Vampire Bats, Fish in Mirrors, and a Lioness Who Broke the Rules

Game theorists had worked out tit for tat on paper. The next question was whether real animals actually play by these rules, and it turned out several of them do, almost exactly.

Vampire Bats Track Who Owes Whom

Vampire bats drink blood and carry it home in a throat pouch to regurgitate for their own babies. Bats in the same colony go further than that: they will also feed a neighbor's babies if that neighbor failed to find food that night, working as a safety net for the whole group.

This starts among close relatives, sisters and cousins, driven by kin selection. Over time it expands to bats that are not related at all, which is reciprocal altruism in its purest form. A bat that repeatedly fails to bring back food, or fails to share what she has, gets fed less generously by the group the next time, and can earn back full cooperation by reliably sharing again. The zoologist Gerald Wilkinson confirmed this by, remarkably, spending years weighing how much each bat regurgitates over time, a real tit for tat system tracked one mouthful at a time.

A Fish That Punishes a Mirror

Stickleback fish are not known for being clever, but a clever experiment revealed something close to tit for tat behavior in them anyway. Put one stickleback in a tank with a mirror, and it treats its own reflection as a rival invading its territory, attacking the glass over and over.

Add a second mirror at an angle, and the fish sees what looks like a second real fish helping it defend against the invader. Tilt that second mirror slightly, so the "helping" reflection appears to hold back and attack less aggressively than it should. The real fish reliably responds by holding back itself the next time it sees that same "partner," effectively punishing the fish that appeared to let it do all the work. A fish with a tiny brain is still tracking whether a partner pulled its weight.

The Lions That Broke the Pattern

Just as this tidy picture was coming together, researchers Craig Packer and Anne Pusey, studying lions in the Serengeti, found a case that did not fit. Using the same playback technique, playing a recording of a threatening stranger's roar, they noticed one particular lioness who consistently hung back instead of joining the group's defense.

By the tit for tat logic that worked so well in bats and fish, the rest of the pride should have punished her, less grooming, less food sharing, some clear cost for her freeloading. They did nothing of the sort. She was never punished at all.

The explanation turned out to be simple once researchers looked closer: she was a poor defender, but the best hunter in the entire group. She was already contributing enormously to the pride, just through a completely different kind of cooperation than the one being measured. A single social relationship can involve several different games running at once, and an individual who looks like a cheater in one of them may be paying their way in full through another.

That complexity, multiple overlapping forms of cooperation happening between the same individuals at the same time, turns out to be the rule rather than the exception once you look closely at any real social species, including our own.

Vampire Bats, Fish in Mirrors, and a Lioness Who Broke the Rules

Vampire Bats Track Who Owes Whom

A Fish That Punishes a Mirror

The Lions That Broke the Pattern

Further Reading