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In animal pairs, males are often more vigilant than females. This is generally assumed to result from mate guarding (either against predators or other males). However, when males have conspicuous secondary sexual characteristics, they could be constrained to be more vigilant because of a higher predation risk than females. We attempted to distinguish between the ‘‘male constraint hypothesis’’ and two variations of the mate-guarding hypothesis by studying the vigilance behavior of the sexually dimorphic wigeon during early winter, when some males are in breeding plumage and some are not and when not all males are paired. The proportion of time spent vigilant by paired males in breeding plumage was five times higher than any other category of males or females. We found no significant differences between the vigilance levels of unpaired male wigeon in cryptic and in breeding plumage and therefore rejected the male constraint hypothesis. As vigilance levels of paired and unpaired females did not differ either, we rejected the hypothesis that paired males invest in vigilance to reduce their mate’s need to be vigilant to predation risks. Paired females interacted less frequently with other wigeon than unpaired ones, and it is probably to protect their female from other males that paired male wigeon increase their vigilance times. Key words: Anas penelope, Eurasian wigeon, mate guarding, predation risk, sexual differences, vigilance. [Behav Ecol 14:724–729 (2003)]