Evolutionary loss of threat display in more social species: phylogenetic comparisons, natural interactions in the wild, and experiments with models

What is it about?

Many species perform threat displays, which are aggressive signals that are cheap to produce and keep unwanted individuals away. However, it remains unclear why threat displays would ever be lost evolutionarily. Here, we show that across the evolutionary tree of hermit crabs, more social species have ceased producing threat displays. To investigate this evolutionary loss, we conducted detailed observations in the wild of both a more social and a less social species. We found that the more social species, despite having higher rates of social interaction, surprisingly never produced threat displays during its interactions. So we decided to experimentally reanimate threat displays in the more social species, using postured models to test how live recipients would respond to threat displays. Our experiments revealed that recipients from the more social species were undeterred by models postured in threat displays, effectively showing no responsiveness. This evolutionary loss in both production of and response to threat displays appears linked to the extreme social dependence that individuals from the more social species have for fellow conspecifics: only from conspecifics can they acquire the shells they need to reproduce. Shells are therefore so valuable they have ultimately driven a “desperado effect” in which desperation causes individuals to pursue these resources at all costs, no longer heeding threat displays, and thereby causing a collapse of communication.

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