Winter ecology of the Porcupine caribou herd, Yukon: Part III, Role of day length in determining activity pattern and estimating percent lying

D E Russell,A M Martell

doi:10.7557/2.6.2.656

Abstract

Data on the activity pattern, proportion of time spent lying and the length of active and lying periods in winter are presented from a 3 year study on the Porcupine caribou herd. Animals were most active at sunrise and sunset resulting in from one (late fall, early and mid winter) to two (early fall and late winter) to three (spring) intervening lying periods. Mean active/lying cycle length decreased from late fall (298 mm) to early winter (238 min), increased to a peak in mid winter (340 min) then declined in late winter (305 min) and again in spring (240 min). Mean length of the lying period increased throughout the 3 winter months from 56 min m early winter to 114 min in mid winter and 153 min in late winter. The percent of the day animals spent lying decreased from fall to early winter, increased throughout the winter and declined in spring. This pattern was related, in part, to day length and was used to compare percent lying among herds. The relationship is suggested to be a means of comparing quality of winter ranges.

Highlights

Since activity pattern appeared greatly influenced by day length, we examined the effect of daily observation time on our estimates of percent lying (Table 2)
Other authors have noted that caribou cue their activity to sunrise producing a conspicuous
The pattern of active and bedded cvcles for the remainder of the day, is constrained by dav length since y caribou appear to exhibit another peak in activity at sunset

Summary

Introduction

The winter activity of Rangifer has been the subject of numerous studies in recent years (Segal, 1962; Gaare etui, 1975; Skogland, 1978; Roby, 1978, 1980; Boertje, 1981; Thing, 1984)and comparisons among herds are beginning to emerge (Roby 1978; Russell and Martell, 1984.)Knowledge of the activity pattern (the shortterm alternation between active and resting periods) and activity budget (the tabulation of the proportion of time in various activities) enables researchers or managers to assess the relative condition of the winter range, determine the energetic relationship and nutritional status of the herd and, by understanding the factors that determine activity budget, more accurately analyze the influence of human disturbance (Klein and White, 1978).It is generally accepted that the proportion ot time spent lying is inversely related to rangeRangifer, Special Issue N o . 1, 1986 quality in relation to either forage quality or availability (Gaare ct al, 1975; Roby, 1978; Boertje, 1981; Russell and Martelh 1984).Animals ingesting poor quality forage require longer periods for rumination than those ingesting high quality forage (Blaxter, 1962).When food is scarce (because of either low biomass or adverse snow conditions), animals face an energetic trade-off between the high energy expenditure ot obtaining food and low energy intake by increasing King time. 1, 1986 quality in relation to either forage quality or availability (Gaare ct al, 1975; Roby, 1978; Boertje, 1981; Russell and Martelh 1984). Bv comparing percent lying (percent of the observation period animals spend K ing) among herds or temporally tor the same herd, an indirect method ot assessing range quality is available. One factor that complicates a direct comparison of percent lying among studies is the observation that Rangifer cue their activity to sunrise (Gaare et al, 1975; Thomson, 1977; Roby, 1978; Erriksson etui, 1981), and that day length (percent of a day occurring between sunrise and sunset) affects activity pattern (Erriksson et ai, 1981). Since winter ranges are located at different latitudes and winter studies have occurred at different times, our objective in this paper is to define the relationship between day length and percent lying and compare studies based on this relationship

Methods

Results

Conclusion