Humans, of all species, should know that there are some big advantages to social living. Groups of animals can hunt together, defend one another, and keep each other clean and safe. But there are downsides to living in groups too. Animals in groups have to share food, space and mates. Group living also makes it easier to pass parasites between individuals. If larger groups of animals always suffer from more parasites than smaller groups, this relationship may even determine why flocks, herds and prides assume the sizes they do.
In a recently published meta-analysis of 69 different studies on group size and parasites, researchers at Harvard and Duke found that bigger groups of animals did indeed tend to have more parasites. This could be for all sorts of reasons. Many parasites are specific to a species, so an animal who lives alone might never pass its parasites along. On the other hand, group behavior like sharing food, mating and grooming could also encourage parasite swaps.
But beyond simply confirming that bigger groups got more parasites, they found something surprising, says Joanna Rifkin, the study's lead author. “What we really weren't expecting was that there was such a big difference between different types of animals,” she says.
Those differences were most obvious between birds and mammals. As the number of birds in a group goes up, the number of parasites go up sharply. In mammals, the relationship between flock size and parasite numbers is much weaker. Rifkin says it's too early to tell why, but it could have to do with how species interact amongst themselves. Mammals often live in smaller family groups that interact with one another more than groups of birds do. Compare a family of prairie dogs to a flock of swallows. There might be ten times the number of swallows than prairie dogs, and while swallow doesn’t really physically interact with its flock mates, it brushes by thousands of them. Prairie dogs, on the other hand, share food, play with one another, and groom each other.
This explanation makes sense to any human parent, says Colin Chapman, an anthropologist at McGill University who studies primate parasites. "It's like when your kids go through childcare, when they start meeting other kids and they get sick," he says. At home, they were only exposed to the germs of their family and friends who came over. At school they interact with all sorts of new kids with new germs.
Understanding how group size affects parasite numbers helps us understand what determines group size in the first place, says Thomas Gillespie, a biologist at Emory University who works on parasite loads in both humans and primates. Perhaps each species has a sweet spot for group size: just big enough to get food and ward off predators, but not too big to attract nasty bugs. And understanding how parasites interact with group size could also help people setting up conservation areas or new populations. Keeping a new population of wild animals from being wiped out by parasites requires understanding how big that population should be.
But it can be difficult to determine how many parasites are affecting a group. For example, consider a summer barbeque. By the end of the night, everyone is itching at mosquito bites. But it's hard to know whether more mosquitoes have joined the party or whether the same few mosquitoes are biting everyone. The same is true in groups of animals in the wild. It’s hard to tell whether a sleeping herd of bison is being tormented by waves of mosquitoes or just a few tenacious ones.
How does the size of human groups affect the number of parasites we get? According to Rifkin, we're probably more like prairie dogs than we are like sparrows. While we live in close contact with groups of friends and coworkers, we try to keep other humans at more of a distance.
But there are a lot of variables, even within human populations, Gillespie says. After screening 183 people for parasites in one rural village in Madagascar, they found that every single person was infected with a parasite. Yet people who live in urban cities with high-quality health care probably don't have any parasites at all. "From a biological perspective, your situation in New York City is extremely novel," he says. The fact that most New Yorkers don't have any parasites flies in the face of the environment that our immune systems evolved in—one full of all sorts of parasites.