In the animal kingdom, females usually invest more energy than males in reproduction and parental care of offspring – in biology, this is known as the Bateman principle. Scientists attributed the unequal efforts to the asymmetry of gametes, in so far as eggs are energetically expensive and limited by internal cycles, while sperm is abundant and cheap. As a result, a traditional understanding of sexual selection emerged, with males competing with one another for a limited resource, and females being extra choosy when searching for a mate.
Seahorses are black swans (statistically, not taxonomically)—they deviate from our understanding of what is normally expected, engaging in a profound biological role reversal. When mating, a female deposits up to 1,500 eggs into the brood patch of the male. The male then releases sperm into the water to fertilize the eggs. For the next several weeks, the female will visit the male for only a few minutes each day, leaving her partner to the parental duties, such as producing sufficient prolactin and oxygen. If successful, muscular contractions occur, and the male seahorse dispels the offspring; within a day or two, he is ready to act as Mr. Mom once again.
As one would hypothesize from the Bateman principle, Texas A&M University evolutionary biology researcher Dr. Adam Jones and colleagues have shown that the male pregnancy results in a reversal in mating behavior, in which the females compete for males and the males end up being choosy.
But, years earlier, Dr. Amanda C.J. Vincent, the Research Chair in Marine Conservation in the Fisheries Centre at the University of British Columbia, Canada argued that the sex-role reversal “had been tacitly and explicitly assumed.” Upon placing male and female seahorses in an aquarium, she observed that only the males tail-wrestle with one another and were behaviorally more aggressive than females. In other words, it appeared as though the males were paradoxically putting more effort into acquiring a mate, despite appearing to invest more energy in producing offspring.
However, Dr. Heather D. Masonjones, now at The University of Tampa, measured the oxygen concentrations of seahorses during pregnancy and demonstrated that males contribute half as much energy as females do when producing offspring. The study confirmed the standard hypothesis, that male seahorses fit the more common mating system of the more-invested sex being the more-choosy one.
A more recent study by Dr. Beat Mattle and Dr. Tony Wilson from the Zoological Museum at the University of Zurich in Switzerland also showed that seahorses have conventional sex roles, with choosy females and attention-seeking males. By exploring the importance of partner body size in courtship and mate selection, the researchers observed that the male seahorses were highly active, showing a clear preference for larger partners; in contrast, females were less discriminatory, showing an ambiguous mating affinity.
For now, the evolutionary origins and consequences of the mating strategy remain partially unknown, but the mystique and marvel of the seahorse’s behavior will always remain.