Nutritional state and collective motion: from individuals to mass migration

Abstract

In order to move effectively in unpredictable or heterogeneous environments animals must make appropriate decisions in response to internal and external cues. Identifying the link between these components remains a challenge for movement ecology and is important in understanding the mechanisms driving both individual and collective motion. One accessible way of examining how internal state influences an individual's motion is to consider the nutritional state of an animal. Our experimental results reveal that nutritional state exerts a relatively minor influence on the motion of isolated individuals, but large group-level differences emerge from diet affecting inter-individual interactions. This supports the idea that mass movement in locusts may be driven by cannibalism. To estimate how these findings are likely to impact collective migration of locust hopper bands, we create an experimentally parametrized model of locust interactions and motion. Our model supports our hypothesis that nutrient-dependent social interactions can lead to the collective motion seen in our experiments and predicts a transition in the mean speed and the degree of coordination of bands with increasing insect density. Furthermore, increasing the interaction strength (representing greater protein deprivation) dramatically reduces the critical density at which this transition occurs, demonstrating that individuals' nutritional state could have a major impact on large-scale migration.

Figure 1.

The mean speed of a locust when alone. (a) The time series showing the mean speed (cm s⁻¹) of a locust when alone in the arena for each of the diet treatments: high protein (red), balanced (black) and low protein (blue). Thirty experimental trials were carried out for each treatment. Error bars show ± one s.d. (b–e) The frequency distributions for the mean speed (cm s⁻¹) of a locust fed on each diet calculated over different two-minute time-windows during the experiment: (b) 8–10 min, (c) 58–60 min, (d) 238–240 min and (e) 478–480 min, illustrating how the speeds of individuals fed on different diets change over time.

Figure 2.

The mean speed of a locust when in a group. (a) The time series showing the mean speed (cm s⁻¹) of a locust when in a group of individuals fed on different diet treatments: high protein (red), balanced (black) and low protein (blue). Thirty experimental trials were carried out for each treatment. Error bars show ± one s.d. (b–e) The frequency distributions for the mean speed (cm s⁻¹) of a locust in a group fed on different diets calculated over two-minute time-windows at different points during the experiment: (b) 8–10 min, (c) 58–60 min, (d) 238–240 min and (e) 478–480 min, illustrating how the speeds of individuals fed on different diets change over time.

Figure 3.

(a) Probability speed distributions of individuals for different diets (red: high protein; black: balanced; blue: low protein) obtained from experiments for t > 300 min and (b) from simulations for different response strengths (red: χ_high = 4.0; black: χ_balanced = 8.0; blue: χ_low = 12.0). The probability speed distribution is the frequency of the different speeds (histogram) normalized by the total number of speed measurements. The dashed line represents ν = 1.5 cm s⁻¹. The insets show the corresponding probability speed distributions of only moving individuals (which we define as ν > 1.5 cm s⁻¹).

Figure 4.

Simulation results for different densities. The (a) global migration speed and (b) order parameter as a function of density for individuals with different response strengths (red: χ_high = 4.0; black: χ_balanced = 8.0; blue: χ_low = 12.0). Error bars show ± one s.d. The dashed line in (b) represents the threshold of the order parameter Φ = 0.5, which we define as the critical density. Each point was calculated as an average over time series from 10 independent simulations. The time series of each run was recorded for time interval Δt = 2000 after the system reached a stationary state (numerical time step dt = 0.006).