An interaction mechanism for the maintenance of fission-fusion dynamics under different individual densities

David Bierbach; Stefan Krause; Pawel Romanczuk; Juliane Lukas; Lenin Arias-Rodriguez; Jens Krause

doi:10.7717/peerj.8974

An interaction mechanism for the maintenance of fission-fusion dynamics under different individual densities

PeerJ. 2020 May 14:8:e8974. doi: 10.7717/peerj.8974. eCollection 2020.

Authors

David Bierbach^{1

2}, Stefan Krause³, Pawel Romanczuk^{4

5}, Juliane Lukas^{1

2}, Lenin Arias-Rodriguez⁶, Jens Krause^{1

2}

Affiliations

¹ Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany.
² Faculty of Life Sciences, Thaer Institute, Humboldt Universität Berlin, Berlin, Germany.
³ Department of Electrical Engineering and Computer Science, Lübeck University of Applied Sciences, Lübeck, Germany.
⁴ Department of Biology, Institute for Theoretical Biology, Humboldt Universität Berlin, Berlin, Germany.
⁵ Bernstein Center for Computational Neuroscience, Humboldt Universität Berlin, Berlin, Germany.
⁶ División Académica de Ciencias Biológicas, Universidad Juárez Autónoma de Tabasco, Villahermosa, Mexico.

Abstract

Animals often show high consistency in their social organisation despite facing changing environmental conditions. Especially in shoaling fish, fission-fusion dynamics that describe for which periods individuals are solitary or social have been found to remain unaltered even when density changed. This compensatory ability is assumed to be an adaptation towards constant predation pressure, but the mechanism through which individuals can actively compensate for density changes is yet unknown. The aim of the current study is to identify behavioural patterns that enable this active compensation. We compared the fission-fusion dynamics of two populations of the live-bearing Atlantic molly (Poecilia mexicana) that live in adjacent habitats with very different predator regimes: cave mollies that inhabit a low-predation environment inside a sulfidic cave with a low density of predatory water bugs (Belostoma sp.), and mollies that live directly outside the cave (henceforth called "surface" mollies) in a high-predation environment. We analysed their fission-fusion dynamics under two different fish densities of 12 and 6 fish per 0.36 m². As expected, surface mollies spent more time being social than cave mollies, and this difference in social time was a result of surface mollies being less likely to discontinue social contact (once they had a social partner) and being more likely to resume social contact (once alone) than cave mollies. Interestingly, surface mollies were also less likely to switch among social partners than cave mollies. A random walk simulation predicted each population to show reduced social encounters in the low density treatment. While cave mollies largely followed this prediction, surface mollies maintained their interaction probabilities even at low density. Surface mollies achieved this by a reduction in the size of a convex polygon formed by the group as density decreased. This may allow them to largely maintain their fission-fusion dynamics while still being able to visit large parts of the available area as a group. A slight reduction (21%) in the area visited at low densities was also observed but insufficient to explain how the fish maintained their fission-fusion dynamics. Finally, we discuss potential movement rules that could account for the reduction of polygon size and test their performance.

Keywords: Cave fish; Fission–fusion society; Markov chain; Poecilia; Social network analysis.

Grants and funding

We received funding from the DFG (BI 1828/2-1 and BI 1828/3-1 to David Bierbach, RO 4766/2-1 to Pawel Romanczuk, and EXC 2002/1 “Science of Intelligence” –project number 390523135 to Jens Krause, Pawel Romanczuk and David Bierbach) as well as the IGB seed money program (to Jens Krause) and the Leibniz-Competition (SAW-2013-IGB-2 to Jens Krause). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.