8 December 2011 Last updated at 07:41

How penguins 'time' a deep dive

Emperor penguins "time" their dives by the number of flaps they can manage with their wings.

This is according to a new study published in the Journal of Experimental Biology.

It aimed to show how the birds reached the "decision" that it was time to stop feeding and return to the surface to breathe.

Tracking the birds revealed that they flapped their wings, on average, 237 times on each dive.

The study was led by Dr Kozue Shiomi, from the University of Tokyo, Japan.

Dr Shiomi and his team think that the penguins' decision to end their foraging dive and return to the surface is constrained by how much power their muscles can produce after every pre-dive breath. This "flying" motion propels the birds forwards, allowing them to swim quickly through the water, gulping fish.

Using data collected from diving penguins on previous field trips, the team analysed the patterns of more than 15,000 penguin dives.

They studied 10 free-ranging birds and three birds that were foraging through a hole in the ice.

Timing the penguins' dives revealed that free-ranging birds began their final ascent to the surface about 5.7 minutes into their dive. But penguins diving through the ice hole often dived for longer before performing a U-turn and returning up through the same ice hole.

Examining the acceleration patterns of the penguins as they dived, the team managed to calculate that all the birds used, on average, 237 wing flaps before starting their ascent.

"We suggest", the team concluded in their paper, "that the decision [to return] was constrained not by elapsed time, but by the number of strokes and, thus, perhaps cumulative muscle work."