Swarming strategies for cooperative species.

In this paper, we propose a model to investigate the relative efficiency of simple swarming strategies based on the interplay between spontaneous and recruitment-based emigration. We conduct a dynamical study of the model which combines inverse density dependence, saturation effects and induced vs. diffusion-like population transfer. The influence of the most relevant parameters is explored on a systematic basis, and transition values for which qualitative changes occur in the system's behaviour are given. The model is then used to study colonization of a multiple sites environment, as well as confrontation between species featuring different swarming strategies. Simulation results indicate that cooperative organisms should have an interest in evolving recruitment-based emigration. The corresponding population transfer patterns prove more efficient in invading new territories, eliminating competitors in the process. We suggest that this advantage could have promoted a simple form of coordinated swarming in species featuring a primitive type of cooperation.

Exploratory recruitment plasticity in a social spider (Anelosimus eximius).

Exploratory recruitment was investigated in an artificial experimental set-up on location in French Guyana. Groups of 200 freshly collected spiders of the neotropical social theridiid Anelosimus eximius were released on an open circular surface and offered a choice between two accessible shelters. Results indicated that a clear-cut collective decision was not always reached, unlike what we found using a different set-up in another set of experiments. Simulations were conducted using available information in order to explore the potential causes for this difference. Theoretical projections fit experimental data and strongly suggest that variability in the collective response results from a combination between modifications of the environment's properties and alteration of the recruitment procedure. Multiple variants of the theoretical set-up (including external bias) are investigated and emphasize plasticity in the collective response. New experimental studies are suggested and adaptative value of exploratory recruitment in social spiders is briefly discussed.

Collective decision-making in social spiders: dragline-mediated amplification process acts as a recruitment mechanism

Amplification mechanisms involved in group cohesion and coordination of several individuals' activities are a major research topic in social arthropod biology. In this paper, we investigate how recruitment processes can be mediated by the use of silk draglines in the case of social spiders. Our intent is to demonstrate how a behavioural feature common to all spider species can induce positive feedback, potentially leading to collective decision-making in a social context. Dragline-mediated amplification mechanisms are investigated in a simulated "Y" choice set-up. Numerical experiments involve two distinct models: a simplified one, devoted to the exploration of the most basic amplifying properties of the system, and a more complex simulation platform, taking into account the geometrical properties of a growing network (two-dimensional web). The effect of hypothetical subpopulations, as well as variations in silk attractivity in the case of mixed populations (originating from multiple nests) are also discussed. Results fit experimental data and demonstrate that spiders' behaviour exhibits very strong amplifying properties that can play a crucial part in the organization of social life. Copyright 1999 Academic Press.