Status signalling in harris sparrows: Experimental deceptions achieved

A new hypothesis to explain the evolutionary stability of subordinance in this system is proposed: dominants and subordinates are adapted, respectively, to defensible and indefensible resource bases, and equal fitness for them is achieved through frequency-dependent selection.

Abstract

Harris sparrows (Zonotrichia querula) signal their social dominance by variations in the extent of black feathering on their throats and crowns. Earlier deception experiments showed sub-ordinates dyed blacker to be socially persecuted. Two explanations of this behaviour are here distinguished: (1) that cheating on the signalling system is socially controlled and (2) that the persecution resulted from an incongruence between signal and behaviour. The incongruence hypothesis is supported. Subordinates that were both dyed and administered testosterone show dramatic increases in social status; thus the evolutionary stability of the signalling system seems not to depend upon social control of cheating. A new hypothesis to explain the evolutionary stability of subordinance in this system is proposed: dominants and subordinates are adapted, respectively, to defensible and indefensible resource bases, and equal fitness for them is achieved through frequency-dependent selection.