Increased pheromone signaling by small male sea lamprey has distinct effects on female mate search and courtship

Male body size affects access to mates in many animals. Attributes of sexual signals often correlate with body size due to physiological constraints on signal production. Larger males generally produce larger signals, but costs of being large or compensation by small males can result in smaller males producing signals of equal or greater magnitude. Female choice following multiple male traits with different relationships to size might further complicate the effect of male body size on access to mates. We report the relationship between male body size and pheromone signaling, and the effects on female mate search and courtship in the sea lamprey (Petromyzon marinus). We predicted that pheromone production in the liver and the liver mass to body mass ratio would remain constant across sizes, resulting in similar mass-adjusted pheromone release rates across sizes but a positive relationship between absolute pheromone release and body mass. Our results confirmed positive relationships between body mass and liver mass, and liver mass and the magnitude of the pheromone signal. Surprisingly, decreasing body mass was correlated with higher pheromone concentrations in the liver, liver mass to body mass ratios, and mass-adjusted pheromone release rates. In a natural stream, females more often entered nests treated with small versus large male odors. However, close-proximity courtship behaviors were similar in nests treated with small or large male odors. We conclude that small males exhibit increased release of the main pheromone component, but female discrimination of male pheromones follows several axes of variation with different relationships to size. Significance statement Large males often produce signals that are more attractive to females. We determined the relationship between male body size and pheromone signaling in the sea lamprey (Petromyzon marinus). We predicted that larger males, who have a larger pheromone-producing organ, would have a larger pheromone signal and that females, who prefer larger pheromone signals, would prefer the odor of large males. Surprisingly, we found that mass-adjusted pheromone release rates increase with decreasing male body mass due to higher rates of pheromone synthesis and a slightly larger liver proportional to total body mass. In a natural stream, small male odors attracted more females but elicited similar courtship behaviors compared to large male odors.