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It once was commonly believed that temperature controlled the antler (and breeding) cycle of the deer family and most other species. A study in Poland in 1954 was the first to demonstrate that the antler growth cycle was controlled by change in daylight hours throughout the year (photoperiod). Shortly after the start of antler growth in spring, a group of red deer (a very close relative of our elk) were shut in a dark building from 4 p.m. – 8 a.m. each day. This change from the lengthening daylight hours of spring to the daylight hours of winter caused the antlers to stop growing, harden, and shed their velvet that summer. The deer soon became so belligerent it was impossible to make them go into the building, so they remained outdoors with long summertime day lengths. The deer then shed their stunted antlers and grew a second set that same year.
The finding that deer antlers are so strongly affected by photoperiod has resulted in a wide variety of studies. Many of these involved sika deer and unheated barns with electric lights controlled by a light switch. When day length was made to grow longer in winter and shorter in summer (phase reversal), the deer followed the light cycle, not the temperature difference between winter and summer. This only applied to those deer undergoing antler replacement as lighting conditions did not affect a fawn’s first set of antlers. However, when the deer were born under reversed lighting conditions, they grew their first antlers at 1 year of age despite that the artificial day lengths were decreasing. They then replaced these antlers at age 1 ½ years rather that at age of 2 years. Their antler cycle was then in synch with the other deer. This demonstrated that day length controls the shedding of old antlers and the growth of new antlers regardless of a deer’s age. Many years ago, a number of deer species (including moose) were relocated to New Zealand. After a short time, they adapted to the reversed seasons of the southern hemisphere.
Frequency modification, which involves increasing or decreasing the number of days to complete one calendar year has also been studied. By skipping every other day’s light-dark cycle, a year’s light-dark cycle was complete in only six months and the deer grew and shed two sets of full-size antlers in one calendar year. When a year’s light-dark cycle was complete in four months, they grew and shed three sets of ½ size antlers in one year. At three-month cycles, the antlers barely started to grow before being shed and after two cycles they skipped one cycle before synchronizing again. The minimum time needed for an antler cycle is probably close to three months. At 24-month cycles, adults replaced their antlers at 12-month intervals, but yearlings replaced their antlers at 24 months. No extra-large antlers were grown.
When the annual light-dark cycle was made to take only two months the deer did not respond, they grew new antlers the following year when it was spring outside. This was the first evidence that deer may not need photoperiodic stimuli to keep track of one year of time.
We are used to the long day lengths of summer and shorter day lengths in winter. However, near the equator there is very little difference in day length duration throughout the year. As a result, these deer grow antlers, breed, and give birth year-round with no synchronization. Males replace their antlers at 12-month intervals, but which month is determined by in what month the deer was born. Other unusual (to us) details include records of some individual tropical deer keeping the same set of antlers for three years. Among the variety of deer species in the tropics are three genera (Axis, Rucervus, and Rusa) which evolved in, and never left, the tropics. These deer are unable to synchronize their breeding and antler cycles to that of northern deer even if they are transported to the boreal zone.
A number of studies have examined the reaction of temperate zone deer to tropical zone photoperiods.
When these deer were subject to constant 12-hour light/12-hour dark cycles, the results depended on when the experiment was started. If it started after the winter solstice, it did not affect the timing of shedding of old antlers and growth of new antlers. If it started in the fall, many did not replace the antlers the next year. In both cases, some did not replace the antlers for 3 to 4 years. However, deer held under constant 8 L/16 D 16 L/8 D, or 24 L/0 D replaced their antlers at irregular intervals, but not in phase with each other. These antler cycles averaged about 10 months in length. In light of these constant light cycles resulting in antler cycles, it is unknown why the deer under constant 12 L/12 D did not replace antlers.
How unequal do light and dark periods need to be to result in antler cycles? Experiments begun after the fall equinox found that yearlings did not replace their antlers the following spring when held under constant lighting of 12 L/ 12 D, 12 ¼ L/11 ¾ D, 12 ½ L/11 ½ D, and 12two¾ L/11 ¼ D. However, when held under constant 13 L/11 D, they did replace them. Apparently, there must be at least a two-hour difference between the lengths of the light and dark periods for deer antler cycles to be in synchrony.
The above study states the latitude at which the day length at the two solstices is 11 or 13 hours is approximately 18 degrees from the equator. Mexico City is at 17 degrees North latitude and white-tailed deer do replace antlers at the same time as in the north. In northern South America there is no synchrony of antler and reproductive cycles. In Honduras (13 to 16 degrees North latitude) there is hint of synchrony as most deer breeding is between July and November. The study suggests the change from asynchrony to synchrony is at about 14 to 17 degrees North latitude, but it is unknown if this is gradual or abrupt.
The next column will deal with nervous system regulation of antlers and antler asymmetry.
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