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Image copyright Carin Bondar, 'The Nature of Human Nature'
It’s a common topic of conversation among new (human) mothers: how our population would cease to exist if males had to bear the children. The physical costs (including a 9 month gestation, followed by giving birth to a 6-10 pound live young) are extremely daunting and may not be as readily undertaken even if the physiology of the human male permitted it. It doesn’t end there. The duties of lactation and child care are generally responsibilities of the female parent, and such tasks involve a great deal of time and energy that could otherwise be spent creating more offspring to represent us in future generations. Human males seem to have it pretty good: biologically speaking they, like the majority of males in the animal kingdom, contribute little more than genetic material to their offspring. Although it may at first seem as though males get off easily when it comes to their ability to contribute to future generations, it’s not all fun and games. Males almost universally compete with each other for sexual partners (and in many cases this leads to the evolution of elaborate physical structures, coloration or behaviors1). In addition, the mere contribution of sperm to the reproductive tract of a female does NOT guarantee that a particular males’ seed will be the successful fertilizer (see ‘Artifical Insemination’). So both males and females have their own difficulties when it comes to procreation, although without question the human female would argue that her male counterpart would not be willing to do the child rearing in her place. Do any females in the animal kingdom have it figured out a little better than us?
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Enter the family Syngnathidae, commonly referred to as pipefishes and seahorses. These fish are ‘sex-role reversed’, which means that males take on the pregnancy and childcare role and females experience more intense competition for mates. Females deposit their eggs into a males’ brooding pouch, and he is therefore guaranteed paternity once he fertilizes them2. The brood pouches found in different species are categorized from simple membranous egg compartments on the males’ ventral side to fully enclosed brooding pouches with placenta-like structures3 (aka male bellies). The ‘pregnant’ males take on the duties of osmoregulating the environment within the brood pouch, aerating the eggs and providing nourishment. However, females don’t get off without some investment of their own. They are faced with an interesting conundrum when it comes to competing with each other for mates: egg production is still required (which has a high energetic cost, unlike the metabolically cheap sperm production), so females do not have the same energetic freedom as males do when it comes to producing expensive sexual ornaments. If such ornaments are produced, they are done so at a potential cost to fecundity, which could make them less attractive to potential mates4. So what can a female sygnathid do to increase her chances to fill a males’ brood pouch? Although they are not as bizarre as structures seen on males in traditional sex roles, females do develop reproductive ornaments that are used both to attain mates and to deter other females4. In addition to the reproductive ornaments, females busy themselves with attempts at ruining the reproductive efforts of other females. ‘Mating disruption’ occurs when large females swim in between a male and female pair while they are mating (how rude!), effectively ending the transfer of eggs5. Large females are effective ‘mating disrupters’, and they have also been shown to influence the behavior of smaller females through intimidation. The mere presence of larger females has been shown to interfere with and substantially decrease reproduction in smaller ones5.
So the overall conclusion is this: when it comes to syngnathid fish males are choosy and females are competitive. Perhaps it’s just the human in me but I feel like all that competing is rather undignified female behavior…it simply isn’t lady-like to disrupt a couple in the throws of passion (or egg transfer)! Although child rearing and care are difficult jobs (and I maintain that the human population would cease to exist should the sex roles be reversed in our species), I think that it is more empowering to be the one who chooses. No offence to all you males out there, but if we (females) are going to do all the work to rear and care for your offspring you’d better be prepared to compete for our affections.
1Darwin, C. (1871). “The descent of man and selection in relation to sex.” Murray, London.
2Jones, A. G., G. Rosenqvist, A. Berglund, and J. C. Avise. 1999. The genetic mating system of a sex-role-reversed pipefish (Syngnathus typhle): a molecular inquiry. Behav. Ecol. Sociobiol. 46:357–365.
3Wilson, A.B., Ahnesjo, I., Vincent, A.C.J. and Meyer, A. 2003. The dynamics of male brooding, mating patterns, and sex roles in pipefishes and seahorses (family Syngnathidae). Evolution 57: 1374-1386.
4Berglund, A. and Rosenqvist, G. 2003. Sex role reversal in pipefish. Advances in the study of behavior 32: 131-167.
5Berglund, A. 1991. Egg competition in a sex role reversed pipefish: Subdominant females tradereproduction for growth. Evolution 45, 770-774.