Whales are all large by any measure, but one group of them in particular, the baleen whales (Mysticeti), are especially large, and, interestingly, this group only became really big relatively recently. Why did they get so big? Ed Yong (on Twitter) writes about the rise of these majestic giants in a series of great articles here and here, based on two separate yet related studies by Slater et al. and Gearty et al..
Baleen whales emerged 35 million years ago, but it was not till less than 4.5 million years ago that three different lineages in this group independently become truly gigantic.
Slater et al. propose that this rapid gigantism was a functional adaptation that evolved to exploit the rich but scattered feeding resources presented by seasonal nutrient upwellings off continental coasts that emerged as the earth entered an ice age. Repeated cycles of glacier expansion and contraction in the northern hemisphere shifted the earth’s climate from continuously warm to seasonally warm and cold. Due to the resulting radical differences in temperature between the northern and southern hemispheres during the extreme seasons, the energy and hence strength of the winds from the south increased markedly. This, in turn, resulted in intensification of upwelling currents in continental coastal waters, brought up much more rich nutrients from the coastal bottom to the surface. The resulting nutrient blooms off the coasts, which were further reinforced by run-offs from the glaciers as they melted, formed a rich feeding bonanza for crustaceans and small fish resulting in their numbers booming as well. And these massive aggregations of crustaceans and small fish, in turn, presented an extraordinarily rich potential resource for whales, which fed on them.
However, these nutrient concentrations were scattered across localized pockets, isolated from each other by large areas of barren ocean. Thus, while the climatic shifts of a few million years ago created a new and extremely lucrative energy resource for oceanic feeders, it was distributed across many small, highly-concentrated yet widely-separate pockets. An organism needed to be able to travel vast distances without feeding to get from one resource pocket to the next, while at the same time needed to be able to rapidly and efficiently consume huge quantities of resources in a very short span of time and space. And that is exactly what the baleen whales eventually evolved to do through their large sizes for the former (which enabled them to make the long and far journeys between feeding areas without eating – blue whales, for example, can travel for up to 6 months without eating) and with a special foraging technique, “lunge feeding”, for the latter.
Gearty et al. add to this story with an explanation of functional constraints as well functional adapation. There is a minimum size required to economically maintain an operational body temperature in the oceans: too small, and your body loses heat too quickly for feeding and metabolism to keep up. At the same time, there is a maximum size that bodies can get up to: too large, and it would be too difficult to find enough food to support it, even in an ocean environment where bouyancy and other factors relieve the graviational burden somewhat. It turns out that the zone between these two limits are relatively small, and the whales are right there in the optimum.
These two explanations complement each other well. Climate change led to a new resource regime that could be efficiently exploited by large bodies, while thermal and energetic constraints acting to balance each other put both a minimum and maximum limit to how large “large” should be.