Male seahorses are sometimes labelled nature’s best fathers, given that they form close, monogamous partnerships with a female and carry the pregnancy from shortly after fertilisation all the way to birth. But female seahorses may see things differently. They forget the long-term bond they share with their male partner if the two are separated even for a few days.
Lined seahorses (Hippocampus erectus) are sexually monogamous, typically mating for life. Like other seahorses, they are unusual in that the males get pregnant, to support hundreds of babies. In the lab, these seahorses usually live about two years, during which time the males can get pregnant 15 to 20 times, says at the Chinese Academy of Fishery Sciences.
“We did this study to provide a case that may help answer the question of whether the broken pair bond of a sexually monogamous species can be repaired,” says Zhang.
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The researchers took captive lined seahorses that had bred successfully in the laboratory and separated the females from the males eight days after mating, while the male was pregnant. They then introduced the female to a new male and waited to see whether she would successfully breed with this new partner. For the females that did so, they again waited eight days and then separated the pair.
Finally, once the two males had given birth, Zhang and his colleagues reintroduced both to the tank with the female – along with a third, unfamiliar male – to see which partner the female would prefer to mate with when given a choice.
They found that the females showed no preference for either of their former partners: instead, females were as likely to partner with the first male, the second male or the third unfamiliar male.
The researchers also found that the females won’t choose their former partner if the latter is unhealthy. In separate tests, females would even mate with a new male while their previous partner was still pregnant.
What makes these findings particularly striking is that the team’s previous investigations, in which female and male couples weren’t separated, resulted in a different outcome. Here, females almost always stuck with their male partner if presented with a new potential mate.
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It is unclear why the females show less fidelity to partners after time apart, says Zhang. It may be that the bond between the male and female weakens in the absence of close proximity. Alternatively, the female may simply no longer recognise and remember her former partner – or partners – after some time apart.
Since seahorse pair bonds can become broken naturally in the wild – even strong currents can break up a long-term relationship, given that these fish are weak swimmers – the authors say that their findings show how normally monogamous females may be presented with opportunities to reassess their choices in life and settle down with a new partner. This may be especially important for females when the male is unfit.
“Seahorse babies are in the male’s pouch. When the male becomes unfit, the health of his babies will suffer,” says Zhang.
Journal of Fish Biology
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