In the icy waters of the Arctic Ocean, a beluga mother gently noses her newborn to the surface for its first breath. These ocean-going mammals are as dependent on air as we are, and must visit the surface regularly throughout their lives. However, many young cetaceans lack the full range of physiological adaptations essential for long duration dives. Adult marine mammals usually have high levels of the oxygen-carrying protein myoglobin in their muscles, which act like a SCUBA tank carrying oxygen that can be released gradually during the course of a dive. In addition, they also have the capacity to neutralise lactic acid released into the muscle when they exhaust their oxygen supply and switch to anaerobic respiration. ‘For bottlenose dolphins it takes up to 3 years for the muscle biochemistry for diving to be fully mature’, says Shawn Noren, from the University of California, Santa Cruz, USA. Yet from an early age, baby belugas must follow their mothers under the sea ice, where air holes are transient and scarce. Could the youngsters develop the essential physical adaptations that would permit them to navigate for lengthy periods beneath the ice at an earlier age than other marine mammals?
Intrigued, Noren and Robert Suydam, from the Alaskan Department of Wildlife Management, collected muscle samples from belugas – from a full-term foetus to mature adults – that had died from natural causes and as part of the Point Lay community annual beluga subsistence hunt in the Chukchi Sea. ‘This collaboration allowed us to obtain sufficient samples within one season’, explains Noren.
Once all of the samples had been assembled, she measured their myoglobin concentrations. Interestingly, the myoglobin concentration in the muscle of newborn beluga calves [1.56 g (100 g wet muscle)−1] was higher than that of other newborn cetaceans [0.13–1.3 g (100 g wet muscle)−1], making belugas better prepared for diving at birth than other species. However, when Noren turned to the samples collected from older calves, she was astonished to see that their myoglobin levels had rocketed and were similar to those of fully grown adults. ‘During the first year, the amount of myoglobin per 100 g of muscle increased by 452%, and mature levels are achieved by 14 months after birth’, she exclaims. Noren also calculated the amount of oxygen that each animal could carry and then used the value to calculate the maximum dive length and depth that they could sustain. Impressively, both the dive length and depth increased dramatically over the first year of life, from 3.6 min and 216 m at birth to 8.54 min and 512 m – in contrast to fully grown adults (>10 years) that perform dives of up to 14 min and can reach depths of 812 m. However, the youngsters’ ability to neutralise acid produced by anaerobic respiration wasn't much greater than that of other young cetaceans.
‘Baby marine mammals are not just small versions of adult marine mammals’, says Noren, who suspects that the rapid maturation of the calves’ diving physiology is driven by the necessity that they accompany their mothers beneath sea ice from an early age. However, she is concerned that the retreat of the sea ice at the North Pole could place these animals under increasing stress. ‘As sea-ice recedes further offshore, cod – which is an important prey of belugas – will follow, because cod prefer living near the ice edge and the belugas may follow. Indeed, there is evidence that when sea-ice cover is low, belugas are further offshore. This range shift may be problematic for immature belugas and females with calves that prefer nearshore habitats and are disadvantaged when competing for resources with larger animals that have greater dive capacities’, Noren explains.