Emergence from torpor rapidly elevates suppressed blood immune parameters in a bat species hibernating in a moderate climate.

Torpor is a state of reduced metabolism that allows animals to conserve energy during periods of limited resources. Critical physiological processes, including the immune function, are downregulated during torpor - a phenomenon that remains relatively understudied in bats, especially in the southern hemisphere. This study examined the effects of torpor on the immune system of the eastern bent-winged bat (Miniopterus orianae oceanensis), an Australian cave-roosting species. We captured 52 bats in austral autumn and winter and housed them in controlled conditions to induce torpor for 2 or 8 h. Blood samples were collected pre-torpor, at the end of the torpor bout and 30 min post-arousal. White blood cell count was measured to assess cellular immunity, and plasma antibacterial capacity was used to evaluate humoral innate antibacterial immunity across time points. We found that total white blood cell count decreased by 23% during torpor, but increased by 75.9% upon arousal, surpassing baseline values by 33.9%. Neutrophils and monocytes were the first cells to be restored, the former making up about 57% of circulating white blood cells after arousal. Antibacterial capacity did not differ between time points. Our results demonstrate rapid restoration of white blood cells after arousal, likely through the release of sequestered immune cells, while short torpor bouts do not impair innate humoral immunity in eastern bent-winged bats. While the rapid restoration of neutrophils could be protective against pathogens accumulated during torpor, it also provides a higher potential for immunopathology and tissue damage upon arousal.