Geisler and colleagues Robert Boessenecker, Mace Brown, and Brian Beatty suspect that over time the spacing between the whale’s teeth became filled with baleen hair. As filter-feeding continued to evolve over millions of years, the baleen got longer, the teeth became smaller, and the former spaces decreased.
In ancient whales of the lineage, teeth might have first helped with grabbing prey, but then were recruited for filtering before evolving into the baleen. The overall gradual process of change fits within a broader pattern showing how body parts that evolve for one function are later co-opted for another use. Another example are the feathers of birds. Many paleontologists think that feathers first evolved for insulation, but then were later recruited for visual signaling and flight.
“Baleen is really strange, and if one only looks at living species, it is hard to envision how one would go from a toothed predator to a filter-feeding whale,” Geisler said. “Coronodon shows that teeth can be functional equivalents to baleen; not as effective but performing a similar role. Since baleen whales evolved from toothed whales, then teeth could form a bridge between whales that used their sharp teeth to snag prey and later toothless whales that had baleen.”
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Although the feeding system is very efficient, it comes with costs. The researchers said blue whale calves, and the calves of many other whales, grow at incredibly fast rates. The calves’ suckling therefore puts a major strain on a mother. As such, a whale mom typically has one calf at a time after a long gestation period.
“With a relatively long life and few offspring," Geisler said, "whales have a difficult time recovering from population crashes, like those that occurred during whaling.”