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Biological Altruism: Why Do Animals Help Each Other?

By Shana McAlexander
Product Developer

Vervet Monkeys

When a firefighter enters a burning building to save an elderly man, his willingness to risk his own life may be attributed at least partly to his desire to help others. We see frequent examples of self-sacrifice by humans, in both professional and spontaneous capacities. What about self-sacrifice among other animals? Evolutionary biologists and animal behaviorists study such behaviors, looking for both immediate and evolutionary explanations.

Rationales for self-sacrificing behavior are discussed and debated across the fields of animal behavior, evolution, ecology, psychology, and philosophy. Most biologists agree on a concept of biological altruism: an act that increases the recipient’s chances for reproductive success at the expense of the perpetrator’s.

Biological altruism presents an evolutionary puzzle. If individuals act under the pressures of self-preservation and the desire to reproduce, then why would 1 organism help another, putting its own reproductive success at risk? Further, if the tendency toward altruism is a heritable trait and individuals with the trait are less reproductively successful, then why is the frequency of altruism relatively high?

Before getting into the changing views of altruism, I will present 3 often-cited examples from altruism research. They may serve as case studies and topics of further research for your class.

Vampire bats

Vampire bats are long-lived, social animals that feed during the night and return to their group for daytime roosting. Gerald Wilkinson’s research team at the University of California, San Diego investigated the altruistic behavior in vampire bat groups in Costa Rica.

Researchers tagged each bat for identification. The bats can survive only 2 or 3 days without feeding. In the early evening, the researchers captured a subset of bats and confined them, reintroducing them to their social group later in the night after the others had returned from feeding. The feeders who donated food to their starving roost-mates potentially compromised their own health.

The researchers tracked the relatedness between those donating and receiving the blood meals. There was a greater frequency of blood sharing between related individuals within the group; however, unrelated bats also exchanged meals. Over time, former recipients were observed feeding former donors, exemplifying “reciprocal altruism,” a behavior associated with long-lived, close-knit animals.

Vervet monkeys

Like some other animals (e.g., prairie dogs), vervet monkeys give warning calls when they sense nearby predators. Calling out a warning is considered an altruistic behavior because the signaler puts itself at greater risk by giving away its own location to the predator.

Robert Seyfarth and Dorothy Cheney at the University of Pennsylvania investigated the system and types of warning calls in a group of vervets. Juvenile callers sometimes overreact (e.g., a windblown leaf may stimulate the “Eagle!” alert), but, as they mature, they learn to distinguish real threats and warn of them exclusively.

European minnows

Many fish species release a specific chemical after their skin has been damaged in some way, as by a predator. This chemical was named Schreckstoff by its discoverer Karl von Frisch in 1938. Von Frisch found that European minnows displayed a fright reaction when exposed to this chemical in the water. He inferred that the Schreckstoff served as a warning to other minnows nearby.

For many people, this example is not as easy to view as altruism since there is no appearance of a “choice” involved; however, the effect is that other individuals of the species survive longer as a result of the production and release of the chemical.

Explaining biological altruism

Mid-twentieth century behaviorists such as Nobel Prize winner Konrad Lorenz believed that altruistic behavior may harm the individual yet benefit the group as a whole, and that a group without altruistic members is less reproductively successful.

The following chart models the reproductive success of a group of monkeys with arbitrarily assigned reproductive success values and theoretical adjustments. Notice that Group 1 has a single altruistic member, while Group 2 is all selfish. Even though the reproductive success of the altruistic Monkey A is low, the reproductive success for Group 1 is greater than for Group 2 because of Monkey A’s sacrifice. The same model applies whether or not members of the group are related (kin).

Group 1

Monkey Gender Altruistic Basic reproductive success value Adjustment for altruistic group Final Reproductive success value
A Female Yes 5 –2 3
B Female No 5 +1 6
C Male No 5 +1 6
D Male No 5 +1 6
Total Group 20 +1 21

Group 2

Monkey Gender Altruistic Basic reproductive success value Adjustment for altruistic group Final Reproductive success value
E Female No 5 0 5
F Female No 5 0 5
G Male No 5 0 5
H Male No 5 0 5
Total Group 20 0 20

In the mid-1960s, evolutionary biologists G.C. Williams and J.M. Smith rejected Lorenz’s assumptions on the basis of the selective pressure that they assumed would work against any altruism trait. The main argument against group-level altruism claims that any selfish (freeloading) individuals of the altruistic group will have a greater probability of reproducing than the altruistic members. Thus, the “selfish gene” will prevail. The mode of inheritance for the altruism trait is not well understood and is probably oversimplified by saying there is a single selfish gene (or a single altruism gene); however, if the altruism trait is inherited, over many generations, the frequency of altruism would be expected to decline within the group.

Current theories, first articulated by William Hamilton in 1964, tend to focus on kin selection. Hamilton predicted that altruism occurs more often between genetically related individuals. If the members of a group are related, a freeloader carries many of the same genes as the altruistic member. Because relatives share genetic makeup, when an altruistic individual helps her relative, she is increasing the chance that their shared genes will be passed on. If we apply this model to a related population containing some altruistic members, the gain in reproductive success to the family group outweighs any loss in reproductive success of the altruistic individual. Consider a situation where there is a diallelic, dominant selfish gene. The recessive, altruistic allele is also carried in some of the selfish population. If the interrelated group survives more successfully as a result of the altruistic members’ behavior, then the recessive gene survives as well.


Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex.

Magurran, A.E., Irving, P.W. and Henderson, P.A. 1996. Is there a fish alarm pheromone? A wild study and critique. Proceedings of the Royal Society B (Biological Sciences) 263: 1551–5.

Okasha, S. 2009. Biological altruism. The Stanford Encyclopedia of Philosophy.

Seyfarth, R.M. and Cheney, D.L. 2000. Social self-awareness in monkeys. American Zoologist 40: 902–9.

Wilkinson, G.S. 1984. Reciprocal food sharing in the vampire bat. Nature 308: 181–4.

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