What Are the Seasonal Physiological Adaptations of Arctic Foxes That Allow Them to Survive Periods of Limited Food Availability?

Research Paper        

11/30/12

What are the seasonal physiological adaptations of arctic foxes that allow them to survive periods of limited food availability?

Introduction

The arctic fox, Alopex lagopus, is a small carnivore native to the Arctic regions in the northern hemisphere. It lives in one of the harshest environments on the planet, with extreme fluctuations in food availability and temperature. It has numerous adaptations for survival in cold temperatures, including thick fur, a circulation system of countercurrent heat exchange in the paws, a good supply of body fat, and a low surface-area to volume ratio to limit the amount of heat that escapes (). In addition, the arctic fox has short ears and a short muzzle, which it can hide from cold ambient temperatures by curling into certain positions or burrowing in the snow.  

Not only do these animals endure frigid temperatures colder than -40°C (Prestrud, 1991), but they also experience seasonal variations in food availability due to the decreased number of prey species present during winter months. Throughout the warmer breeding season, their main food source is small rodents; however, there are not significant sources of small rodents available during the winter. The arctic fox must feed on the carcasses of seals, reindeer, and seabirds, so it is therefore assumed that these animals have very unpredictable access to food in the winter. (Fuglei and Øritsland, 1999) As a result, arctic foxes are subject to periods of starvation due to variable food sources, frequent winter storms, low temperatures, and extended darkness. “When animals go through periods of limited food availability, they have to rely on their body reserves to provide energy for metabolic functions” (Fuglei, Mustonen, & Nieminen, 2004, pg.157). This requires adjustments in fat deposition, metabolic rate, and hormonal regulation, leading to the question: What are the seasonal physiological adaptations of arctic foxes that allow them to survive these periods of limited food availability? 

Previous studies on arctic foxes have led to controversy in regard to what extent the arctic fox can withstand low ambient temperatures without having to increase its metabolic heat production. In addition, it is uncertain as to whether or not the BMR of arctic foxes goes through seasonal changes, due to variations in the results of previous experiments. Two recent studies, aimed at settling the discrepancies between past experiments, focused on the physiological adaptations of arctic foxes in response to seasonal variations in food availability.

Literature Review

Fuglesteg et al. (2006) and Fuglei et al. (2004) ran a number of experiments on arctic foxes to determine which factors operate as physiological adaptations to temporary starvation and low ambient temperature. Both studies experimented on wild foxes, captured in the vicinity of Svalbard, Norway under permit from the Governor of Svalbard and the Norwegian National Animal Research Authority. This location has a high-Arctic desert climate with complete 24-hour darkness from November through January, and temperatures below freezing from September to May. Therefore, the foxes used had intact physiological adaptations to an arctic environment.

Literature Review: Article I

Fuglesteg et al. (2006), examined the seasonal fluctuations in the basal metabolic rate (BMR) of arctic foxes, determined their critical temperature (T1c) and average body core temperature under BMR conditions, and evaluated the changes in body mass in response to periods of forced starvation. The study consisted of two series of experiments, each performed on a different group of arctic foxes. In the first series of experiments, measurements of metabolic rate were conducted in the summer months (July-August), and then again in winter months (January-March). In the second series of experiments, metabolic rate was measured in regular intervals for a period of 14 months (beginning in May). In both sets of experiments, animals were placed in a climatic chamber during the month prior to measurements to avoid high stress levels. Also, food was removed at least 12 hours before the experiments were conducted to ensure measurements reflected post-absorptive conditions. The metabolic rate was found by measuring the rate of oxygen consumption and carbon dioxide production. The values of these gases were determined from “the differences in the concentrations of the two gases in reference air from outside the metabolic cage and air collected from the metabolic cage” (Fuglesteg et al., 2006, pg. 310). Metabolic measurements were performed at fixed Ta’s (six in the summer, eight in the winter), and it was assumed that the T1c point fell within one of the intervals between the fixed Ta that was tested. The T1c of the foxes in summer and winter were estimated as the Ta at which the best-fit regression line (for data points at Ta’s above the assumed T1c) intersected the best-fit mean (for points at Ta’s above the assumed T1c). Body core temperature was measured by use of radio transmitters inside the stomach of the foxes. The impulses from the transmitters were registered by a radio placed inside the climate chamber, and body temperature was determined by counting the number of impulses per minute. Lastly, three starvation periods were performed in February, March, and April, each lasting eight days.  The foxes were weighed 10 days before food was removed, in the morning of days 0, 2, 4, 6, and 8, and after re-feeding on days 11 and 15. These experiments were used to determine body mass and BMR values in response to starvation.

The study showed significant variation in both mass-specific and total BMR from summer to winter months. In fact, average mass-specific BMR was reduced by 37% from summer to winter (2.53 W kg-1 and 1.56 W kg-1, respectively). The summer level of total BMR was 9.53 W and the winter level was 6.97 W, which is a 27% difference. Figure 1 shows the differences in mass-specific BMR at difference ambient temperatures, summer (filled symbols), and winter (open symbols).

[pic 1]

Figure 1: Mass specific metabolic rate (MR; W kg-1) at different ambient temperatures (Ta; °C), summer (filled symbols), and winter (open symbols). Fuglesteg et al., 2006, pg. 313