Gannets have always been thought to be
loyal partners, but it turns out they have a
40 per cent divorce rate.
Image: iStockphoto

Australasian gannets once thought to pair-bond for life have a divorce rate similar to humans, but risk breeding failure with a new partner, according to research from The University of Auckland.

“It’s always been assumed that these birds are monogamous, and they’ve been held up as an example of fidelity for humans,” says Ms Steffi Ismar who led the research as part of her PhD. “In fact, our research has shown that the divorce rate from one breeding season to the next is around 40 percent.”

The novelty of this finding may say as much about how humans view the world as about the birds themselves.

“In the past people have projected their values onto other species and assumed that some species form lifelong partnerships, but as our society has become more liberal our approach to research has changed and we are starting to challenge some of these assumptions,” says Ms Ismar.

The study also showed that individuals who divorced and found a new mate were less likely to successfully raise a chick in their first breeding season compared with those who kept the previous year’s partner.

This may be due to the common phenomenon of breeding failure in newly-formed couples, followed by success at subsequent attempts, amongst species that require teamwork to raise their young.

“Like many animals, Australasian gannets cooperate to raise their chicks, taking turns to incubate and feed their young, and look after themselves,” says Ms Ismar. “Gaining experience of how to co-operate with a new partner may be the critical factor in ensuring that adults can successfully raise their young.”

The high rate of re-partnering, despite being associated with reduced breeding success, suggests that gannets must choose between the lesser of two evils – waiting too long for their former partner to arrive and missing out on breeding altogether, or making the best of a suboptimal breeding opportunity.

“Our results seem consistent with the ‘musical chairs’ hypothesis of adaptive divorce theory, which says that maintaining access to a high quality territory is crucial for mate bond decisions,” says Ms Ismar “Birds who have secured a good territory can’t wait for ever for their old mate to arrive - eventually they just have to get on with it.”

Data from the upcoming breeding season and further analyses will give further insight into why pairs separate, whether the same individuals repeatedly divorce, and whether new pairs really do have greater breeding success in their second year.

The research team studied banded Australasian gannets at Cape Kidnappers during consecutive breeding seasons in 2007 – 2008 and 2008 – 2009. Their results have been published online in the European journal Naturwissenschaften.