Influence of photoperiod and photoperiod reversal on growth, mortality, and indicators of age of Dicrostonyx groenlandicus

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Abstract
After weaning, collared lemmings (Dictrostonyx groenlandicus) were exposed to 3 months of summer (S) photoperiod (18.5L:5.5D) or winter (W) photoperiod (6.5L:17.5D) or to one of four 6-month photoperiod treatments (SS, SW, WW, or WS). Lemmings were weighted every 3 weeks, and at the end of treatment, eye lenses were weighted, and 10 body and 22 skull measurements were taken. At the end of the treatment, W, SW, and WW lemmings did not differ in weight, but were significantly heavier than S, SS, and WS lemmings. Animals in WS were significantly heavier than those in SS and both were significantly heavier than S lemmings. The marked contrast of the photoperiodically induced growth pattern of Dicrostonyx groenlandicus with that of other microtines may be due to a relatively larger investment in winter reproduction by collared lemmings. Lemmings that died during the experiment were significantly larger both in body weight and skull size than those that survived. Eye lens weights of S and W lemmings did not differ, but were significantly smaller than those of SS, SW, WW, and WS lemmings. Inequality of mean lens weights was evident among the 6-month treatments, but nevertheless, eye lens weight was the best univariate age criterion and performed approximately as well as linear combinations of body or skull measurements selected by stepwise discriminant analysis.