Know thy neighbours

I’ve always had a good memory for faces. When I meet someone for the second time, I often need to pretend that I haven’t met them before, to avoid the embarrassment of one-sided recognition. But despite my uncanny ability to tell human faces apart, I struggle to distinguish between non-human individuals. Take lizards, for instance. My current research depends upon distinguishing individual lizards from one another. Could I tell two lizards of the same species apart in the way that I tell two people apart? Absolutely not. Sometimes an individual lizard really stands out — its skin might be more orange than brown, or the diamond patterns on its back may be unusually well-defined. But more often than not, I need a little help. For my research this summer, I’m sewing different coloured beads into the lizards’ tails to be able to distinguish one from the other. It’s like recognising people by the jewellery they wear, instead of their faces.

But as with many sensory skills, other organisms are vastly better at across-species individual recognition than we humans are. A striking example of this skill is the northern mockingbird, a bird that lives in the same place as I currently do. Given how common they are in Gainesville, Florida, mockingbirds attract as much of my attention as crows or pigeons would. I’d be hard-pressed to tell you how many mockingbirds I’ve seen today, let alone whether I’ve seen different individuals. But the mockingbirds I’ve been ignoring have quite possibly been watching me.

Some years ago, biologists at the University of Florida in Gainesville decided to pay attention to the same mockingbirds that I barely notice. Specifically, these researchers were interested in whether these birds, who encounter thousands of humans every day on this busy college campus, can distinguish one person from another. At first, this seems like an impossibly tough question to answer — you can’t sit a mockingbird down to ask who it has met that day. But the researchers found an elegantly simple solution to this problem, by tapping into the mockingbirds’ maternal instincts.

The researchers began by locating mockingbird nests. For the next four days, one of two researchers walked and reached towards each nest in turn, attempting to convince the mockingbird sitting on her eggs that the researcher represented a credible threat. On each day, the nesting mockingbird understandably grew more and more wary of the approaching researcher — she would attack more frequently, issue more alarm calls, and create more of a ruckus in response to the human. But the moment of truth came on day five, when a second researcher approached the nest. Would the mockingbird continue to escalate her response to the approaching person, if she thinks that all humans are the same and therefore equally threatening? Or would she recognise this as a different person, a person who had not spent the last four days trying to vandalise her nest? Time and time again, female mockingbirds’ response to the second intruder was similar to their first response to the first intruder, demonstrating that mockingbirds can not only tell two humans approaching their nest apart but also distinguish them from the hundreds of others walking past!

Birds’ superior identification skills don’t stop at recognising people, but extend into the realm of objects as well. These recognition skills come to the fore when the American coot, a little black-and-white waterbird, is taking care of the most important objects in its life — its eggs.

Coots live in a cut-throat world. On average, half the chicks in a coot nest die of starvation. To guard against losing all their offspring, female coots often sneak into a neighbouring coot’s nest and lay an egg or two. After all, if you’re going to be out-competed for food by a neighbour, you might as well try to trick the neighbour into providing for your offspring too. But the competing coots don’t take this subterfuge lying down. They have evolved the ability to distinguish between their own eggs and their neighbour’s eggs. Unfamiliar eggs are unceremoniously ejected from the comfort of the nest, thus ensuring that the algae and insects painstakingly collected are used to feed its own progeny.

Upon learning about coots’ precise egg recognition skills, I tried to match my own object identification abilities against theirs, and opened up a carton of eggs. Faced with two rows of seemingly identical ovoids, I threw my hands up, both defeated and thoroughly impressed, yet again, by the natural world.

Ambika Kamath studies organismic evolutionary biology at Harvard University