Why Do Vampire Bats Share Blood? Friendship in Bat Colonies

Why missing a meal is so dangerous

The common vampire bat lives only on blood, a food that is mostly water and protein with almost no fat. As a result, it stores very little energy in its body. A bat that fails to feed burns through its reserves fast and can die after about two to three nights, roughly 60–70 hours, without food.

Failing to feed is not rare. In his field study in Costa Rica, published in 1984, biologist Gerald Wilkinson found that on any given night about 7% of adult bats and about a third of young bats under two years old returned to the roost without a meal. Hosts wake up, move away or are guarded, and young bats are still learning to hunt. Over a year, almost every bat will have bad nights.

How vampire bats share blood

Food sharing happens in the roost during the day. A hungry bat approaches a roost-mate, often after grooming it, and licks at its mouth. A well-fed bat then regurgitates part of the blood from its stomach directly into the other bat’s mouth.

The amount given is small for the donor but valuable for the receiver. Because a fed bat has just drunk roughly half its own body weight in blood, as explained in Do vampire bats really drink blood?, it can spare a portion and lose only a few hours of reserve. For a starving bat, the same portion can buy another night to find a host. Scientists describe this as a small cost to the giver and a large benefit to the receiver.

Kin help: mothers, daughters and relatives

Most food sharing in the wild flows from mothers to their own pups. In Wilkinson’s Costa Rica study, about 70% of the sharing events observed were mothers feeding their young, and young bats are fed regurgitated blood from about their second month of life.

Common vampire bats also help other relatives. Female bats often stay for years in the colony where they were born, so roosts contain mothers, daughters, sisters and cousins. Helping relatives survive also passes on the genes they share, a process biologists call kin selection.

Help between non-relatives: reciprocity

Kinship does not explain everything. Vampire bats also feed unrelated roost-mates, and these partners tend to be bats they spend a lot of time with. Wilkinson found that sharing outside the family happened between bats with long, close associations in the roost.

Later experiments with captive bats, led by Gerald Carter and Gerald Wilkinson, tested this directly. Bats were fasted for a night and returned to their group, and the researchers recorded who fed whom. Published in 2013, the results showed that how much a bat had received from a particular partner in the past predicted how much it gave back better than how closely the two were related. This kind of give-and-take is known as reciprocity or reciprocal altruism.

The study also tested an alternative idea, that bats share mainly to stop a hungry neighbor from pestering them. That “harassment” explanation did not fit the data well: donors often approached and fed hungry bats themselves.

Friendship, grooming and social networks

Vampire bats groom each other often, licking and cleaning a partner’s fur. Grooming is closely linked to food sharing: bats that groom each other are more likely to share blood, and grooming may also let a bat judge whether its partner is hungry.

Female bats that feed many different roost-mates seem to be better protected against bad luck. In a 2015 study, bats with a wide network of sharing partners lost less food support when one important donor was taken away than bats that depended on one or two partners. Having many friends works like insurance.

Strangers can become partners, but slowly. Experiments published in 2020 found that unfamiliar bats placed together usually began with grooming and only later moved on to sharing food, as if testing the relationship with small favors first. Another study, published in 2019, tracked bats with tiny sensors after they were released back into the wild in Panama and found that pairs with strong bonds in captivity continued to meet and associate in the wild.

Is vampire bat food sharing unique?

Regurgitating blood for adult roost-mates is unusual among mammals, and the common vampire bat is the best-studied example. The hairy-legged vampire bat also shares food mouth to mouth. Most other bats feed only their own dependent young.

Researchers still debate how much each factor matters, because behavior in the wild is hard to measure. Most agree that kinship, long-term association and reciprocal help all play a part, and that the risk of fast starvation is what makes sharing worth it. Learn more about the roosts where these relationships form in Where do vampire bats live?