A study of social interactions among vampire bats that felt sick suggests family comes first when illness strikes – and may help explain which social interactions are most likely to contribute to disease transmission.

Most social interactions for bats that felt sick diminished, but vampire bat moms maintained close social connections with their kin. They continued to groom their offspring even if the youngsters seemed sick, and mothers that felt sick also kept grooming their healthy offspring.

“Moms everywhere are probably not surprised to find that vampire bat mothers were the most likely to potentially sacrifice their own health to tend to the needs of their offspring,” said Gerald Carter, senior author of the study and assistant professor of evolution, ecology and organismal biology at The Ohio State University.

Carter said the family connection evident in this study bears out what was seen with the initial COVID-19 coronavirus outbreak in China: The disease was spreading mainly within family groups because these social connections will not be reduced when people are sick.

This study examined grooming and food sharing among vampire bats after some were injected with a substance that activated their immune system and made them feel sick for several hours, but didn’t infect them with an actual illness.

Social grooming among unrelated bats that felt sick decreased, but food sharing continued at normal levels – suggesting a social behavior necessary for survival will persist despite the risk for disease and that family members are likely at highest risk of catching another’s illness.

“To take a human analogy, a sick person might completely stop shaking hands, but they might still engage in more necessary social interactions – like preparing food for their families,” Carter said.

The research is published online in the Journal of Animal Ecology.

Vampire bats are highly social animals that huddle closely in their dark, cave-like roosts and, according to previous research led by Carter, show signs of maintaining “friendships” in the wild.

The experiments were conducted in a captive colony of 36 vampire bats – 24 adult females and their 12 captive-born offspring. Such a combination of unrelated mothers mixed with their offspring is common in the wild, said Carter, also a research associate at the Smithsonian Tropical Research Institute (STRI) in Panama, where this study took place.

The project was led by first author Sebastian Stockmaier, a graduate student at the University of Texas at Austin, and advised by Carter and co-authors Daniel Bolnick of the University of Connecticut and Rachel Page of STRI.

Stockmaier injected each bat with both the molecule causing the immune challenge or a placebo in random order and observed the frequency and types of behavior to determine whether they decreased or remained consistent.

He observed that sick bats had fewer grooming partners – but the grooming that did occur (chewing and licking another bat’s body) was equally intense among sick and healthy pairs of bats.

Food sharing, on the other hand, remained constant. Vampire bats commonly regurgitate their blood meals to feed their hungry counterparts, and donor bats continued that behavior even if the hungry bats appeared to be sick.

“We all know that more socially connected people are more likely to transmit disease to each other and that when sick people are less social, that can slow down how a pathogen spreads,” Carter said. “This study demonstrates the type of social connection makes a difference in what social behavior is likely to continue even when people are sick.”

Based on their observations, the researchers attributed changes in the sick vampire bats’ social behavior to feeling lethargic rather than as an infection-control strategy.

“It’s not uncommon for humans who are sick to isolate themselves or avoid each other to prevent disease transmission,” Carter said. “For vampire bats, the instinct to stay strong through social interaction likely outweighs the potential benefits of preventing infections among others in their roost.”