All available evidence indicates that summer food stress, for whatever reason, can contribute to unusually high newborn fawn mortality that would be mistakenly credited to predation.
Many factors can influence a young fawn’s growth rate, general well-being and prospects for survival. However, the mother’s level of nutrition during her last trimester of pregnancy is probably the most critical factor determining fetal development and survival prospects.
Malnourished does invariably give birth to small, weak fawns that die within a few days; some nutritionally stressed does might even abandon large, seemingly healthy newborns. In either case, scavengers such as black bears and coyotes are quick to secure an easy meal, often leading to inflated predation rate estimates.
The nursing doe’s nutritional requirements are probably equally important. Lactation is considered to be one of the most energetically demand- ing factors of maternal care in mammals and can impact the mother’s fitness, behavior and ultimately the young fawns’ chances of surviving. Unfortunately, this aspect of nutrition in white-tailed deer has not been well studied.
The Northern whitetail’s reproductive schedule is closely regulated by seasonal changes in daylight hours (i.e., photoperiod). Most does breed during a relatively short period during early November and give birth to fawns during late May or early June. This generally assures that fawns are born during times of lush new vegetation, giving them ample time to achieve maximum body growth and fatness prior to winter onset.
Hence, natural selection has minimized the occurrence of poorly timed births on Northern whitetail range. For obvious reasons, the timing of breeding and birthing is less critical and more variable in the South.
Female whitetails live within a highly organized matriarchal society. Related females socialize with one another and share ancestral range during most of the year. In spring, the pregnant doe may range quite widely within her ancestral range. However, does with newborn fawns demonstrate a form of maternal defense, or territoriality, for a period of four to six weeks.
Young whitetails are “hiders.” Within a couple of weeks the newborns are introduced to their mother’s fawning range that generally varies in size from 20 to 30 acres. They seldom leave the boundaries of this range, unless led by their mother. Instead, they rely on inactivity and hiding cover separate from their mothers for their first four weeks, while they are almost totally dependent upon their mother’s milk for sustenance. Meanwhile, the doe must find sufficient food within this relatively small area to meet her physical needs as well as the enormous energetic requirements for milk production.
The doe visits, grooms and separately nurses each fawn only two or three times daily (about eight ounces per meal), then leads them to a new bed site. Siblings normally do not bed together until they are about 25 days old — when they are physically capable of fleeing from predators.
Studies conducted at Michigan’s Cusino Wildlife Research Station during the 1960s, revealed that food-stressed does often exhibited delayed lactation or sometimes produced no milk at all, but food shortage did not reduce the doe’s milk nutritive value. Hence, nutritional stress can adversely impact a nursing doe’s total milk yield (and her maternal behavior) but not milk quality.
Generally, this late-spring, early summer period is a time of abundant resources necessary to satisfy the pregnant doe’s high energetic requirements for late-term fetal growth, subsequent maternal care and lactation. However, there are exceptions. Certain social and environmental factors, which vary regionally, can sometimes negatively impact resource availability for whitetails during the time of lactation and result in higher than expected reproductive failure.
For example, although less of a problem on Northern range, drought has frequently resulted in high newborn fawn mortality in Texas. In South Texas, researchers reported that annual fawn mortality rates varied appreciably, depending upon the amount of rainfall, which in turn determined the quality and availability of deer foods and newborn fawn hiding cover during spring and summer. Fawn losses ranged from 10 percent during years of heavy rainfall to as much as 90 percent during years of drought.
In contrast, on Northern range, winter weather severity and length of snow cover determines deer survival and reproductive success. Even relatively short winters (January-March) of only modest winter food shortage might result in 25 percent newborn fawn mortality. If cold temperatures prevail well into mid-spring, delaying snow melt and subsequent green up, the loss might be much higher.
Typically, open areas lose snow cover about two weeks earlier than forested cover, and provide winter- stressed deer their first source of nutritious forage. However, our stud- ies conducted in Upper Michigan revealed that opening green-up might occur as early as the first part of April, but sometimes not until early May. Likewise, vegetation killing frosts are not unusual in June. Based upon fetal examination, nutritionally related newborn fawn mortality rates ranged from 45 percent during years of late (May 3) spring green-up versus only 5 percent during years of unusually early (April 12) opening green-up.
High deer population density, and associated social factors, might also contribute to limited resource availability for the lactating doe. An expanding deer herd can seriously reduce deer forage quantity and quality. Even at densities of over 100 deer per square mile, our studies showed that deer preferred to feed on spring vegetation instead of rich supplement feed. Unfortunately, as deer numbers increased, the quantity of the best forage species decreased. Conversely, other less nutritious plants increased, probably because they were less palatable, masking the true effects of deer overabundance.
Likewise, because does with newborn fawns defend their fawning grounds, when deer density is high, subordinate (generally younger) does must occupy less desirable habitat where they are more poorly nourished and their young are more vulnerable to accidents, predation or abandonment.
Given nutritional shortage, for whatever reason, Quebec researchers led by Jean-Fancois Therrien suggest the doe’s resource needs might no longer be sufficient to cope with high energy needs of lactation, growth and replenishment of body reserves. If so, they theorize that lactating does should adopt a conservation strategy, ensuring their own future reproduction and survival at the expense of their current newborn offspring. Accordingly, they say, “This trade-off in energy allocation could be expressed through changes in behavior. For example, females might decrease nursing activities and increase time spent foraging when food resources decrease.”
Some prior nursing behavior studies suggested that total time spent suckling by the fawn could be an index to the amount of milk the fawn received. However, more recent investigations indicated this is not the case.
