Saturday, May 7, 2016
Note: This program first aired on May 7, 2016.
Walking in the woods in late winter or early spring, if you are very lucky, you may happen upon antlers, either singly or in a pair, dropped from one of two local species of Cervidae or deer family, the white tailed deer or the moose. The sexually mature males of each of these species drop their antlers in the winter after the mating season has ended for the year, to be found by gnawing rodents and the occasional and lucky human.
These species produce a new set of antlers every year, in a feat that pushes the boundaries of mammalian bone growth. Antlers are in fact bone, but have two distinct phases of development that are as different from each other as life and death. The first phase of antler development is the growth phase. This starts in early spring on a male cervid. The initiation of antler production in temperate and high latitude cervids is determined by photoperiod, or day length. The length the day is increasing in the spring, and this signal is picked up through the eyes and transmitted to the pineal gland, which regulates melatonin production. Melatonin has a regulatory effect for sex hormone production, and is involved in both the shedding of the old antlers and the production of the new ones.
Antler development begins on the pedicles, bony knobs that develop on the skulls of young male deer and moose. The surface of all bones in the body is covered by a vascular tissue called the periosteum, and the pedicles have a specialized periosteum that is antlerogenic, meaning it gives rise to the antlers. This specialized tissue is similar to some embryonic tissue or stem cells, and can self differentiate into antler tissue in a similar way that certain embryonic cells turn into bone. In fact, this tissue can be transplanted to other places on the deer’s body, and will result in the growth of an antler to the transplant site (so yes, you could make a unicorn deer). The sex hormones or androgens like testosterone seem to have the biggest impact, female deer can be induced to grow antlers if their hormones are manipulated correctly, and in the wild the female deer that are occasionally seen with antlers are actually hermaphrodites (possessing female genitalia but undescended testicles).
When the antlers first begin to grow they are soft and spongy, almost like cartilage. The tissue is mainly water, and the dry fraction of the growing bone is mostly protein with a small amount of minerals like calcium and phosphorous. The antlers are at this stage living tissue, full of blood vessels and nerves, and covered by a thin hair covered skin like membrane called velvet. Antlers grow like onion roots—from the tip, and at an incredibly fast rate, upwards of ¼ inch a day. The frame of the growing antler is constructed first, and is then slowly filled in as the summer goes on. In late summer as the days begin to shorten noticeably, testosterone levels go up and the antlers begin to calcify. The blood vessels that have nourished the growing bone structure die back and the bone dies but stays attached to the pedicles. Once the bone dies, the velvet dies as well, and bucks and bull moose rub the thin skin off, revealing the smooth mineralized mature bony antler underneath. The antlers are used during the mating season as males fight other males for access to females, and females make themselves available to the males of their choosing. After the fall mating season is complete, a hormonally mediated abscission zone forms at the boundary between the pedicle and the antler, and the bone there erodes away to the point that the antler falls off.
The fast growth rate of antlers, combined with the requirement to regrow the antlers on a yearly basis create a huge nutritional and metabolic burden on these animals. It takes a large amount of mineral nutrients, and an even larger amount of calories. Growing a large set of antlers certainly signals age, as the antlers grow larger each year, and also access to the natural resources required to produce them, poor nutrition can show up as poorly developed antlers.
The ancestors of this antler bearing group of animals didn’t have antlers, in fact they had tusks—enlarged canine teeth that they apparently used for the same purposes as antlers are used today. And whether it is a saber toothed deer or an Irish elk with its 12 foot wide antlers, these show that sexual selection, the selective force that deals only with getting a mate, is what drives some of the most remarkable feats of evolution known.
From the Journal of Anatomy regarding ossification of antler tissue in deer: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571559/
More on deer antlers from the Journal of Anatomy: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571559/
About photoperiod, pineal gland and melatonin http://millar.bio.ed.ac.uk/andrewM/Jo%20Selwood%20site/photoperiodism.htm
On the Irish Elk http://www.iwla-rh.org/html/DGIF_articles/deer_antlers.html