Do female Norway rats form social bonds?
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Social bonds reflect specific and enduring relationships among conspecifics. In some group-living animals, they have been found to generate immediate and long-term fitness benefits. It is currently unclear how important and how widespread social bonds are in animals other than primates. It has been hypothesized that social bonds may help in establishing stable levels of reciprocal cooperation. Norway rats (Rattus norvegicus) reciprocate received help to an unrelated social partner. It is hitherto unknown, however, whether this cooperative behaviour is based on the establishment of social bonds among involved individuals. Norway rats live in social groups that can be very large; hence, without bonds, it may be difficult to keep track of other individuals and their previous behaviour, which is a precondition for generating evolutionarily stable levels of cooperation based on direct reciprocity. Here we tested whether wild-type female rats form bonds among each other, which are stable both over time and across different contexts. In addition, we scrutinized the potential influence of social rank on the establishment of bonds. Despite the fact that the hierarchy structure within groups remained stable over the study period, no stable social bonds were formed between group members. Apparently, social information from consecutive encounters with the same social partner is not accumulated. The lack of long-term social bonds might explain why rats base their decisions to cooperate primarily on the last encounter with a social partner, which may differ from other animals where cooperation is based on the existence of long-term social bonds.
Social bonds have been hypothesized to favour reciprocal cooperation. Norway rats reciprocate help received from a social partner, but it is hitherto unclear whether they form social bonds that might further such cooperative behaviour. Here we tested whether female Norway rats engage in social relationships with a same-sex partner, which are stable over time and across contexts. In contrast to the hypothesized existence of bonds among long-term group members, our results provide no evidence that rats form specific social relationships. Rather than accumulating social information into social bonds, rats apparently base their decision to cooperate merely on the outcome of recent encounters.
KeywordsNorway rats Social bonds Hierarchy Peer relationship
We thank Evi Zwygart for the help with animal care, Valentina Balzarini and Michelle Gygax for the drawings and Joachim G. Frommen for the comments on the manuscript. This study was funded by the SNF-grant 31003A_156152 provided to MT.
MKS and MT conceived and designed the study and wrote the manuscript. JN, KS, AKR and MG collected the data. MKS and LE planned the statistical analyses, which were conducted by MKS. LE wrote the R-code for all Mantel tests. All authors read and commented on the final version of the manuscript.
Compliance with ethical standards
In accordance with the animal welfare regulations of Switzerland (Tierschutzverordnung Schweiz 04/2008) the rats (weight category 300–400 g) were housed in enriched cages (80/50/37.5 cm). Enrichment included a wooden house and board, a channel, a piece of wood to nibble, a loo roll to play, digging-material (wood shavings), nest-building material (hay) and a salt block. Food (conventional rat pellets and corn mix alternating with fresh vegetables or fruits) and water were provided ad libitum according to recommendations of the Federation of Laboratory Animal Science Associations (Forbes et al. 2007). We established small groups with five individuals per cage (Sharp et al. 2003). All rats experienced a handling procedure from early age onwards, so they were well habituated to humans and not stressed while being transported to the experimental cage or by the presence of an observer. No injuries occurred in the experiments. Approval and research permission was granted by Swiss Federal Veterinary Office under licence BE25/14.
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national and/or institutional guidelines for the care and use of animals were followed. This article does not contain any studies with human participants performed by any of the authors.
All data generated or analysed during this study are included in the supplementary information files.
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