Data and code from: 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 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 (Petromyzon marinus). 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 x 10-7 M down to 5 x 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 that male sea lamprey adjust their pheromone signals based on the
presence of rivals and their baseline signal, but not any graded
assessment of competition risk.