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.

Decision accuracy in complex environments is often maximized by small group sizes

“…Despite such groups appearing similar in structure and form, there can often be greatly differing selection pressures that underlie the formation of groups. For example, vast mobile swarms of locusts are on a 'forced march' driven by limiting essential nutrients [47] and cannibalistic interactions [47][48][49].…”

Active Brownian particles

“…However, the situation is improving, due to technological advances in digital tracking and data processing, the empirical study of large scale collective motion in the field has become possible [17,220,54]. Furthermore, an increasing number of controlled laboratory experiments is being performed on collective motion of such different organisms as fish [1], insects [50,26], bacteria [321,322] or keratocytes (active tissue cells) [334]. The resulting data provide the foundation to address the question on actual properties of social interactions in real world swarms and flocks.…”

“…This active fluctuations are a pure far-from equilibrium phenomenon and are relevant in the motion of an biological agents or artificial self-propelled particles. The origin of these fluctuations can be for example variations in the propulsion of chemically powered colloids [260,164,295], complex intra-cellular processes in cell motility [313,41] or unresolved internal decision processes in animals [245,195,26]. For example, the motion of a macroscopic animal moving in a homogeneous environment, where the fluctuations due the environment can be assumed as negligible, may nevertheless appear random to an external observer.…”

“…Motivated by recent observations on cannibalistic behavior in locusts and crickets [319,24], we introduced a model of individuals (active particles) responding to others with escape and pursuit behavior [289,26]. This biologically motivated interactions cannot be expressed simply in terms of an interaction potential as discussed in Section 6.1 and do not include a velocity alignment term as discussed in Sections 6.1 & 6.2, but constitute selective, velocity dependent attraction and repulsion interactions.…”

The nature of nutrition: a unifying framework

“…The consequence of migration is a greatly increased chance of encountering fresh nutritional resources but, meanwhile, the crickets are 'travelling with their lunch' (Hansen et al 2011). Bazazi et al (2008Bazazi et al ( , 2011 subsequently showed that protein deprivation is also associated with enhanced marching in groups of cannibalistic desert locust nymphs.…”