Population Health and Evaluation:
Reproductive Success
Reproductive success is the single most important parameter regulating long-term population status and so, indices to this process can be quite valuable to a manager. The reproductive process can be broken into two components which provide more interpretive value: reproductive potential and reproductive attainment.

Reproductive potential, the number of fawns which could be produced in a given year, is best measured by counting the number of fetuses present in a sample of ten or more adult (2.5 years of age or older) does. Fetuses can not be measured easily until about 1 month after the breeding season; this technique can not be used with hunter-harvested deer in the Cross Timbers region because harvest seasons coincide with the breeding season. Reproductive potential can also be estimated by measuring the number of ovulation sites on the ovaries. Each ovulation site forms a distinct tissue called a corpus luteum (CL). Although there may be a loss of up to 10% of ovulated eggs that never implant as fetuses, CL counts can be used to estimate reproductive potential for the coming year. Corpora lutea counts can be used only if all does in the sample have completed breeding, which would be sometime in late December for most does. A representation of CL location and tissue morphology is provided in Figure 7.

Corpora lutea counts give an index to nutritional conditions during the 2 months prior to the breeding season. Average CL counts of adults range from 1.8-1.9 for populations in excellent condition down to 1.0-1.3 for populations in poor condition. Usually a doe breeds for the first time as a yearling and will produce only one CL. As relative condition declines, the prevalence of breeding by yearlings can be expected to decline as well. On the otherhand, doe fawns will breed if conditions are optimum, so ovaries from fawns can be checked to provide a fine tuning index under such conditions. An interpretive consideration is that the first time a doe breeds she usually produces one CL and becomes receptive to breeding later than older does. If doe harvest takes place prior to or during the breeding season, then the prevalence of breeding by doe fawns during the previous year can be estimated by recording the prevalence of previous lactation among 1.5-year old does by examining their teats. An Oklahoma deer manager would have difficulty using CL indices because the current firearms hunting seasons close prior to the end of the breeding season. However, a Texas manager could take advantage of CL indices by scheduling doe harvest as late in the deer season as possible, because Texas’ hunting season often runs into early January.

Reproductive attainment, the number of fawns added to a population (i.e.,net recruitment), is measured by estimating the relative success of adult-sized does at rearing fawns. The "fawn crop" or reproductive attainment commonly is estimated by conducting fawn-at-heel counts during fall cruise and/or spotlight surveys. Fawn crop is calculated by dividing the number of fawns by the number of adult-sized does and multiplying by 100. Yearling does are included as adult-sized does because they can not be reliably discriminated based on body size during surveys.

Fawn crop is determined by a host of interacting factors acting on the deer population during the spring and summer as well as the factors affecting egg production during the previous fall. The mother’s nutrition during gestation affects the fawn’s body weight at birth. Heavier fawns stand a much greater chance of surviving the rigors of early life compared to lighter fawns. The newborn’s survival is further impacted by availability of cover for hiding from predators, adequacy of nutrition to support lactation by the doe, and other mortality factors, such as the high rate of tick-related mortality that sometimes occurs in southeastern Oklahoma. Late summer forage quality could affect development and survival, as fawns begin grazing by 6 weeks of age.

Fawn crop is one of the most important population characteristics because it directly impacts long-term stability. For a population to remain at a stable level it must recruit animals into the population at a rate equal to that at which animals are lost due to natural mortality, dispersion, and harvest. If a population of 100 deer loses 10 deer per year to natural mortality, legal harvest, poaching, and accidents, then 10 fawns must be produced and recruited into the population to insure stability. Populations living in small blocks of habitat or in areas where annual weather patterns cause wide variations in habitat quality are particularly susceptible to population instability.

Spotlight surveys are used to estimate fawn crop, as well as sex ratio and population density. Details on establishing and conducting spotlight surveys are discussed briefly elsewhere in this publication. A wildlife biologist with the state wildlife agency, extension service, Soil Conservation Service, Noble Foundation, or a private consulting firm can provide assistance with this technique.

Data collected at the NFWU can be used as an example of how herd health indices can be interpreted and used in making management decisions. Seasonal changes in the relative health of the NFWU deer herd were evaluated by Texas Tech University graduate student Tom DeLiberto using over 25 physiological variables. Most of these variables are not practical for management programs because they require specialized sampling procedures or are costly to evaluate. Those variables with direct interpretive value that are discussed here include: reproductive potential, reproductive attainment, breeding season, kidney fat index, and dressed weight. No reference data are available for antler measurements or other physiological variables for males because only females were sampled in the study.

