You’re at a party and you’re single. Unfortunately, so are most of your friends, and they have already paired up with the most attractive people in the room. Do you try your chances with the ones who are left, even though you don’t really like them?
Animals face these sorts of decisions all the time. For example, the vast majority of birds stay with their mates for at least one breeding season. Some even mate for life. None of them have an infinite choice of mates, and some individuals will inevitably end up with an inferior partner. And according to Simon Griffith from Macquarie University, that can be a very stressful experience.
Griffith worked with the stunning Gouldian finch. It’s hard to think that any bird so beautiful could be unattractive but even for them, looks matter. The birds have either red or black heads, and all of them prefer to mate with partners that share the same colour. If a mixed pair gets together, their chicks are 40-80% more likely to die early. So for a female, it pays to be choosy about her mates.
Griffith set up a mass speed-dating event by introducing large groups of Gouldian finches, all strangers, into six aviaries. Eventually, they all paired up and settled down in a set of next boxes. If females ended up with males of the opposite head colour, they laid their first egg nearly a month later than those who landed a male with the same colour. Their blood also contained around three times as much corticosterone, a hormone produced in stressful situations.
If Griffith forced the females into arranged partnerships with specific males, he found the same thing. Females who were saddled with unattractive males laid their eggs later and had more corticosterone. Griffith found that their hormones spiked just 12 hours after they met their partner and stayed high for several weeks. The speed of the spike suggests that the male didn’t do anything to boost the female’s hormones. She was just reacting to his presence.
Griffith writes, “These females are making the best of a bad situation and are dissatisfied with their partner although he does represent a better option than not breeding at all.” So why feel stressed about it? Here, it would be too easy to anthropomorphise the birds, by picturing them sulking in their nest boxes. But remember that Griffith measured the levels of a hormone; he didn’t interview the birds about their emotions.
Griffith thinks that a surge of stress hormones could actually help the finches. Stressed animals often put less effort into reproduction. After all, when conditions are tough, it might make sense to focus on surviving rather than breeding. And if finches are stuck with an incompatible partner, they could also benefit from belaying their resources until they find a better one. The hormones could even affect a female’s behaviour, making her more likely to seek out a better partner for some sneaky sex on the side. A female who’s stuck with a poor male isn’t out of options.
Reference: Griffith, Pryke & Buttemer. 2011. Constrained mate choice in social monogamy and the stress of having an unattractive partner. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.2672