Flash Expansion Threshold in Whirligig Swarms

<div><p>In the selfish herd hypothesis, prey animals move toward each other to avoid the likelihood of being selected by a predator. However, many grouped animals move <i>away</i> from each other the moment before a predator attacks. Very little is known about this phenomenon, called flash expansion, such as whether it is triggered by one individual or a threshold and how information is transferred between group members. We performed a controlled experiment with whirligig beetles in which the ratio of sighted to unsighted individuals was systematically varied and emergent flash expansion was measured. Specifically, we examined: the percentage of individuals in a group that startled, the resulting group area, and the longevity of the flash expansion. We found that one or two sighted beetles in a group of 24 was not enough to cause a flash expansion after a predator stimulus, but four sighted beetles usually initiated a flash expansion. Also, the more beetles that were sighted the larger the resulting group area and the longer duration of the flash expansion. We conclude that flash expansion is best described as a threshold event whose adaptive value is to prevent energetically costly false alarms while quickly mobilizing an emergent predator avoidance response. This is one of the first controlled experiments of flash expansion, an important emergent property that has applications to understanding collective motion in swarms, schools, flocks, and human crowds. Also, our study is a convincing demonstration of social contagion, how the actions of one individual can pass through a group.</p></div

Collision Avoidance During Group Evasive Manoeuvres: A Comparison of Real Versus Simulated Swarms With Manipulated Vision And Surface Wave Detectors

Coordinated group motion has been studied extensively both in real systems (flocks, swarms and schools) and in simulations (self-propelled particle (SPP) models using attraction and repulsion rules). Rarely are attraction and repulsion rules manipulated, and the resulting emergent behaviours of real and simulation systems are compared. We compare swarms of sensory-deprived whirligig beetles with matching simulation models. Whirligigs live at the water's surface and coordinate their grouping using their eyes and antennae. We filmed groups of beetles in which antennae or eyes had been unilaterally obstructed and measured individual and group behaviours. We then developed and compared eight SPP simulation models. Eye-less beetles formed larger diameter resting groups than antenna-less or control groups. Antenna-less groups collided more often with each other during evasive group movements than did eye-less or control groups. Simulations of antenna-less individuals produced no difference from a control (or a slight decrease) in group diameter. Simulations of eye-less individuals produced an increase in group diameter. Our study is important in (i) differentiating between group attraction and repulsion rules, (ii) directly comparing emergent properties of real and simulated groups, and (iii) exploring a new sensory modality (surface wave detection) to coordinate group movement

Number of Sighted Beetles in Tank vs. Startled: Box plot (mean, quartile, and range) of the total number of sighted whirligigs in a population of 24 with the percentage of accelerating (startled) in a group.

<p>(N = 14 groups for each category from 1–24, N = 7 for category 0 with no sighted beetles).</p

Scatterplot of Sighted vs. Startled beetles: Scatterplot of actual sighted whirligigs joining a group in a population of 24 on the mean percentage of whirligigs to startle after a predator stimulus (n = 105 groups, statistics are shown in Table 1).

<p>The fitted Sumpter-Pratt equation described in the data analysis section is shown (T = 1.704, K = 1.835).</p

Statistical influence of the number of sighted individuals (independent variable) in a group on various group properties (DV = Dependent Variable).

<p>n.s. = not significant,</p><p>*** is P<0.001</p><p>The nonlinear function fitted is the Sumpter and Pratt (2009) model described in the text with parameters T (threshold) and K (step strength). The parameter m (slope) is shown for those models best fit with a linear regression.</p

Development Time: Scatterplot of actual sighted whirligigs in a population of 24 on FE development (time to expand during FE).

<p>Development time is given in mean frames (1/30 s) between first startle to the time of maximum expansion (n = 105 groups, statistics are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136467#pone.0136467.t001" target="_blank">Table 1</a>). The linear regression line is shown (Y = 1.6*X + 46.9).</p

Rapid growth of a deep-sea wood-boring bivalve

The growth of Xylophaga atlantica Richards, a deep-sea wood-boring bivalve, was studied by recovering a 1-year time series of oak and pine panels deployed at depths of 100 and 200 m at the edge of the continental shelf, south of Cape Cod. Change in shell height between samples was used to assess growth rate. At the 100 m site, the first individuals to settle grew much faster, on average, than those that settled later in the year on the same panels (0.085 mm day-1 vs 0.031 mm day-1, respectively). The growth rate of the maximum sized individuals was 0.027 mm day-1 whereas the modal growth rate was half that at 0.015 mm day-1. The modal growth rate of those recovered from 200 m was much greater at 0.246 mm day-1 and is thought to be due to the warmer average temperatures there. Differences in growth rate due to season, substrate and previous density were also apparent. © 1994

Longevity: Scatterplot of actual sighted whirligigs in a population of 24 on the longevity of an FE (mean frames (1/30 s)) from peak group area until they have settled (n = 105 groups, statistics are shown in Table 1).

<p>The linear regression line is shown (Y = 16.0*X + 62.0).</p

Optimal individual positions within animal groups

Animal groups are highly variable in their spatial structure, and individual fitness is strongly associated with the spatial position of an animal within a group. Predation risk and food gains are often higher at the group peripheries; thus, animals must trade-off predation costs and foraging benefits when choosing a position. Assuming this is the case, we first use simulation models to demonstrate how predation risk and food gains differ for different positions within a group. Second, we use the patterns from the simulation to develop a novel model of the trade-off between the costs and the benefits of occupying different positions and predict the optimal location for an animal in a group. A variety of testable patterns emerge. As expected, increasing levels of satiation and vulnerability to predators and increasing predation risk result in increased preferences for central positions, likely to lead to increased competition or more tightly packed groups. As food availability increases, individuals should first prefer center positions, then edge, and returning to central positions under highest food levels. Increasing group size and/or density lead to more uniform preferences across individuals. Finally, we predict some situations where individuals differing in satiation and vulnerability prefer a range of different locations and other situations where there is an abrupt dichotomy between central and edge positions, dependent on the levels of monopolization of food by peripheral individuals. We discuss the implications of our findings for the structure of groups and the levels of competition within them and make suggestions for empirical tests. Copyright 2008, Oxford University Press.