Fighting fish 'take a breather'
Siamese fighting fish take gulps of air from above water so they can continue to clash, say scientists.
Males of the species, known for their aggressive territorial displays, can also take in oxygen from both the air and water.
Scientists analysed how the fish harness this ability in order to maintain energy during a fighting bout.
They found that males incorporate visits to the surface into their battles to boost their oxygen uptake.
"It seems their smaller gills, a result of living in low oxygenated water, cannot keep up with the vigour of the fight, and more air breathing is required," explained Dr Steven Portugal from the Royal Veterinary College, London.
He worked with colleagues from the University of Queensland, Australia on the study published in the journal Comparative Biology and Physiology Part A.
Siamese fighting fish (Betta splendens) are found throughout south-east Asia where they live in low-oxygenated pools and rice paddies.
The fish are from the unusual Anabantodei group that can take in oxygen from the air via a specialised organ as well as from the water through their gills and skin.
"The males of the species are ornate and very aggressive towards members of the same sex," Dr Portugal told BBC Nature.
"Historically, the local people of south-east Asia have taken advantage of this aspect of their behaviour, and fought them against each in tank arenas, in a similar fashion to cock-fighting."
Since they were first removed from the wild for such organised bouts the fish have become popular aquarium species, with breeders concentrating on intensifying their colours and attitude.
Dr Portugal however focussed his research on the energetic costs of this display behaviour.
In the lab the team introduced two males to a tank where they performed their famous fighting behaviour.
The fish were in separate bottles within the tank but could see each other through the glass and scientists reported that they began displaying almost immediately following their introduction.
The researchers analysed the gases within the tanks before and after a fight to understand how much energy the fish used during a clash, and where they gained their oxygen from.
"It seems that during fights, they simply cannot get enough oxygen from the water, they must increase their visits to the surface to breathe more oxygen from the air," said Dr Portugal, "In a sense, they have to take a breather."Catch a breath
But the fish were not resting at the surface. The team observed that both males surface at the same time so that they can continue to fight.
"Synchronous air-breathing in fish is typically an anti-predator defence mechanism," Dr Portugal told BBC Nature.
"Coming to the surface alone makes you vulnerable to predation, and synchronous air breathing can function to reduce an individual's susceptibility to attack."
End Quote Dr Steven Portugal Royal Veterinary College, London
It seems they can't even take in more oxygen per breath, so these fights are seriously demanding for the fish”
Dr Portugal explained that for the fighting fish this "seemingly gentleman-like behaviour" is purely tactical.
"If your foe needs to breathe first, you might be forgiven for thinking this is the best time to strike - he's weak enough to have to breath first, turns his back to surface for air, [it's] seemingly the best time to attack.
However, if your attack at this point is not successful, your opponent comes back to carry on fighting you with plenty of oxygen... Eventually, you will have to surface to breath too, and potentially suffer the same fate of being attacked.
Therefore, by both surface-breathing at the same time, neither of you are risking being attacked by the other during the ascent and descent from the surface."
Dr Portugal told BBC Nature that he was surprised to find the males were "operating so close to their limits" during these confrontations.
The team found that nearly all the additional oxygen needed for the fight was provided by frequent trips to the surface.
"It seems they can't even take in more oxygen per breath, so these fights are seriously demanding for the fish," he commented.