Humpback whales may look like gentle giants, but each year they undertake nature's most extreme crash diet, shedding around 36% of their mass in less than two months – somehow avoiding the tissue breakdown that comes with starvation in other species. Now, new insights into their epic migration has uncovered just how massive their fat loss is.
Researchers from Australia's Griffith University used drones to follow 103 adult humpback whales (Megaptera novaeangliae) across the southern hemisphere, from their cold-water feeding grounds on the Western Antarctic Peninsula to the warmer climes of their breeding site off the coast of Colombia.
Of particular interest was seeing how this 5,000-mile (8,000-km) one-way journey transformed their huge bodies, and how they managed to burn through blubber but remained healthy enough by the time they reached the tropics for the females to birth calves.
The researchers found that the mammals burn 24,250 lb (11,000 kg) of blubber – the weight of two adult African elephants – on their marathon swim, downsizing their bulk by more than a third. All without stopping for even a snack on the way. To power this, they need the equivalent of around 125,700 lb (57,000 kg) of krill, enough to fill a cement mixer full of the tiny crustaceans, a load that outweighs an Airbus A320 jet at takeoff. That's an estimated 28.5 million individual krill for each migrating whale.
It's the metabolic equivalent of a 200-lb (90-kg) person losing more than 70 lb (30 kg) in less than two months – without any negative health impacts.
“Southern hemisphere humpback whales depend on Antarctic krill for their annual energy requirements, fueling their long migrations between feeding and breeding grounds,” said lead researcher Alexandre Bernier-Graveline. “We found the whales were at their fattest in early autumn (fall) – March-May – and slimmest by late spring – August-December – showing a dramatic seasonal change in body condition.
“Our study quantifies the whales’ extreme ‘feast and fast’ lifestyle, and the critical role of Antarctic krill in their survival and migratory life-history strategy," he added.
This extreme fat loss provides new insights into the animal's robust physiology and metabolism – and how much fuel is needed to complete this long-haul trip. The energy they burn on their six-to-eight-week journey, Bernier-Graveline estimates, is roughly all the calories that an average human consumes in 62 years.
Unlike humans, whale blubber is a more readily available fuel to burn than what we store in our fat cells. As such, their extreme fasting regimen doesn't impact their overall health or damage organ systems. However, one strain on their finely tuned physiology is very much linked to us: Ice loss in Antarctica.
Both rapid climate changes in the region, as well as a pattern of overfishing, have seen krill numbers trend downward. This also affects other species including chinstrap (Pygoscelis antarcticus) and gentoo (P. papua) penguins, but is likely to be felt the most in large mammals like humpbacks that require millions of these shrimp-like individuals to bulk up each year.
The new research helps scientists better understand the requirements of humpbacks and how the availability of keystone species krill (Euphausia superba) could directly impact whale numbers. It also shows how technological advances – in this case, drone-based photogrammetry – can provide crucial information about the life cycles and behaviors of elusive marine mammal species.
"By linking migration and reproductive energy costs to krill biomass, our findings provide critical ecological context for understanding how environmental changes such as krill population fluctuations could impact whale populations," the researchers noted.
The research was published in the journal Marine Mammal Science.
Source: Griffith University
