Condition dependence of (un)predictability in escape behavior of a grasshopper species

Abstract (Un)predictability has only recently been recognized as an important dimension of animal behavior. Currently, we neither know if (un)predictability encompasses one or multiple traits nor how (un)predictability is dependent on individual conditions. Knowledge about condition dependence, in particular, could inform us about whether predictability or unpredictability is costly in a specific context. Here, we study the condition dependence of (un)predictability in the escape behavior of the steppe grasshopper Chorthippus dorsatus. Predator–prey interactions represent a behavioral context in which we expect unpredictability to be particularly beneficial. By exposing grasshoppers to an immune challenge, we explore if individuals in poor condition become more or less predictable. We quantified three aspects of escape behavior (flight initiation distance, jump distance, and jump angle) in a standardized setup and analyzed the data using a multivariate double-hierarchical generalized linear model. The immune challenge did not affect (un)predictability in flight initiation distance and jump angle, but decreased unpredictability in jump distances, suggesting that unpredictability can be costly. Variance decomposition shows that 3–7% of the total phenotypic variance was explained by individual differences in (un)predictability. Covariation between traits was found both among averages and among unpredictabilities for one of the three trait pairs. The latter might suggest an (un)predictability syndrome, but the lack of (un)predictability correlation in the third trait suggests modularity. Our results indicated condition dependence of (un)predictability in grasshopper escape behavior in one of the traits, and illustrate the value of mean and residual variance decomposition for analyzing animal behavior.

Individuals were taken from their home cages on the evening before phenotyping.They were placed in cylindrical vials of dimensions 8 cm height and 5 cm diameter containing 2-3 leaves of grass and their individual and pair identities were recorded on their vials.Vials of paired individuals with the same home cage, age, and sex were kept together at all times.The vials were then placed inside a tray and put in the fridge at 8ᵒC.
Phenotyping happened the following morning.Every 30 minutes a randomly chosen pair was taken from the fridge.One randomly chosen individual of the pair was anesthetized and injected with a solution as described in the Materials and Methods section.Both individuals were then marked with a yellow paint marker (Edding 750).One randomly chosen individual of the pair was marked with an "i" on the dorsal side of the wings and pronotum, while the other individual was marked with an "I" (Figure S1).Since the type of marking was randomly assigned independent of the treatment, the observer was blind regarding the condition treatment of individuals, while individuals could be easily distinguished.
Both grasshoppers were placed in individual cages under a heating lamp for 1h, with water and food available.The pair was then taken to the arena to acclimatize.One individual was placed in the center of the left side of the arena, while the other individual was placed in the center of the right side.The temperature in the arena was maintained at approximately 27ᵒC, using air conditioning and heat lamps placed above the arena.Trials started after an acclimatization period of 5 min.
A camera on a tripod with wheels (Figure S2) was slowly moved and placed on top of the grasshopper on the left side of the arena.The "chaser" (Figure S3) was then placed on the floor behind the grasshopper at approximately 40-50 cm of distance.Using a smartphone to remotely control the software Ethovision on the computer, the video started to be recorded and a 2 minutes timer was set.The chaser was then moved toward the grasshopper until it jumped.After the jump, the recording was interrupted and the camera was slowly moved on top of the grasshopper on the right side of the arena, where the same procedure was repeated.After the timer of the first individual ended, the second trial of that individual was started.The same procedure was followed alternatively until both individuals had 10 chases recorded.
When a grasshopper jumped away from the arena, it was gently captured with a vial and placed in the original spot in the center of the left or right side of the arena.This happened in about 15% of the cases.After the 10 trials of each individual were completed, the same protocol was followed for the second pair of grasshoppers, until all pairs from the fridge had been processed.

Figure S1 :
Figure S1: Grasshopper marked with the yellow marker.

Figure S3 :
Figure S3: Chaser used to trigger escape jumps.

Figure S4 :
Figure S4: Camera point of view.

Figure S6 :
Figure S6: Example of chaser and grasshopper tracks in the software Ethovision.

Figure S7 :
Figure S7: Distribution of the repeatabilities in the random effects.

Table S1 :
Model estimates based on a multivariate double-hierarchical generalized linear model applied to three aspects of escape behavior in steppe grasshoppers Chorthippus dorsatus.