Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Burrowing owl (Athene cunicularia)
Habitat: Old mammal burrows throughout North and South America
What's out of sight is out of mind. It's perhaps not surprising that relatively rare predatory birds like owls are faithful to their sexual partners. After all, if they don't come into contact with many other owls, any temptation to stray would be easier to resist.
For one species of owl, though, the lure of a different sexual partner can be just a few steps away. The burrowing owls of Argentina have embraced town life, nesting in large numbers in urban areas near cities including Bahía Blanca. While owl nests in rural areas can be separated by as much as 15 kilometres, urban nests may be little more than 10 metres apart. So do urban owls stay faithful, like their country cousins?
Burrowing owls get their name for their tendency to set up home in abandoned mammal burrows – the only owl known to do so. They stand a little over 20 centimetres tall, on a pair of strikingly long legs. Those come in handy during hunts, during which the owls often run down insects and small mammals on foot rather than swooping on them from above. These are not your typical night owls either: they are often active during the day, feeding at dawn or dusk.
Burrowing owls are found throughout the Americas, although northern populations are dwindling as their preferred grassland habitats are being destroyed by human activity. In South America, the owls are still common, and recently they have begun moving into urban areas.
In 2011, Martina Carrete and José Tella of the Doñana Biological Station in Seville, Spain, found out that the owls' larger brains appear to lead to more behavioural variation within the species, with owls that are less fearful of humans being the ones most likely to live in urban environments around Bahía Blanca.
We know that urban life can also influence the sex lives of birds. For example, blackbirds exposed to the bright lights of the city instead of rural darkness develop to sexual maturity earlier. Curious to see whether there were any effects on the sexual behaviour of urban owls, the team returned to Argentina.
Over six consecutive breeding seasons the team captured and took blood samples from a total of 1100 owls – both chicks and adults – from 61 urban and 7 rural nests. Genetic analyses showed that all rural chicks were the offspring of the two adults looking after the nest.
Despite the urban owls' greater opportunity to stray, the same applied in urban settings too, even though birds in other species are known to cheat on their partner when population density increases. Only in one case were the chicks not related to the adult male looking after the nest. Either female owls only have eyes for their partners, or male owls in urban settings watch their partners very carefully, the researchers conclude.
Dieter Lukas at the University of Cambridge, who studies the evolution of monogamy in mammals, thinks the results make sense. "While differences in breeding density are likely to directly affect changes in the social behaviour, mating behaviour is likely affected by a number of additional factors," he says. "For example, for some species of birds it has been suggested that the need for paternal care might constrain both partners to remain faithful even if [population] densities change."
Peter Dunn at the University of Wisconsin-Milwaukee agrees that other factors are important. He says the owl's longevity may come into play. "The basic idea is that if you are long-lived, you have other opportunities to breed, and so males are more likely to abandon a brood if their paternity is in question."
Greater population densities probably do test animals' faithfulness if those densities become really high, though. Populations of breeding mammals, for instance, are usually 10 times as dense as birds of similar body size, says Lukas. "Some researchers have suggested that this could help to explain why social monogamy is much more widespread among birds than among mammals."
Last week, the owl monkey was identified as a mammalian exception to the rule: genetics show it is the only primate known to be truly monogamous. Perhaps that's because baby owl monkeys need the care of two parents if they are to survive, says Lukas. Or perhaps there's something about the word "owl" that just instantly encourages fidelity.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0091314
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