Reproductive ecology of seahorses
Seahorses provide an ideal opportunity to test our understanding of the evolution of sex differences because of their unique specialization for parental care. Male seahorses undergo a lengthy “pregnancy” during which developing embryos are aerated, osmoregulated and nourished in an abdominal pouch, while females provide no care.
In most animals, females limit male reproductive rate and males compete to mate with females, while being relatively indiscriminate in their choice of partners. The consequent pressures of sexual selection are thought to explain why males are larger, more colourful and more conspicuous than females. Thus, in role-reversed species where males limit female reproductive rate and females compete for mates, females would be expected to show “male” traits.
Experimental manipulations in the laboratory and field work in Australia on several species of seahorse show that seahorses, despite intense male care, are not role-reversed. Although males appear to limit female reproductive rate, females remain faithful to one male. The same pairs mate repeatedly, with the male and female pairs coming together every morning to “Greet” one another. Pair bonding has the effect of increasing reproductive efficiency in seahorses: experimental manipulation showed that pairs which Greeted daily had shorter interbrood intervals and larger broods than males and females placed randomly together. Pair bonding has probably evolved as a result of the relative immobility and sparse distribution of seahorses.
Despite male limitation of female reproductive rate, male seahorses still compete more for mates. This unexpected result is probably explained by the male-biased short-term operational sex ratio: females take longer to prepare to mate and lose receptivity sooner than males. Although larger males win competitions, seahorses are essentially monomorphic, probably because competition is rare in the wild, or because size does not affect reproductive success differently.
The evolution of parental care in seahorses can be followed through a linear sequence of male pouch development found across the pipefish genera of the same family (Syngnathidae), culminating in the seahorse pouch. Comparative analyses of syngnathid reproduction indicate that role-reversal is probably the ancestral state in seahorses.
Vincent, A.C.J. (1990). Reproductive ecology of seahorses. Doctoral dissertation, Cambridge University, UK.