Start typing “dog shares...” into a search engine and instant cuteness results. In one clip, a yellow Labrador brings its toys to the fence to share with the neighbor dog.
The clips are charming, but are the animals actually sharing? Is it a behavior that animals even know?
Through observation, scientists have found examples of sharing in the animal world. Often, it’s based on reciprocity, which means there could be a future expectation the giver will one day be the receiver.
Animals May Be Altruistic
Sharing is considered an altruistic behavior. Altruism is an action that benefits the recipient but not the giver.
Donating blood, for example, can be considered an altruistic behavior because the donor sacrifices a vital bodily fluid. But other than feeling proud, the donor doesn’t receive anything in exchange for their sacrifice.
Scholars have long studied and questioned why humans evolved to be altruistic. Humans are social creatures and rely on each other for survival, but some scholars have debated whether humans are genuinely altruistic or have other motivations, such as the need to feel good about themselves.
Scholars have also debated the role of altruism in animal behavior. Scientist Charles Darwin struggled to fit altruism into his theory of natural selection because he knew some species made sacrifices without any personal benefit. With honeybees, for example, some bees held off on reproducing their own offspring in order to support the hives’ young.
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Puzzled by the behavior, Darwin described altruism in animals as a type of evolutionary snag. In 1964, another naturalist proposed an explanation — animals were more likely to share with others who were genetically related. Known as “Hamilton’s Rule,” the theory offered a formula for predicting altruistic behavior based on both the recipient’s need and its genetic relation to the donor.
Thus, the theory held there was an innate drive to pass on genetic material, and that drive motivated altruistic behavior in sharing resources with kin in need.
Hamilton’s Rule has been used to explain altruistic behavior in animals, as well as humans. Experts have debated the theory, and one criticism is that it fails to factor in how hierarchies might force sharing behavior.
Similarly, other studies have found that genetics can only motivate sharing to a certain extent. Simply put, some animals don’t like moochers.
In a 1984 study in Nature, biologist Gerald S. Wilkinson observed how wild vampire bats (Desmodus rotundus) regurgitated blood for other bats that had failed to feed that night.
“The premium for them if they fail to feed on any given night is high. Their risk of starvation is elevated substantially,” says Wilkinson, now a professor and associate dean at the University of Maryland.
In his observation, Wilkinson banded 184 vampire bats in Costa Rica. During a 26-month study, he learned how the bats organized themselves socially. He also saw how bats that successfully fed each night were engorged, and it was obvious when a bat returned to the roost hungry.
“If they [hadn’t eaten], their stomachs were shrunken,” Wilkinson says.
It’s not uncommon for vampire bats to fail on an evening hunt. They drink the blood of larger animals like cows and horses, both of which find the little bats bothersome and can throw them off.
When a failed bat returns to the roost, they may receive a regurgitated blood donation. Or they might ask and get told to talk to the wing because the face isn’t listening. If refused, the bat being asked to donate will push the other bat away or simply turn away. But if the bat is willing to make a donation, the two will put their heads together so the hungry bat can lap blood from the other’s mouth.
However, Females only shared with females, while Males were not included unless they were still young. Stingy bats weren't part of the sharing network and were often turned away if they asked for blood. So what makes a bat open wide and regurgitate? Reciprocation. “The advantage of sharing increases the potential there will be someone to repay them in the future,” Wilkinson says.
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Predicting Sharing Behavior
In a 2013 follow-up study in Proceedings of the Royal Society B: Biological Sciences, Wilkinson and his then-graduate student, Gerald G. Carter, worked with captive vampire bats that were intermittently fasted for two years.
The researchers were curious if kinship played a role in whether a fed bat shared with a fasted bat. They also wondered if “harassment” was a factor, meaning did a hungry bat essentially bully a fed bat into giving them the bat equivalent of their lunch money.
The study found that fed bats were more likely to seek out a fasted bat, which meant harassment wasn’t a factor. Most of the sharing dyads, almost 67 percent, were unrelated, so kinship also wasn’t a driving factor.
The biggest predictor was whether the fasted bat had shared food in the past, and it was 8.5 times more important than being related.
“There are not many examples quite like the vampire bat where it’s a natural behavior,” Wilkinson says. “They do this on their own, and it has significant consequences from there on.”
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Annual Reviews. The Evolution of Altruism in Humans
Charles Daniel Batson. Altruism in Humans
JSTOR Daily. Why Do Animals Share?
Journal of Theoretical Biology. Hamilton's rule
Ethology. Helping those who help others: The roles of indirect reciprocity and relatedness
Research Gate. The kin/group selection controversy. 5. Hamilton’s rule is a flawed concept
Proceedings B. Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment
Emilie Lucchesi has written for some of the country's largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor's degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, A Light in the Dark: Surviving More Than Ted Bundy, releases October 3, 2023, from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.
