How Animals’ Personalities Can Shape Ecosystems

“Mutualisms” are mutually-beneficial interactions between individuals of two different species that lay the foundations for healthy ecosystems. Pollination and cleaner fishes who feed on the parasites and dead skin cells of other fishes are two common examples of mutualisms in nature. Nearly every species is involved in some sort of mutualistic behavior, including humans. 

Mutualisms are usually studied at the species level (such as one species of ant defending one species of orchid in return for nectar). While the species-level approach helps explain some trends, few studies have analyzed how mutualistic behavior can differ between individuals of the same species. In other words, while an entire ant species might defend an orchid species, do some individual ants play a bigger role than others? 

Seed dispersal is an interesting example of mutualistic behavior. Typically, certain animals disperse a plant’s seeds by carrying them away from the plant, usually with the purpose of hoarding and consuming them. This behavior is typically classified as “mutualistic” because each unconsumed and dispersed seed can potentially result in new plant growth (as long as the seed isn’t damaged in some way). However, when the animal consumes the seed, this benefits the animal at the expense of the plant, so the researchers label such interactions “antagonistic.” On average, these costs and benefits play out to the advantage of both species. 

In reality, each individual animal consumes and disperses a different proportion of seeds, making some individuals more mutualistic than others. One implication of this is that some individuals may be more important than others when it comes to seed dispersal (from a conservation standpoint). In this paper, researchers explore how personality factors influence the dispersal behavior of deer mice, seeking to understand the relationship between an individual’s personality traits and their place on the antagonistic-mutualistic continuum.

To carry out their study, the researchers trapped deer mice in an experimental forest in Maine. They marked each mouse and tested them for personality traits including boldness, anxiety, docility, and activity level. From there, mice were given access to seed stations with a mix of white pine seeds, red oak acorns, and beech seeds. The seed stations were dusted with fluorescent powder so researchers could track seed dispersal routes, and the seeds were painted with fluorescent paint and marked for later identification. Researchers later cross-checked the dispersal routes and seeds with video footage of the seed stations to learn which seeds were taken by which mice. They assessed whether each individual seed interaction was mutualistic (meaning the seed was dispersed alive and intact) or antagonistic (meaning the mouse consumed the seed or stored it below ground where germination was unlikely to happen). 

The results were as follows:

• Out of 934 observed seed interactions by tagged deer mice, the researchers were able to determine the outcome of 532 seeds. 349 of these observations involved white pine seeds, 135 involved beech seeds, and 48 involved red oak acorns. 
• On average, mice were more antagonistic than mutualistic toward all three seeds.
• For red oak seed interactions, an individual’s timidness predicted the individual’s antagonism. Specifically, bolder individuals were more mutualistic toward red oak seeds, meaning they were more likely to remove and disperse them either on or below the surface. 
• For white pine and beech seed interactions, bolder individuals were more antagonistic, though mutualism was more strongly predicted by body condition (calculated using body mass and the scaled-mass index) and forest type. Specifically, individuals with higher body condition tended to be more antagonistic.

With acorn (red oak) seed interactions, the researchers found that timid mice were more likely to stow acorns in an underground burrow or engage in immediate seed consumption. They believe this is because acorns are typically too large to fit in a deer mouse’s cheek pouch, making it difficult for a mouse to see and move optimally while carrying them. As a result, bolder mice are more likely to engage in the risky behavior of caching an acorn outside of a burrow.

For white pine seeds and beech seeds, on the other hand, bolder mice were more antagonistic. Specifically, bolder individuals were more likely to consume white pine seeds immediately at the seed station, and they typically ate beech seeds immediately after taking them (or brought them into their burrows). The authors hypothesize this has something to do with bolder animals prioritizing current energy needs over future energy reserves, but they call for more research on this topic. 

It’s important to note that the general trend of mice acting antagonistically toward seeds does not mean the entire act of seed dispersal is not mutualistic. The authors note that it only takes a minority of individual mice acting as seed spreaders to help seeds germinate, thus benefiting the wider environment. An implication of this is that positive interactions (dispersing whole seeds) are disproportionately more important to seed dispersal than negative interactions (eating or bringing seeds to one’s burrow); the consequences of the positive interactions outweigh those of the negative ones.

Understanding the impact of individual animals’ personality types on their mutualistic behavior is important for a number of reasons. As human activities alter natural environments, it is vital to understand the potential impact a certain environmental change can have, and certain environmental changes may favor particular personality types in animals. For example, bolder mice are more likely to venture into open areas in search of food, which means that tree clearing may disproportionately affect them. This presents another factor humans must consider before encroaching on the environment. 

What’s more, conservation decisions are usually made at the species level. However, if only a minority of individuals are doing the majority of mutualistic behaviors (for example, if only the boldest mice are spreading most of the seeds of a given tree species), these mice might be considered “keystone individuals” for conservation projects. For example, previous research has demonstrated that bolder, more active individuals tend to be removed from environments at higher rates from hunting and fishing; understanding how these individuals impact the environment can increase our understanding of the consequences of such activities. While more research is needed on this topic, the study offers another reason why it’s important to value animals for their individual traits rather than their species membership alone.