Body temperature
T
b of three animals increased with the increase of T
a and showed significantly positive correlations with T
a. The linear regression equation for
A. chevrieri was T
b = 33.830+0.110 T
a (F = 185.075, P<0.01, Fig 1A) and for
E. miletus was T
b = 32.236 + 0.164 T
a (F = 361.916, P<0.01, Fig 1B); and for
E. olitor was T
b = 33.625+0.094 T
a (F=90.939, P<0.01, Fig 1C). Within the TNZ, T
b of
A. chevrieri,
E. miletus and
E. olitor were 36.90± 0.10°C, 36.49 ± 0.17°C and 36.44 ± 0.13°C, respectively (Fig 1).
Thermogenic effect of T
a is considered to be the most influential and direct factor in inducing significant physiological changes in animal behavior and energy balance, particularly in animals exposed to cold or high temperatures
(Scholander et al., 1950). Maintenance of mammalian body temperature depends on the metabolic rate levels and thermal conductance (
McNab, 2009). Under high temperature acclimation,
Tupaia belangeri adapts to its environment by increasing T
b (Feng et al., 2022). When mice were acutely exposed to an environment of 42°C, their T
b raised sharply
(Miova et al., 2008). It our previously study, Tb in
E. miletus,
A. chevrieri and
E. olitor decreased under cold exposure or in winter
(Zhu et al., 2009; Zhu et al., 2008; Zhu and Wang, 2015). In this study, high temperature acclimation increased T
b of the three animals, which may be an adaptation strategy for high temperature, increasing T
b was beneficial for narrowing the difference with T
a while increasing heat dissipation and reducing metabolic rate
(Yang et al., 2008; Vejmìlka et al., 2021). Moreover, all three species increased their T
b under high temperature conditions, reflecting convergent adaptation (Table 1). Comparing the amplitude of T
b changes of three animals based on the slope of the linear regression equation,
E. olitor was less affected by high temperature, which may be related to the thicker fur, which increased its heat insulation effect
(Yang et al., 2021). Although
E. olitor and
E. miletus belong to the same genus, however, the range of T
b change of
E. olitor was lower than that of
E. miletus, which may be related to the habitat environmental temperature of its field distribution, the ambient temperature of the habitat in
E. miletus was higher than that of
E. olitor. T
b in
A. chevrieri was higher than that of the two species of Eothenomys, which may be related to its higher RMR
(Wang et al., 2006; Yang et al., 2021).
Resting metabolic rate and thermal neutral zone
RMR of
A. chevrieri showed significant differences in different T
a (F =27.850, P<0.01). According to the definition of TNZ, there was no significant difference in RMR between temperatures of 25°C and 30°C during high temperature acclimation, indicating that TNZ for this species is estimated to be within the range of 25-30°C. The regression equation for lower critical temperature was RMR =7.930-0.231 T
a and the regression equation for the upper critical temperature was RMR = -4.604+0.255 T
a (Fig 2A). RMR of
E. miletus was significantly affected by T
a (F = 65.770, P<0.01). Differences in RMR between 25°C and 30°C were not significant, so the TNZ of
E. miletus was 25-30°C. The regression equation for the lower critical temperature was RMR = 7.238 - 0.214 T
a and the regression equation for the upper critical temperature was RMR = -3.549+0.191 T
a (Fig 2B). RMR of
E. olitor showed similar trends to
A. chevrieri and
E. miletus (F = 80.214, P<0.01). TNZ in
E. olitor was 25-30°C. The regression equation for lower critical temperature and upper critical temperature were RMR = 7.045 - 0.199 T
a and RMR = -1.755+0.140 T
a, respectively (Fig 2C).
RMR represents the minimal energy expenditure necessary for an animal’s survival (
Janelle and Ayres, 2020). It plays an important regulatory role in animals facing different environments
(Feierabend et al., 2015). Temperature is an important environmental factor affecting RMR
(Chen et al., 2020). RMR in
C. barabensis acclimated at high temperature was significantly lower than that acclimated at low temperature
(Xu et al., 2014). High temperature reduced RMR and non-shivering thermogenesis in
M. unguiculatus (Guo et al., 2020). In present study, RMR of all three animals decreased with increasing Ta within the temperature range of 5 to 25°C, which increased above 30°C. RMR of the three animals in our study were lower than that of in normal temperature, when facing high temperature environments, the three species in this study chose the similar adaptation strategy (Table 1)
(Zhu et al., 2009; Zhu et al., 2008; Zhu and Wang, 2015). Through comparison, it was found that three species all decreased RMR under high temperature, indicating that high temperature can reduce heat production to regulate body temperature.
TNZ is a crucial concept in the study of energy strategies in physiological ecology, primarily referring to a range of environmental temperature fluctuations within which mammals can maintain their lowest metabolic rate (
Bligh and Johnson, 1973). Although there are many factors that affect TNZ, T
a is one of the most important factors that cannot be ignored
(Yang et al., 2021). For same species, TNZ can change with T
a, it showed that there had a decrease in the lower critical point and an increase in TNZ width under cold temperature, while increasing the lower critical point and decreasing the width of TNZ under high temperature
(Zhu et al., 2016). For example,
Dipus sagitta had a narrower TNZ in summer and it had a wider TNZ in winter
(Bao et al., 2000). For different species, TNZ is closely related to altitude or temperature
(Zhu et al., 2022). Based on our previous researches, it found that high temperature acclimation narrowed the TNZ and the lower critical temperature point increased in three rodents
(Yang et al., 2021; Zhu et al., 2008; Zhu and Wang, 2012). Narrowing TNZ might be beneficial for reducing energy consumption and is an adaptation method for high temperature acclimation (Table 1)
(Zhang et al., 2007).
Thermal conductance
C values in three animals showed significant differences under different Ta (
E. miletus: F=117.764, P<0.01;
A. chevrieri: F=35.533, P<0.01;
E. olitor: F=346.111, P<0.01; Fig 3A, B, C). All of them remained C values stable within the range of 5 to 25°C, which increased with increasing temperature within the range of 25 to 35°C, the linear regression equations were C =-3.171+0.123Ta (
E. miletus: F=155.516, P<0.01); C =-3.116+0.121Ta (
A. chevrieri: F=148.715, P<0.01); C =-0398+0.120Ta (
E. olitor: F=146.419, P<0.01), respectively.
Thermal conductance affects the energy balance and is one of the important factors affecting animal’s energy consumption in small mammals
(Naya et al., 2013; Meyer et al., 2010). The limited size of small mammals makes it difficult for them to change the thickness of their fur to adapt to different environmental temperatures; it is therefore particularly important to change the C value
(Yang et al., 2021). In our study, it was found that the C values of three animals remained stable below the TNZ, while it increased with the increase of T
a above 25°C, showing that three animals need to enhance their heat dissipation ability under high temperature
(XuanYuan et al., 2023). Moreover, according to the slope of the correlation linear regression, the change ranges of C from large to small is
E. miletus,
A. chevrieri and
E. olitor, suggesting that the C value may be related to the cave depth. The cave depth of
A. chevrieri was deeper and the temperature in the cave is lower. Therefore, the C value of
A. chevrieri was small, which is beneficial for reducing energy loss
(Luo et al., 2000). Because the living environment temperature of
E. olitor is lower than that of
E. miletus and
A. chevrieri and its fur was thick and its heat insulation effect was strong. Therefore, C value of
E. olitor was lower than that of
E. miletus and
A. chevrieri (Luo et al., 2000; Li et al., 2012).
