After spawning their first nest, female chinock salmon Oncorhynchus tshawytscha paired with small males (c. 46% of female weight) spent an average of 16·2 h between spawning of successive nests compared with 9·6 h for females paired with large males (c. 112% of female weight) (P< 0·05). Neither frequencies of female nest construction behaviours (digging and probing) nor male courtship behaviour (crossovers and quivers) differed between large- and small-male pairs. Male quivering frequencies were correlated significantly with female digging and probing frequencies, whereas the crossover frequencies were not. It is suggested that delayed spawning by females in the presence of relatively small males is a primary mechanism by which females in the genus Oncorhynchus exhibit mate choice.
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