Learning motherhood -- a challenge for whitetails
Nutrition Sets The Doe's Reproductive Schedule...
July 01, 2010
|Matriarch does defend the same areas annually, first-time mothers establish fawning grounds close to their mothers, and second-time mothers establish new fawning grounds some distance away. Rick Baetsen photo|
When it comes to raising fawns, experience counts. Compared to maternally experienced does, first-time mothers simply do a lousy job. On average, first-time mothers not only conceive fewer fawns, they also tend to lose a greater percentage of their newborn.
In whitetails, instinct and learning come into play in determining a doe's basic fawn-rearing skills. However, maternal behavior is controlled by certain hormones, such as prolactin and progesterone. Nutritional shortage and social stress can disrupt normal physiological rhythms and contribute to reproductive failure -- more frequently a problem among subordinate first-time mothers.
Most female whitetails breed at yearling age (1.5 years old) and produce their first fawns when two years old. However, malnourished does may not produce their first fawns until they are 3 years old.
In contrast, roughly one-half of the doe fawns living on the Midwest farm lands exhibit an advanced rate of maturity, breed and conceive fawns when 7 to 9 months old and give birth to their first fawns when one year old.
Therefore, various environmental factors can impact doe fawn-rearing success. Unfavorable climate and poor nutrition can delay or prevent breeding. Likewise, poorly fed pregnant does commonly experience impaired fetal development and give birth to stunted fawns that die soon after birth. Even dominance-submissive interactions and resultant social stress can cause physiological problems and reproductive failure.
Regardless of the pressures or problems involved, however, young first-time mothers are more likely to suffer such reproductive consequences -- as our research at Michigan's Cusino Wildlife Research has so clearly shown.
In order to appreciate more fully the trials and tribulations involved in fawn-rearing, one must first understand whitetail social organization. Remember, deer exhibit something referred to as sex segregation. That is, the adult sexes live separately most of the year -- a common trait of all hooved mammals.
Related females share a common ancestral range and live in a matriarchal society, composed of mothers, daughters, grandmothers, great-grandmothers and so forth. These cohesive social groups of related individuals occupy overlapping ranges during much of the year. In contrast, males are inclined to disperse from their birth range and form fraternal or bachelor groups.
Also, remember that such social behavior patterns evolved because they are adaptive -- they improve survival prospects. Therefore, since the whitetail is a prey species, the close alignment of related does on their traditional range must have certain reproductive benefits. It also means, of course, that excessive or insufficient mortality, for whatever reason, could disrupt these social patterns and jeopardize reproductive success.
The Role of Nutrition
The role of nutrition in the reproductive performance of yearling does was brought out rather clearly during our studies evaluating the benefits of supplemental feeding.
Prior to supplemental feeding, about one-third of the yearling does in our square-mile Cusino enclosure failed to conceive and none bore twins; inutero yearling productivity amounted to a mere 0.65 fetuses per doe. With supplemental nutrition, virtually all of them bred, twins were common, and their productivity rose to 1.29 fawns per doe -- even at high herd density while the herd was provided supplemental feed.
Although adult does exhibited relatively low productivity while subsisting upon natural forage few of them were barren. With the improved diet, fetal rates increased 50 percent among 2.5-year-olds and 21 percent among older does.
In addition, before diet supplementation, one-third of the fawns died soon after birth. This death rate was similar to that recorded for penned deer severely malnourished in winter but on good diet during spring.
So, without doubt, nutrition is the primary factor setting the female whitetail's reproductive stage. Beginning at a young age, favorable nutrition is required if females are to maintain good physical development, achieve sexual maturity, have timely conceptions, and give birth to healthy fawns. Otherwise, an entire fawn crop might be lost if does are malnourished throughout pregnancy.
In 1982, I published an article in the Journal Wildlife Management showing that female whitetails isolate themselves and aggressively defend a fawning area (i.e., territory) for about 4 weeks following birth of their fawns -- an idea not previously reported and not well received by some scientists.
Generally, matriarch does defend the same areas annually, first-time mothers establish fawning grounds close to their mothers, and second-time mothers establish new fawning grounds some distance away.
