← Back to the game

The real science · vampire bat

Blood, friendship, and bookkeeping.

Everything in the game comes from real studies of real bats. Here's the natural history behind each mechanic, and where the science is still open.

The diet

The common vampire bat (Desmodus rotundus) feeds only on blood, mostly from sleeping livestock, taking a few teaspoons in a 20-minute meal. The part that makes it really tough on the bats? Blood is nearly all water and protein, so bats can't store much energy. Gerald Wilkinson's foundational field study in Costa Rica found that adult bats come home hungry on about 7% of nights (young bats fail far more, on up to a third of nights), and a bat that goes roughly 3 nights without a meal starves to death1. Every bat in a colony lives two bad nights away from risking death.

The bonds

Wilkinson's 1984 Nature paper reported something remarkable: bats that fed regurgitate part of their blood meal to roost-mates that didn't. The donation costs the donor but can buy the recipient about another 12 hours of life1. Crucially, sharing wasn't limited to mothers and pups or close kin. Unrelated adults shared too, and they shared preferentially with the individuals who had shared with them before. It became one of the textbook cases of reciprocity in animals.

The ones who invested are the ones who get rescued

Thirty years later, Gerry Carter and Wilkinson tested the reciprocity claim with controlled fasting experiments in a captive colony. They withheld food from one bat at a time and watched who fed it. The best predictor of receiving food was not relatedness, but the amount of food and grooming that bat had previously given to its potential donors 2. In the game, your rescue probability is built from your bonds (P = 0.02 + 0.60 × bond, counted over each roost-mate that fed that night). Help comes back from the relationships you've built, and only a bat with blood can give any away.

Grooming

Vampire bats groom each other far more than the hygienic baseline other species show4. Grooming removes ectoparasites like bat flies and mites5 (the game's mite mechanic), but it's also a social cue. When Carter introduced strangers to each other in captivity, new relationships developed by escalation. Bats began with low-cost grooming, "testing the water," and only later graduated to food sharing if that bat groomed them back3. That's what the game rewards when grooming builds the bond that later makes sharing (and rescue) likely.

Nights with a bright moon

Vampire bats are creatures of the dark in a strict sense. Field observations in México found that Desmodus forages far less on bright moonlit nights, concentrating activity into the darkest hours6. This is a pattern called lunar phobia, seen across many Neotropical bats, most plausibly as predator avoidance. The game's moon cycle comes from that finding. The direction of the effect is sourced; the size of the penalty on a bright night is the game's own tuning, and the storm nights that close each season are dramatized. In reality, there is no "end" for feeding at the end of the season, these parts just make the game more fun from a user perspective.

How much to give?

Donations aren't all-or-nothing. Donors regurgitate for varying durations, and the amount transferred varies with the relationship and the donor's own condition 1,2. The sip / meal / full-crop choice in the game reflects the bat's choice in generosity. Bigger gifts build bonds faster but cost more of your own safety margin, so are more of a gamble!

What we still don't know

The open questions are the fun ones. Should a bat concentrate on a few strong bonds or spread across many weak ones? How do reciprocity and fitness interdependence (having a genuine stake in a partner's survival) interact? What happens to a relationship after a partner defects? These are live research questions, and they're the ones my own work tackles with long-term colony data, social-network models, and manipulative experiments. The game's dilemmas are the real dilemmas. The optimal strategy is genuinely unsolved.

← Play the game

References

  1. Wilkinson, G.S. (1984). Reciprocal food sharing in the vampire bat. Nature 308, 181–184. doi:10.1038/308181a0
  2. Carter, G.G. & Wilkinson, G.S. (2013). Food sharing in vampire bats: reciprocal help predicts donations more than relatedness or harassment. Proceedings of the Royal Society B 280, 20122573. doi:10.1098/rspb.2012.2573
  3. Carter, G.G., Farine, D.R., Crisp, R.J., Vrtilek, J.K., Ripperger, S.P. & Page, R.A. (2020). Development of new food-sharing relationships in vampire bats. Current Biology 30, 1275–1279. doi:10.1016/j.cub.2020.01.055
  4. Carter, G.G. & Leffer, L. (2015). Social grooming in bats: are vampire bats exceptional? PLoS ONE 10, e0138430. doi:10.1371/journal.pone.0138430
  5. Wilkinson, G.S. (1986). Social grooming in the common vampire bat, Desmodus rotundus. Animal Behaviour 34, 1880–1889. doi:10.1016/S0003-3472(86)80274-3
  6. Crespo, R.F., Linhart, S.B., Burns, R.J. & Mitchell, G.C. (1972). Foraging behavior of the common vampire bat related to moonlight. Journal of Mammalogy 53, 366–368. doi:10.2307/1379175