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 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.

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Animal pigmentation plays a key role in many biological interactions, including courtship and predator avoidance. Sympatric benthic and limnetic ecotypes of threespine stickleback (Gasterosteus aculeatus) exhibit divergent pigment patterns. To test whether differential predation by cutthroat trout contributes to the differences in pigmentation seen between the ecotypes, we used a within-generation selection experiment on F benthic-limnetic hybrids. After 10 months of differential selection, we compared the pigmentation of fish under trout predation to control fish not exposed to trout predation. We found that stickleback exhibited more lateral barring in ponds with trout predation. Ponds with trout were also less turbid, and a greater degree of barring was negatively correlated with the magnitude of turbidity across pond replicates. A more benthic diet, a proxy for habitat use, was also correlated with greater lateral barring and green dorsal pigmentation. These patterns suggest that differential exposure to cutthroat trout predation may explain the divergence in body pigmentation between benthic and limnetic ecotypes.

The aim of this study was to assess the effect of perch positioning on laying hens’ locomotion and the resulting energy experienced at the keel. Twenty Nick Chick and 20 Brown Nick hens were trained to transition from a platform to a perch in several configurations. Three variables of perch positioning were tested in a 2 × 2 × 2 factorial design: direction (upward vs. downward), angle (flat vs. steep), and distance (50 cm vs. 100 cm). All hens were tested for five jumps of each treatment combination at 27–28 weeks of age. As predicted, we found steep angles and long distances to result in higher peak forces and impulse during take-off, flight, and landing; longer latency to jump; a higher likelihood to perform balancing movements; and a longer latency to peck at the provided food reward. The effect of perch positioning on locomotion and force at the keel during downwards jumps and flight was more pronounced in Brown Nick hens than in Nick Chick hens. Although we cannot state how the observed forces at the keel relate to the risk for keel bone fractures, our results indicated that optimizing perch positioning can reduce accumulated forced at the keel and consequent risk for fracture due to unsuccessful transitions.