A vampire bat\u2019s habit of regurgitating some of its bloody stomach contents to hungry roost-mates is a textbook example of reciprocal altruism \u2013 \u201cI\u2019ll scratch your back if you scratch mine.\u201d<\/p>\n
But it now seems that this sort of co-operation is more subtle than simple tit-for-tat.<\/p>\n
Vampire bats are most likely to share their blood meals with close relatives, but they also donate food to unrelated individuals at risk of starvation.<\/p>\n
When biologists split up family members within a roost, bats were most likely to cope with lean times if they had donated blood to many non-relatives in the past.<\/p>\n
Feeding non-kin expands a bat\u2019s social network, making it more likely that it will itself be helped in times of trouble.<\/p>\n
The University of Maryland\u2019s Gerald Carter, who led the research, writes on his blog that, like human friendships, vampire bats\u2019 social connections might be based on their overall quality, rather than whether a particular individual co-operated during the last encounter.<\/p>\n