Breath-hold divers face a dilemma: remain submerged to exploit rich hunting opportunities or return to the surface to replenish the air supply. ‘When to end a dive may not be a straightforward decision’, say Kozue Shiomi and colleagues from The University of Tokyo, Japan, and Scripps Institution of Oceanography, USA. Deep-diving animals, such as emperor penguins, decide to terminate a dive long before they make it back to the top, so what factors determine when a diving animal decides to begin ascending? Suspecting that the muscle work expended until they decide to return to the surface rather than submergence time might trigger an emperor penguin's return journey Shiomi, Katsufumi Sato and Paul Ponganis decided to analyse the dive profiles of emperor penguins diving freely in open sea and from an ice hole to find out what triggers the birds' decision to end a dive (p. 135).

Using data collected from diving penguins on previous field trips, the trio analysed 15,978 dive profiles from 10 free-ranging birds and 495 dives from 3 birds foraging through an ice hole. Calculating the time when each penguin began its final ascent to the surface, the team realised that almost all free-ranging birds began their final ascent around 5.7min into the dive. However, penguins diving through the ice hole often dived for longer before performing an U-turn and returning to the ice hole, so some other factor must be sending them back to the surface.

Analysing the birds' acceleration patterns to calculate the number of wing beats taken before turning to return to the ice hole, the team realised that the birds used on average 237 wing beats before embarking on their return. ‘We suggest the decision [to return to the surface] was constrained not by elapsed time, but by the number of strokes, and thus, perhaps cumulative muscle work’, say Shiomi and colleagues.

P. J.
Point of no return in diving emperor penguins: is decision to return time limited by the number of strokes?
J. Exp. Biol.