Climate change could influence many aspects of an organism’s energy budget.
For animals that rely on torpor (short-term hypothermia) to save energy
overnight, increasing night time temperatures could be problematic since
they would limit the degree to which metabolic rate could be lowered. Most
studies on the use of torpor explore how it is affected by decreased
environmental temperatures, and have associated its use with decreased
ambient temperature. Climate change presents a challenge where organisms
face increased, rather than decreased, ambient temperatures. Because torpid
organisms reduce their body temperature and metabolic rates as ambient
temperatures get colder, warming temperatures could reduce the efficiency,
and potentially the use of hypothermia. Here, we first defined new ways of
comparing torpor use across species. We then compared measurements of
torpor in hummingbirds across different natural temperature regimes to
evaluate the possible effects of rising temperatures on energy management.
We found that hummingbirds save an average of 82% of their energy per
hour they used torpor. We also found that temperate birds used torpor
more often at a colder than at a warmer site, supporting our hypothesis
that warming could reduce torpor use. In contrast, tropical hummingbirds at
a slightly warmer site used torpor more often than hummingbirds at a
slightly colder site, suggesting that climate change could affect tropical
and temperate birds differently.