How Personality Affects Species Interactions
From sea anemones to chimpanzees, animals tend to exhibit consistent individual behavioral tendencies. These consistent behavioral differences are better known as animal personality, and they shape many aspects of animals’ day-to-day lives.
For example, aggressive water striders chase away docile rivals and mates alike, and bold guppies bravely stare down predators rather than fleeing. One aggressive individual in a pond’s water strider personality mixture chases away all the females, preventing everyone from mating, and all guppies enjoy safety after bold guppies in a stream’s behavioral mixture chase away predators. These mixtures of personality types in groups of interacting animals determine the outcome of (1) interactions between members of one species and (2) members from different species.
Interactions within and between species might seem arcane and abstract, but the outcomes of these interactions shape many of ecology’s twists and turns. When the rats in your uncles’ basement grow overcrowded, they begin to fight each other for food or nest space, resulting in a population crash. Lakes fill with tasty perch when they manage to outcompete minnows as juveniles. We aimed to test how something as innocuous as mixtures of personality traits could explain the outcome of interactions within and between species because species interactions shape ecological outcomes.
Insects make great subjects for probing interactions between and within species because you can catch hundreds of them in an afternoon. We collected thousands of invasive meadow froghoppers (Philaenus spumarius) and local katydids from the genus Scudderia. These two insects share a host plant across the eastern US, suggesting that they interact over a huge geographic range. We measured three of their behavioral traits to predict the outcome of their interactions. These katydids and froghopper insects exhibit consistent differences in activity level (how much they move around), boldness (whether they tolerate threats), and perch height (where they spend their time). To describe the mixtures of these personality traits of insect groups we employed a powerful metric of personality diversity: behavioral hypervolumes.
Behavioral hypervolumes are the behavioral space occupied by a group of animals. Behavioral space is a space in which each dimension is animals’ performance on a behavioral test. We build hypervolumes by running animals through three or more behavioral tests and plotting them in a space according to their performance on these behavioral tests. The hypervolume is the volume of the shape enclosed by these animals in behavioral space. This means that groups of animals that are similar in their behavior form small hypervolumes, and groups with behaviorally different animals form very large hypervolumes. Behavioral hypervolumes are consequently a metric of behavioral diversity. Single behaviors determine how animals live, so we suspect that the behavioral diversity of a group across of three traits can shape interactions within and between groups. We tested this with an experiment focusing on interactions within groups and a second focusing on interactions between groups.
We gauged the effect of behavioral hypervolumes on interactions within groups by putting groups of froghoppers and katydids of known hypervolumes together in indoor enclosures for a day. This allowed us to observe how their behavior related to their tendencies to cluster in groups of their own species. We found that groups of katydids with larger behavioral hypervolumes clustered together more. This suggests that that behaviorally different katydids better tolerated each other. This might help them to coexist at a larger scale, but that is more than uncertain.
To evaluate how behavioral hypervolumes shape interactions between species, we assembled groups of katydids and froghoppers and put them together in outdoor enclosures with either a high or low density of froghoppers. After ten days, we opened the enclosures and counted the surviving insects. No more than two katydids died per enclosure; however, the froghoppers varied widely in their death rates. When katydids were behaviorally diverse, more froghoppers died. At high froghopper densities, froghoppers died much less when both species’ groups were behaviorally very homogenous. We suspect that large hypervolume katydid groups clustered together on these insects’ shared host plant, repelling froghoppers to the edge of the cages where there is no food. These findings suggest that behavioral diversity intensifies the interactions between katydids and froghoppers.
Behavioral diversity in katydids increased their ability to tolerate each other and resulted in the deaths of invasive froghoppers. This conveys that behaviorally diverse insect groups better tolerate each other, but may have lower tolerances for other species. You can think of personalities as the strategies animals employ to et and avoid being eaten. When animals within one species employ different strategies, they overlap with each other less, allowing them to better tolerate each other. However, if they employ a diverse array of strategies, they could also be more likely to conflict with other species.
Insect personality traits become relevant when they determine how animals survive, grow, and reproduce. Whether or not some katydid is bold, aggressive, or lethargic might seem trivial. However, the mixture of behavioral tendencies of katydids in aggregate could determine whether they harm invasive froghoppers. This keeps us coming back to measure the personality of insects for years and years.
These findings are described in the article entitled The multidimensional behavioural hypervolumes of two interacting species predict their space use and survival, recently published in the journal Animal Behaviour. This work was conducted by James L.L. Lichtenstein, Colin M. Wright from the University of California Santa Barbara, Brendan McEwen and Jonathan N. Pruitt from McMaster University, and Noa Pinter-Wollman from the University of California Los Angeles.