Deer scientists have known for decades about the powerful influence of light on deer behavior and physical characteristics like antler growth. Early theories about latitude affecting things like rut timing were close – but incorrect. It isn’t latitude but different lengths of daylight at different latitudes that trigger certain cycles. Put a doe in an enclosure with a controlled light source, and you can make her enter estrus by manipulating day length to fool her brain about the season, at any latitude! Put a buck in that enclosure, and you can make him grow and shed multiple sets of antlers in a year with the same trick.
But what if a deer has no way to perceive day length, also known as “photoperiod”? What if a deer is born without eyes, so its brain has no way of receiving cues about photoperiod through the optic nerves? We know the answer thanks to a Mississippi buck born in 1981. Due to a birth defect known as anopthalmia, the buck had no eyes at all.
First 5 Years: Observation
Biologists captured the buck in Copiah County in southwest Mississippi on November 3, 1981, and estimated him to be at least 6 weeks old (A fawn that young in November is not unusual in this area of Mississippi where the rut peak is later than in most areas of the country). Other than missing eyes, the buck was in good physical shape. Dr. Harry Jacobson, now retired, recognized the research potential of the blind buck and brought him to the Mississippi State University Deer Lab for study.
For the first five years of the buck’s life at the MSU Deer Lab, he went through the normal antler cycle of growth, velvet shedding, and eventual antler casting. But each year he shed his antlers a few days later than the year before, and he shed the velvet of his new set even more days later each year. He averaged 378 days between antler casting and 373 days between velvet shedding. The two events grew slightly closer together but also later each year as time went on.
The blind buck was also out of sync with other bucks at the research facility that had normal vision. Those bucks cast their antlers on April 9 on average, but by 1986 the blind buck shed his antlers on May 19. While the average velvet-shedding date at the lab was September 24, the blind buck did not shed velvet until November 24, 1986.
The fact the blind buck was almost growing and shedding his antlers on a normal annual rhythm already revealed something to Harry Jacobson.
The moon cycle has no relationship to the seasons and cannot serve as a reliable cue. Weather doesn’t work either. Only day length is chained to the Earth’s orbit around the sun and thus connected to the ultimate driver of seasons.
“This showed that there’s an endogenous rhythm built in to the animal, genetically,” said Harry. “But that internal cycle requires information about photoperiod change for fine-tuning and to align the antler cycle with breeding dates. That endogenous cycle is still basically a year plus a week or two, so it gets later every year without photoperiod detection.”
A clock knows there are 24 hours in a day, but it doesn’t know what time it is. The clock requires an external input of the correct time, such as you setting the clock, or a cell tower telling your smartphone you just traveled across a timezone boundary. A deer has an internal clock that knows the approximate length of a year, but it needs the input of actual light through vision to set that clock to the correct time.
Interestingly, Harry and his co-researchers also noted that the buck’s molting cycle – shedding his winter coat for lighter summer hair and growing a thicker coat again in fall – shifted later each year in the same pattern as its antler cycle.
Phase 2: Restoring “Vision”
To confirm that photoperiod information would reset these cycles, Harry and veterinarian Dr. Steven Waldhalm devised a test.
In deer with normal vision, darkness stimulates the pineal gland in the brain to produce the hormone melatonin. Human brains also do this, which is why some people experiencing insomnia take melatonin supplements to tell the brain it is time to sleep. In deer, it is the changing daily duration of the melatonin “signal” that helps set the timing of antler growth, testosterone cycles, and breeding dates. To simulate the reception of light, researchers gave the blind buck a melatonin blocker beginning in early 1987.
Immediately, the buck’s antler cycle shifted earlier and became consistent for two consecutive years. Two years in a row he shed his velvet on the same day, November 11, and two years in a row he cast those antlers on the same day, May 5. Though the effect was not the same as the buck receiving actual photoperiod information, the test confirmed that manipulating melatonin levels – just as regular daylight cycles would – could adjust the timing of antler growth.
New types of tests were underway in April 1990 when feral dogs broke into the blind buck’s enclosure and killed it. The buck was 9½ years old when it died, ending the experiment.
Throughout its life, the buck grew unusual antlers, according to the scientific report published in 1992. As you can see in the photo below, the beams swept backward, and the tines projected backward as well, suggesting that vision and light may also play some role in normal antler shape and growth, though I’m not aware of any other research in this area.
What We Learned
What did modern science and deer hunters learn from the blind Mississippi buck?
First, deer have a built-in calendar that roughly approximates a solar year, and in the absence of light, a buck will still go through a nearly normal cycle of antler growth.
Second, it is day length, sensed through cycles of melatonin production, that sets the built-in calendar and tunes it to the seasons. The guidelines for the best local trigger dates are encoded in genetics, which is why breeding dates and antler growth timing varies among deer populations by region.
And that’s really the bigger lesson for many questions about the timing of deer behaviors: It’s photoperiod, period. For antler growth, testosterone cycles in bucks, and estrus timing in does, leading to advantageous timing of the fawn drop, it is photoperiod. It’s not the moon, the weather, or any other human-imagined force of nature.
The timing of the fawn drop is a crucial survival factor in many areas, especially in the North where harsh winter weather can kill fawns born too early or too late. The moon cycle has no relationship to the seasons and cannot serve as a reliable cue. Weather doesn’t work either. Though it certainly gets colder in winter and warmer in summer, unusual weather events like unseasonably cold or hot weather at inconsistent times reduce the reliability of temperature as a timing cue. Only day length is chained to the Earth’s orbit around the sun and thus connected to the ultimate driver of seasons.
Next time you see moon-based rut predictions or hear that cold fronts trigger the rut, remember the blind Mississippi buck. He showed that changing length of daylight tied to the reliable solar cycle is the ultimate clock for deer breeding and antler growth.