Peanut the Dog (Nut, for short) lives in my house. Being a behavioral ecologist, I’ve kept some mental field notes on her behavior. For example, I’ve noticed that during her nightly rumpus in the dogpark, Nut is almost always mild-mannered when interacting with other dogs. I’ve also noticed that her other dog friends are also often behaving predictably. Vaayu, Nut’s basenji/blue heeler-mix friend, for instance, is reliably (and neurotically) fixated on his tennis ball. Wilson, the black boxer-mix who sometimes comes to visit, consistently attempts matings with the couch cushions on our living room sofa (his love for Nut is unrequited, and I suppose cushions are approximately Peanut-shaped). These behaviors have happened repeatedly. They are consistent. In fact, I’d say that these dogs have personalities.

Personality in dogs is no surprise to dog owners. But what about other animals? It turns out that animal personality is not rare. From fruit flies¹ to trout² to bighorn sheep³, personality has been found in nearly every species where scientists have looked.  But what exactly is personality? Personality is simply defined as individual differences in behaviors that are consistent across time and/or contexts. This distinction has been called other things in the behavioral ecology literature—‘behavioral syndrome’ is a common synonym—but, whatever you call it, this consistency of behavior among individuals in a group seems to matter in ecology.

The existence of personality in animal populations is important for a contemporary and ever-increasing phenomenon: biological invasion.

Imagine you have a population of fish that all exhibit the same behaviors with essentially no variation (Scenario 1 in the figure). We would say that this group of fish does not exhibit personality. These fish are all highly sociable, and equally so. Being sociable also means that they tend not to disperse much, since they’re more interested in hanging out with each other rather than exploring new (fishless) territory. Thus, these fish are not considered to be highly invasive when placed in a new environment.

Now imagine a second population of fish that are the same in all other qualities, except that a few of these fish don’t mind being alone and are consistently bold and exploratory (the green fish in Scenario 2). Still, most of the other fish are highly sociable and tend not to venture off much unless other fish have already seeded a sizable group elsewhere. This population exhibits personality—individuals differ consistently in their behaviors. And, importantly, the fact that some few fish are ‘leader’ fish means that this population more quickly explores new territory. This works through successive waves of bold then shier fish moving through the environment: bold, less sociable fish set out to explore, thus establishing a group upstream big enough to attract some of the more shy, sociable fish. The new arrivals of these shy, sociable fish then come swimming along and push the bold, less social fish out into yet more unexplored territory. Bold fish then thus claim still more territory that shier fish will soon join in to exploit…and so forth. It turns out that the existence of personality—and the appropriate mix of personalities within a group—could make all the difference in a species invasion.

By now, behavioral biologists have looked at many invasive populations and observed that similar processes may be driving lots of invasions. Crayfish are bolder in their invaded range compared to their native range⁴; highly invasive mosquitofish individuals that are bolder and more exploratory tend to disperse further⁵; a few invasive bird species are more aggressive and less shy at the edge of their invasion^6,7. The examples are accruing, but still, we aren’t sure to what extent animal personality plays a role in successful biological invasions in general. So as always, there’s more science to do!

References:

1. Kain, J. S., Stokes, C. & de Bivort, B. L. Phototactic personality in fruit flies and its suppression by serotonin and white. Proc. Natl. Acad. Sci. U.S.A. 109, 19834–19839 (2012).

2. Frost, A. J., Winrow-Giffen, A., Ashley, P. J. & Sneddon, L. U. Plasticity in animal personality traits: does prior experience alter the degree of boldness? Proc. Biol. Sci. 274, 333–339 (2007).

3. Réale, D., Gallant, B. Y., Leblanc, M. & Festa-Bianchet, M. Consistency of temperament in bighorn ewes and correlates with behaviour and life history. Animal Behaviour 60, 589–597 (2000).

4. Pintor, L. M., Sih, A. & Bauer, M. L. Differences in aggression, activity and boldness between native and introduced populations of an invasive crayfish. Oikos 117, 1629–1636 (2008).

5. Cote, J., Fogarty, S., Weinersmith, K., Brodin, T. & Sih, A. Personality traits and dispersal tendency in the invasive mosquitofish (Gambusia affinis). Proceedings of the Royal Society B: Biological Sciences 277, 1571–1579 (2010).

6. Duckworth, R. A. Adaptive Dispersal Strategies and the Dynamics of a Range Expansion. The American Naturalist 172, S4-S17 (2008).

7. Liebl, A. L. & Martin, L. B. Living on the edge: range edge birds consume novel foods sooner than established ones. Behavioral Ecology 25, 1089–1096 (2014).

Finally, if you’d like to read more about the mathematical model that formalizes the fish scenarios described above, or seminal scientific literature on animal personality in general, read:

Fogarty, S., Cote, J. & Sih, A. Social Personality Polymorphism and the Spread of Invasive Species: A Model. The American Naturalist 177, 273–287 (2011).

Sih, A., Bell, A. & Johnson, J. C. Behavioral syndromes: an ecological and evolutionary overview. Trends in Ecology & Evolution 19, 372–378 (2004).

Sih, A., Cote, J., Evans, M., Fogarty, S. & Pruitt, J. Ecological implications of behavioural syndromes. Ecology Letters 15, 278–289 (2012).