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 two
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.