The magnificent pronged antlers growing from the skull of a male red deer are billboards. Their size reveals his strength and fighting ability to other males, and his health and quality as a mate to females. Similar horns, antlers and crests adorn the skulls of other animals, including giraffes, antelope, goats, cows, sheep, and dinosaurs like Triceratops. They’re all highly visible, as befitting their role in signalling. What use, after all, is a billboard that you can’t see?
But what if that wasn’t necessary? What if an animal had a way of “seeing” inside a rival’s body? Would antlers need to be visible at all?
Pavel Gol’din doesn’t think so. A zoologist at the National Academy of Sciences of Ukraine, Gol’din has been studying beaked whales—an elusive group of mammals known for their incredibly deep dives and their bizarre skulls. They look like someone glued on a variety of crests, ridges, and domes onto the skulls of more vanilla whales. Many of these structures are found only in males, and are signatures of single species. A few are made of spongy tissue; most are made of incredibly dense and compact bone.
Gol’din thinks that these structures are internal versions of a red deer’s antlers. They serve a similar purpose, signalling a male’s size, strength and health. The big difference is that, unlike antlers, these structures are largely invisible from the outside.
But that’s not a problem for the beaked whales. They navigate with echolocation, making high-pitched calls and seeing the world in the rebounding echoes. If they aimed this sonar at their peers, they should be able to distinguish the compact bone of the skull from the soft overlying tissues. They could effectively become living ultrasound scanners, seeing through each other’s heads and visualising the bones within. In this way, they could signal to each other using features that are hidden from sight.
Gol’din’s idea is still a hypothesis, and one that he hasn’t tested. But other scientists are certainly intrigued. “My first reaction was: Wow, what a cool idea! Why didn’t we think about that before?” says Olivier Lambert from the Royal Belgian Institute of Natural Sciences.
Lambert thinks that Pavel’s idea may not apply to all beaked whales, but seems convincing for many species, like the extinct Globicetus and Tusciziphius. “I clearly remember conversations with other colleagues about these crests. We were asking if a part of them could be visible from the outside for visual display. But this is not necessary for such specialized echolocating animals.”
These inner structures don’t wreck the whales’ streamlined bodies, as horns or external ornaments surely would. That’s important given how frequently they dive. With internal antlers, they could get the advertising space of a bus and the profile of a Ferrari at the same time.
Colin Macleod from the University of Aberdeen also likes the idea and has an unfinished paper in a drawer that explored a similar concept. “Pavel seems to have independently come to similar conclusions—always a good thing!” he says. But he notes that deer antlers aren’t just for display; males also use them to fight each other when stand-offs turn sour. Beaked whales may do something similar, using their skulls as weapons. “But because of echolocation, their skull structures can still serve a display function in the process of escalating aggression, despite being internal.”
“The multitude of scratches that appear on male beaked whales—often with just the right spacing to be from the tusks of another male—suggest that they use their teeth for fighting,” adds John Hildebrand from the University of California, San Diego. “Examining the interior of your rival’s skull with sonar may be another way to judge his ability to put up a good fight.”
Both ideas—fighting and signalling—certainly seem more plausible than the alternatives. Some scientists have suggested that the structures weigh the skulls down when the whales dive, but there’s no evidence that the males diver deeper than the females or youngsters. Others have hypothesised that the structures are like brass instruments that help the whales to make calls, but some of them sit in front of the sound-producing organs and would get in the way if anything.
Whales actually evolved from artiodactyls—the family of even-toed hoofed mammals that includes deer, cows, sheep, and antelopes. It’s not far-fetched to think that the same evolutionary forces that outfitted rams with their horns or stags with their antlers were also responsible for the textured skulls of beaked whales.
“But it will be a challenging hypothesis to test, and it may not be logistically possible at the moment,” says Nick Pyenson from the Smithsonian Institution. Museum specimens are few and far between, and “their deep-diving ecology makes them very tough to study at sea. It takes researchers going far off-shore and to remote places to even encounter them,” he says. For example, after scientists first found the bones of the spade-toothed beaked whale, it took 150 years before someone finally saw it in the (dead) flesh.
MacLeod agrees. “They are so difficult to observe that it’s hard to carry out the required observations to actually test them. We might never know for certain exactly how many of these structures are actually used.”
Reference: Gol’din. 2014. ‘Antlers inside’: are the skull structures of beaked whales (Cetacea: Ziphiidae) used for echoic imaging and visual display? Biological Journal of the Linnean Society. http://dx.doi.org/10.1111/bij.12337