Whale lungs are so large and specialized that they present their own quandaries. They must be able to collapse quickly enough to avoid rupturing when the whales dive deep (as some toothed whales do), but also to reinflate rapidly at the surface after two hours underwater. Blue whales don’t dive anywhere close to the depths you’d expect for their body size. In part it’s because their prey live near the light, but it also seems that it takes too much energy to breathe all the oxygen necessary for a deeper plunge.

As organisms scale up, physics dictates what’s possible for any kind of movement and function, be it blood flow, digestion or locomotion. Sauropod dinosaurs, for example, had limbs like columns to support their massive weight, yet their load was most likely lightened by an avian-like respiration system, which permeated their skeleton with air sacs.

Whales obviously haven’t had to deal with the force of gravity since they became fully aquatic; underwater, they are essentially weightless. Instead, forces such as drag have shaped their bodies, especially when feeding. When scientists used allometry to calculate drag on mathematical models of different-size whales, they found that beyond lengths of 110 feet a blue whale would not be able to close its mouth fast enough around quickly escaping prey. Others have found that a whale that big wouldn’t gain enough calories from the mouthful to make up for the energy lost from the act.

In other words, the largest whales ever measured, at 109 feet, are theoretically the largest whales that can exist.

Of course, physics isn’t the only factor imposing limits on these leviathans. Whaling is estimated to have killed nearly three million whales in the 20th century alone.

Human hands have imperiled other cetaceans. Not a whistle or splash of the Yangtze River dolphin has been recorded since the first decade of the 21st century. Responsibility for the extinction of this species can be placed squarely on our shoulders: We dammed the only river in which it lived. Other species such as the vaquita, a small porpoise that has never been spotted outside the Gulf of California, remain on the extinction watch list; there are only one or two dozen left.

The news isn’t all dire: Some whale species, such as humpbacks, have rebounded from the brink; gray whales, icons of the West Coast, are even expanding to new habitats as climate and oceans change.