For many animals, siblings are a key component of their social environment during early life. Previous research has shown that the early social environment is important, but it has not yet been clear whether the number of siblings or the nature of their interactions is the decisive factor. "The early social environment is often treated as a single, uniform factor," says Bruno Camargo dos Santos, behavioral ecologist of Wageningen University & Research and lead author of a new study published in the Proceedings of the National Academy of Sciences. "We wanted to experimentally disentangle what exactly makes the difference."