Whale in Alaska (Source: Flickr)
By Anna Brinks ’21
The largest mammal on Earth, the blue whale, is one of the many gentle goliaths populating our oceans. With a tongue that weighs as much as an elephant, this aquatic mammal consistently has had a larger average size than its terrestrial counterparts. The driving factors behind this incredible size difference, however, are still disputed.
The four living lineages of Sirenia, Cetacea, Pinnipedia, and Lutrinae have entered and diversified in the ocean (Gearty, McClain & Payne, 2018, p.1). Previous studies on the size increase between the terrestrial and aquatic animals in these clades usually emphasize a release from size constraints enabled by the unique qualities of a marine environment. Theories centered on increased buoyancy, a larger habitat area, heat regulation demands, and increased protein availability all predict an increase in aquatic mammals’ average body size: buoyancy enables skeletons to more easily support a heavy body mass, the expansive marine habitat allows plenty of room for growth, large mammals retain heat more efficiently than smaller ones in cold aquatic environments, and the extensive ocean life cultivates many food source options.
These theories, however, cannot explain the rapid selection towards a size attractor (or a common, optimal size) of approximately 500 kg for three of the four aquatic mammal clades (with the exception of Lutrinae) (Gearty et. al., 2018, p.1). While the terrestrial relatives in these clades display significant variation in size, this variance is far less apparent in their aquatic counterparts. This convergent and limiting weight likely arises from pressures that are uniform across the clades and could involve a combination of these theories or a novel concept.
A study published on March 26, 2018 in the journal PNAS follows the evolutionary trajectory of 3,859 living and 2,999 fossil mammal species to quantitatively test the relative contributions of ecological, biomechanical and physiological controls on size (Gearty et. al., 2018, p.1). Craig McClain (a part of the Louisiana University Marine Consortium), Jonathan Payne (a professor of geological sciences at Stanford’s School of Earth, Energy, and Environmental Sciences), and William Gearty (a graduate student at Stanford Earth) created a model of surplus energy that factors in the intake of chemical energy by feeding (F), basal metabolism (M), and heat loss to the environment (H):
E = F – M – H (Gearty et. al., 2018, p.3)
This model accurately predicts an increase in average size for marine mammals and corresponds closely with their observed size. Furthermore, it indicates that limitations of feeding efficiency and basal metabolism demands constrain the maximum size of aquatic mammals while thermoregulatory costs play an important role in constraining the minimum size. In fact, the minimum size possible for a terrestrial endotherm is more than three orders of magnitude smaller than a comparable aquatic endotherm due to the greater energetic cost of living in water that is below body temperature (Gearty et. al., 2018, p.4).
There are some exceptions to the size attractor of 500 kg. Otters, in the Lutrinae clade, are much smaller and have an average weight of approximately 10 pounds. This is likely because they are not fully aquatic and therefore do not face the same thermoregulatory constraints (Gearty et. al., 2018, p.2). Baleen whales, conversely, are larger than this size attractor. The filter feeding strategy utilized by baleen whales is more efficient than the feeding of the other three toothed aquatic groups, allowing surplus energy to be used for growth (Gearty et. al., 2018, p.4).
Ultimately, the study demonstrates that rather than being released from size constraints, aquatic mammals face stricter size limitations than their terrestrial relatives. The energetic cost model successfully explains the observed increase in body size, decrease in size variance, and rapid evolution of toothed, aquatic mammals (Gearty et. al., 2018, p.5). Understanding this model and exploring its application to other aquatic animals may help provide insight into the unique evolutionary trajectories and selective pressures that marine environments foster.
References
- William Gearty, Craig R. McClain and Jonathan L. Payne. Energetic tradeoffs control the size distribution of aquatic mammals. PNAS, 2018 DOI: 1073/pnas.1712629115
- National Geographic. (2017, August 07). Blue Whale. Retrieved April 04, 2018, from https://www.nationalgeographic.com/animals/mammals/b/blue-whale/