TOEFL Reading: ETS-TOEFL阅读机经 - N_4M35Y0DI137P76L$

Many hypotheses have been suggested to explain why meerkats take turns acting as sentinels, warning other group members of any threats from predators. A. Kin selection is thought to explain sentinel behavior, but this hypothesis does not fully explain how unrelated immigrants can be part of a group. B. Even though sentinel behavior can be explained with reciprocal altruism, there is lack of evidence that group members are ever held responsible for not taking their turn to guard. C. Guard duty increases the personal safety of every group member, but the increase is at its least for the member who actually stands guard. D. The proposed theory that sentinel behavior increases the safety of only family members is contradicted by the sentinel behavior of immigrant group members. E. A plausible reason for sentinel behavior may be to allow other meerkats a chance to forage for food in groups rather than as individuals. F. Sentinel behavior is most likely the result of self-protection by individuals and only secondarily a means to protect the group.