In polygynous mammals, the adult sex ratio is often female-biased due to higher male mortality. Sex-biased mortality may begin during the juvenile period if sex-specific growth rates or behavior result in differential mortality. I examined the causes of male-biased mortality in juvenile thirteen-lined ground squirrels, a polygynous species with a strongly female-biased adult sex ratio (1M:7 F) and little adult size dimorphism. Average litter size at emergence was 6.9 +/- 0.4 juveniles and the sex ratio was even. Mothers left their litters 17.0 +/- 1.3 days after emergence, and all juveniles slept alone 28.9 +/- 2.6 days after emergence. There was no juvenile size or growth rate dimorphism. On average, only 26% of all juvenile males survived their first summer in contrast to 34% of juvenile females. Three weeks after emergence the number of juvenile males that disappeared increased dramatically, and by the end of the summer, the sex ratio was 1M:1.7F. Radio-tracking of 33 males and 33 females confirmed that 71.2% of all juvenile deaths were due to predation. Natal dispersal did not substantially contribute to the large proportion of males that disappeared over the summer. There was no significant difference between distances males and females were active from their burrows, their home range size, or home range overlap with maternal and sibling ranges. The rate of range expansion did not differ between the sexes at any time during the summer. Behavioral observations indicated a tendency for males to spend more time in social interactions, particularly play, than females. An experimental test of fearlessness indicated that males were less wary than females. Males had significantly shorter flight initiation distances and resurfaced significantly faster than did females after a frightening stimulus. Resurfacing times were not significantly different between the sexes in a second experiment, but the overall behavioral pattern was similar to the first. Reduced fear could increase exposure to predation and therefore elevate mortality of juvenile males.