Aggression Is Induced by Resource Limitation in the Monarch Caterpillar

Abstract

Food represents a limiting resource for the growth and developmental progression of many animal species. As a consequence, competition over food, space, or other resources can trigger territoriality and aggressive behavior. In the monarch butterfly, Danaus plexippus, caterpillars feed predominantly on milkweed, raising the possibility that access to milkweed is critical for growth and survival. Here, we characterize the role of food availability on aggression in monarch caterpillars and find that monarch caterpillars display stereotyped aggressive lunges that increase during development, peaking during the fourth and fifth instar stages. The number of lunges toward a conspecific caterpillar was significantly increased under conditions of low food availability, suggesting resource defense may trigger aggression. These findings establish monarch caterpillars as a model for investigating interactions between resource availability and aggressive behavior under ecologically relevant conditions and set the stage for future investigations into the neuroethology of aggression in this system.

Keywords: Behavioral Neuroscience; Biological Sciences; Entomology.

Graphical abstract

Figure 1

Characterization of Aggressive Behavior in fourth and fifth Instar Monarch Caterpillars Example time lapse of a stereotypical aggressive encounter, characterized by a lateral lunge, between two (A) fourth instar or (B) fifth instar conspecifics. Red arrows indicate a change in caterpillar head movement as it progresses from recognition, initiation, contact, and then cessation of a single attack. The timestamp on each image denotes the duration of the encounter, in milliseconds. Scale bar represents 2cm.

Figure 2

Aggressive Behavior Increases in Both fourth and fifth Instar Caterpillars with Decreasing Food Density (A) Asclepias curassavica of low, intermediate, or high density was provided to fourth and fifth instar caterpillars and aggressive behavior was observed. (B) There was a significant effect of food quantity on the number of aggressive attacks in fourth instar caterpillars (F_{2, 45} = 8.84; p < 0.0006). Post hoc analyses revealed a significant increase in the number of aggressive attacks on low food density relative to both intermediate (p < 0.0109) and high food densities (p < 0.0006), while no significant difference between intermediate and high food densities was observed (p < 0.3002). (C) There was a significant effect of food quantity on the number of aggressive attacks in fifth instar caterpillars (F_{2, 50} = 15.34; p < 0.0001). Post hoc analyses revealed a significant increase in the number of aggressive attacks on low food density relative to both intermediate (p < 0.0027) and high food densities (p < 0.0001). The number of attacks were also significantly higher on intermediate food density in comparison to high food density (p < 0.0375). Data are represented as mean ± SEM. ∗ = p < 0.05; ∗∗ = p < 0.01; ∗∗∗ = p < 0.001; ∗∗∗∗ = p < 0.0001.

Figure 3

Ethogram of Four Representative fifth Instar Caterpillars on Intermediate Food Density during a 10-Min Video For each caterpillar, the time at which an aggressive encounter occurs was recorded, as well as its activity and location, shown as fractions of time. Aggressive encounters were classified as either attacking (black) or being attacked (red). Activities were categorized into foraging (orange), feeding (bule), or resting (purple). The location of each caterpillar was also recorded (on the leaf: green; off the leaf: yellow).

Figure 4

Aggressive Behaviors Tend to Occur on the Leaf during Feeding In fourth (A) and fifth (B) instar caterpillars, there were significantly more aggressive attacks on the leaf than off the leaf (fourth instar: t₁₈ = 5.102; p < 0.0001; fifth instar: t₁₈ = 4.876; p < 0.0001). (C) There was a significant effect of behavioral state on the number of aggressive attacks in fourth instar caterpillars (F_2,27 = 8.047; p < 0.0018). Post hoc analyses revealed that significantly more aggressive attacks occur during feeding relative to resting (p < 0.0012). No significant difference between feeding and foraging (p < 0.1903) or between foraging and resting was observed (p < 0.0876). (D) There was a significant effect of behavioral state on the number of aggressive attacks in fifth instar caterpillars (F_2,27 = 14.98; p < 0.0001). Post hoc analyses revealed that significantly more aggressive attacks occur during feeding relative to both foraging (p < 0.0191) and resting (p < 0.0001). The number of aggressive attacks is also significantly higher during foraging in comparison to during rest (p < 0.0418). (E) At the fourth instar stage, the aggressive caterpillar is significantly more likely to stay on the leaf (t₁₈ = 4.936; p < 0.0001), while the attacked caterpillar is significantly more likely to leave the leaf (t₁₈ = 2.830; p < 0.0111). (F) At the fifth instar stage, the aggressive caterpillar is significantly more likely to stay on the leaf (t₁₈ = 7.325; p < 0.0001), but the attacked caterpillar is equally as likely to leave the leaf as they are to stay (t₁₈ = 1.671; p < 0.1120). Data are represented as mean ± SEM. ∗ = p < 0.05; ∗∗ = p < 0.01; ∗∗∗ = p < 0.001; ∗∗∗∗ = p < 0.0001.

Figure 5

Lighting and Tentacle Laceration Have No Effect on Aggressive Behavior (A) In the experimental treatment, lights were turned off 1 min prior to testing. The effect of lighting on aggressive behavior was then measured in arenas with low food availability. (B) Caterpillars were equally as aggressive when the lights are off as they are in lighted conditions (t₁₆ = 1.054. p < 0.3076). (C) In the experimental treatment, tentacles from each caterpillar were removed 24 hr prior to testing. The effect of tentacle laceration was then measured in arenas with low food availability. (D) Caterpillars were equally as aggressive without tentacles as they are with intact tentacles (t₁₄ = 0.9964. p < 0.3360). Data are represented as mean ± SEM.