We examined reproductive tracts of 253 female Mongolian gerbils (Meriones unguiculatus) and recorded the uterine locations (relative to siblings of opposite sex) in which 812 male and 823 female fetuses were found. Within-litter comparisons revealed that sexes were not distributed randomly across uterine horns. The percentage of males in right horns (55.0%) was greater than the percentage of males in left horns (41.8%) and the percentage of females in right horns (45.0%) was significantly less than the percentage of females (58.2%) in left horns. We did not find differences in the total number of fetuses in left and right uterine horns or a sex bias in the total sample of 1635 fetuses. Results were discussed in terms of (a) effects of sexual segregation of fetuses on expected probabilities of fetuses developing adjacent to 0, I , or 2 fetuses of opposite sex and (b) consequent alterations in expected frequencies of behavioral phenotypes in populations of gerbils.
The uterine locations of male and female fetuses, relative to fetuses of the same or opposite sex, are associated with important differences in the physiblogical, morphological, and behavioral characteristics of adult members of all litterbearing rodent species (mice, rats, hamsters, and gerbils) that have been examined to date (Clark &