- July 08, 2026
- By Jennifer S. Holland M.S. ’98
A new University of Maryland study focused on wild bats provides fresh insight into the effects of evolutionary pressures, finding that an animal’s immune system is influenced not only by its need to fight infections, but by sex, age and competition for mates.
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 greater understanding of how animals naturally allocate resources could ultimately help explain why immune systems vary so widely across species.
While some scientists have suggested that an animal’s mating strategy isn’t likely to affect immune system investment, the new study found the opposite. Particularly for male bats, competition for mating opportunities, along with age, strongly influences immune strategy.
“It really drives home the way biological trade-offs have helped sculpt the evolution of immune defense,” Wilkinson said.
For this study, the researchers examined adult greater spear-nosed bats, Phyllostomus hastatus, a long-lived tropical species found in Central and South America, over the course of decades. 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.
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 used a measure that 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. Higher neutrophil levels indicate 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 neutrophil-to-lymphocyte ratio (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 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.”
In addition to Wilkinson, the paper’s co-authors are alums Jillian Kaiser ’25 and Ana Cunningham’25, ’25, biological sciences doctoral students Katherine Armenta and Alexis Lawson, and former biology postdoctoral associates Severine Hex and Jack Rayner.