‘United we stand, divided we fall’: intertwining as evidence of joint actions in pea plants

Abstract In life, it is common for almost every kind of organism to interact with one another. In the human realm, such interactions are at the basis of joint actions, when two or more agents syntonize their actions to achieve a common goal. Shared intentionality is the theoretical construct referring to the suite of abilities that enable such coordinated and collaborative interactions. While shared intentionality has become an important concept in research on social cognition, there is controversy surrounding its evolutionary origins. An aspect still unexplored but promising to bring new insights into this open debate is the study of aneural organisms. To fill this gap, here we investigate whether climbing plants can act jointly to achieve a common goal, i.e. reaching the light. We examined Pisum Sativum plants growing intertwined when there is a need to climb but a potential support is not present in the environment. Three-dimensional kinematic analysis of their movement revealed a coordinated and complementary behaviour. They tend to coordinate their movement in time and space to achieve a joint climbing. By deliberately extending the context in which a joint action takes place, we pay tribute to the complex nature of this social phenomenon. The next challenge for the field of joint action is to generate a perspective that links coordination mechanisms to an evolutionary framework across taxa. Through the mathematical description of movement (i.e. kinematics), we demonstrate that pea plants are able to act jointly for the achievement of a common goal, i.e. to reach the maximum exposure to light. When a support is not available, they organize a braided structure that supports their ascent towards the light. Of relevance is that their movement is highly coordinated in time and space as evidenced by correlational analyses. The movement exhibited by the two plants acting together is completely different from the one exhibited by a plant acting towards a static and inanimate support (i.e. a wooden pole). This suggests that plants are able to adapt their movement depending on the kind of element they are interacting with. Our findings provide an important contribution to the study of coordinated and collaborative interactions in climbing plants.