Variation in Urinary Amino Acids in the Mozambique Tilapia: A Potential Signal of Dominance or Individuality?

Abstract

The urine of male tilapia (Oreochromis mossambicus) contains a pheromone which is—at least in part—composed of two steroid glucuronates. However, the polar fraction of urine (not containing steroids) also contains potent odorants, the identity and relevance of which are unknown. Amino acids are polar, potent odorants for fish, and are often found at high concentrations in fish urine. We therefore measured urinary amino acid concentrations (by gas-chromatography/mass-spectrometry) and assessed their contribution to the odor of male urine (by electro-olfactogram)—do amino acids form part of the “dominance pheromone” in tilapia? Urine was taken from males of different social status (dominant and subordinate) and passed through C18 solid-phase extraction cartridges. The polar fraction (aqueous flow-through) contained highly variable levels (up to 17 mM) of l-arginine, l-glutamate, and l-phenylalanine. Urinary [l-arginine] was higher, and [l-phenylalanine] lower, in urine from dominant males than that from subordinates. Although no statistical differences were seen for urinary [l-glutamate], a weak positive correlation with the donor’s social rank was found. In addition, comparison of olfactory responses to an artificial mixture of amino acids, based on the concentrations in a pool of urine from dominant males, showed that these three constitute most—but not all—of the olfactory potency of the polar fraction. These results show that (1) urinary amino acids constitute part of the olfactory potency of male urine, (2) levels of l-arginine, l-phenylalanine and, to a lesser extent l-glutamate, depend on social status, and (3) odorants other than amino acids are present in the aqueous polar fraction. We suggest that amino acids contribute to a urinary “signature mixture”; this could be important in individual recognition during aggressive male–male interactions, wherein urine has previously been shown to play a modulatory role.