Sex Role

in animals

Francesco Cavraro

     In the field of reproductive ethology, a classic behavioural pattern sees males of one species competing for access to the partner (Anderson, 1994). To win this competition, during evolution, male individuals developed a series of somatic traits. These traits are called secondary sexual characters and include striking colours and other ornaments, or threatening armaments. Despite this, in some animal species, females compete more intensely than males for access to partners. This phenomenon is called sex-role reversal.

     Usually, it is the female sex that invests most in reproduction and, possibly, in the care of offspring. In addition, the reproductive success of females is usually limited by gamete production, while the success of males is limited by the availability of partners (Bateman, 1948). But sometimes things go in the opposite direction. In species where sex-role reversal is present, besides an increased competition between females for access to the partner, there is also a stronger selection of the partner by males and the presence in females of secondary sexual traits. The best documented cases of sex-role reversal have been described in fish and birds.

The reproductive success of females is generally limited by the production of gametes, while the success of males is limited by the availability of partners.

     In most species, females are the most ‘interested’ to mate with a quality partner, which ensures the transmission of good genes to future generations, but not in species that show sex-role reversal. A famous example is the jacana, a sort of South American moorhen, which spends most of its life scampering with its very long tapered fingers on the leaves of water lilies. On the water lilies the jacana also builds its nest. Being a rather risky placement, the female of this bird cannot afford to invest everything in a single clutch, as a female duck would do, thus here comes the sex-role reversal: in the social organization of the jacana an adult female defends a territory, hosting a harem of males, against the intrusions of other females, potential competitors. Each male of the harem builds a nest (with little help from the female), cares for the eggs that are laid there and also takes care of the chicks until independence, under the “supervision” of the dominant female. This female, unlike most bird species, is larger than the male (weighs up to 50 % more) and possesses more striking secondary sexual traits.

     In fish, although paternal parental care is widespread, males usually compete more intensely than females for the access to the partner. Cases of sex-role reversal in this group mainly concern species where males carry eggs or embryos on their bodies, and consequently the time and space available for egg care are limited. A classic example of sex-role reversal is found in pipefish. We have already seen in a previous article how Syngnathidae (seahorses and pipefish) are a particular family of fish in which females lay eggs on a special pouch present in the abdomen of males. In two pipefish species, Nerophis ophidion and Syngnathus typhle, females are the main competitors for the access to the partner, while males are more selective than females in accepting mating. Males benefit from their selectivity by choosing larger females, which will lay larger and higher quality eggs, ensuring a greater probability of survival of the offspring. In addition, in both species of pipefish, females exhibit ornaments that constitute secondary sexual traits, which are used during competition with other females or during the mating ritual with the partner. These secondary sexual traits represent an ‘honest signal’ of the quality of the female carrying them.

     Sex-role reversal in fish can also be observed in species where parental care is particularly long or requires a specific nest, difficult to find or able to accommodate only a few eggs. Normally, males of the bavosa Salaria pavo compete for females, but also practice parental care, using natural cavities as nests. When a female approaches the nest, the male begins the courtship. If the courtship is successful, after spawning and fertilization, the female leaves, while the male remains to guard and take care of the embryos until hatching. Almada et al. (1995) however, noted that, when the availability of nests is scarce, females also compete for access to the partner, and it is always the females who take the initiative and begin courtship. Sex-role reversal in a species is therefore not something determined a priori, but can only be present in some populations, as a consequence of particular environmental conditions.

[1] Almada V.C., Goncalves E.J., Oliveira R.F., Santos A.J. (1995) Courting females: ecological constraints affect sex roles in a natural population of the blenniid fish Salaria pavo. Anim. Behav. 49, 1125–1127

[2] Andersson M., 1994. Sexual selection. Princeton University Press, Princeton, NJ, pp. 599

[3] Bateman A.J., 1948. Intra-sexual selection in Drosophila. Heredity 2, 349–368

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