So-called silent estrus - is it the "Chase Phase" trigger?
July 01, 2009
For whitetails, fawn-rearing and antler growth dominate the summer season. Following several months of sexual segregation, early autumn marks a period of behavioral change. It is a time of social integration — a time of mixing and complex social interaction.
On northern range, the whitetail pre-rut starts with velvet stripping, generally in early September. Depending upon latitude, it may last for about six weeks, finally giving way to that brief, hectic period Charles Alshemier has coined the "chase phase," before serious breeding commences.
The pre-rut is characterized by greatly increased deer activity, intense socialization, mixing of the sexes, early stages of signposting, and sparring among bucks. Despite the intensity and complexity of deer behavior at this time of year, however, deer interactions involve highly ritualized and stereotyped patterns of behavior. An atmosphere of tolerance, social order and even predictability prevails, as one senses a strict social system molded by centuries of adaptation to adverse environmental pressure.
By comparison, during the latter part of October, everything changes. Suddenly, almost overnight, bucks seem to go bonkers. Scrape making activity literally explodes, and bucks become intolerant of male companionship as they begin to wildly chase every doe in sight. All semblance of social order tends to vanish — or so it seems — as belligerent bucks scurry about in every direction, day and night, testing every doe they encounter.
Why the sudden change? What is the mysterious force that triggers such excitement and wild behavior, during this brief period we deer observers refer to as the chase phase of the rut? Is it the first breedable doe? Or is there some other signal that forewarns bucks and alerts them to breeding opportunities that soon will develop?
In the Upper Midwest, the whitetail's "breeding window" is relatively narrow, and tightly controlled by decreasing photoperiod. Changes in deer reproductive behavior are abrupt and easily recognized, at least to the trained eye.
If you plot the breeding dates of does in any given northern area, you find a truncated pattern, not a normal curve. That is, the first does will generally breed during the last week of October, or maybe not until the first few days of November. Thereafter, the frequency of breeding escalates rapidly, peaks during mid-month, but tails-off gradually, with some breeding occurring in December, and even January in areas where doe fawns breed.
On northern range, the chase phase occurs at about the same time, year after year — starting around the middle of October. Therefore, given the timing of this annual event, it is tempting to credit the first breedable doe as being the unique force causing bucks to suddenly go berserk. To the contrary, however, I suspect the chase phase is just that: much chasing and testing of does by energetic bucks, but little or no breeding.
There is a phenomenon referred to as "quite heat" or "silent estrus," wherein the female exhibits all the histological and physiological manifestations of estrus, including ovulation, but the mating response (psychological heat) is lacking. This condition is especially common in cows and mares, but has also been reported in elk and deer. Some researchers, including myself, believe this may be the mysterious stimulus that turns whitetail autumn into a frenzy of activity — a couple of weeks before breeding commences.
In other words, a doe experiencing a silent ovulation may emit important pheromones that alert bucks, but she will not exhibit a mating response and will flee from his advances. It takes little imagination to envision the havoc that could develop should several does reach such an attractive, but illusive, state simultaneously.
However, according to Karl Miller, the term "estrus" denotes a period of heat and willingness to mate. Therefore, technically speaking, there is no such thing as a silent estrus. "More correctly," says Miller, "such a condition should be referred
to as a silent ovulation."
Not all whitetail does exhibit a silent ovulation, but some do, generally two or three weeks before successful breeding. The female sex hormone estrogen is responsible for the doe's receptive breeding behavior, but trace amounts of another female hormone, progesterone, are also necessary. It seems as though the need of estrogen is greater in some individuals than in others, and silent ovulation may be caused by insufficient secretion of estrogen by the ovaries.
In some cases, however, stress may cause excessive progesterone production (from the adrenal glands) and block the stimulating effects of estrogen. Hence, a proper balance of hormones is essential to prompt a psychological estrus and the doe's mating response.
Quite by accident, our studies at the Cusino Wildlife Research Station, located in Upper Michigan, revealed that female deer become unusually active the day or two before they breed. Many species apparently demonstrate such restlessness, which is presumably due to increased secretion of estrogen. Young does breeding for the first time, in particular, tend to exhibit long periods of hyperactivity, sometimes lasting 40 hours or more, prior to breeding.
We used this knowledge to conduct controlled breeding studies with penned deer, by monitoring doe pacing activity with mechanical counters and introducing the buck during periods of peak activity to pin-point the time of breeding. This system also allowed us to calculate the estrus cycles of does, and to conduct detailed studies of deer blood chemistry around the time of estrus.
While monitoring hormonal changes in the does reproductive system during autumn, we observed that some does showed elevated blood levels of progesterone 12 to 23 days before their first behavioral estrus of the season. Generally, a slight increase in progesterone secretion is associated with release of eggs from the ovaries (ovulation), whereas high levels occur with pregnancy. However, since the does would not mate, we interpreted these findings to mean that some does ovulated, and in fact had a so-called silent estrus, prior to successful breeding (in November).
Researchers studying a high-density deer herd in northern Ohio also concluded that most does exhibited a silent ovulation in late October or early November. This was followed by a second, true estrus and successful breeding in less than 15 days.
Similar observations were made with black-tailed deer. Based upon histological examination of deer ovaries, it was concluded that does ovulated at intervals of eight to nine days until they successfully bred. The researchers also noted that the first ovulation of the season either consisted of a silent ovulation or did not result in a lasting pregnancy. (Failed pregnancies are sometimes due to insufficient progesterone production.)
Given the physiological conditions involved, the blacktail researchers speculated that an initial silent ovulation might stimulate one or both sexes, thereby helping to synchronize the sexes — bringing both into breeding condition at about
the same time.
