Body mass is usually positively associated with reproductive output in females, with larger females having more offspring. However, in gliding or flying species, large body mass is constrained as a result of wing loading, defined as the ratio of body mass to the surface area available to generate lift. Gliding animals may be particularly susceptible to increased wing loading resulting from added mass during pregnancy. We compiled reproductive and morphological data for gliding and related nongliding mammals in all extant taxa where this type of locomotion occurs to test the predictions that gliders will have proportionally lighter litters than related nongliders, and relative litter mass will be negatively associated with wing loading. Contrary to our 1st prediction, gliders had heavier offspring than did nongliders in all taxa examined. Consistent with our 2nd prediction, however, greater relative litter mass was associated with lower wing loading. Maintaining the ability to glide when pregnant may influence investment in reproduction. However, gliding locomotion appears to be associated with increased litter mass, perhaps because of smaller litters of heavy young. Thus, we suggest that gliders may be characterized by K-selected life-history traits such as low mortality, slow metabolism, and fewer offspring per litter, but increased investment in individual young. The findings of this comparative study highlight how major shifts in locomotor mode can have a profound influence on patterns of life history.