Seasonal changes in heart rate and food intake in reindeer (Rangifer tarandus tarandus)

This study tested the hypothesis that the annual cycle in heart rate (HR) i n reindeer is, at least in part, a consequence of seasonal fluctuation in v oluntary-food intake. Heart rate and daily dry matter voluntary-food intake (DDMVFI) were recorded in two captive female reindeer (Rangifer tarandus t arandus) from April 1995 to August 1996. Heart rate was measured continuous ly in each animal for 20-24 h for 7 days each month using Polar (R) Sport T esters (PST); DDMVFI was measured in each animal daily for 17 months. Modal daily heart rate (MDHR) and DDMVFI fluctuated seasonally in close synchron y, both reaching maxima in July and minima in January. The relationship bet ween HR and DDMVFI was investigated experimentally by manipulating the leve l of feeding in a stepwise manner in May, when appetite was low and in Augu st, when DDMVFI was close to maximum. Heart rate showed stepwise changes in close synchrony with the changes in levels of feeding. These results sugge st that the seasonal increase in HR in summer is a consequence of increased food intake and, likewise, decreased HR in winter is a consequence of redu ced food intake. The observed relationship between food intake and HR presu mably reflects changes in cardiac output and/or the rate of flow of blood t o the gastrointestinal tract which are influenced by meal size.