Those fierce-looking scales on a crocodile's cranium are actually just cracks in the animal's armor. Researchers have always assumed that the deep lines traversing the face and jaws of a crocodile marked the boundaries between thick scales that covered the animal's head. But those lines are actually simple physical cracks, like fissures in dried-out mud, rather than the developmentally programmed scales that cover the rest of the crocodile's body. Understanding the physical process by which the cracks form illuminates the biology of the crocodile, scientists say—and helps scientists track the interplay between physical and genetic forces during embryonic development.
"This is a completely different mechanism than is normally used in development," says evolutionary biologist Michel Milinkovitch of the University of Geneva in Switzerland who was the lead author of the new paper.
Each scale on most reptiles' bodies develops from a discrete developmental unit, called a scale primordium. When fully developed, the scales generally form symmetrical or regular patterns—many scales share the same size and shape, and scales on either side of the body or head tend to exactly mirror each other. But Milinkovitch noticed while studying other aspects of crocodiles that the so-called scales on their heads didn't form any obvious symmetry.
"I was really surprised by these chaotic patterns of scales when I looked closely at the crocodile's head," Milinkovitch says. "There were all different sizes and different shapes. Comparing the left and the right, they were very different, and comparing different individuals, they were also very different."
So Milinkovitch and his colleagues took a closer look. They first captured high-resolution photos of every angle of the heads of 15 young Nile crocodiles (Crocodylus niloticus) and created three-dimensional models of the heads. Then, they removed all details from the models except the borders between scales, and analyzed the lines using pattern recognition techniques.
"The signatures that we found in the lines all indicated cracking," Milinkovitch says. "Exactly as if we'd done the same exercise with an artfully cracked Chinese pot."
The scientists then followed the embryonic development of crocodiles in nine eggs to see how such patterns could emerge. On the bodies of the growing crocodiles, scale primordia developed as expected. But on the heads, primordia never appeared and the developmental genes normally upregulated during scale formation remained off. Instead, mounds of thicker skin first developed throughout the head. These structures contain receptors known to help crocodiles sense vibrations in the water around them.
Then, in late embryonic development, grooves began to appear. They began as tiny rifts, but soon grew longer and deeper, branched, and interconnected. The patterns of lines were shaped both by randomness as well as the placement of the thicker domes; the cracks avoided these areas. Milinkovitch hypothesizes that this mechanism allows a thick skin to form quickly during development at the same time as the head is still growing.
"This paper illustrates a really interesting concept," says biologist Cheng-Ming Chuong of the University of Southern California in Los Angeles who was not involved in the work. "When we observe biological development, sometimes its genetics that plays an important role, sometimes it's these physical-chemical processes, and sometimes it's a combination."
The findings and the methods could help scientists understand other aspects of development that integrate physical force, Chuong says, such as the formation of fingerprints (which are different in identical twins). It also could shed light on the formation of wrinkles or skin cracks in adulthood. "We need to integrate the genetic research with the physical research to fully understand all this," he says.
"It is likely that with all the attention to genetic determinism, we have underestimated this kind of physical process," Milinkovitch agrees.