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

Area of Science:

• Ethology
• Neurobiology
• Animal Behavior

Background:

• Morphological size differences typically reduce aggression in smaller individuals during contests.
• Central serotonin (5-HT) influences aggression levels, potentially overriding size-based dominance.
• Stalk-eyed fly contests show escalation proportional to size similarity.

Purpose of the Study:

• To investigate if elevated serotonin in smaller male stalk-eyed flies can increase aggression against larger opponents in size-mismatched contests.
• To determine if enhanced aggression in smaller flies can counteract the typical suppression of contest intensity due to size disparity.

Main Methods:

• Size-mismatched male stalk-eyed flies (Teleopsis dalmanni) with >5% eyestalk length difference were used.
• The smaller fly in each pair received either a control or a pharmacologically elevated serotonin treatment.
• Forced fights were conducted to observe aggressive behaviors and contest outcomes.

Main Results:

• Smaller flies with elevated serotonin exhibited significantly increased high-intensity aggressive behaviors.
• Despite increased aggression, the probability of winning for the smaller, treated flies did not change.
• Larger, untreated opponents adapted their fighting strategies to match the increased aggression of smaller opponents.

Conclusions:

• Elevated serotonin can increase aggression in smaller individuals facing larger opponents but does not overcome size-based advantages.
• Contest outcomes are not solely determined by size; opponents demonstrate behavioral flexibility based on perceived aggression.
• Serotonergic activity plays a role in modulating aggression intensity, influencing behavioral responses in social interactions.