While investigating the breeding behavior of whitetails at Cusino, we found that does bred, on average, eight to nine days earlier when confined with a mature buck as opposed to does exposed to a buck only once daily. As I discussed in past articles, the exact mechanisms were unknown, but some type of biostimulation must have been involved. We hypothesized that the unusually early breeding activity was induced by the constant (unnatural) presence of a buck and relatively crowded pen conditions to which they were subjected.
We also theorized that the normally silent first ovulation of the season, which typically occurs one to two weeks prior to breeding, was in fact transformed into a fertile estrus through the effects of biostimulation. However, studies conducted in Georgia, under the guidance of Karl Miller, indicated this probably was not the case. According to Miller, it is more likely that buck-doe interaction or excitation in the Cusino studies actually advanced the does' first behavioral estrus of the season.
The reader should be cautioned, however, not to conclude that breeding is advanced in naturally occurring high-density deer herds. Sometimes the opposite is true. Instead, social stress due to crowding may cause certain hormonal imbalances and delay estrus in young does by as much as two weeks, or possibly prevent breeding altogether — especially when accompanied by overbrowsing, nutritional shortage, and resultant sub-par physical condition.
It's also important to note that not all does exhibit a silent ovulation. In the Cusino studies, for example, only four of 10 does sampled around the time of estrus exhibited blood hormone levels indicative of silent ovulation. (Unfortunately, we did not identify the doe ages.) In fact, research conducted at the University of Georgia suggests only young does, approaching their first breeding season experience a silent ovulation. In most regions of the country these would be one and one-half year old females, but could include doe fawns or older does in some cases.
Studies led by Matt Knox, at the University of Georgia, showed that female hormone secretion the month before breeding varied considerably, depending upon the doe's age. Four of five young does, that had never bred before, exhibited blood hormone values indicative of silent ovulation, seven to 29 days before breeding. Older sexually experienced does did not.
These observations led the researchers to conclude that silent ovulation in whitetails may play an important role when young females achieve puberty. On the other hand, silent ovulation may not be critical in adults as they shift from sexual dormancy to breeding condition during autumn.
Probably one of the most compelling reasons to associate the chase phase with the so-called silent estrus is the dramatic change evident in buck behavior during the two week interval before the first doe breeds. Some stimuli — presumably involving releaser and/or priming pheromones — trigger the wild behavior in yearling bucks as they vacate their birth range, coincident with a sharp rise in formation of new scrapes by mature individuals. But even in the absence of older bucks, yearlings in the Cusino studies suddenly began to make scrapes at this time.
The important pheromones that signal a doe's pending breeding condition are present in the mucus of her vaginal secretions. These attractants are carried with her, of course, but are also deposited in her urine. The exact chemical compounds involved are unknown. However, recent studies led by Jemiolo, from Indiana University, and Miller, indicate that certain volatile compounds in vaginal secretions and doe urine are dependent upon ovarian hormones, and are present only during
the estrous period.
For the first time in scientific literature, Jemiolo and her co-workers documented that the composition of volatile chemical compounds emitted by female whitetails changes with the stage of their reproductive condition.
According to the investigators, "Estrous females discharged (vaginal) mucus that was richer in the characteristic volatiles than that of females [not in estrus]. Nine of 14 characteristic compounds (64%) were produced only by estrous females, whereas only two (14%) were produced by females [not in estrus]. Certain aromatic hydrocarbons, whose presence may or may not be significant, along with two alcohols and two ketones occurred only in estrous vaginal samples. Since estrous vaginal secretion, but not mid-cycle (non-estrus) secretion, has been shown to elicit courtship behavior in male white-tailed deer, these compounds may represent behaviorally important chemosignals."
Jemiolo and her co-workers emphasize, however, "Further biological and behavior research is needed to determine which chemical compounds identified in this study (if any) are important in communicating sexual/reproductive status."
Therefore, it seems logical that certain of these chemical compounds function as pheromones signaling estrus to the buck. Also, since these compounds are dependent upon ovarian activity, the same signaling agents could be present during silent ovulation as well — only additional research will clarify this point.
To my knowledge, the behavioral response of bucks to does experiencing silent ovulation have not been studied.
There also seems to be some uncertainty whether adult does exhibit silent ovulation. If such an estrus warning system is linked solely to young (pubertal) does, then range nutritional conditions and sexual maturity rates could also be important factors governing the intensity and timing of pursuit behavior by bucks, during the period we call the chase phase of the rut. Poor range and slowed maturity could contribute to decreased rutting activity, even in the presence of mature bucks.
While I'm not an advocate of the "second rut" theory, there are some important considerations here that may justify the position of those who support such theory. That is, at least here along the southern shore of Lake Superior, I see no such thing as a second rut — the rut is intense, brief, and simply dwindles away during late November. In milder environments, however, breeding patterns could be very different.
If important pheromones (whatever their source) accompany the silent ovulation in young, pubertal does, then there may be additional late season excitation signals that rekindle the pursuit response of bucks. Maybe those who hunt more southerly areas, where a fairly high percentage of the doe fawns breed — generally in December — in fact, witness a resurgence of chasing behavior during late November or early December, when doe fawns would be expected to exhibit silent ovulation.
Further, given the lateness of the season, and potential for stress
brought on by unfavorable environmental conditions, or other factors, some doe fawns might exhibit silent ovulation (during late November or early December) but never achieve behavioral estrus. If so, such a situation could cause unusual late season excitement among rutting bucks,
despite minimal breeding activity.
Currently, there is no concrete evidence supporting my contention that important pheromones accompany silent ovulation in white-tailed deer. But circumstantial evidence certainly points in that direction. If
some does consistently exhibit silent ovulation each autumn, prior to the main breeding season, and important chemical signals accompany this physiological change, then it's my guess that such a condition is the magical force triggering the chase phase of the whitetail rut.