Sex, Age, and Skull Morphology Effects on Eye Size and Refractive Errors in Reindeer

Eye size influences captured light and visual resolution and is used as an indicator of the relative importance of vision. Although vision-related sexual dimorphism is subtle among vertebrates, Cervids exhibit pronounced sexual dimorphism in body size, antler development, and reproductive strategies. We tested the hypothesis that male reindeer (Rangifer tarandus tarandus) with larger skeletal dimensions would have larger eyes and narrower binocular overlap but similar refractive errors compared to females. We collected skull morphological, ocular biometric, and refractive error data from 69 semi-domesticated reindeer (0.5-12.5-year-old; 27 females, 42 males) living year-round under extreme high-latitude seasonal variation in photoperiod in south-central Norway. The relationships between eye size, mandible length, interorbital distance, and orbital diameter were analyzed, and binocular overlap and anterior blind zones were calculated. Eye size increased with measures of skull size, while refractive error remained minimal across all ages. Although male calves had significantly larger mandibles and interorbital distances, their orbital diameters were the same as in female calves. However, in adults, males also had larger orbital diameters. After controlling for differences in skeletal measures, males were found to have relatively smaller eyes than females, whereas binocular overlap and blind zones were similar. These findings demonstrate vision-related sexual dimorphism in reindeer.