Male swordtails wee more often to attract the ladies

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Peeing upstream is a good way to get you the ladies, according to research published in a recently issue of the online journal PLoS ONE.

Gil Rosenthal and coauthors studied the release system of male pheromones in the Sheepshead swordtail (Xiphophorus birchmanni), and found that male fish released pheromones in their urine, achieving spatial and temporal control in their release previously not thought possible in fish.

Pheromones are chemical messengers involved in a wide variety of social interactions (including mating), and fish pheromones are thought to be passively released, given that almost all fishes lack specialized scent glands or scent-marking behaviours.

The authors used intraperitoneal fluorescein dye injections to visualise pulsed urine release in the male swordtails, and then conducted a series of experiments to study the mechanics of urine release and its effects on females.

They first studied the effects of male urine on females by presenting female swordtails with three chemical cues in a Y-maze apparatus: 1) male urine collected during the fluorescein treatment, 2) "male water" that had housed males for three hours (containing urine and any other secretions), and 3) carbon-filtered water as a negative control. 

The authors found that female fish preferred both male urine and "male water" over the control, indicating the presence of pheromones in the male urine.

The authors then monitored male urination patterns in the presence and absence of females to determine whether males urinate more often in close proximity to females.  They placed a male fish in an observation tank that had been marked into quadrants, and presented it with five female fish (or none at all) placed in a stimulus tank that abutted the observation tank.  The number of times the male urinated and the length of time it spent in the quadrant closest to the stimulus tank were recorded and analysed.

The authors finally used a digital video camera to record courtship interactions in X. birchmanni–X. malinche hybrids in the Río Calnali in México, and scored both total courtship time and the amount of time that males were positioned upstream of females.

The results indicated that the male swordtails target the release of their urine to maximise detection by females, having evolved sophisticated temporal and spatial control of pheromone release comparable to that found in terrestrial animals.  This was achieved by both substantially increasing their urination rate in the presence of females, and spending significantly more time upstream of the females (to ensure that their pheromones will be detected by females downstream).

For more information, see the paper: Rosenthal GG, JN Fitzsimmons, KU Woods, G Gerlach and HS Fisher (2011) Tactical release of a sexually-selected pheromone in a swordtail fish. PLoS ONE 6(2): e16994.