杏吧原创

The dominant males in one species of Amazonian fish have developed a simple way to grab mating opportunities. They hide while smaller, less-dominant males work hard to persuade a female to lay eggs, then they come forward and steal the receptive female鈥檚 attention.

Lower-ranking male sailfin tetras (Crenuchus spilurus) put in a lot of time and effort to court females. They may spend several days circling aquatic weeds with their colourful fins fully extended.

at the National Institute for Amazonian Research in Amazonas, Brazil, witnessed the displays while snorkelling in streams in the Amazon forest, collecting data for a different study on the species鈥� behaviour.

But then he noticed something else: as soon as a male began leading the now-receptive female to a nesting area, a more dominant male emerged from hiding 鈥� and the female would choose to mate with him instead. The phenomenon, called 鈥渃ourtship piracy鈥�, might be unique to Chrenuchus fish, but it is also possible that other animals are courtship 鈥減irates鈥� and that their tactic hasn鈥檛 yet been noticed, he says.

鈥淭his isn鈥檛 something mean the dominant males are doing,鈥� says da Silva Pires. 鈥淭hey鈥檙e just being lazy and safe, sheltered from predators and protecting their nesting site, and waiting to take advantage of another male.鈥�

To study the behaviour in more detail, da Silva Pires and his colleagues collected fish and brought them back to the lab. They placed three fish 鈥� one large male, one small male and one female 鈥� in each of 22 laboratory aquariums. Each aquarium contained plastic plants for courting around and a PVC pipe that resembles the sailfin tetra鈥檚 usual nesting sites, rolled up leaves from the buriti palm (Mauritia flexuosa).

In all 22 aquariums, the larger male took possession of the pipe within the first 5 minutes and hid there, while the smaller male made 鈥渆nergetic鈥� efforts to court the female, says da Silva Pires.

Eventually, the female would start mimicking the courting male鈥檚 swimming patterns and her body would darken in colour, signalling she was receptive to spawning. At that point, the hard-working male would lead the female to the nesting area, the PVC pipe.

But in all of the aquariums in which spawning occurred 鈥� 19 of the 22 鈥� it was the dominant male who the female actually mated with. Just as spawning was about to occur, the dominant male would swim out of the pipe, and the female would choose to mate with him.

Occasionally, the smaller male would try to fight the larger male, says da Silva Pires, but more often he seemed to accept that all of his hard work had been in vain.

鈥淭he whole time they鈥檙e courting, [the smaller, non-dominant males] keep their fins spread out,鈥� he says. 鈥淏ut as soon as that larger male comes out of hiding, they drop their fins and tilt sideways to show submission.鈥�

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