Extreme Seasonal Growth in Arctic Deer: Comparisons and Control Mechanisms

Arctic ungulates have the capacity to voluntarily restrict growth to times of the year of predictable high quality seasonal herbage abundance. The constraints of the arctic environment mean that not only must the timing of this growth seasonality be accurate but growth rate must be maximised for a short period of time. It is known that daylength is a critical cue for the timing of seasonal rhythms of growth; a manipulated photoperiod of 16L:8D stimulates out of season growth in reindeer during winter and this growth is associated with an increase in the plasma levels of insulin like growth factor 1 (IGF 1). In contrast a manipulated photoperiod of 8L: 16D delays the spring rise in growth. There is good evidence from a boreal cervid, the red deer, that the number of hours of day influences growth rate also and this is associated with IGF 1. Thus daylength may have a dual role in growth seasonality, timing and amplitude. Although the mechanisms underlying the timing of rhythms by daylength are partly understood a putative role for daylength in controlling the rate of an event is a new concept.