Measures of growth, development, and sexual maturation in mongolian gerbils (Meriones unguiculatus): Effects of photic period during ontogeny

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Abstract
We examined, in Mongolian gerbils, the adequacy of two commonly used indices of rate of development, age at eye‐opening and age at vaginal introitus, as predictors of biologically important events: initiation of feeding on solid food and age at first parturition. We then enquired as to the interrelationships among body weight, eye‐opening, and vaginal introitus in Mongolian gerbils whose rates of development were manipulated by altering the extent of their exposure to illumination during ontogeny. We found that (1) age at vaginal introitus was significantly, but weakly, correlated with age at first parturition (Experiment I); (2) age at eye‐opening strongly predicted age of initiation of gnawing on solid food (Experiment II); (3) eye‐opening occurred at markedly different ages, but at the same body weight in gerbils reared under 0/24‐, 6/18‐, 12/12‐, and 24/0‐hr day–night cycles (Experiment III); and (4) vaginal introitus occurred at both different ages and different body weights in females reared under different day–night cycles (Experiment III).