Ruby-throated hummingbirds suppress ATP demand to save energy during torpor

The cellular mechanisms that underlie hummingbird torpor are only beginning to be studied. We are the first to report two ATP-demand pathways downregulated in torpid ruby-throated hummingbirds. We characterized SERCA and Na + /K + ATPase activity in normothermia and torpor. SERCA activity was 10–13× higher than in songbirds. Sarcolipin was more phosphorylated in torpor, reducing heat production, and Na + /K + ATPase activity fell 43%. These shifts lower ATP demand and conserve energy during torpor.