The evolutionary transition from solitary to social life is driven by direct and indirect fitness benefits of social interactions. Understanding the conditions promoting the early evolution of social life therefore requires identification of these benefits in nonderived social systems, such as animal families where offspring are mobile and able to disperse and will survive independently. Family life is well known to provide benefits to offspring through parental care, but research on sibling interactions generally focused on fitness costs to offspring due to competitive behaviors. Here we show experimentally that sibling interactions also reflect cooperative behaviors in the form of food sharing in nonderived families of the European earwig, Forficula auricularia. Food ingested by individual offspring was transferred to their siblings through mouth-to-anus contacts and active allo-coprophagy. These transfers occurred in both the presence and the absence of the tending mothers, even though the direct contact with the mothers limited sibling food sharing. Neither food deprivation or relatedness influenced the total amount of transferred food, but relatedness affected frass release and the behavioral mechanisms mediating food sharing. Related offspring obtained food predominately through allo-coprophagy, whereas unrelated offspring obtained food through mouth-to-anus contacts. Overall, this study emphasizes that sibling cooperation may be a key process promoting the early evolution of social life.
Temperature and photoperiod are important environmental parameters for organisms. The present study tests the hypothesis that, during embryogenesis, temperature and photoperiod influence pre-and post-eyespot development time in dragonflies of the family Libellulidae differently. Eggs are used from eight species (five different genera, from Africa/Europe, and lentic/lotic habitat preferences). The eggs are reared under different constant or fluctuating temperature and light conditions. There are no general species-specific degree-days for pre-or the post-eyespot development in these species. In all study species, the variance within and between the treatments of the duration in days and the degree-days of pre-eyespot development is lower than that of post-eyespot development. Pre-eyespot development appears to be less flexible in its reaction to environmental influences. By contrast, post-eyespot development appears to react more flexibly to environmental influences. All eight species show the same pattern. This indicates strongly that this flexibility is a general pattern in Libellulidae that might help the species within this family to cope successfully with variations in environmental conditions. Because eyespot development and katatrepsis occur close to each other, the above-described pattern might also appear in other odonates and in other insect groups that exhibit katatrepsis. For all of them, it is essential for survival to match the time of hatching with adequate external temperature and photoperiodic conditions.
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