What's the point of falling in love? If you've been through a rough breakup, you've probably asked yourself that question. Long-term relationships (by that we mean reproductive relationships that last for more than one breeding attempt) are pretty rare in animals overall. Humans do this, of course, but our closest relatives, the chimpanzees and bonobos, don't. Neither do most mammals, amphibians, fish, non-avian reptiles, or insects, although there are a handful of representatives that have this breeding strategy in all of these groups, meaning it evolved independently multiple times. The exception is birds. Birds are super cool because among the more longer-lived species, long-term partnerships are very common, which is surprising especially considering how rare this strategy is throughout most of the tree of life.

The core question our lab is interested in is: What is it about keeping a mate year after year that is so beneficial that it would be so common in birds, and would have evolved many different times? There are probably many reasons, but a potential option is that it allows partners to trust each other. Offspring care by two parents (biparental care in nerd speak) tends to go hand in hand with long-term partnerships, suggesting that a lot of it has to do with child-rearing. 

For many birds, laying an egg is a big deal, and previous work Dr. Amy has done suggests that it is enough to hamper a female's ability to be a good mom. This puts her in a tough spot: there are benefits to putting more energy into the egg because chicks from bigger eggs tend to be stronger and healthier. However, a really big egg will make it harder for her to care for the chick down the road. The solution to her problem is the male- if she can rely on him to be a good dad, then she is free to put plenty of energy into the egg and trust that her partner can pick up any slack. But can she trust him? It is risky to rely on him too much the first time they breed together because she doesn't know if he will do a good job. After their first breeding season together, she might be more likely to lay a larger egg that hatches into a healthier chick, relying on her partner to do a bit more of the childcare. Among species that form long-term bonds there is something termed the "mate familiarity effect", where we tend to see that pairs who have been together for longer tend to have more, healthier offspring than pairs who are breeding together for the first time. This is irrespective of the individual experience of the parents independently of each other- time spent together is important. This idea of female investment we call the "trust hypothesis"- that the mate familiarity effect is at least partly due to sex-specific division of labor after the first breeding attempt where females put more energy into the egg and males put more energy into childcare. 

We have some evidence for this hypothesis in Cassin's auklet from Dr. Amy's Ph.D. work but now we want to know if trusting your partner is a reason we see these types of long term relationships evolving again and again. We are using two other species that form long-term pair bonds, the Atlantic puffin and Eastern screech-owl, to answer this question. By combining behavioral recording of nest attendance in both male and female, physiological measurement of the stress and health of both parents and chick, monitoring chick growth, and measuring eggs, we hope we will be able to build a more general theory of what makes it worth it to fall in love.