Squamate metabolic rates decrease in winter beyond the effect of temperature

The reptilian form of hibernation (brumation) is much less studied than
its mammalian and insect equivalents. Hibernation and brumation share some
basic features but may differ in others. Evidence for hypometabolism in
brumating reptiles beyond the effect of temperature is sporadic and often
ignored. We calculated the standard metabolic rates (SMR, oxygen uptake
during inactivity), in winter and/or summer, of 156 individuals
representing 59 species of Israeli squamates across all 17 local families.
For 32 species, we measured the same individuals during both seasons.
 We measured gas exchange continuously in a dark metabolic
chamber, under the average January high and low temperatures (20°C and
12°C), during daytime and nighttime. We examined how SMR changes with
season, biome, body size, temperature, and time of day, using phylogenetic
mixed models. Metabolic rates increased at sunrise in the diurnal species,
despite no light or other external cues, while in nocturnal species the
metabolic rates did not increase. Cathemeral species shifted from a
diurnal-like diel pattern in winter to a nocturnal-like pattern in summer.
Regardless of season, Mediterranean species SMRs were 30% higher than
similar-sized desert species. Summer SMR of all species together scaled
with body size with an exponent of 0.84 but dropped to 0.71 during
brumation. Individuals measured during both seasons decreased their SMR
between summer and winter by 47%, on average, at 20°C and by 70%
at 12°C. Q10 was 1.75 times higher in winter than in summer, possibly
indicating an active suppression of metabolic processes under cold
temperatures. Our results challenge the commonly held perception that
squamate physiology is mainly shaped by temperature, with little role for
intrinsic metabolic regulation. The patterns we describe indicate that
seasonal, diel, and geographic factors can trigger remarkable shifts in
metabolism across squamate species.