Detecting collective behaviour in animal relocation data, with application to migrating caribou

Abstract

Summary Collective behaviour can allow populations to have emergent responses to uncertain environments, driven by simple interactions among nearby individuals. High‐throughput ethological studies, where individual behaviour is closely observed in each member of a population (typically in the laboratory or by simulation), have revealed that collective behaviour in populations requires only rudimentary cognitive abilities in individuals and could therefore represent a widespread adaptation to life in an uncertain world. However, the ecological significance of collective behaviour is not yet well understood, as most studies to date have been confined to specialized situations that allow intensive monitoring of individual behaviour. Here, we describe a way to screen for collective behaviour in ecological data that is sampled at a coarser resolution than the underlying behavioural processes. We develop and test the method in the context of a well‐studied model for collective movement in a noisy environmental gradient. The large‐scale distribution patterns associated with collective behaviour are difficult to distinguish from the aggregated responses of independent individuals in this setting because independent individuals also align to track the gradient. However, we show that collective idiosyncratic deviations from the mean gradient direction have high predictive value for detecting collective behaviour. We describe a method of testing for these deviations using the average normalized velocity of the population. We demonstrate the method using data from satellite tracking collars on the migration patterns of caribou (Rangifer tarandus), recovering evidence that collective behaviour is a key driver of caribou migration patterns. We find moreover that the relative importance of collective behaviour fluctuates seasonally, concurrent with the timing of migration and reproduction. Collective behaviour is a potentially widespread dynamic property of populations that can, in some cases, be detected in coarsely sampled ecological data.

Tactical departures and strategic arrivals: Divergent effects of climate and weather on caribou spring migrations

“…Furthermore, caribou are social animals: Despite variability in their wintering and post‐calving summering areas, they exhibit largely collective migrations to their calving grounds (Skoog , Dalziel et al. ).…”

“…Statistical tools for identifying migrations and range shifts are almost entirely based on analysis of individual trajectories (B€ orger and Fryxell 2012, Cagnacci et al 2016, Gurarie et al 2017, Spitz et al 2017; though see Dalziel et al 2016, Calabrese et al 2018. While effective, these tools require some individual supervision, a prohibitive task when a single movement dataset contains hundreds of individuals with diverse sampling regimes across multiple decades.…”

“…While effective, these tools require some individual supervision, a prohibitive task when a single movement dataset contains hundreds of individuals with diverse sampling regimes across multiple decades. Furthermore, caribou are social animals: Despite variability in their wintering and post-calving summering areas, they exhibit largely collective migrations to their calving grounds (Skoog 1968, Dalziel et al 2016.…”

A framework for modelling range shifts and migrations: asking when, whither, whether and will it return

“…By isolating 'residual' movements from ranging and migratory models, interactions between individuals and environmental covariates and interactions among individuals (i.e. collective behaviours) can yield deeper insights into the cognitive mechanisms of migration (Delgado et al 2014;Dalziel et al 2016).…”

Collective animal navigation and migratory culture: from theoretical models to empirical evidence

“…Although the mechanisms of animal navigation have fascinated researchers for decades, focus has primarily been at the level of the individual (Gould and Gould, 2012). However, many migratory species are known to move in large groups (Milner-Gulland et al, 2011) and social interactions can alter migratory movement decisions (Dalziel et al, 2016;Torney et al, 2018b). How individual navigation ability is affected by social interactions, and what unique navigational abilities can emerge at the collective level, has been far less studied, although a growing body of theoretical and empirical results supports the hypothesis that social interactions during collective navigation can lead to improved navigational ability (Fig.…”