What’s Love Got to Do With It? The Truth About Seahorse Monogamy
“Unlike most other fish, they [seahorses] are monogamous and mate for life.” – National Geographic
It’s a theme repeated over and over again in the media and in popular culture. The idea that seahorses are monogamous and mate for life is one that excites the imagination and has helped them gain recognition and notoriety as the most romantic of fish. But is it true?
Seahorses are an ever popular symbol of the mystical and unique. And starting in the late nineties, their popularity grew as tales of their dedicated fidelity spread through news agencies like wildfire. The Shedd aquarium in Chicago opened their Seahorse Symphony exhibit in 1998, the most ambitious seahorse display of its kind. News outlets and laypersons were equally enamored with the fascinating seahorses and news outlets were happily passing on every bit of information about seahorses they could. Seahorses have a head like a horse, a prehensile tail like a monkey, and they fall in love! What more could the public want?
The seahorse true love meme seems to go back to a study done by Amanda Vincent and Laila Sadler about pair bonding in the seahorse Hippocampus whitei in November 1991 to April 1992. Their results were published in the paper “Faithful pair bonds in wild seahorses, Hippocampus whitei” in 1995. In that, they detailed their findings where the 98 seahorses they studied in the wild were part of monogamous pairs that stayed faithful to one another even if there were opportunities for mate switching. The only time new mate were found was if one disappeared, which only happened in a very small number of cases.
This was very big news. Monogamy is generally thought of something that occurs in higher animals, not fish*. Because of the findings, the paper hypothesized that pairing may endure over the winter, as it had been shown in a species of pipefish, a close relative of the seahorse.
Because seahorses are the media darling of the fish world, journalists leapt at the chance to write about the little fish that falls in love. And to most people, monogamy usually means a single mate for life, so the faithful pair-bonding seahorse soon became the fish that mates for life and with and dies without a mate. The public ate it up. This was great PR for conservation groups, because the perception is if one seahorse is taken from the ocean, its mate will die of loneliness, thus killing all future generations too. In short, it was a great tragedy that everyone loved.
There was a problem with this; the study that showed this was specific to one species of seahorse only, and covered a limited time frame. The study speculated that pairs may persist outside of the breeding season, but it was not actually observed. And while the study ran through most of the breeding season of H. whitei, it missed the beginning of the season so initial pairing was not observed.
The conclusion that they’d only have one mate and would not find another if one died came about on its own, most likely due to anthropomorphizing the ideals of this monogamous fish. But that actually ran contrary to the findings of the study. What it actually showed is that widowed seahorses pair up with the next available mate as soon as they can. However, this one fact continually escaped mention both from conservation group’s press releases and the media in general.
That wasn’t the only problem with the study. Many aquaculturists, aquarium professionals and hobbyists from all over the world who have had first hand experience with seahorses disputed the notion that seahorses mated for life. At least in captive environments, seahorses were not monogamous – that is, not in the mate-for-life sense. Many observed that some species, such as Hippocampus abdominalis don’t form pairs at all, while others formed pairs but switched partners willy-nilly; some seahorses being more pair oriented, others preferring new partners as often as they could.
Slowly, new studies began to emerge disputing the notion of exclusivity and monogamy in seahorses. In 2000, a study of Hippocampus subelongatus showed that male seahorses had a propensity for mate switching. While many were monogamous, almost half switched partners between broods. And less than 10% could be attributed to mate loss from things like predation. In most cases, the seahorses simply found other mates. Another study, published in 2004, opened up additional questions about the case for seahorse monogamy . The study took a look at the social structure of Hippocampus breviceps, a small seahorse native to Australia. These seahorses were observed living in groups, and mating with any available seahorse, with no exclusivity or pair bonding.
H. abdominalis, another Australian native, were observed in the wild for 5 years from 2000 to 2005, and observed to be polygamous, with one male being courted by several females. In this species, it appear that there is sex role reversal, with females competing for the affections of the male. These finding were published in a 2007 study confusingly named “Genetic monogamy despite social promiscuity in the pot-bellied seahorse (Hippocampus abdominalis)” . The aforementioned name refers specifically to the males only carrying a brood from one female at a time. Males switched partners between broods. A later study also revealed that the males preferred larger females, which may be a motivating factor in switching mates. In that study it was also shown that the females are the gender that competes for mates, and males play a role in the sexual selection of females. This gender role reversal has been well documented in various species of pipefish so it wasn’t surprising to find it in the closely related seahorses.
There was another study in 2009 that looked at a different species of seahorse, this time from European waters.  Hippocampus guttulatus showed that not only did the seahorses pairs didn’t last, but if an established pair existed, and the male was presented with a larger female, he would almost always choose the larger female, leaving behind his former mate.  In this species, the male was still the suitor, but still displayed similarities in mate preference to H. abdominalis.
There are many species of seahorses, and only a handful have been studied. However, the growing body of evidence shows that seahorses are not nearly as romantically inclined as we’d like to believe. Of the species mating activity studied in the wild, only one has shown any lengthy fidelity. Some species pair bond, and of those species, the bond is likely not permanent. It may last through the breeding season, or it may only last a couple matings until a bigger, sexier seahorse comes along.
While it is disappointing to the romantic in us all, it turns out to be great news for conservation efforts. It means if one mate is removed from a pair, it doesn’t necessarily doom the other, or the species as a whole. Seahorse will seek out another mate. That doesn’t mean that the disappearance of a mate is a trivial matter though, as different species will wait varying time spans before seeking out a new partner. And because a seahorses range is limited, as is their mobility, finding a suitible partner may be an arduous task. But what that does mean is that the misguided attempts such as this story of a lone seahorse should be re-evaluated.
2) Kvarnemo C, Moore GI, Jones AG, Nelson WS, Avise JC (2000) Monogamous pair bonds and mate switching in the Western Australian seahorse Hippocampus subelongatus. Journal of Evolutionary Biology, 13, 882-888.
3) Moreau, M.-A. & Vincent, A. C. J. (2004). Social structure and space use in a wild population of the Australian short-headed seahorse, Hippocampus breviceps Peters 1869. Marine and Freshwater Research.
6) Naud M.J, Curtis J.M.R, Woodall L.C, & Gaspar M.B (2009) Mate choice, operational sex ratio, and social promiscuity in a wild population of the long-snouted seahorse Hippocampus guttulatus. Behav Ecol 20:160-164
*And while it may be the case that people don’t think of monogamy in fish, there are, in fact, several other species of fish that are, in fact, monogamous.