Typically, the matriarch gives birth before her daughters, hence assuring proper alignment of adjacent fawn-rearing territories.
Initially, boundary disputes tend to involve some rather violent, aggressive interactions. This is one time during the year that a doe will readily attack a buck twice her size. Thereafter, some form of scent-marking (possibly via urine and feces) by the territorial doe likely delineates her area -- the bounds of which even the newborn soon recognizes.
It's important to note that first-time mothers establish fawning territories bordering her mothers. Since predation of newborn fawns quite likely played an instrumental role in shaping this behavior, the young mother's fawns probably gain certain survival benefits from this alignment -- possibly even outright protection by the matriarch doe in some cases.
Also, young does tend to be lower in dominance and not so hormonally-charged, as compared to experienced does. As a result, they probably have difficulty establishing territories when competing with unrelated older does and can not easily trespass upon the property of strange does when moving or attempting to retrieve fawns.
Since does defend a fawning territory, generally 10 to 30 acres in size, space is limited at high herd density. When space is contested, the older, most dominant does tend to control favorable fawning grounds. Subordinate, first-time mothers on the other hand are more likely relegated poor quality habitat (or possibly none at all) -- their fawns are doomed.
Once herd density surpassed 100 deer per square mile in our enclosure studies, we saw a sharp rise in newborn fawn mortality among young does. Despite unlimited high quality (supplemental) feed, first-time mothers lost 63 percent of their newborn. By comparison, prime-age does lost only 6 percent of their fawns.
Indirect evidence suggested that most fawns died shortly after birth and that losses were related to territorial behavior associated with fawn rearing. Our best evidence indicated that crowding at peak herd density limited fawn-rearing space and disrupted maternal behavior.
We concluded that heavy fawn mortality resulted either because of mother-infant imprinting failure or outright abandonment of otherwise healthy offspring by socially stressed young mothers. As in the case of nutritional stress, abandonment often arises because of insufficient production of prolactin, a hormone that induces milk production and promotes maternal instinct.
Most newborn fawn mortality occurs during the first 4 weeks. Therefore, proper maternal care during this early period, when the young fawn's chief defense against predators is hiding, is especially critical to fawn survival -- a phase we studied intensively by radio-collaring mothers and their newborn during a 6-year study. Quite by accident, black bears entered the enclosure during 3 of the 6 years.
Compared to experienced does, we found that first-time mothers demonstrated certain poor maternal care traits, especially during the first 4 weeks. For example, first-time mothers and their fawns exhibited large home ranges (46 acres and 27 acres, respectively) compared to experienced mothers and their fawns (27 acres and 14 acres, respectively) during the critical first
This suggests that young does had difficulty establishing an exclusive fawn-rearing territory. Such constant shifting about in unfamiliar habitat probably increased the risk of imprinting failure, led to accidents, disrupted normal maternal vigilance, and generally decreased fawn survival prospects.
Compared to experienced does, first-time mothers also stayed about 60 feet closer to their bedded fawns and moved their fawns shorter distances from one bed to the next (312 feet versus 423 feet for experienced mothers). On average, they also spaced twin fawns closer together (247 feet apart compared to 504 feet apart for experienced mothers), making it easier for alert predators to find and kill one or both fawns.
Wide spacing of siblings, mother-infant separation, and an appreciable distance between consecutive bedding sites during the young fawn's hiding phase are sound antipredator strategies common to all ungulates. Among white-tailed deer such behavior logically minimizes predator attraction and increases the likelihood that one fawn will survive should a predator kill its littermate.
The differences we noted in maternal care traits, relative to doe age, at least partially explains why older does are more successful in rearing fawns and rarely lose an entire litter to predators.
Among the maternal care strategies employed by female whitetails, outright defense of their young may be as important as any factor influencing newborn fawn survival. Prime-age does, in particular, commonly show complex distraction behavior that tends to lead predators away from their fawns. They've even been known to strike at coyotes and domestic dogs in defense of their young.
