Why are whales so massive and why do the biggest of them prey on such small animals?

Why are whales so gigantic?

Being big in the ocean is a big advantage for three main reasons:

1) it reduces competition and predation by other species;  

2) largely increases species’ home ranges, both horizontally and vertically which, in turn, permits access to enormous foraging areas;

3) greatly increases their heat retention capacity.

               However, these enormous proportions are only possible in the ocean, as their great body masses are supported by buoyant forces. On land, physical constraints, such as gravity and bone mechanics, limit maximum sizes, as the animals need to be able to support their own weight. Thus, cetaceans’ body sizes are limited not by physical factors but by both energetics and food availability.

               As mammals, cetaceans are air-breathers, just like us, and for this reason they must balance their demand for oxygen at the surface with their need for higher quality food at depth. One way evolution found to overcome this problem was precisely through gigantism: as size increases, mass-specific oxygen usage decreases, meaning that larger air-breathers possess greater diving capacities, enabling them to feed for longer periods at depth. This results in higher feeding rates and, therefore, greater energy intake per dive with increasing body size.

               It is thought that toothed whales (Odontocetes), such as sperm whales and beaked whales, evolved larger body sizes to improve their diving ability and better exploit deep-sea prey using powerful biosonars. Whereas, baleen whales (Mysticetes) large sizes, besides improving their diving capacity also permitted a more efficient exploitation of abundant and patchily distributed small prey.

               Nonetheless, to maintain energetic efficiency with such massive body sizes, these animals must eat accordingly, making them some of the most voracious animals in the oceans. To meet this large demand for food, toothed and baleen whales evolved different strategies: while Odontocetes forage on larger, single prey, Mysticetes have evolved a filter-feeding system that allows them to explore dense prey patches off small-bodied prey, such as small schooling fishes and even smaller zooplankton.

               Although all whales appear to have enhanced their foraging capacities with increased body size, it seems that this trait was even more beneficial for baleen whales (Figure 1). For toothed whales, increased body sizes essentially provide greater diving capacity and prey detection range (more acute biosonar structures), but do not ensure the capture of larger prey, as they also incur way greater foraging costs.

On the other hand, in baleen whales, there is a linear correlation between size and the amount of food intake, as the bigger they are the largest is the volume of prey-laden water that they can engulf – the largest rorquals are able to engulf 100 to 160% of their own body volume.

Why do the biggest whales prey on such small animals?

               Odontocetes and Mysticetes have been estimated to have diverged one from another around 35 million years ago, in the late Eocene. It is thought that the biggest drivers for this divergence were related with food availability and habitat preferences.

               While toothed whales evolved very acute biosonars, that allowed them to better explore the vast pelagic biomass in more offshore waters, baleen whales developed a filter-feeding strategy that permitted them to efficiently explore a vast, underutilized prey resource: high-density aggregations of small-bodied species that explode in numbers in regions of intense wind-driven coastal up-welling (areas of high turbulence that cause the upward movement of great quantities of nutrients, resulting in phytoplankton blooms which will, in turn, attract small grazers, generating high feeding opportunities for a wide variety of species).

               As paradoxal as it might seem, it was this evolution towards small-bodied prey that enabled baleen whale to get so gigantic. By developing this filter-feeding technique these whales not only achieved higher foraging efficiency compared to single-prey-feeding toothed whales but they also gained access to huge energy resources as are lower trophic level organisms. For instance, the blue whale, the biggest animal ever to live in our planet, reaching up to 33 m in length, feeds almost exclusively on krill, a small shrimp-like crustacean measuring only 2-3 cm long.

The reason behind this strange relation has to do with energy flow along the food chain. When an animal (or plant) is ingested by another only 10% of the energy stored in the first is passed along to the latter. Therefore, it is more advantageous to prey on lower trophic levels, which have more energy available to be taken in (Figure 2). This is also the reason why maximum size of carnivores in often less than that of herbivores (for instance, the largest terrestrial mammal hunter, the polar bear, has a maximum weigh of 800 kg, while the largest grazing herbivore, the African elephant, can weigh up to 6000 kg).

Article written by the biologist Diogo Costa.

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