The blue whale—the largest living animal in the world—is the largest it has ever been. Blue whales and other baleen whales haven't always been as large as they are now. It turns out that baleen whales grew this large only relatively recently.
By “recently”, we of course mean two to three million years ago. In the 30 million years of baleen whale evolution, this figure does indeed seem “recent”. Two to three million years ago, the whales began growing to over 10 meters in length. Thus, they've only been as large as they are for about a tenth of their evolutionary history.
There were a few theories on why baleen whales became so large. Scientists thought that maybe this gigantism came about because of falling temperatures in the world's oceans. Another theory points to the evolution of filter feeding or large predators. However, a new study has found what may be the real reason behind the emergence of the largest living animal.
New research has found that a change in the distribution of whale prey in the ocean drove the increase in the size of whales. When ice sheets began growing larger in the Northern Hemisphere, it changed the distribution of krill in the oceans. Before this, krill had been distributed mostly evenly across the waters. As glaciers and ice caps formed, however, krill began to gather in dense patches in different parts of the oceans depending on the season.
This worked to the advantage of whales. Whales ingest krill by filtering them through their baleen. The dense patches of food thus made filter feeding—the way whales feed on prey—more efficient. Plus, the large size of the whales made it easier for them to migrate long distances to where their prey is for the season. It seems that the evolution of the whales' baleen and filter-feeding systems 30 million years ago made it possible for the whales to grow to become giants.
Thus, the blue whale in particular became the largest living animal in the world due in part to the development of its 30 million-year-old feeding system. This feeding system allowed the whale to be able to eat as much as they need while expending little energy.
This study provides insight into how our world's oceans can support such gigantic creatures. However, rapid changes in the world's climate and ecological systems may affect the capacity of the world's oceans to sustain their giant residents. In these modern times, the largest living animal as well as its closest relatives may have to contend with more and more threats to their survival. Shifts in climate, which gave rise to the whales' large size, may now pose a danger to the continuation of their species.
With these changes in climate, will our oceans be able to sustain both these giant mammals as well as a growing human population? Scientists say that we may be able to find the answer in fossil records. However, it's safe to say that if we act on climate change, we might not even have to look for the answer to the question.
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