Seasonal patterns of body temperature daily rhythms in group-living Cape ground squirrels Xerus inauris

Abstract

Organisms respond to cyclical environmental conditions by entraining their endogenous biological rhythms. Such physiological responses are expected to be substantial for species inhabiting arid environments which incur large variations in daily and seasonal ambient temperature (T(a)). We measured core body temperature (T(b)) daily rhythms of Cape ground squirrels Xerus inauris inhabiting an area of Kalahari grassland for six months from the Austral winter through to the summer. Squirrels inhabited two different areas: an exposed flood plain and a nearby wooded, shady area, and occurred in different social group sizes, defined by the number of individuals that shared a sleeping burrow. Of a suite of environmental variables measured, maximal daily T(a) provided the greatest explanatory power for mean T(b) whereas sunrise had greatest power for T(b) acrophase. There were significant changes in mean T(b) and T(b) acrophase over time with mean T(b) increasing and T(b) acrophase becoming earlier as the season progressed. Squirrels also emerged from their burrows earlier and returned to them later over the measurement period. Greater increases in T(b), sometimes in excess of 5°C, were noted during the first hour post emergence, after which T(b) remained relatively constant. This is consistent with observations that squirrels entered their burrows during the day to 'offload' heat. In addition, greater T(b) amplitude values were noted in individuals inhabiting the flood plain compared with the woodland suggesting that squirrels dealt with increased environmental variability by attempting to reduce their T(a)-T(b) gradient. Finally, there were significant effects of age and group size on T(b) with a lower and less variable T(b) in younger individuals and those from larger group sizes. These data indicate that Cape ground squirrels have a labile T(b) which is sensitive to a number of abiotic and biotic factors and which enables them to be active in a harsh and variable environment.

Figure 1. Body temperature (T_b) daily rhythm of an adult Cape ground squirrel (605 g) for the first (21 to 28 May), eighth (09 to 16 July), fifteenth (27 August to 03 September) and twenty-second week (15 to 22 October) of a 23-week measurement period.

‘M’ indicates the mesor (37.41°C), ‘A’ the amplitude (0.92°C) and ‘Ø’ the acrophase (189.11° or 12∶36 h) of the fitted cosine curve. SR and SS show times of sunrise and sunset.

Figure 2. Mean ±SE daily rhythm parameters of eight Cape ground squirrels during the 23 week measurement period for: (a) T_b Mesor (°C); (b) T_b Amplitude (°C); (c) T_b Acrophase (time of day and degrees).

Individuals inhabiting the flood plain and the woodland are denoted by solid and open circles. Maximum, minimum and mean T_a values are shown in (d) as top, middle and lower lines.

Figure 3. Mean ±SE immergence and emergence times in the flood plain (solid circles and bold line) and woodland (open circles and light line).

Mean number of animals observed at any one time was 8.1±4.5 at emergence and 5.6±2.6 at immergence.

Figure 4. T_b (open circles) and T_a (solid circles) and fitted cosine curves for a Cape ground squirrel during the 9^th week of the sampling period illustrating the variation in T_a and T_b.

The difference between the lowest T_b value recorded (33.39°C at 19:08) and the highest T_b during the previous day (39.32°C at 16:08) was 5.93°C. Over the 23 week period, extreme changes in T_b included one individual that decreased in T_b by 5.56°C and another that increased in T_b by 5.98°C in one hour.

Figure 5. Mean ±SE T_b changes between successive hours across all eight individuals during the first, eighth, fifteenth and twenty-second weeks of the measurement period.

Grey bars represent the mean ±SE times of emergence (left-hand bar) and immergence (right-hand bar).

Figure 6. Mean ±SE T_b of the eight individuals for the first, eighth, fifteenth and twenty-second weeks of the sampling period.

T_b values rose rapidly in the morning before reaching a plateau during the day.

Figure 7. Mean ±SE values of the mesor, amplitude and acrophase shown per age class (subadults and adults) and for different group sizes (1, 3, 4, 5 and 9).

The number of individuals in each category is indicated above the error bars. The parameters have been averaged for the level of individual (per category) and then for all weeks, hence SE is non-zero even when only data from one individual is presented.