Adoption in vampire bats

We have a new short note about nonkin adoption in vampire bats published in Royal Society Open Science. I never thought we would write a paper based on a single observation, but we have such interesting longitudinal data leading up to this event. Here’s the story:

One of the “facts” I often hear about vampire bats is that they will adopt a baby bat if the mother dies, even if the pup is unrelated. This behavior makes no evolutionary sense, so I thought it was just a myth or an anecdote. It actually comes from a talk given at the Fifth International Bat Research Conference later published in the conference proceedings in 1980. The German bat biologist Uwe Schmidt kept a captive study colony of vampire bats in Germany and during the 1970s, he and his lab witnessed several cases where a female adopted an unrelated orphaned pup and even began lactating to feed it. It’s not clear from the literature how many times he saw this, but we think it occurred at least four times. Interestingly, this was also around the time that Schmidt first reported regurgitated food sharing in captivity, before it was later studied in the wild by Jerry Wilkinson.

Imran Razik, a graduate student in my lab, spent the summer of 2019 doing experiments with a captive colony of vampire bats at the Smithsonian Tropical Research Institute in Panama. He also saw a case of nonkin adoption. However, Imran also knew the complete social history of the mother, the pup, and the adopter since the moment these bats first met. He had watched the pup being born and introduced the two females in captivity.

The adopting female named “BD” was actually a bat we first captured in 2015, then we studied her in captivity for about two years to see how she and others formed relationships, then we released her back to the wild to study how she reintegrated with her wild colony, and how she and others maintained her relationships from captivity. Using proximity loggers, we tracked her relationships across days and inside and outside the roost.

It gets better. Two years later in 2019, we recaptured her and brought her back into our lab, where we again studied how she formed new bonds, this time with a completely different group of strangers.

Imran feeding his vampires
BD feeding Lilith
BD feeding Lilith’s orphaned pup

So it turns out that in 2019 BD formed a uniquely strong bond with one particular bat named Lilith, who we captured about 340 km away on the other side of Panama. Increasingly over time, BD groomed and fed Lilith more than any other bat in the colony and Lilith also groomed BD almost the same amount. Imran watched this relationship form. Then, when Lilith got sick and died (which was very sad for Imran), he watched BD adopt Lilith’s pup, and feed the pup with regurgitated blood and milk.

We don’t know if or how often this adoption happens in the wild. We also don’t know why it happens. It’s not the easiest thing to study because it can only happen when a mother bat dies and her young pup survives.

But what we do know is that the adoption was explained by the social history between Lilith and BD. Imran had watched the social interactions in this colony for 6 hours a day, every day, for 4 months! I think this study shows how we can put rare events into context by monitoring long-term social history.

It is important to note that, unlike non-kin food sharing, there’s no evidence that nonkin adoption is an adaptive trait, and it might not be.

It might simply be a maladaptive byproduct of maternal care. Behavioral ecologists tend to focus on cooperative behaviors that are adaptive and important in nature, not strange and rare non-adaptive behaviors like a dolphin saving a drowning person, a seal trying to feed a dead penguin to a human diver, a deer befriending a dog, or a captive gorilla taking care of a kitten. However, studying adoption might give us insight into what immediate factors in the brain or environment affect parental care decisions. As a new parent myself, I have come to realize the utter power of baby cuteness! I feel that my brain has been completely re-wired. Most of us can understand the strong desire to adopt and care for a cute puppy or kitten, or to take on the ultimate responsibility of adopting a child. Regardless of why these traits exist, it is inherently fascinating to consider the neuroendocrine mechanisms that underlie them, the stimuli that trigger them, how they differ across species or individuals, and how these traits might even be pre-adaptations for other forms of cooperation.

Who could turn down this face?

Razik I, Brown BKG, Page RA, Carter GG. 2021. Nonkin adoption in the common vampire batRoyal Society Open Science. https://royalsocietypublishing.org/doi/full/10.1098/rsos.201927

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