Increased vigilance of paired males in sexually dimorphic species: distinguishing between alternative explanations in wintering Eurasian wigeon

In animal pairs, males are often more vigilant than females.This is generally assumed to result from mate guarding (eitheragainst predators or other males). However, when males haveconspicuous secondary sexual characteristics, they could beconstrained to be more vigilant because of a higher predationrisk than females. We attempted to distinguish between the "maleconstraint hypothesis" and two variations of the mate-guardinghypothesis by studying the vigilance behavior of the sexuallydimorphic wigeon during early winter, when some males are inbreeding plumage and some are not and when not all males arepaired. The proportion of time spent vigilant by paired malesin breeding plumage was five times higher than any other categoryof males or females. We found no significant differences betweenthe vigilance levels of unpaired male wigeon in cryptic andin breeding plumage and therefore rejected the male constrainthypothesis. As vigilance levels of paired and unpaired femalesdid not differ either, we rejected the hypothesis that pairedmales invest in vigilance to reduce their mate's need to bevigilant to predation risks. Paired females interacted lessfrequently with other wigeon than unpaired ones, and it is probablyto protect their female from other males that paired male wigeonincrease their vigilance times.