasis on female attractiveness compared to male attractiveness. This posed a problem to the notion that humans exhibit traditional sex roles, as male extravagance and female drabness would be expected under these roles, rather than the reverse. Gotschall (2007), who attempted to explain this anomaly, proposed that Homo sapiens is a partially sex-role reversed species. He attempts explains the emphasis on female attractiveness with the proposition that females have genetic traits that are highly variable and detectable by males, and which signal fecundity or the genetic quality of the female. Furthermore, this is not the only anomaly in human sex-roles, as a study by Weerth and Kalma (1993) looking at sexual jealousy in humans found that females were self-reportedly more likely to be aggressive if they became of the “victim” of partner infidelity. They had predicted that males would be more aggressive in such a situation, as the greater paternity uncertainty in males was expected to be associated with greater aggression towards partner infidelity. They attribute this unexpected result to the decrease in paternity uncertainty brought by the increased use of contraceptives by females, causing a reversal of traditional sex-roles in this facet. These are examples of partial sex-role reversal in humans.
Conclusion
Reversal of traditional sex-roles occurs in numerous taxa, including birds, insects and fish.
Studies show a link between female-biased OSR and sex-role reversal, as well as between
male-biased ASR and sex-role reversal. These results seem contradictory but may be explained by differences in OSR and ASR caused by differences in the time which each sex spends in the breeding pool. While sex-role reversal on this level has not been observed in humans, the greater emphasis on female attractiveness cross-culturally may be best explained by humans being a partially sex-role reversed species. Finally, there is also evidence of reversal of traditional sex-roles when looking at aggressive reactions in response to partner infidelity in humans.
References Cited
Berglund, A. (1994). The operational sex ratio influences choosiness in a pipefish. Behavioral Ecology, 5(3), 254-258.
de Weerth, C., & Kalma, A. (1993). Female aggression as a response to sexual jealousy: A sex role reversal?. Aggressive Behavior, 19(4), 265-279.
Fritzsche, K., Booksmythe, I., & Arnqvist, G. (2016). Sex Ratio Bias Leads to the Evolution of Sex Role Reversal in Honey Locust Beetles. Current Biology, 26(18), 2522-2526.
Gottschall, J. (2007). Greater Emphasis on Female Attractiveness in Homo Sapiens: A Revised Solution to an Old Evolutionary Riddle. Evolutionary Psychology, 5(2), 347-357.
Liker, A., Freckleton, R., & Székely, T. (2013). The evolution of sex roles in birds is related to adult sex ratio. Nature Communications, 4(1).