The NFWU deer herd was in good health during the course of the study. Its reproductive potential was excellent, based on fetal counts: both adult does (greater than or equal to 2.5 years old ) and 1.5-year age-class does exhibited almost maximum reproductive potential, with an average of 1.9 fetuses per doe. The presence of two fetuses in 9 of 10 1.5-year age-class animals indicates that either they had bred as fawns the previous year or conceived two fetuses at their first breeding. Both of these alternatives are indicative of excellent nutritional conditions during the preceeding summer and fall.

Reproductive attainment or fawn crop was 47% in 1985 and 124% in 1986 at the NFWU. Deer densities were considerably higher during 1984 and 1985 compared to 1986. Evidently, the deer population was abundant enough during 1984-1985 to depress the 1985 fawn crop. The lower deer density during 1986 was due to removal of does during the course of the research project combined with the normally heavy hunting pressure on surrounding properties.

The timing of the breeding and fawning seasons was documented by measuring fetuses and extrapolating back to the breeding season and forward to the fawning season. These measurements indicate that breeding took place during November 5 - December 15, with a peak around November 27. This is the period during which the "rut" and all of the behavioral peculiarities associated with it are most obvious. Fawns usually breed several weeks later than older does, so the actual breeding season was longer if fawns were, in fact, breeding. Assuming a 205-day gestation period and the above breeding dates, fawning took place during May 27 - July 10, with a peak around June 22. If the previous year’s fawns were included in the sample, then the fawning season probably would stretch into early August, with a peak during late June.

In addition to the obvious information on the reproductive cycle of the deer herd, analysis of reproductive seasons can aid in the interpretation of other indices. For example, an extended fawning season could be reflected in decreased yearling body weights the following year. Late fawns generally will not weigh as much as early fawns when they reach the yearling age-class.

Fat deposition is a process that is closely tied to the reproductive season of the female white-tailed deer. Late gestation and lactation demand such a nutritional commitment from a doe that she does not put on significant fat deposits until lactation is almost complete. Gestation and lactation by an adult doe increase her energy requirements by 15% and her protein requirements by 46%. These percentages would be even higher for normally smaller fawn and yearling does, because the calculations are based solely on body weight. One negative aspect of breeding by younger animals is the potential impact that the extra nutritional demands might have on the young does’ own physical development. Animals that breed at 6 months of age can be expected to weigh less as adults than similarly aged animals that did not breed until they were 1.5 years of age.

The fat deposition process in the NFWU deer herd was documented using the kidney fat index (KFI). The KFI accurately represents fat levels elsewhere in the body. If severe nutritional stress causes KFI values to drop below 10-15%, then additional requirements are met by utilizing energy stored as fat in the bone marrow. The basic seasonal pattern of KFI was similar during both years of the study (Figure 8). Low fat levels during summer were associated with the increased nutritional demands of lactation and the potentially lower quality of forages available to deer during this season. Fat levels increased during fall, as nutritional demands of lactation faded and acorns and other forages provided additional sources of energy. Fat levels probably peaked sometime between the fall and winter sampling periods, as the adult does prepared themselves for the generally low quality and quantity of forage available during late winter and the pending nutritional demands of gestation during the spring. The lower KFI during fall 1985 compared to fall 1986 probably was due to more intense competition for food associated with the higher deer density during 1985 compared to 1986.

Dressed body weights of adult female deer collected on the NFWU did not differ between seasons at the normal level of statistical significance (Figure 8). This lack of statistical significance is important, because it indicates that the normal seasonal variations in forage quality and quantity on the NFWU were not significant enough to cause major changes in dressed weight. In other words, forage supplies on the NFWU did not decline to the point where they caused significant nutritional stress. The high variability evident in these seasonal samples of 3 to 9 deer emphasizes the earlier point concerning adequacy of sample size.

In spite of the nonsignificant seasonal variation, the apparent annual variation in dressed weight has potential interpretive value. Dressed weights during summer and fall appeared higher during 1986 than during 1985. This probably was due to the negative effects of the more abundant deer population. Herd health, as indicated by the dressed weight index, probably was relatively lower during 1985 and then increased during the second year as decreased competition improved the forage supply for the average deer.

Examination of these physiological indices provided insight into the health of the NFWU deer herd. Each variable lends its own unique contribution to our understanding of the herd health puzzle. As with any puzzle, individual pieces are of limited value until they are integrated into a larger context. For example, lower than expected average dressed weights of 1.5-year old does could be considered indicative of a health problem. However, knowledge of the potential breeding by fawns and the subsequent negative impact on dressed weights of 1.5-year old does, would help explain this apparent discrepancy. Additionally, the full value of any health index will be realized only when several years of data are examined for apparent trends in herd health. Comprehending the applications and limitations of physiological indices to herd health will allow realization of their full value in a management program.