UMaine research shows animal personalities change what role critters play in building the world around them

Just like people, animals have personalities — and, like people, these individual differences in behavior may determine what role an animal plays in building the world around it.

“Mutualisms” are symbiotic relationships between animals: clownfish keeping stinging sea anemones clean in exchange for protection, or bats providing nutritious guano to pitcher plants as they provide a place to rest. These relationships not only benefit the species involved, but also provide the foundation for developing biodiversity through co-evolution and gene flow within populations. To understand how these relationships shape ecosystems, scientists need to understand what drives them in the first place. 

Animal personality is often overlooked in studies of ecology, but researchers at the University of Maine decided to see how unique behavioral types affect mutualistic interactions — specifically, between deer mice and the tree seeds that they hoard to eat, and disperse among the forest to germinate along the way. 

In a study published in the journal PNAS, UMaine scientists observed the behavior of deer mice at over 200 stations with red oak, white pine and beech seeds. The researchers classified the behavior as either negative or positive, depending on whether it gave the seeds a chance to germinate. For example, consuming the seed at the site was a negative interaction, while removing the seed and moving it intact at another site was considered positive. The mice were then put on a continuum, with more antagonistic individuals at the negative end of the spectrum and mutualistic individuals at the other. The researchers then looked to see whether individuals classified as “antagonistic” had personality traits (such as boldness measured in standard behavioral tests) in common. For example, an individual mouse’s timidness or boldness was measured by the amount of time spent at the end of an emergence tunnel before emerging.

The results showed that while deer mice were found to be generally antagonistic toward all the varieties of seeds — that is their food, after all — individual mice were found to have far more mutualistic mannerisms than others depending on some other personality traits that they exhibited. 

Timid mice tended to be more antagonistic than their bold counterparts when it came to their behavior with large seeds like acorns, opting for the “safer” choice of taking the seed deep underground into a burrow where it can’t germinate or taking it to a secluded location to eat it instead of caching the seed on the surface. Bold mice, on the other hand, were more likely to behave antagonistically toward the smaller white pine and beech seeds, eating the seeds on-site or shortly thereafter rather than caching them for later. The different hoarding techniques of bold and timid mice suggest that their personalities affect their survival strategies — and the trees that grow in their wake.

“These results are exciting as they show that each individual plays a slightly different role in an ecosystem, and this role is determined by their unique personality. Every mouse counts and its mind can potentially have cascading effects on ecosystems! This implies that we need to start rethinking the way we conserve and manage ecosystems, we need to start considering the role played by each individual,” says Alessio Mortelliti, an associate professor in the UMaine Department of Wildlife, Fisheries and Conservation Biology.

The significance of animal personalities in these ecosystem-building mutualisms show that removing individuals with certain personality traits can have more of an impact on the dynamics of a habitat than previously thought — even beyond deer mice and their seeds. Previous research has shown that bolder individuals are more likely to be removed from populations via hunting or fishing. Studies have also shown that such personality traits might impact an animal’s ability to cope with changes in habitat like urbanization. 

“As far as we know, our study is the first to examine how personality traits might impact the strength of mutualisms. One of the reasons why this hasn’t been done before is probably because mutualisms, like most ecological processes, are so complex. I think it will be important to examine the role that personalities might play in other mutualisms, such as pollination. I think there is also a lot of potential for future studies examining the impact of personality traits on the seed dispersal mutualism. Specifically, it would be important to learn more about how personality might play into cache recovery and pilfering behavior,” says Allison Brehm, a UMaine Ph.D. candidate in the Department of Wildlife, Fisheries, and Conservation Biology and principal author of the journal article. 

The study was funded by a National Science Foundation (NSF) Faculty Early Career Development (CAREER) grant and the Maine Agricultural and Forest Experiment Station (MAFES).

Contact: Sam Schipani,