Imagine yourself swimming in the dark, your favorite food everywhere within easy reach, but since you can’t see it, catching it is hard – and moreover…you’re never sure if something is eying you for its dinner. Morning comes and you watch hungrily as your hoped for breakfast descends to the depths below, following the line of darkness.
That is the scenario recently put together by evolutionary biologists at U.C. Berkeley (Things that go bump in the night: evolutionary interactions between cephalopods and cetaceans in the tertiary. Lindberg and Pyenson) to illustrate the most compelling theory on cetacean development of sonar. The scientists reason that a branch of ancestral cetaceans switched from grazing on plants to the abundant nautilus type of invertebrate very early in their evolution, and because the nautilus has a large shell that reflects sound easily, these early cetaceans began to use a very basic and rudimentary form of echolocation to catch them. With the help of a few loud clicks, the early whales were able to find their prey in the dark, and even to follow them down to darkness where the nautilus sought refuge in the early morning.
As the nautilus became less abundant in the oceans, the whales began to fine-tune their biosonar in order to hunt other species of squid. These squid are harder to find because they are basically bags of water with just a beak and a structural ‘pen’ to reflect the sonar, so the whales developed more sophisticated biosonar. Those ancestral whales began to specialize on what they hunted, and species divergence occurred rapidly to the species of toothed whales and dolphins that are present today.
You might be wondering how the first early whales figured out how to use echolocation, after all, they had not developed the special hearing system needed to process the sound yet. However, brains are very ‘plastic’, meaning they are able to adapt and make sense of information that comes in a variety of ways. This is shown by an amazing person in this video:
Or this group of visually impaired mountain bikers: (see Mountain Biking with the Blind)
Yet there was one more ingredient that needed to be in place for early cetaceans to make the quantum jump to the high-pitched sonar systems that they went on to develop – a change in the gene structure in the hearing apparatus also independently evolved in bats, as the researchers explain in Convergent sequence evolution between echolocating bats and dolphins:
* Cases of convergent evolution – where different lineages have evolved similar traits independently – are common and have proven central to our understanding of selection. Yet convincing examples of adaptive convergence at the sequence level are exceptionally rare . The motor protein Prestin is expressed in mammalian outer hair cells (OHCs) and is thought to confer high frequency sensitivity and selectivity in the mammalian auditory system . We previously reported that the Prestin gene has undergone sequence convergence among unrelated lineages of echolocating bat . Here we report that this gene has also undergone convergent amino acid substitutions in echolocating dolphins, which group with echolocating bats in a phylogenetic tree of Prestin. Furthermore, we find evidence that these changes were driven by natural selection.
But what about the baleen whales? Do they echolocate too? The next post will look into how those largest of animals manage to find and dine on some of the smallest organisms in the ocean.