Some male bats have immune readings that would send a human doctor into emergency mode — yet these bats are perfectly healthy, even dominant. That's one of the surprising findings from a new study of wild bats in Central and South America, and researchers say it reveals just how much lifestyle shapes the immune system.
Scientists at the University of Maryland examined 511 greater spear-nosed bats, a tropical species where males guard groups of up to 25 females. The researchers measured different types of white blood cells and compared them with stress hormones and the bats' ages. They wanted to understand whether competition for mates changes how animals invest in their immune defenses.
The answer was yes — especially for males. Male bats relied more heavily on what scientists call rapid-response immune defenses, the kind that jump into action immediately against new threats. This reliance grew even stronger as males aged. During mating season, both male and female bats showed their highest readings for this fast-acting immune response, suggesting that reproduction itself shifts how the body defends itself.
"Our work offers clarity and detail that was missing from the immunity story," said Gerald Wilkinson, a biology professor at the University of Maryland who led the study. "Notably, some scientists have suggested that an animal's mating strategy isn't likely to affect immune system investment, but we found the opposite."
The researchers were struck by another discovery: some male bats had immune readings that would indicate serious illness in humans. Yet these bats showed no signs of disease — they were actually the most successful members of the group, the ones successfully defending females and fathering offspring. This raises questions about how bats tolerate stress that would be dangerous in other mammals.
"Other studies have shown that bats can be exposed to pathogens without generating the cytokine storm — the immune overreaction — that leads to major inflammation in humans," Wilkinson said. "It's an immune puzzle that perhaps bats can one day help us solve."
Female greater spear-nosed bats can live 20 years or more, while males face much higher mortality. This difference likely explains why their immune strategies diverged so dramatically over evolution. The study, published in the Journal of Animal Ecology, drew on decades of tracking data to estimate the bats' ages — giving researchers an unusually detailed picture of how immunity changes across a lifetime.
Wilkinson said understanding how animals naturally balance immune defenses with other needs could ultimately help explain why immune systems vary so widely across species. For now, the study adds to growing evidence that mating competition and short lifespans have deeply shaped how these bats evolved to fight disease.