Starting out the size of a hippo some 50 million years ago, whales have since evolved into the largest animals on Earth. But their growth wasn’t steady over the millennia; instead, filter-feeding whales like the blue whale only ballooned in size starting about 2.5 million years ago. Whales’ grass-gobbling relatives, such as sea cows, also expanded in size during this time.
The similar pathways to enormous size among whales and sea cows provide new insights into the history of the ocean’s food supply, according to paleontologists Nick Pyenson, curator of Fossil Mammals at the Smithsonian National Museum of Natural History, and Geerat Vermeij, distinguished professor of paleontology at the University of California, Davis.
Pyenson and Vermeij tracked marine mammal size as a barometer of ocean productivity and food availability. The idea is that as food availability goes up, so does size. “I think that maximum size is as good a proxy for ocean productivity as we’re ever going to get,” Vermeij said.
The rise of large whales, sea cows and other leviathans corresponds with the start of Earth’s most recent glacial epoch, Pyenson and Vermeij reported July 5 in Biology Letters. The researchers suspect glacial runoff — sediments scoured off the land by glaciers — sparked a significant increase in ocean productivity, which launched whales and other marine mammals to massive proportions. This finding correlates with previous studies by Pyenson, Vermeij and others, which found similar timing in patterns of body size expansion in marine invertebrates in nearshore and coastal habitats. “To me, this is an astounding result,” Vermeij said.
In the same study, Pyenson and Vermeij also discovered a size discrepancy among large marine herbivorous mammals, the Sirenia and Desmostylia. The herbivores in the northern Pacific Ocean are larger than their counterparts in the Atlantic Ocean. The researchers attribute the size difference to seaweed. Large, fast-growing kelps in the Pacific Ocean provide more food for grazers than the Atlantic Ocean’s slow-growing seagrasses, the researchers said.
Not only was there abundant food in the Pacific Ocean as grazers grew in size, these aquatic plants were defenseless against predators. In fact, most aquatic plants lack the arsenal of defenses developed by land plants, Vermeij reported in a separate study published April 18 in the Annals of Botany.
The contrast between land and aquatic plant defenses leads to many interesting questions, he said, including how animals, in addition to herbivores, shaped the evolution of plants. For example, spines, hairs and chemical and visual signaling are common in land plants but rare in their aquatic cousins. Vermeij attributes the differences to predators and pollinators, both of which often rely on long-distance visual and scent cues on land. “It seems to me that animals, including herbivores, have an enormous amount to do with plant defenses,” he said. “I would argue that plants influence herbivores less than herbivores influence plants.”
— Becky Oskin, content strategist in the UC Davis College of Letters and Science