Rare and Opportunistic Use of Torpor in Mammals-An Echo from the Past?

Abstract

Torpor was traditionally seen as a winter survival mechanism employed by animals living in cold and highly seasonal habitats. Although we now know that torpor is also used by tropical and subtropical species, and in response to a variety of triggers, torpor is still largely viewed as a highly controlled, seasonal mechanism shown by Northern hemisphere species. To scrutinize this view, we report data from a macroanalysis in which we characterized the type and seasonality of torpor use from mammal species currently known to use torpor. Our findings suggest that predictable, seasonal torpor patterns reported for Northern temperate and polar species are highly derived forms of torpor expression, whereas the more opportunistic and variable forms of torpor that we see in tropical and subtropical species are likely closer to the patterns expressed by ancestral mammals. Our data emphasize that the torpor patterns observed in the tropics and subtropics should be considered the norm and not the exception.

Fig. 1

Global distribution of torpor patterns, based on the maximum duration of bouts. (A) Daily torpor (n = 117); (B) Prolonged torpor (n = 21); and (C) Hibernation (n = 109). (D) Frequency distribution of torpor patterns (maximum duration) in relation to the four climate zones (tropical: 0–23.5°N/S, subtropical: 23.5–40°N/S, temperate: 40–60°N/S, or polar: 60–90°N/S). Black: data deficient (n = 28); dark blue: hibernation; middle blue: prolonged torpor; and light blue: daily torpor. Sample sizes for the four climate zones are: polar (n = 2), temperate (n = 60), subtropical (n = 118), and tropical (n = 95). (E) The phylogenetic relationships of species with known torpor use type. The tip labels represent the longest torpor type observed in the species, using the same color scheme as in (D). Pie charts at the nodes were estimated using ancestral state estimation, and the proportion of each color represents the likelihood that the shared ancestor used that particular torpor type.

Fig. 2

(A) Global distribution of seasonality in torpor use and whether torpor is used seasonal (blue, n = 44/275) or non-seasonal (red, n = 70/275); data deficient is not depicted (n = 161/275). (B) Frequency distribution of seasonality of torpor use based on climate zone. Blue: strictly seasonal use; red: torpor use not restricted to one season; black: data deficient. Sample sizes for the four climate zones (tropical: 0–23.5°N/S, subtropical: 23.5–40°N/S, temperate: 40–60°N/S, or polar: 60–90°N/S) are polar (2), temperate (60), subtropical (118), and tropical (95). (C) The phylogenetic relationships of species with data on torpor use over multiple seasons. Pie charts at the nodes were estimated using ancestral state estimation, and the proportion of each color represents the likelihood that the shared ancestor used non-seasonal (red) or seasonal torpor (blue).