Behavioural time budgets of organisms reflect their reaction norms to environmental conditions and, as such, indicate their behavioural plasticity. In environments which limit the activity time of organisms, efficient organization of behaviours is critical to enhancing performance to achieve necessary functions and minimize time and/or energetic costs. We investigated variations in behavioural time budgets of a common deposit-feeding crab, Scopimera intermedia, to the spatio-temporal scales of environmental changes experienced on sandy shores in the monsoonal tropics, where activity times are restricted by the tides. Activity levels of the crabs varied strongly with air temperature, with crabs emerging from the sediments only when air temperature exceeded ~ 20 °C. Behavioural water uptake increased in frequency with both feeding rate and environmental temperature, and was often followed by waving displays. Water uptake and waving, therefore, appear to be multifunctional behaviours, as not only do the crabs replenish water for feeding and wave their chelipeds to court females, but these behaviours can also contribute to thermoregulation, enhancing cooling when taking up water from the sediment surface. Such plastic and multifunctional behavioural traits are likely a widespread solution for survival and reproductive success in harsh and dynamic environments, where specialized traits are energetically costly and time-inefficient to optimize performance.
Surviving in extreme and variable environments is challenging due to the potentially substantial energetic cost animals need to undertake in order to maximize performances under contrasting conditions. This study reveals a multitasking strategy adopted by the sand-bubbler crab to survive hot and variable temperatures within a limited activity time on tropical sandy shores. Behaviours of the crabs are strongly temperature dependent; and when active whilst the tide is out the crabs flexibly adjust their activity times to feed, stay cool and attract mates using a limited set of multifunctional behaviours. Specialized traits are therefore not necessary to achieve these critical functions and, as such, this study highlights the importance of this plastic, multitasking strategy as a common solution for animals to endure highly variable environments with minimal cost.
The authors would like to thank Mark Denny (Stanford University) for stimulating discussions over the possible functions of waving in crabs, and Stefano Cannicci (SWIMS, The University of Hong Kong) for constructive comments in improving the manuscript. T.Y.H. would also like to thank Mr. Camille Chung for demonstrating how to use Arduino and Ms. Cecily Law (SWIMS, The University of Hong Kong) for providing technical assistance. The authors would also like to thank four anonymous reviewers for their constructive comments and criticisms.
The research was supported by a Postgraduate Studentship from the University of Hong Kong awarded to T.Y.H.
- Multifunctional behaviours