understudy

To study or know a role to such an extent as to be able to replace the normal performer when required.

To act as an understudy (to someone).

To act in a similar manner to some known person.

That lean gentleman had apparently understudied Pharaoh of old and hardened his heart.

A performer who understudies; a standby for a study (9).

For Wynne-Edwards, the role of the outcasts in the welfare of the group is to wait in the wings as understudies, ready to step into the shoes of any territory holder who dies on the main stage of group reproduction.