Why past favors predict sharing better than family ties
The most direct test is to compare predictors of sharing within pairs of bats. When researchers fasted 20 vampire bats over two years and recorded 48 days of induced sharing, the amount a bat had previously received was the best predictor of how much it gave, and it was 8.5 times more important than relatedness [2]. That is a strong signal for reciprocity: bats appear to keep track of who has helped them and return the favor.
The same study found that 64 percent of sharing pairs were unrelated, close to the 67 percent you would expect if bats were not favoring relatives at all [2]. Donors also initiated sharing more often than recipients, which argues against the idea that bats give mainly to stop harassment [2]. Together, these patterns make reciprocal altruism the better explanation for day-to-day sharing decisions.
Kin selection is not absent, just not the whole story
Kin selection is the idea that helping relatives can spread because it passes on shared genes. The original explanation for vampire bat blood sharing invoked both direct benefits to the giver and indirect benefits through relatives [2]. The evidence does not show that kinship is irrelevant; it shows that relatedness is a weaker predictor of donations than past food received [2].
A review of the behavior's evolution proposes that food sharing began as a form of extended maternal care, rooted in kin selection, and was later co-opted to produce reciprocal benefits [3]. In that view, kin selection and reciprocity are not rival explanations for every donation. Kin selection may explain how the behavior got started, while reciprocity helps explain why it persists and how partners are chosen [3].
Helping strangers can pay off by building a donor network
If reciprocity is important, helping non-kin can be useful because it widens the circle of bats that might later return the favor. In an experiment that fasted 14 adult females and measured donations from 28 possible donors, bats that had fed more non-kin in previous years had more donors and received more food when their usual partners were unavailable [1]. That is consistent with the idea that non-kin sharing builds a backup network.
The same experiment found that donors are not interchangeable: females received less food when a past donor was prevented from giving than in a control condition [1]. The treatment also increased variation in how much different donors contributed, which the authors suggest could reflect different responses to a partner's inability to reciprocate [1]. This points to partner choice and switching, both hallmarks of reciprocal relationships rather than simple family favoritism.
What the evidence can and cannot settle
The separation is strongest at the level of predicting individual donations: past reciprocity outperforms relatedness, and unrelated pairs are common [2]. But the evolutionary origin of the behavior is a different question, and the review evidence suggests kin-based maternal care was later co-opted for reciprocal benefits [3]. That means the two mechanisms may operate at different levels rather than one simply replacing the other.
Some evidence comes from male bats and from vocalizations around food sharing, where relatedness is described as low within the colony [4]. Contact calls have also been linked to past food-sharing partners, consistent with reciprocity rather than kinship as the main driver of donation rates [5]. These findings support the reciprocity picture, but they do not by themselves rule out kin effects in other groups or contexts.
Sources used in this answer
Social benefits of non-kin food sharing by female vampire bats
Female vampire bats that fed more non-kin in previous years had more donors and received more food when past donors were unavailable, suggesting that helping non-kin expands a bat's donor network.
Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment
Past food received predicted food given 8.5 times more strongly than relatedness, and 64 percent of sharing pairs were unrelated, supporting reciprocal altruism over kin selection or harassment as the main explanation for sharing decisions.
Co‐option and the evolution of food sharing in vampire bats
Food sharing in vampire bats may have evolved as extended maternal care rooted in kin selection and later been co-opted to yield reciprocal benefits stabilized by partner choice.
Comprehensive Investigation of Male Vampire Bats: Unique Vocalizations Associated with Reciprocal Altruism and Social Behavior in Captivity
Male vampire bats in captivity produce vocalizations associated with reciprocal altruism around food sharing, in a colony described as having low relatedness.
Common vampire bat contact calls attract past food-sharing partners
Contact calls attract past food-sharing partners, consistent with reciprocal food sharing being about eight times more important than kinship for predicting donation rates.