Such behavior may account for the selection of clearings as bedsites
for fawns reared by prime-age does, a situation where watchful mothers may more readily detect a predator's presence and rush to the fawn's aid when necessary. In this open habitat the fawn also might have a better chance of outrunning an attacking predator. Conversely, first-time mothers more often stashed their fawns in dense cover, probably leaving them to fend for themselves.
Furthermore, because nursing does commonly respond to the distress call of fawns other than their own, first-time mothers and their young may derive some unexpected help from nearby older does in fending off predators.
Regardless of the mother's age, handled or accidentally disturbed fawns traveled 2 to 3 times farther before rebedding when accompanied by the mother. Sometimes, fawns led by prime-age does traveled nearly one-half mile outside of their normal territory when disturbed. However,
fawns of younger does always rebedded within the mother's territory.
As part of this study, I tested the behavioral response of does to a simulated predator threat by alternately depositing coyote urine and distilled water (the control) near bedded radio-collared fawns. Then I relocated fawns the next morning to determine their movement distance between bed sites.
Only the fawns of prime-age does moved much further away in response to coyote urine versus the water (813 feet versus 403, respectively). Consequently, maternal guidance must have accounted for this difference, suggesting that the doe's maternal skills improve considerably with experience.
During our 6-year study, 43 (19 percent) of 226 newborn fawns died before 1 month of age, including 7 (18 percent) of 40 radio-collared fawns. We found 14 dead fawns, 4 were killed by black bears, 4 were abandoned by the mothers, 3 died in accidents (including 2 drownings), 2 were stillborn, and 1 died of unknown cause.
Fawn losses averaged 32 percent for 3 years when bears were present in the enclosure, compared to 10 percent mortality when these predators were absent. In other words, bears apparently killed about 22 percent of the fawns. Interestingly, prime-age does lost only slightly more fawns when bears were present versus absent (17 percent and 11 percent, respectively). However 2- and 3-year-old mothers suffered heavier fawn mortality when bears were present (32 percent and 58 percent, respectively), versus absent (13 percent and 4 percent, respectively).
These findings also hint that outright defense of fawns by multiple does (including the experienced matriarch) lessens predation and lowers newborn fawn mortality rates for first-time mothers. By comparison, the dispersing 3-year-old doe has less chance of such matriarchal support in fending off predators and runs the risk of greater fawn loss.
Other Social Effects
In whitetails, maternal success largely determines the young doe's social standing in the herd. Those who fail to rear their first fawns revert to subordinate yearling behavior by seeking their mother's leadership.
Since such domination has potential physiological and reproductive consequences, I examined the performance of 73 young does in detail during their first and second breeding seasons; 55 successfully reared their first litter whereas 18 failed.
Although there were no differences in physical condition or breeding history among does at yearling age, certain aspects of their performance differed when they were 2.5 years old. For example, although the two groups were equally productivity, the maternally unsuccessful does bred later as compared to the successful mothers during their second breeding season (25 November versus 17 November, respectively). Does that failed to raise their first litter also bore more male progeny the second season, as compared to the successful mothers (64.7 percent males versus 38.9 percent males, respectively).
These differences undoubtedly have adaptive significance. Because a successful 2-year-old doe tends to disperse to new range to raise her second litter, it is important for her to give birth early in the season so as to claim a favorable fawning territory. Giving birth to female progeny also enables her to form her own family group.
In contrast, the unsuccessful 2-year-old doe quite likely remains on ancestral range for another year. The delayed birth of her second litter and tendency to conceive males (which ultimately disperse) would minimize future competition with the matriarch for territorial fawning space.
Nutrition clearly sets the doe's reproductive schedule. Sometimes, however, social factors and learning behavior can be just as important in governing newborn fawn survival prospects.
Our findings indicate that the newborn fawn's bedsite habitat, movement patterns, social and/or spatial relationships, and evasive tactics when threatened by predators are largely maternally controlled. Because a doe's fawn-rearing skills improve with experience, older mothers are more successful than younger ones, especially when social crowding or predation seriously threatens newborn fawn survival. Regardless of prevailing environmental constraints, maximum fawn-rearing success will occur when physically mature, maternally experienced does comprise the bulk of the breeding herd.