Summary: New research shows that healthy animals living with sick companions develop bodily odor changes that resemble the odors of sick animals. These odor changes may influence social behavior and affect how disease spreads within groups.
Source: Monell Chemical Senses Center
Odors provide rich information about individuals, including clues to health. Researchers at the Monell Center report that when healthy animals share an environment with sick conspecifics, their body odors shift and begin to resemble those of the sick animals.
These findings imply that odor-based sickness signals can trigger physiological changes in otherwise healthy animals. Those changes could alter social interactions and may influence patterns of pathogen transmission in social groups.
“Exposure to the odors of sick individuals may prompt protective or preparative responses in their social partners, reducing the risk of infection,” explained study lead author Stephanie Gervasi, PhD, a chemical ecologist at Monell.
Earlier work from Monell had already shown that immune activation and inflammation change bodily odors, suggesting that these scent changes function as natural signals of health status. The current study extends that idea by demonstrating that the mere presence of sickness odors in the environment can modify the odors produced by healthy animals.
To test this, the researchers used laboratory mice and induced sickness-like inflammation by injecting some animals with lipopolysaccharide (LPS), a non-infectious bacterial endotoxin that activates the immune system and produces symptoms similar to sickness. LPS-injected animals served as the “sick” group, and were then housed with otherwise healthy cage-mates.
Behavioral bioassays were conducted using “sniffer” mice trained to discriminate urine odors from sick versus healthy animals. These trained mice reliably distinguished urine from healthy animals housed with sick partners from urine of healthy animals housed only with healthy partners. In other words, the healthy cage-mates of sick mice smelled more like sick animals to odor-discriminating mice.
Analytical chemistry and statistical predictive modeling of urinary volatile compounds produced results that paralleled the behavioral assays. Chemical profiles of urine from healthy mice housed with sick partners were more often classified by the model as “sick” than profiles from healthy mice housed only with other healthy animals.
The research team repeated the experiment using cages divided by perforated partitions that allowed odors to flow between compartments without physical contact. The same odor shifts occurred in healthy animals separated from sick animals by the partition, strongly indicating that the effect was not due to transfer of physical material but rather to exposure to airborne odor cues.
“This work shows not only that odors communicate disease, but that they can directly affect the physiology of animals that perceive them,” said Monell behavioral biologist Gary Beauchamp, PhD, one of the senior authors. “It is a striking example of information transfer by olfaction that could shape physiology, social behavior, and ultimately disease dynamics in many species.”
Bruce Kimball, PhD, a USDA National Wildlife Research Center research chemist based at Monell and another senior author on the paper, emphasized the potential relevance for wildlife. “If healthy animals begin to emit sickness-associated odors after exposure to sick conspecifics, that insight could improve our understanding of pathogen spread and help inform monitoring and management strategies in wild populations,” he said.
The USDA National Wildlife Research Center has maintained a field station at Monell for decades, contributing to a large body of work on chemical senses in birds and wildlife. The Monell–USDA affiliation has produced extensive research on how odor cues affect behavior and ecology in a variety of animal species, and the current study adds new evidence that odor-based signals can produce physiological and social consequences.
Funding: The study team included Marianne Opiekun and Talia Martin of Monell. Research support came from USDA APHIS-WS-NWRC and grants from the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health (grant numbers DC000014 and DC020296). Additional private support was provided by Dr. James Albrecht. The authors are responsible for the content and the views expressed do not necessarily reflect the official positions of the funding agencies.
Research summary (abstract)
Body odors change with health status, and odors from sick animals can induce avoidance in healthy conspecifics. The researchers hypothesized that exposure to sickness odors, even from non-infectious sources, could alter the physiology and scent profiles of healthy animals. Using LPS to induce immune activation in mice, the team combined behavioral odor discrimination assays with analytical chemistry and predictive classification modeling to compare urinary volatiles from healthy mice housed with healthy partners versus healthy mice housed with sick partners. Trained mice discriminated between these odor types, and chemical analyses confirmed that urine from healthy animals co-housed with sick conspecifics was more likely to be classified as “sick.” These results suggest that sickness-related odors can cascade into neuroendocrine or immune changes in healthy individuals, with potential consequences for individual behavior, social dynamics, and pathogen transmission.
This report summarizes findings from a peer-reviewed study led by Stephanie S. Gervasi, Maryanne Opiekun, Talia Martin, Gary K. Beauchamp, and Bruce A. Kimball, originally published in Scientific Reports. The work highlights the role of odor communication in health signaling and its possible impacts on disease ecology and animal behavior.