A new study of wild bats reveals that an animal's immune system is shaped by more than its need to fight infections. Researchers from the University of Maryland found that sex, age and competition for mates influence how bats balance different types of immune defenses—providing new insight into how evolutionary pressures shape immune strategy.
The findings, published June 30 in the Journal of Animal Ecology, highlight a fundamental challenge faced by all animals: Maintaining a strong immune system requires energy and resources that could otherwise be invested in growth, reproduction or survival. But rather than simply evolving the strongest possible immune response, animals appear to evolve immune strategies that match the demands of their lifestyles.
"Our work offers clarity and detail that was missing from the immunity story," said the study's lead author, UMD Biology Professor Gerald Wilkinson , noting that understanding how animals naturally allocate resources to immunity and other biological needs could ultimately help explain why immune systems vary so widely across species.
"Notably, some scientists have suggested that an animal's mating strategy isn't likely to affect immune system investment, but we found the opposite," Wilkinson said. "For male bats especially, we found that competition for mating opportunities, in addition to age, strongly influences immune strategy. It really drives home the way biological trade-offs have helped sculpt the evolution of immune defense."
For this study, the researchers examined a population of adult greater spear-nosed bats, Phyllostomus hastatus, a long-lived tropical species found in Central and South America. The species has an extreme mating system in which dominant males vigorously defend groups of up to 25 females, and successful males may father dozens of offspring over their reproductive lives. Yet males have much higher mortality than females, which can sometimes live twice as long—20 years or more.
"These characteristics make the species ideal for asking whether intense reproductive competition and shorter male lifespan are associated with different immune strategies," Wilkinson said. "Also, our team collected data on the population for decades, could estimate individuals' ages and could repeatedly test hundreds of animals—an amazing sample for this type of project."
The researchers analyzed blood samples from 511 bats, measuring different types of white blood cells and comparing those measurements with urinary cortisol levels, an indicator of physiological stress. They also assessed body condition and estimated the bats' ages using long-term tracking data and biological markers of aging.
To understand how the bats balanced different immune defenses, the researchers calculated the neutrophil-to-lymphocyte ratio (NLR). This measure compares two major types of white blood cells: neutrophils, which provide a rapid first-line defense against threats, and lymphocytes, which support more targeted immune responses that can adapt to specific pathogens. A higher NLR indicates a greater reliance on rapid-response immune defenses.
"By scoring many of the bats multiple times over several years, we were able to show that immune investment changes predictably under different reproductive situations," Wilkinson said.
The researchers identified several key patterns. The male bats showed greater reliance than females on rapid-response immune defenses. The NLR also increased with age, especially in males, suggesting that older bats shift toward rapid immune defenses as immunity that develops over time in response to pathogens declines. In addition, both male and female bats showed their highest NLR values during the mating season, indicating that reproduction itself causes a shift toward rapid-response immune defense.
"One surprise was the fact that some of these male bats had NLR measurements that, had they come from a human blood test, would have indicated a pathological condition," Wilkinson noted. "Yet the animals showed no outward signs of ill health, and in fact were the best-functioning bats—the ones successfully defending females."
The finding raises new questions about how bats tolerate physiological stress that would be concerning 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."
The new study adds to a growing understanding that immune systems are shaped not only by exposure to disease, but also by the demands of an animal's life history—including reproduction, aging and social behavior.
"Our findings stress the role of the mating system in developing immune strategy—a relationship not widely appreciated based on studies of other animals," Wilkinson said.
The substantial differences between the sexes, he added, inspire him to consider what trade-offs the males and females of our own species have made.
"The bats remind us that immunity is not one-size-fits-all, including in humans," Wilkinson said. "I hope our findings stimulate further research on how this crucial biological system has evolved across the animal kingdom."
