Male sea lamprey countersignal relative to their baseline pheromone but not the intensity of rivals’ signals

Animals signalling to potential mates inadvertently reveal information to sexual rivals. In species that communicate with visual or acoustic signals, rivals are well documented to use this information to optimize their own signalling strategy based on the current level of competitive risk. We studied how males fine-tune their signals after exposure to varying levels of simulated competition in a species that relies on chemical signals, the sea lamprey (<i>Petromyzon marinus</i>). Sea lamprey aggregate on spawning grounds in streams, where males each build a nest and signal to females using the sex pheromone 3-keto petromyzonol sulphate (3kPZS). We hypothesized that males use the concentration of environmental 3kPZS to infer the level of competitive risk and adjust their 3kPZS release proportionally. Males increased 3kPZS release after exposure to 3kPZS but, contrary to our hypothesis, the change in release was similar across concentrations from 5 × 10^-7 M down to 5 × 10^-13 M. Interestingly, the increase in 3kPZS release after exposure to 3kPZS was negatively correlated with baseline release rates. Taken together, our results indicate male sea lamprey adjust their pheromone signals based on the presence of rivals and their own baseline signal but not any graded assessment of competition risk.