Popular hunting literature is filled with claims regarding the attractive powers of certain deer lures. “Estrous urine” lures, in particular, are advertised as being packed full of “hormones” and “pheromones,” reputed to be irresistible to a buck in rut.
Just what is a so-called pheromone? And what do scientists have to say about this intriguing subject, referred to as chemical communication, used by white-tailed deer during the rut?
According to Georgia-based researchers Larry Marchinton and Karl Miller, the communicative odors produced by deer might include urine, vaginal secretions, secretions from certain skin glands and probably saliva. Feces also serve as a means of odor communication in some mammals, and probably does in whitetails, too, but this has not been documented.
Seven types of skin glands have been identified in whitetails that likely play some role in scent communication. These include the forehead, preorbital and nasal glands on the head; the tarsal, metatarsal and interdigital glands on the legs; and the preputial gland on the bucks’ penis sheath.
What’s a Pheromone?
“Chemical signals that relay information among animals are called pheromones,” Miller said. “This term was originally coined to describe chemical sex attractants in insects, but has since been expanded to include any chemical produced by one individual that transfers information to another member of the same species.”
Some researchers reserve the term pheromones for insects and use chemical signals when referring to mammals.
“Whatever the terminology,” Miller said, “these signals include releaser pheromones, which evoke an immediate behavioral response; priming pheromones, which result in a physiological response; and informer pheromones, which relay information but generally do not result in a behavioral or physiological response.”
There are two avenues by which a buck can receive chemical signals deposited in doe urine: One pathway is through the nose and main olfactory system, and the other is by way of what is referred to as the vomeronasal organ. It appears these two systems serve different behavioral and physiological functions.
Odor detection through the main olfactory system is accomplished by receiving airborne chemicals through the nose. Nerve fibers in the olfactory epithelium of the nose carry the sensation of smell along the olfactory nerve to the olfactory lobe of the brain. The odor information is then transmitted to various parts of the brain and processed.
Logically, then, if the chemical signals included releaser pheromones that signal a doe’s estrus condition, the process would evoke a mating pursuit response by the buck.
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The vomeronasal serves a different function. If you examine the roof of a deer’s mouth, you will see a small opening near the center. This is the opening of the vomeronasal organ, which apparently is used to analyze deer urine. In this case, a buck will not only sniff a doe’s urine but will also taste it, oftentimes performing a lip-curl, or flehmen. In doing so, he holds his neck and chin upward at about a 45-degree angle, opens his mouth slightly, curls his upper lip and closes his nostrils. This pumps some urine aroma into the vomeronasal organ for analysis.
However, chemical signals obtained through the vomeronasal organ do not follow nerve pathways to the same parts of the brain as those that enter through the nose.
Instead, Miller and Marchinton said, “Nerves from the vomeronasal organ travel through the accessory olfactory system, in which nerves are connected via a single synapse in the accessory olfactory bulb to a part of the brain called the amygdala, which in turn has direct connections to the hypothalamus. This is important, since the hypothalamus is the part of the brain that controls the reproductive physiology of the deer through the production of hormones.”
In other words, priming pheromones received from the doe’s urine likely pass through the vomeronasal system and have a direct effect on the buck’s reproductive physiology, but they probably do not evoke an immediate behavioral response as in the case of releaser pheromones. This essentially suggests that a buck does not use the flehmen response to determine if a doe is in estrus.
How It Works
Another major difference between the nose and vomeronasal systems, as pointed out by Miller and Marchinton “… Is that the main olfactory (nose) system appears to be used to analyze smaller, more volatile, airborne molecules, and the vomeronasal system to analyze larger, less volatile molecules that are in solution in some liquid, such as urine.”
In other words, the vomeronasal system is used to determine whether the doe is approaching estrus, whereas bucks use their nose to detect volatile chemical compounds indicating that a doe is in estrus.
If so, then bucks and does use the vomeronasal system to ensure reproductive synchrony. Females likely receive the all-important priming pheromones from glandular secretions and urine deposited by bucks at their rubs and scrapes. Bucks receive the critical messages through analysis of doe urine, after inducing them to urinate by using their ritualized mating approach.
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