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When you studied animal behaviour in year 13 you probably learned about the different mating systems: polygamy (polygyny & polyandry), promiscuity – & monogamy: a bond between a single male & a single female. You may also have heard that in some species, such as swans, that bond is life-long.

It turns out things are more complex than that. My first inkling of this came back when I was working on the literature survey for my PhD on behaviour in black swans, & found (rather to my surprise – I was probably too naive for my own good back then) that swans indulged in some definitely non-standard breeding practices, with more than one paper describing a menage a trois (one female, 2 males). Cuckoldry wasn’t unknown, either. Plus, our own stitchbirds are known to practice polyandry, polygyny, and monogamy as well; homosexuality’s been documented in a large number of animal species; & of course there’s that recent example of oral sex in fruit bats. Anyway, the concept of variations on monogamy in swans got me thinking & I realised that for monogamy the idea of life-long fidelity probably didn’t hold either – after all, if your current partner is incapable of producing offspring, or turns out to be a lousy parent, it’s probably worth your while (in genetic & evolutionary terms) to divorce them & find a new one. Plus, if one partner dies, the other is unlikely to sit around doing nothing (in the reproductive sense) for the rest of their life. But it wasn’t something I investigated further at the time.

A new paper by Stefanie Ismar & her colleagues (Ismar et al. 2009 – based on Ismar’s PhD research) looks at the frequency & costs of divorce in Australasian gannets. (Not the costs of a settlement – the reproductive & hence evolutionary costs!). Gannets have long been another example of life-long fidelity in animals. They nest in large colonies (the best-known in New Zealand are probably those at Cape Kidnappers & Muriwai beach) & are highly territorial within those colonies, aggressively defending the small area immediately around their nest site. Each breeding season the males arrive at the breeding colony first, re-establishing their territories & waiting for the females to turn up – the birds are philopatric, which means that they are strongly attached to the colony where they were hatched & will return there to breed themselves. Although this set-up is described as monogamous, Ismar et al. cite genetic research showing at least some chicks have ‘extra-pair parentage’ – they’re produced by matings between one partner & an outsider (I’d suggest sneaky copulations between the female & a roaming, possibly neighbouring, male, as it would be harder for a strange female to come in & lay eggs in an established pair’s nest).

The study population comprised individually-banded gannets in part of the Cape Kidnappers colony. Ismar recorded the presence/absence of individuals, the presence of pair bonds, & the reproductive success of breeding pairs (where possible comparing her data with those from previous breeding seasons). From this the team was able to determine if members of a pair had divorced (one living with a new partner but the previous mate also present in the colony) or if a mate had been lost (ie didn’t reappear at all in the subsequent breeding season).

They found that divorce rates ranged between 40-43% – so much for gannets faithful unto death :-) This sounds high – were the birds accruing any reproductive advantage through their fickleness? Well, over the 2-year study period, pairs that remained together had significantly higher reproductive success (they fledged more chicks) than individuals who’d lost a mate & subsequently re-mated. Similarly, divorcees fledged fewer chicks than birds who retained their mates from the previous season (although the difference was statistically significant only for the 2008-09 season). These results suggest that there’s a benefit to sticking with the same mate when you can, perhaps because it takes a bit of experience to learn to cooperate with a partner in rearing the chicks. This would explain why first-time breeding pairs often have a lower breeding success than long-established pairs.

So why the high divorce rate? The researchers suggest that this may reflect delays in partners arriving at the colony. A bird who waits too long for their beloved to turn up may end up not breeding at all, so divorce could represent a compromise that offers at least a chance of reproductive success (the entertainingly-named ‘musical chairs hypothesis’). However, it’s still too early to completely rule out the idea of divorce as a way of minimising mate incompatibility, or perhaps minimising inbreeding  - as the authors say, more research is needed to unweave the ecological & behavioural determinants of breeding success in long-lived seabirds. 

Ismar, S., Daniel, C., Stephenson, B., & Hauber, M. (2009). Mate replacement entails a fitness cost for a socially monogamous seabird Naturwissenschaften DOI: 10.1007/s00114-009-0618-6