Why are whales so big, but not bigger?

Whales are huge mammals weighing from 20 to 200 tonnes. The blue whale, the largest animal ever known to have existed, reaches more than 30 metres in length and its tongue alone is as heavy as an elephant.

But how did they get that big, what sets the smaller whales apart from the larger whales, and why aren’t they bigger? These questions have long fascinated scientists, but the answers have remained elusive.

Now, a large research team from the US and Europe has made some progress – with the help of new technologies – in determining how much energy species of different size expend to capture prey, and which feeding methods bring the greatest rewards.

“Energy is a key currency for all life, and we wanted to know how energy gain compares to energy use in foraging whales with different body sizes and feeding strategies,” says team leader Jeremy Goldbogen from Stanford University, US.

“The ratio of energy gain relative to energy use reveals a whale’s foraging efficiency and that provides clues as to why different whales are big and why they aren’t bigger.”

In a paper published in the journal Science, the team reveals that specialised feeding mechanisms enable whales to exploit prey resources in unique ways, yet their gigantism is limited by prey availability and foraging efficiency.

In particular, the filter feeding strategy evolved by baleen whales rewards and drives them to achieve the largest body sizes to have ever evolved on Earth.  

Goldbogen and colleagues analysed thousands of foraging dives and 53,000 feeding events by hundreds of whales, porpoises and dolphins from 11 different species from Greenland to Antarctica.

Suction-cup tags fitted with various sensors enabled them to measure feeding rates, while active acoustics in baleen whales and the stomach contents of toothed whales provided data on prey quality from which they calculated energy intake.

This information was integrated with their data on body size across the species, giving a comprehensive picture of whale energetics spanning the smallest to the largest creatures.

The toothed group, which use echolocation to forage, are limited to feeding on one prey target at a time and expend more energy to dive the water’s depths for large prey such as giant squid and resurface for air.

This works well for sperm whales (Physeter macrocephalus), the largest toothed predators, which have developed powerful biosonar to find large squid in the deep, dark ocean, says Goldbogen.

“Their energy surplus isn’t as great as other whales, but they clearly do just fine within their energy niche.” 

However, they are operating at their biological limit, according to the team’s calculations. Filter feeders, on the other hand, can consume considerably more food at higher rates with far more efficiency.

Blue (Balaenoptera musculus), humpback (Megaptera novaeangliae), fin (B. physalus), minke (B. acutorostrata) and other filter feeding whales gulp huge volumes of water containing small prey then use rows of flexible comb-like plates, called baleen, to strain the water out.

This breaks the common model that equates predator size with their prey, showing that “the largest predators in the oceans paradoxically feed on the smallest prey”, notes Terrie Williams from the University of California-Santa Cruz in a related commentary.

While energy expenditure doesn’t limit their body size, krill and similar foraging resources are seasonal and could therefore be limiting these baleen giants from becoming any bigger. 

The study highlights the delicate tightrope that whales tread.

“You have to wonder just how perilous it is for whales living on an energetic knife’s edge,” says Nicholas Pyenson from Smithsonian’s National Museum of Natural History.

“If you’re a blue whale and your only prey item is krill, and something causes krill populations to go into decline, then you are at an evolutionary dead end because you would not be able to eat enough to sustain yourself.”

There is much more to understand about the seasonality and dynamics of prey resources, and blue whales’ size could face other constraints such as their heart rate which the team recently discovered appears to beat near maximum capacity.

Nonetheless, Goldbogen says defining an energy niche for different sized whales sheds light on how much prey whales need to survive, in turn informing their conservation and management – particularly for whales that consume prey targeted by fisheries.

Whale conservation has critical implications for the ocean’s entire ecosystem, notes Williams.

“In addition to removing astonishing amounts of prey in a single gulp, waste products from their food processing fertilise the oceans, thereby enhancing the primary production of prey that they and other marine animals rely upon,” she writes.

“Even in death, their enormous bodies feed members of marine ecosystems at nearly every trophic level, from killer whales that hunt them to highly mobile scavengers and microbial assemblages that devour their carcasses.”

This underscores the importance of increasing conservation and research efforts by investigating and helping them meet their needs, she adds. “Such efforts begin with appreciating the biology of big.”