Behavioural adjustments enable the colonization of subterranean environments

Specialized subterranean species are iconic examples of convergent evolution driven by environmental constraints, representing an ideal model system for eco-evolutionary studies. However, scientific research on the behavioural adaptations of subterranean organisms has lagged and is biased mostly towards a few model species. Through a systematic literature review, we aimed to assess quantitatively whether a congruent evolution of behavioural traits among subterranean species exists. We considered four types of behaviours (exploratory, feeding, social and anti-predator) and tested predictions about their occurrence among species with different degrees of adaptation to subterranean environments. We argue that these behaviours constitute the earliest adaptations to three main ecological factors characterizing subterranean habitats, namely the constant darkness, the limited food resources and the reduced predator pressure. Darkness selects for individuals that rely on non-visual cues to explore their surroundings, track food resources and locate potential sexual partners. The reduced availability of food promotes high foraging plasticity and deters species from adopting aggressive behaviours towards conspecifics. The reduced predator pressure allows species to abandon specific anti-predator behaviours and permits safer execution of their biological activities. Our analyses show that these four behaviours are likely to represent the result of an early convergent adaptation to the subterranean realm.