Vibrating Aggression: Spider Males Perform an Unusual Assessment Strategy During Contest Displays

A recurrent question in animal contests is whether individuals adopt a self or mutual assessment rule to decide to withdraw from a contest. However, many empirical studies fail to find conclusive support for one of these two possibilities. A possible explanation is that assessment strategies vary between individuals. In the contests of the orb-web spider Trichonephila clavipes, males perform a vibrational display on webs that may escalate to physical contact. Since all individuals perform the vibrational phase and only some of them escalate, we proposed 2 hypotheses: (1) all individuals perform mutual assessment during the vibrational phase, or (2) some individuals that do not escalate adopt self-assessment, while individuals that escalated adopt mutual assessment. To evaluate these hypotheses, we investigated the relationship between the duration of the vibrational phase and frontal leg length (a proxy of male fight capacity) of loser and winner males in contests that escalated and did not escalate to the physical contact phase. We found a non-significant relationship between duration and losers leg length for both contests that escalate and did not escalate. While we found a positive relationship between duration and winners leg length, particularly in contests that did not escalate. These results do not provide support for mutual assessment or for a mix of different assessment rules among individuals. We suggest that in T. clavipes, the dynamics of the vibrational phase may be explained by two different contest strategies (opponent-only assessment or size-based aggressiveness) that are dependent on intruder motivation to escalate. During a contest, rivals may decide to keep fighting or run away. To reach a decision, each rival may use its own fighting capacity or compare its strength with its rival. We investigated fights between males of an orb-web spider to show that the decision to withdraw in the initial contest phase depends mostly on the opponent fighting capacity, indicating that rivals may use one between two unusual fighting strategies.