Scientific evidence of sharks sleeping has been documented in Australia for the first time.
A team of scientists has observed sleep in draughtsboard sharks, a bottom-dwelling species endemic to Australia, by analysing the animals’ metabolism and posture.
The study’s lead author, Dr Michael Kelly of La Trobe University, said there was previously anecdotal evidence of sleep in sharks, mostly from observations by scuba divers.
“From a scientific standpoint, quantitatively, this had never been looked at,” he said.
The team monitored seven draughtsboard sharks over a 24-hour period, and found that the sharks’ metabolism – as measured through oxygen consumption – dropped during periods of inactivity longer than five minutes.
“We know that a drop in metabolism is a telltale sign of sleep in many, many other animals,” Kelly said.
When the sharks were inactive for five minutes or longer, the animals’ metabolic rates dropped lower than when they were when awake but resting, or actively swimming. In addition, the animals also changed their body posture to lying flat while sleeping.
Draughtsboard sharks are ambush predators. When awake but inactive, the sharks stick out their pectoral fins and sit with their heads up, Kelly said.
The sharks sometimes slept with their eyes shut, but the researchers noted this happened more often during the day. They surmise the shut-eye was associated with light levels rather than sleep.
Kelly’s previous work has investigated behavioural indicators of sleep in the sharks, such as a rousing threshold. “When these animals are inactive for five minutes or longer, it takes a higher amount of stimulation to elicit a reaction from them,” he said.
Behavioural indicators of sleep are not considered definitive, as it’s difficult for researchers to tell whether an animal is truly asleep or merely playing dead or in quiet wakefulness.
Draughtsboard sharks are a species of buccal pumping sharks, which manually push water over their gills so they can respire even while still. Obligate ram ventilators, on the other hand – which includes great whites and tiger sharks – must keep swimming to ventilate their gills, and die from a lack of oxygen if they stop.
“The idea was … those animals aren’t sleeping because they move continuously,” Kelly said, adding the theory had been debunked in other animals. “Staying still is not a criteria for sleep. We see that in marine mammals – whales and dolphins are able to keep swimming and sleep … We know that birds are able to keep flying and sleep.”
Kelly plans to monitor brain activity in draughtsboard sharks, and says further research on sleep in ram ventilators is needed.
The study was published in the journal Biology Letters.