David vs. Goliath: Serotonin modulates opponent perception between smaller and larger rivals

Abstract

During agonistic encounters, the perception of a larger opponent through morphological signaling typically suppresses aggression in the smaller individual, preventing contest intensity escalation. However, non-morphological factors such as central serotonin (5-HT) activity can influence individual aggression, potentially altering contest intensity despite initial size discrepancies. When male stalk-eyed flies (Teleopsis dalmanni) fight, contest escalation is directly proportional to similarity in body size, with escalation being lower in size-mismatched contests. We have shown that both high-intensity aggression and the probability of winning are increased in males with pharmacologically elevated 5-HT relative to size-matched non-treated opponents. Here, we hypothesized that, in size-mismatched contests, increasing brain 5-HT in the smaller opponent could similarly increase aggression and counteract the low contest intensity normally driven by size discrepancy. Size-mismatched male pairs (greater than 5% difference in eyestalk length) engaged in a forced fight paradigm, with the smaller fly either untreated or with pharmacologically elevated 5-HT levels. The expression of high-intensity aggressive behaviors was significantly increased in smaller treated opponents, but the probability of winning was not altered. This suggests that while elevated serotonergic activity can increase aggression and intensity despite perception of a larger opponent, this is not sufficient to overcome size biases with respect to contest outcome. However, the fact that larger opponents continued to win against smaller treated flies was not simply a function of size. Instead, untreated larger males adjusted their fighting strategy to match the increased aggression of their smaller treated opponent, suggesting contextual flexibility in behavior based on individual opponent assessment.

Neuromodulation of Nestmate Recognition Decisions by Pavement Ants

“…Ants were rapidly decapitated with micro-scissors and prepped for brain removal following previous methods conducted on other small insects [26–27, 36]. Heads were visualized under a dissection microscope and held stationary by micro-tweezers.…”

“…The biogenic amines OA, DA, and 5-HT were quantified using high performance liquid chromatography with electrochemical detection following a previously developed method with slight modifications [26–27; 35]. The brain samples were thawed, briefly sonicated and centrifuged at 17,000 rpms and 50 μL of the supernatant was injected into a Waters Alliance e2695 separations module.…”

Assessment strategies and fighting patterns in animal contests: a role for serotonin?

“…This is remarkably similar to the pattern shown by winning opponents in contests of varying size disparity, and suggests that endogenous brain levels of 5-HT, and possibly other biogenic amines such as OA, may be responsible for the escalation patterns shown by winners during longer duration fights. We have previously demonstrated that raising 5-HT in a smaller competitor will lead to increased willingness to engage in HI encounters with larger opponents in stalk-eyed fly males ( Bubak et al. 2015 ).…”

Serotonin has opposite effects on the aggressiveness of crayfish, facing either a smaller or a larger rival: alteration of size perception

“…This property is probably widespread in animals as risk taking is also controlled in higher vertebrates by 5-HT (Macoveanu et al, 2013;Blanchard and Meyza, 2017), which can selectively shape social behavior (Crockett et al, 2010). Previous observations in stalk-eyed flies have shown that 5-HT can increase the aggressiveness of a smaller rival by counteracting the perception of its size discrepancy with a larger opponent (Bubak et al, 2015). Our results confirm and extend these observations in crayfish by showing that 5-HT is also able to decrease the aggressiveness of a larger opponent by affecting its evaluation of a smaller rival.…”

“…In flies, aggressiveness is increased by the administration of a 5-HT precursor (Bubak et al, 2014) or by selective 5-HT neuron activation (Hoopfer, 2016). Interestingly, recent observations in flies have also shown that 5-HT has a much more complex role in aggressiveness by modulating the perception of a rival (Bubak et al, 2015).…”