On the other hand, the rate at which offspring solicit their mother’s suckling and rate of rejected attempts by the mother could provide information on offspring hunger and motivation. This could also reveal how other aspects of maternal care, such as the mother’s willingness to nurse, change in response to nutrition.
The Canadian researchers fail to acknowledge that outright fawn abandonment by wild food-stressed does can be a serious factor, ultimately leading to the fawn’s death from starvation or predation.
CAPTIVE DEER STUDY
Using captive deer, Therrien and his cohorts manipulated food availability to simulate nursing doe food shortage during summer. At the same time, they monitored maternal care traits and fawn behavior when fawns have high energy needs for growth and survival and are almost totally dependent upon their mother for sustenance.
Eighteen prime-age (3 to 8 years old) female whitetails were used in the study during two consecu- tive years. All were fed ad libitum diets except during summer, when one group of nine does was fed full rations but the other food-restricted group was fed only 80 percent as much from late May through August. Fawns had access to special feeder troughs and were weighed as often as possible on electronic platform scales.
Starting when fawns were about 5 days old, the researchers observed the behavior of both groups, until fawns were about 80 days old.
The most striking differences in lactation behavior between the two groups occurred during the first 30 days, when fawns relied almost entirely upon maternal milk. Time spent suckling decreased sharply in both groups after 30 days. Although mothers and fawns from the food-restricted spent more time foraging than controls, they obviously could not compensate for shortages in the artificial diet.
Fawns from the food-restricted group also spent more time suckling and soliciting suckling from their mothers than fawns from the well-fed mothers, suggesting they tried harder to get milk. Even so, fawns from food-restricted mothers grew slower and suffered higher mortality.
After the first month, differences between treatment in suckling frequency and total suckling time decreased, probably because all mothers reduced their energy allocation to milk production. However, fawns in the food-restricted group still solicited their mothers and were rejected more often than control fawns, suggesting they were still trying to compensate for milk short- age earlier in life.
Food-restricted mothers also spent 21 percent more time foraging than well-fed does, in an apparent attempt to compensate for energy shortage. This supports the energy conservation theory.
MORE QUESTIONS THAN ANSWERS
Note that this was a behavioral study, conducted under highly artificial penned conditions, the results of which may or may not apply directly to whitetails living in the wild.
This study was also plagued with rather low conception rates (1.2 fetuses/doe) and exceptionally high newborn fawn mortality from the start, despite does presumably fed full nutritious rations during most of the year. Of 42 fawns born, 10 (24 percent) died prior to 3 days of age.
A total of 16 fawns also died during the study, 14 of them in the food-restricted group, all after 16 days of age. Assuming equal distribution of fawns according to the mother’s diet, this amounted to 88 percent mortality, versus only 13 percent mortality in the control group.
Although researchers collected and examined dead fawns, they could not determine if fawns died from disease, starvation, or both. Clearly, however, compared to controls, food-restricted fawns gained 26 percent less body mass from birth to 80 days, and suffered extremely high mortality.
Despite the shortcomings of the referenced Therrien study, the evidence indicates that summer nutrition can be an important factor determining white-tailed deer welfare and productivity. In addition to high (24 percent) newborn fawn mortality at birth, I find it especially interesting that food-restricted mothers suffered such high (88 percent) loss after fawns were 16 days of age. This rather clearly indicates that stressed mothers sacrificed maternal care for their own good.
As I’ve discussed in numerous other Deer & Deer Hunting articles, it’s my personal view that limitations in nutrition — especially during late stages of fetal development and within a few days after birth — can retard fetal development, disrupt maternal care traits, and ultimately lead to abandonment and death of newborn fawns. If study animals in the above mentioned study had not been pen confined, I suspect outright abandonment and fawn deaths would have occurred sooner.
Most importantly, all available evidence indicates that summer food stress, for whatever reason, can contribute to unusually high newborn fawn mortality that would be mistakenly credited to predation.
From a management standpoint, these findings also indicate that certain measures could be taken to minimize such loss. For example, on Northern range, early green-up plant species should be encouraged, as should drought-resistent plants on Southern range. In either case, control- ling deer density is important so that the abundance of preferred plant species is not reduced, or possibly even eliminated, due to overgrazing.
— Langenau, E.F. and J.M. Lerg. 1976. “The Effect of Winter Nutritional Stress on Maternal and Neonatal Behavior in Penned White-tailed Deer.” Applied Animal Ethology 2:207-223.
—Ozoga, J.J., and L.J. Verme. 1986. “Relation of Maternal Age to Fawn-Rearing Success
in White-tailed Deer.” Journal of Wildlife Management, 50:480-486.
— Ozoga, J.J., L.J. Verme and C.S. Bienz. 1982. “Parturition Behavior and Territoriality in White-tailed Deer: Impact on Neonatal.” Journal of Wildlife Management, 46:1-11.
— Smith, W.P. 1987. “Maternal Defense in Columbian White-tailed Deer: When is It Worth It?” American Naturalist, 130:310-316.
— Therriern, J.F., et.al. 2008. “Maternal Care in White-tailed Deer: Trade-off Between Maintenance and Reproduction Under Food Restriction.” Animal Behavior 75:235-243.
— Verme, L.J. 1962. “Mortality of White-tailed Deer Fawns in Relation to Nutrition.” Pages 15-38 in Proceedings of the First National White-tailed Deer Disease Symposium. University of Georgia.
— Youatt, W.G., L.J. Verme and D.E. Ullrey. 1965. “Composition of Milk and Blood in Nursing White-tailed Does and Blood Composition of Their Fawns.” Journal of Wildlife Management, 29:79-84.
— John Ozoga has been D&DH’s top research contributor for more than 20 years. He is a retired deer research biologist.