No Optimal Weight to Survive the Winter in a Northern Island Population of Water Voles Arvicola amphibius

Growth and body size of mammals are commonly correlated with many life history strategies, including those related to survival and reproduction. However, in certain circumstances suboptimal growth rates and smaller size may be advantageous and adaptive. The water vole Arvicola amphibius is a large vole, about three times the size of a field vole Microtus agrestis, but with similar ecological and reproductive characteristics. Island populations were studied on the coast of northern Norway, just below the Arctic Circle, during 2003-2018, by capture-mark-recapture. The main aims were to study growth rates, asymptotic weight and survival, expecting that the “optimal” weight for surviving the winter would be 140-160 g. The smallest juveniles caught weighed only 21 g and were assumed, based on data from the literature, to be around 14 days old. This age was used as starting point for the growth curve. This, however, may have been about one week too early, as juveniles are more likely not fully weaned and leave their nest of birth until 30-40 g. Initial growth rates in juveniles was relatively high but declined from around 100-120 g or 40-50 days old. The asymptotic weight was not clearly defined, but its maximum was around 150-160 g. Most juveniles that survived the winter weighed between 100 and 160 g in their first summer. Large individual variations in growth rates were found. Overwintered subadults in spring weighed about the same as juveniles did in the autumn but grew quickly in April and May to reach adult size. A specific “optimal” weight for juveniles that survived the winter was not found. The range could be given as 100-160 g, too broad to define an “optimal” weight range. However, those that survived tended to be slightly heavier than those that died. Reproducing adults generally weighed 180-220 g and did not reduce their weight toward the autumn, i.e., to increase winter survival, but very few adults survived even the summer and almost none survived their second winter. Juveniles postponed reproduction until next spring, most likely to take advantage of fresh vegetation growth and less competition.