Like dogs laying claim to territory, male Mozambique tilapia (Oreochromis mossambicus) use urine to assert their dominance. The scents, called pheromones, that dominant males release in their urine keep subordinate males docile and prime females to lay eggs. But male social status and urinary pheromones are just part of the elaborate process of tilapia mating. After males establish their dominance over the subordinate males, the fish dig and defend pits in which the attracted fertile females lay eggs. Females, which choose their mates, also need to communicate they are ready to reproduce so that dominant males know when to strut their stuff. Samyar Ashouri, José Da Silva, Adelino Canário and Peter Hubbard from the University of Algarve, Portugal, suspected that tilapia may release additional scents in their feces to communicate their fertility, so they investigated whether females started defecating any attractive aromas when they were ready to mate.

The researchers created groups of fish with two males and four females in each group, then placed each group into its own tank. They tracked the behaviors of the males for a week and decided which one was dominant based on its tendency to bite, dig and mate. They also tracked the reproductive cycles of the females for 3 months to determine when the females were about to ovulate and most likely to draw the attention of males. After establishing which male was dominant and each female's fertility, the researchers collected feces from the fish to analyze their contents; they found that dominant males and fertile females had higher quantities of several amino acids and two bile acids – chemicals which help digest fats and other nutrients – in their feces. The team suggested that these differences are due to reproductive hormones, rather than dietary changes associated with motherhood, because they occurred in both sexes.

Still, the amino acids and bile acids can only signal that the females are ready to reproduce if other fish can smell them. To check whether the males could smell the scents from the female's feces, the researchers put male fish in small tanks, pumped different substances into the water and measured whether the nerves in their noses responded to the scents. Males picked up the scent of the amino acids and bile acids in the feces of the ready to reproduce female fish, and when the scientists pumped the two bile acids simultaneously over the fish's nose, the resulting electrical activity in the nerves suggested that males even have the capacity to recognize each bile acid individually.

Confident that the male fish could smell the chemicals, the researchers checked to see whether the scents caused any behavioral changes in the males. First, they placed a male tilapia in front of a mirror to fool the fish into thinking he had a competitor to fight, and then pumped in the chemicals from the female's feces to see whether the males mellowed. Because none of the amino acids or bile acids reduced the male's aggressiveness, the researchers concluded that the fecal odors don't act like male urinary scents. However, when the scientists placed males in tanks with three distinct chambers that the fish could swim into, they observed that pumping feces from fertile females into one of the chambers attracted the males and prompted them to start digging more frequently in preparation for mating. When the team pumped the bile acids alone into the tank, the males also swam into the scented section and dug more, but not if the researchers only pumped in the amino acids. This suggests that there is a connection between a female tilapia's fertility and the bile acids she produces, and a male's nose knows it.

Ashouri
,
S.
,
Da Silva
,
J.
,
Canário
,
A.
and
Hubbard
,
P.
(
2023
).
Bile acids as putative social signals in Mozambique tilapia (Oreochromis mossambicus)
.
Physiol. Behav.
272
,
114378
.