A risk-tolerant immune system may enable house sparrows’ wanderlust

For animals exploring new territory, taking risks is key to survival. But eating unfamiliar foods can be dicey, since they might contain new pathogens and parasites. One avian immune system, however, seems to have a way of rolling with the punches.

Researchers found a link between the willingness of female house sparrows to eat weird food — specifically, seed spiked with chicken poop — and the expression level of a gut immunity gene, TLR4. The higher the expression of this gene, the more poop-laced food the birds ate, the team reports in the July Brain, Behavior, and Immunity.

TLR4 produces a protein that warns the immune system of bacterial invaders in the gut, says study coauthor Lynn “Marty” Martin, a disease ecologist at the University of South Florida in Tampa. High gene expression means the gut is on alert, he says, so the birds seem to take more foraging risks. The findings could help to explain why house sparrows are so adept at colonizing new parts of the world, even when at risk of encountering unfamiliar parasites and pathogens.  

Martin’s lab has previously shown that sparrows on the fringe of populations expanding their range are more inclined to try new foods and tend to express high levels of TLR4. He and colleagues want to know whether the level of gene expression is linked to a “flexible” immune system that can seemingly fine-tune itself on the fly, adjusting expression levels like turning knobs on a radio. This flexibility could enable the immune system to counterattack new threats more readily than an immune system hardwired to defend against old, familiar enemies, Martin says.

To begin to test the idea, he and colleagues caught 37 wild house sparrows, Passer domesticus, in the Tampa area, where the birds are an introduced species, and gave them two feeding tests. In one test, sparrows received seed mixed with sterilized chicken poop — mimicking the kind of unsavory material sparrows often encounter as ground feeders in cities and on farms. In the other, sparrows could choose between poop-spiked seed and normal seed. Birds were hidden from one another during the tests because sparrows are social eaters, taking cues from one another on which foods to eat and which to avoid. Afterward, researchers analyzed TLR4 expression in the birds’ gut tissue.

The greater the level of TLR4 expression in female sparrows, the more spiked food they ate. “Those individuals have guts prepared to deal with more bad guys flooding in,” Martin says. “As the female takes risks, they’re going to increase the chances that a bad guy gets through.”

Sparrows that expressed high levels of TLR4 ate more food than those with lower expression of the gene, but dietary decisions differed by sex: High TLR4 females ate large amounts of both spiked and unspiked food while high TLR4 males opted for more unspiked food. And when offered a choice, both females and males showed a marked preference for the poop-free seed.

The difference in female and male feeding choices may have been due to the timing of the experiment, Martin says. The researchers tested sparrows during the breeding season when females’ energy needs increase as they ramp up egg production and prepare to care for nestlings. Female sparrows generally ate more food and ate faster than males in the experiment.

The findings add more nuance to the growing field of behavioral immunology, which examines the interplay between the immune system and behaviors that can help animals to either avoid or reduce health threats. Martin and colleagues’ work “is exciting because they are showing this mechanistic underpinning that may explain variation in behavior — risk-taking or risk-avoidance behavior,” says physiological ecologist Sarah DuRant of the University of Arkansas in Fayetteville. “There aren’t a lot of studies really connecting those underlying physiological mechanisms.” 

DuRant’s own work in canaries has shown that the sight of a sick neighbor can trigger a preemptive immune response in healthy birds (SN: 6/8/21). On reading the new study, DuRant wondered whether house sparrows’ gut gene expression is shaped by their environment, or if the birds are “pre-programmed” one way or another. “I’m not sure that’s totally resolved at this point, but I think they’re getting there,” she says.

While sparrows are unlikely to be conscious of adjustments in gut gene expression, one question is how these tweaks result in observable behavior changes. In other words, how does a sparrow “know” it can make risky food choices? 

“Do they plan the way that their guts work? Probably not,” Martin says. “But they are a lot more brainy than little things are often given credit.”