In a classic set of studies over a ten-year period, biologist Gerald Wilkinson found that, when vampire bats return to their communal nests from a successful night’s foraging, they frequently vomit blood and share it with other nest-mates, including even non-relatives. The reason, it turns out, is that blood-sharing greatly improves each bat’s chances of survival. A bat that fails to feed for two nights is likely to die. Wilkinson showed that the blood donors are typically sharing their surpluses and, in so doing, are saving unsuccessful foragers that are close to starvation. So the costs are relatively low and the benefits are relatively high. Since no bat can be certain of success on any given night, it is likely that the donor will itself eventually need help from some nest-mate. In effect, the vampire bats have created a kind of (      ).

* forage: 먹이를 찾아다니다

① complex social hierarchy 

② ecological diversity 

③ mutual insurance system 

④ parasitic relationship 

⑤ effective reproduction process