Extreme Seasonal Growth in Arctic Deer: Comparisons and Control Mechanisms

Arctic ungulates have the capacity to voluntarily restrict growthto times of the year of predictable high quality seasonal herbageabundance. The constraints of the arctic environment mean thatnot only must the timing of this growth seasonality be accuratebut growth rate must be maximised for a short period of time.It is known that daylength is a critical cue for the timingof seasonal rhythms of growth; a manipulated photoperiod of16L:8D stimulates out of season growth in reindeer during winterand this growth is associated with an increase in the plasmalevels of insulin like growth factor 1 (IGF 1). In contrasta manipulated photoperiod of 8L: 16D delays the spring risein growth. There is good evidence from a boreal cervid, thered deer, that the number of hours of day influences growthrate also and this is associated with IGF 1. Thus daylengthmay have a dual role in growth seasonality, timing and amplitude.Although the mechanisms underlying the timing of rhythms bydaylength are partly understood a putative role for daylengthin controlling the rate of an event is a new concept.