Introduction to the Flehmen Response Across Wildlife Species

Have you ever seen a white-tailed deer buck chasing a doe, or a bull elk pushing around his harem of cows, or a bighorn ram sniff the urine of a ewe? You’ll often see them stop and make a similar face across species and then get a sudden burst of energy with a focusing interest in the females. This wide-eyed smiling appearance erupts when the male’s upper lip curls up as he inhales through his mouth. This is the Flehmen response, sometimes called the Flehmen posture. But what exactly is it and what does it mean?

What is the Flehmen Response and How Does it Work?

Asking the question– “What is the Flehmen response?”– is basically asking– “What is the rut?”– because they are so intertwined. The Flehmen response is a physiological process by which an animal smells pheromones and hormones, causing the animal’s upper lip to curl up while orally inhaling to funnel air filled with pheromones and hormones to the vomeronasal organ, otherwise known as the Jacobson organ. The Jacobson organ is a sensory organ located in the roof of the mouth near the throat. Both male and females can exhibit Flehmen behavior but it is much more common among males, especially among hooved species. When a female is hot (in estrus), she releases hormones and pheromones that indicate she is ready for breeding from her genetalia, feces, urine, and other excretory pathways. These pheromones and hormones are interpreted by the male’s Jacobson organ, which then triggers the males libido (male sexual reproductive drive). The burst of energy that follows the Flehmen response is the physical manifestation of the sex drive and causes the male to seek out the female with the lone goal of reproducing offspring. This is the rut.

Will harvesting “spike” white-tailed deer help grow bigger bucks? The foundation of many wildlife management plans are often built on one side of the other of the argument with solid reasoning either way. This age-old question among white-tailed deer hunters, ranchers, and managers, is actually a question of two parts– 1. are spike white-tailed bucks genetically inferior and, if so, 2. will culling spike bucks improve the antler size among the white-tailed deer herd. To thoroughly but briefly deconstruct the sides, we must clearly define some terminology, clarify some misnomers on the topic, and breakdown when and how the reasons ‘to cull or not to cull’ situationally apply.

Using the Texas Parks & Wildlife Department definition, a spike in the case of this writing is a white-tailed deer buck with at least one unbranched antler.¹ Our definition of an unbranched antler is a single beam or tine and does not include “cactus”, perruque, or broken antlers. We are excluding these from our criteria because cactus or perruque antlered bucks are often associated with testicular and/or testosterone related issues and broken antlers are often environmentally related (fighting, hit by a car, etc.). Deer with antlers that have not broke through the skin of the scalp are often called “nubbin” or “button” bucks are considered antlerless, not spike bucks. Culling is the act of selectively removing an animal from the population as a means of controlling or removing undesirable characteristics (bad genetics, diseases, etc.). And lastly, just so we’re clear, bigger bucks is not indicative of body size or mass… Although good body condition can be an indication of good health, our definition of bigger bucks is white tailed deer with larger, better scoring bone (antlers) on their heads.

Myths

Now to dispel a couple of myths. One common misconception about white-tailed deer spikes is this idea that “once a spike, always a spike”. This is simply not true; not that a spike couldn’t be a spike it’s whole life, but most in fact are only a spike for their first set of antlers and progressively get larger, better scoring antlers each year with age and nutrition.² A study in south Texas conducted by Stephen F. Austin State University researchers found that the average spike with 3 legal points or less at 1.5 years old will be at or above the herd average number of points and Boone and Crocket score by 3.5–5.5 years old.² Another major misconception is that all (or most) spikes are caused by poor antler genetics. The genetic aspect certainly plays a role, but in actuality, there are numerous reasons that can cause spike antlers among white-tailed deer at any stage of life. This may include injuries or infections at the antler pedicle (the base of the skull where antlers regenerate from each antler growing season) from fighting, motor vehicle collisions, or casting previous year’s antlers, opposite hind limb injuries, nutritional deficiencies, antler breakage, testicular injuries or infections, testosterone issues, harsh weather conditions during buck fawn gestation, or simply age.^1,3,4 In fact, a large portion of single spike-antlered bucks (one antler has branches and one is a spike) are caused by good ol’ nature and nurture (environmental and/or health) rather than genetics.

One study conducted in Alabama found that 62% of single spike-antlered bucks were caused by external factors like those described above.³ Furthermore, 92% of white-tailed bucks with two spike antlers are a yearling (about 1.5 years old) with their first set of antlers.¹ But, just because most double-sided spike bucks are yearlings does not mean most yearlings are spikes; only about 25%–33% of yearlings are spikes.¹

White-tailed deer are an icon of Texas hunting and are the most sought-after big game animal in the state (and the world for that matter). This is mostly due to the sheer prevalence of white-tailed deer and deer hunters– a result of the success story of white-tailed deer management which brought the species back from a couple-hundred-thousand in the early 1900’s to well over five million currently in Texas⁵. Part of effective white-tailed deer management relies on understanding the animals, both the requirements to grow healthy deer and the struggles the animals face that inhibit them from reaching their full potential (antler growth, reproductive success, body weight and composition, etc.)

The three things that are required to grow healthy deer with great antlers is genetics, age, and nutrition. Though respectable deer can be grown and managed with two of the three criteria, it takes all three to make an exceptional deer herd. Nutrition may be the most important and influenceable factor of the three. It is the most influenceable because any ranch can improve the availability of nutritious feed; some ranches do not have the opportunity to change the existing genetics and low fence properties cannot control the age of the deer their neighbors harvest. The reason nutrition is also most important is studies have shown that the effect of maternal nutrition on a fawn in utero (while the fawn is in the doe’s womb) can be more impactful on that deer’s future antler growth than any amount of nutrition after birth^2,4. This means that a ranch with malnutritioned or undernourished does during the pregnancy period between early spring and late summer could result in an entire generation of nutritionally stunted fawns; fawns that will grow up to be does that are smaller in body size with a lesser reproductive success, and bucks that are unable to grow trophy antlers.

Another important aspect of good “off-season” nutrition for a doe is with regards to the lactation (milk production) period. Lactation is arguably one of the most physiologically stressful and demanding periods of a female deer’s life. It occurs in Texas when temperatures are frequently and consistently in the triple digits and the amount of available nutritious forage is often dried and declining, forcing deer to shift to second and third choice browse selection. If a doe is unable to satisfy the dietary requirements for her own survival, she may even abandon her fawn (or likely fawns since white-tailed deer often have a fawning rate of 70+%). Increased fawn mortality means a greater number of carcasses on the landscape, which has been shown to increase coyote activity³, and may further exacerbate fawn mortalities by promoting opportunistic predation.

Spring and summer nutrition is also directly important for bucks. Some of the most nutritionally and energy demanding times for bucks are during the antler growing period, which occurs immediately after the breeding stress of the rut. Bucks can go days without food or water while trailing a doe in estrus, burning an amazing amount of unreplenished calories to sexual competition and reproduction, leading to dramatic loss of weight and body condition.