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.
References:
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
