When an attractive doe saunters by, male deer need to be ready to fight their peers for a chance to mate with her. And to battle other bucks, these ruminants have developed some highly specialized appendages: antlers, which must be both tough—to absorb the impact—and stiff—to win a pushing war.
"It is very difficult to make anything that is both stiff and tough, but it seems that dueling deer solved the problem eons ago," John Currey, of the Department of Biology at the U.K.’s University of York, said in a prepared statement. Dry bone would splinter under such pressure, and previous tests had shown that when wet—like internal bones—antlers underperformed in laboratory strength tests. So how do they do it?
To find out more about these impressive antlers and how they become optimal fighting apparatuses, the researchers collected samples of red deer (Cervus elaphus) bones and their antlers at different times of the year to test in the laboratory. "Antlers look as if they are dry, but no one knew if they really are dry when used in contests," said Currey, who led the research project.
Currey and his team found that the secret to the finely tuned tools is, indeed, in the antlers' moisture content. In the early stages of development, a stag's antlers are relatively moist, but as the year wears on and the velvet covering them wears off, the moisture levels seem to drop to relative humidity outdoors. By the time mating season arrives in the late summer, the males' antlers are quite dry, the group discovered.
"We have determined that during the time of year when antlers are being used in earnest they are nearly as dry as they can get," wrote the authors of the paper, which was published online November 27 in the Journal of Experimental Biology.
Testing 40 millimeter-long samples of dry deer antler and wet deer femur, the researchers found that the antler was nearly as stiff and unyielding to bending pressure as the bone. But its toughness was what surprised them. By applying pressure to the middle of the antler, the researchers found that the deer antler was 2.4 times tougher than the wet bone. And when testing blunt impact, the antler could withstand six-times the force that broke the femur bone.
Debate continues as to whether this species' antlers are "living" or "dead" when rutting begins. But the data presented in this study support the idea that most all growth and nutrient supply has stopped by then, as the authors noted that it is "extremely unlikely" that the antlers "had a functioning blood supply," rendering the antlers fairly dry down to the core.
The authors were most surprised by how well suited the dry antler is to its function. These specially adapted weapons are designed "to absorb the shocks of the original encounter during a fight, yet to have a reasonably high modulus of elasticity and static mending strength to keep the antler from bending too much, or breaking, in the pushing match that follows the first clash of antlers." And better understanding of super-effective materials like these can help engineers and medical researchers develop better, stronger materials in the future.
Image of a male red deer during rutting courtesy of Bill Ebbesen via Wikimedia Commons