Rats give more generously in response to the smell of hunger

Reciprocal cooperation among unrelated individuals is widespread in the animal kingdom. For example, Norway rats (Rattus norvegicus) exchange food reciprocally and take into account both the cost of helping and the potential benefit to the receiver. Rats show their need for food through solicitation, which increases the chances they will receive help. But communication through calls and gestures may not honestly reflect the actual need of the recipient, and might instead be used to trick a potential donor into helping. In the new study, Schneeberger and colleagues used Norway rats to investigate odor as a potentially "honest cue" by which prospective donors can assess others' need for food.

The researchers provided rats with odor cues from hungry or well-fed rats located in a different room. They found that the rats were quicker to provide help (by pulling a food tray within reaching distance of another rat) when they received odor cues from a hungry rat than from a well-fed one.

The authors then analyzed the air from around the rats, revealing seven different volatile organic chemicals that differed significantly in their abundance between hungry and satiated rats. According to the authors, the olfactory cues may result directly from recently ingested food sources, from metabolic processes involved in digestion, or from a putative pheromone that indicates hunger. This "smell of hunger" can serve as a reliable cue of need in reciprocal cooperation, supporting the hypothesis of honest signaling.

The authors add: "Rats donate food preferably to social partners in urgent need."