l -Kynurenine, an amino acid identified as a sex pheromone in the urine of ovulated female masu salmon

Many animals employ sex pheromones to find mating partners during their reproductive seasons. However, most sex pheromones of vertebrates remain to be identified. Over the past 20 years, steroids and prostaglandins have been identified as sex pheromones in several fishes. These pheromones are broadly termed “hormonal pheromones” because they or their precursors act as hormones in these fishes. Hitherto, no other type of sex pheromone has been unambiguously identified in teleost fish. Here we report the identification of a “nonhormonal pheromone” in teleost fish. The urine of the reproductively mature female masu salmon ( Oncorhynchus masou ) contains a male-attracting pheromone. Bioassay-guided fractionation yielded an active compound that was identical to l -kynurenine in spectral and chromatographic properties. l -Kynurenine is a major metabolite of l -tryptophan in vertebrates. This pheromone elicits a male-specific behavior at even picomolar concentrations; its electrophysiological threshold is 10 −14 M. l -Kynurenine is a reasonable substance for female masu salmon to advertise their readiness for mating.