ORIGINAL ARTICLE
Long-term roosting data reveal a unimodular social network in large
fission-fusion society of the colony-living Natterer’sbat
(Myotis nattereri)
Veronika M. Zeus
1
&
Christine Reusch
1
&
Gerald Kerth
1
Received: 20 December 2017 /Revised: 11 May 2018 / Accepted: 23 May 2018 / Published online: 31 May 2018
#
Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
In many social animals, groups recurrently split into subgroups that regularly re-merge. Such fission-fusion behavior allows
individuals to better balance the cost and benefits of group living. However, maintaining a large number of close social links in
groups with fission-fusion dynamics may be difficult. It has been suggested that this is the reason why in several species, large
groups show more subunits (higher modularity) than do small ones. Many bat species exhibit fission-fusion dynamics in their
colonies. This makes them well suited to investigate the proposed link between group size, stability of social links, and group
modularity. We studied the daily roosting associations of a Natterer’s bat colony (Myotis nattereri), where up to 80 members
carried individual RFID-tags. Based on more than 10,000 individual recordings, we analyzed the influence of relatedness, age,
sex, and breeding status on the colony’s social network structure during three breeding seasons. We found an almost fully
connected social network with very low modularity and generally weak pairwise associations. Nevertheless, the relative strengths
of associations between individuals remained stable across years. Sex, age, and breeding status significantly influenced the
strength of an individual’s associations and determined the influence of individuals in the network. In general, associations
between bats that were similar in all abovementioned traits were stronger than those between dissimilar individuals. Our results
show that despite high fission-fusion dynamics, large colony sizes, and low modularity of their social network, Natterer’sbats
were able to maintain stable long-term associations.
Significance statement
For a variety of social and ecological reasons, large social groups often consist of several communities with stronger individual
bonds within and weaker individual bonds between such social subunits. Unlike that predicted for its relatively large size, the
studied Natterer’s bat colony that consisted of up to 80 individually marked bats was not subdivided into communities. Despite
the fully connected network, the individual associations were not random. Instead, their strength was mainly driven by related-
ness and similarity in age and breeding status of the colony members. Moreover, we found stable long-term pairwise relationships
between individuals across several years. Our study shows that despite the strong fission-fusion behavior and large size of their
colony, Natterer’s bats formed a fully connected, unimodal social network.
Keywords Bats
.
Social network
.
Association
.
Assortativity
.
Fission-fusion
.
Modularity
Introduction
In social animals, the strength of individual associations be-
tween group members often show a non-random pattern
(Krause and Ruxton 2002; Krause et al. 2007). In general,
animals may be passively associated with each other in space
and time, for example because they share strategies for use of
habitat and resources (Mitani et al. 1991; Chaverri et al. 2007;
Aplin et al. 2013). Alternatively, they may be actively
attracted to—or repelled by—certain other individuals. In this
Communicated by G. S. Wilkinson
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s00265-018-2516-4) contains supplementary
material, which is available to authorized users.
* Veronika M. Zeus
veronika.zeus@gmail.com
1
Zoological Institute and Museum, Greifswald University,
Loitzerstrasse 26, 17489 Greifswald, Germany
Behavioral Ecology and Sociobiology (2018) 72: 99
https://doi.org/10.1007/s00265-018-2516-4
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