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. 2023 Aug 17;20(1):27.
doi: 10.1186/s12983-023-00501-3.

Body mass is associated with hibernation length, body temperature, and heart rate in free-ranging brown bears

Alina L Evans #  1 , Boris Fuchs #  2 , Navinder J Singh  3 , Alexandra Thiel  2 , Sylvain Giroud  4   5 , Stephane Blanc  6 , Timothy G Laske  7 , Ole Frobert  8 , Andrea Friebe  9 , Jon E Swenson  10 , Jon M Arnemo  2   3
Affiliations

Body mass is associated with hibernation length, body temperature, and heart rate in free-ranging brown bears

Alina L Evans et al. Front Zool. .

Abstract

Background: Despite centuries of research, debate remains on the scaling of metabolic rate to mass especially for intraspecific cases. The high variation of body mass within brown bears presents a unique opportunity to study the intraspecific effects of body mass on physiological variables. The amplitude of metabolic rate reduction in hibernators is dependent on body mass of the species. Small hibernators have high metabolic rates when euthermic but experience a drastic decrease in body temperature during torpor, which is necessary to reach a very low metabolic rate. Conversely, large hibernators, such as the brown bear (Ursus arctos), show a moderate decrease in temperature during hibernation, thought to be related to the bear's large size. We studied body mass, abdominal body temperature, heart rate, and accelerometer-derived activity from 63 free-ranging brown bears (1-15 years old, 15-233 kg). We tested for relationships between body mass and body temperature, heart rate, and hibernation duration.

Results: The smallest individuals maintained lower body temperatures during hibernation, hibernated longer, and ended hibernation later than large bears. Unlike body temperature, winter heart rates were not associated with body mass. In summer, the opposite pattern was found, with smaller individuals having higher body temperature and daytime heart rates. Body mass was associated with body temperature in the winter hypometabolic state, even in a large hibernating mammal. Smaller bears, which are known to have higher thermal conductance, reached lower body temperatures during hibernation. During summer, smaller bears had higher body temperatures and daytime heart rates, a phenomenon not previously documented within a single mammalian species.

Conclusion: We conclude that the smallest bears hibernated more deeply and longer than large bears, likely from a combined effect of basic thermodynamics, the higher need for energy savings, and a lower cost of warming up a smaller body.

Keywords: Brown bears; Heart rate; Hibernation; Metabolic rate; Thermal conductance; Thermoregulation.

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Conflict of interest statement

Timothy Laske is an employee of Medtronic Inc. Remaining Authors have no Competing interest.

Figures

Fig. 1
Fig. 1
Sample sizes for each analysis. Some brown bears had only body temperature (Tb) or heart rate (HR) logging, and not both, and some were excluded from HR or Tb, due to failure of a specific logger or from one season, due to pregnancy, or in one case, killed by another bear. Some bears from summer were not available in winter, because they were killed during the autumn bear hunt
Fig. 2
Fig. 2
Spring body mass and age for female (dots) and male (triangles) bears from the heart rate dataset. Generalized additive model of the form spring body mass ~ s(age) with a cubic regression spline and basis dimension 3 predicting age based on body mass for females (solid line) and males (dashed line)
Fig. 3
Fig. 3
Annual variation in body temperature and heart rate (HR) across size groups; A The analysed daily mean body temperature data of brown bears in Sweden measured between 2010 and 2016 pooled in to three body mass groups: Small” (orange) was defined as < 60 kg (N = 25), “medium” (blue) as 60–120 kg (N = 11), and “large” (black) as 120–240 kg (N = 10). B Mean daytime HR of brown bears in Sweden measured between 2011 and 2016, pooled in to three body mass groups: “Small” (orange) < 60 kg (N = 32), “medium” (blue) 60–120 kg (N = 31), and “large” (black) 120–240 kg (N = 11). To illustrate the full annual cycle, the same data is partly shown twice (February to June). Sample sizes are for an individual in a given year, body mass measured in either February, May or June
Fig. 4
Fig. 4
Heart rate (HR) in winter: Predicted mean daytime HR for hibernating brown bears with body mass of 40 kg (orange solid line), bears with body mass of 120 kg (blue dashed line), and mean smoothed values for each bear winter (grey solid line). Shaded areas display 95% confidence intervals. HR data from 1 December to 23 February
Fig. 5
Fig. 5
Body temperature in winter: Predicted mean daytime body temperature (Tb) for hibernating brown bears with body mass of 40 kg (orange solid line), and bears with body mass of 120 kg (blue dashed line) and mean smoothed values for each bear winter (grey solid line). Shaded areas display 95% confidence intervals. Tb from 1 December to 23 February
Fig. 6
Fig. 6
Daytime summer heart rate (HR): Predicted mean HR (HR) for brown bears during summer and during day time with body mass of 40 kg (orange solid line), and bears with body mass of 120 kg (blue dashed line) and mean smoothed values for each bear summer (gray solid lines). Shaded areas display 95% confidence intervals. HR data from 1 June to 21 August measured between 10:00 and 22:00
Fig. 7
Fig. 7
Nighttime summer heart rate (HR): Predicted mean HR (HR) for brown bears during summer and at night with body mass of 40 kg (orange solid line), and bears with body mass of 120 kg (blue dashed line) and mean smoothed values for each bear summer (grey solid line). Shaded areas display 95% confidence intervals. HR data from 1 June to 20 August measured between 02:00 and 06:00
Fig. 8
Fig. 8
Passive summer body temperature: Predicted daily mean body temperature (Tb) for brown bears during inactivity in summer with body mass of 40 kg (orange solid line), and bears with body mass of 120 kg (blue dashed line) and mean smoothed values for each bear summer (grey solid line). Shaded areas display 95% confidence intervals. Tb data from 1 June to 21 August
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