Glenn D. DelGiudice

Winter severity, survival, and cause-specific mortality of female white-tailed deer in north-central Minnesota

Knowledge of age-specific survival of deer (Odocoileus spp.) and the underlying causes of mortality are essential to our understanding of their population dynamics. We examined age-specific survival and cause-specific mortality of female white-tailed deer (O. virginianus) in an area where wolves (Canis lupus) recently had become reestablished and assessed the influence of winter severity during 1991-1996 using new survival analysis procedures. Sufficient data permitted rigorous survival analyses on 153 of 179 radiocollared females ≥0.6 years old, whose age distribution remained stable with annual median ages of 5.9-6.7 years old during the last 5 years of the study. Winter severities ranged from unusually mild to historically severe; 84% (81 of 97) of all mortalities occurred during November-May. The median age of deer survival according to the iterative Nelson estimator (INE) was 3.6 years (90% confidence limits [CL] = 2.4, 4.0). The smoothed estimate of the hazard function was a U-shaped curve with an elevated risk of mortality at 0.6 years old, which declined progressively until 5 years old, then increased steadily with the highest age-specific risk of death for deer ≥10 years old. Fits of the Anderson-Gill (AG) proportional hazards model to our data indicated that survival was negatively associated (P < 0.0001) with winter severity (primarily snow depth), but not with site of winter capture or body mass at capture. The risk of death increased progressively over the course of each winter, with maximum risk occurring in winters of greatest snow depth. Estimated life-time mortality rates (90% CL) due to hunting, wolf predation, and miscellaneous causes were 43.3% (29.5, 57.1%), 33.1% (20.1, 45.3%), and 23.6% (13.2, 34.0%), respectively. Within 5-11 years of reestablishment in our area, wolf predation became a leading cause of mortality for female deer, but its importance relative to hunting depended on annual fluctuations in winter severity. Competing risk analyses permitted comparison of age-specific hazards associated with hunter harvest, wolf predation, and miscellaneous causes of mortality. The nature of the observed interaction between the cause-specific hazards and the effects of winter severity appears to validate the practice of regulating the issuance of antlerless deer permits based on population condition. Collective evidence suggests that as agencies formulate management strategies relative to wolf reintroduction or range expansion, the frequency of severe winters, snow depth, deer population goals, and the use of antlerless permits should be primary considerations.

Effects of winter undernutrition on body composition and physiological profiles of white-tailed deer

We examined the effects of undernutrition and recovery on body composition and blood and urinary profiles of 6 captive white-tailed deer (Odocoileus virginianus) between 18 December 1984 and 3 May 1985. Deer were weighed, and blood and urine were collected every 2 weeks from 10 January to 3 May. At weeks 2, 8, and 14, body composition was estimated by the dilution of tritiated water technique and standard predictive equations. Feed intake decreased and cumulative mass loss increased during nutritional restriction. Baseline body composition included 62.1±0.9 (SE)% water, 11.9±1.0% fat, 20.5±0.7% protein, and 4.5±0.0% ash. Percent protein loss was linearly related (r 2 =0.91, P<0.001) to percent mass loss. Peak mass loss from the beginning of the study (12.8±2.0%) occurred at Week 12; estimated protein loss was 12.5%. Fat reserves were 85% depleted from Week 2 to Week 14. Elevated packed cell volume (PCV), serum calcium (Ca), cholesterol, triglycerides, and cortisol; and diminished serum urea nitrogen, thyroxine (T 4 ), urinary urea nitrogen: creatinine and potassium: creatinine were associated with reduced food intake, characteristics reflected initiation of nutritional recovery after nutrition improved. Sequential data collection and the use of a combination of indices in blood or urine will yield the most useful assessments of animal nutrition and condition

A Long-Term Age-Specific Survival Analysis of Female White-Tailed Deer

Abstract We conducted a 13-year survival (i.e., time survived since birth) and cause-specific mortality study, divided into 2 phases (Phase I = years 1–6; Phase II = years 7–13), of 302 female white-tailed deer (Odocoileus virginianus) ≥0.6 years old at capture. The study spanned a period of extreme variability in winter severity (maximum winter severity indexes [WSI] of 45–195) and hunting pressure. Most studies of survival and cause-specific mortality of northern deer have assumed constant survival rates for adults of each sex (≥1.0 yr old pooled) and examined fawns (0.6 ≤ x ≤ 1.0 yr old) separately. We observed U-shaped hazard (i.e., instantaneous risk of death) curves for both phases of the study, indicating that risk of death is highest for younger and older individuals. The estimated hazard for Phase II was generally lower and relatively constant for adults 2–10 years old compared to Phase I, where the instantaneous risk of death began to increase at age 6 years. This difference likely reflected differences in winter severities, associated changes in magnitude of wolf (Canis lupus) predation, and changes in hunting pressure between the 2 phases. The age distribution of our study cohort was relatively stable over the study period. Subsequently, when we included 76 neonates (i.e., ≤0.6 yr old) in the study cohort, the descending arm of the all-causes hazard began its descent at a hazard rate of 2.3 (vs. 1.0 without neonates), clearly demonstrating that the greatest risk of mortality occurs in the first year of life. We compared cumulative survival estimates for these data using the generalized Kaplan–Meier (GKM) and the iterative Nelson estimator (INE), and we illustrate the potential for bias when applying the GKM to left-truncated data. Median age of survival for females was 0.83 years old (90% CI = 0.79–1.45 yr old) using the INE and 0.43 years old (90% CI = 0.17–0.78 yr old) using the GKM. Lastly, we used a simulation approach to examine the potential for bias resulting from pooling adults. These simulations suggest that models using the constructed discrete time variable give nearly unbiased survival estimates and provide support for researchers and managers applying age-specific hazards derived during study periods to determine the reliability of adult age-pooled survival estimates. As indicated by our data, it is important to consider environmental variation and its interactions with natural mortality forces (e.g., predation) and age distribution of the population when setting harvest goals.

Surplus Killing of White-Tailed Deer by Wolves in Northcentral Minnesota

During winters (November–May) of 1990–1991 to 1995–1996, I investigated factors that may predispose white-tailed deer ( Odocoileus virginianus ) to surplus and excessive killing by gray wolves ( Canis lupus ). Each year, 22–75 female deer were monitored with radiotelemetry (total = 172). Forty-one deer with radiocollars were killed by wolves; annual mortality rate in November–May ranged from 3.3% (1992–1993) to 22.7% (1995–1996). Surplus and excessive killing of deer by wolves was observed only during winter 1995–1996 (11 of 17 deer investigated). There was no difference in the number of days elapsed (3–4) between estimated dates of death and ground examination of the kill site during winters of 1990–1991 to 1994–1995 versus 1995–1996; median ages of wolf-killed deer were similar (9.8 versus 7.8 years). Median marrow fat in femurs was lower during winter 1995–1996 (8.0%) than during winters when surplus and excessive killing did not occur (84.1%). Winter 1995–1996 was historically severe with mean weekly snow depths ≥70 cm from late January to mid-March and ≥60 cm through early April. Evidence indicates that nutrition and extreme deterioration of condition of deer is the major link between winter severity (penetrability and depth of snow) and excessive killing by wolves, which may be predicted when snow depth exceeds 70 cm for 4–8 weeks.

Effects of feeding and fasting on wolf blood and urine characteristics

Feeding and fasting trials were conducted with 2 groups (A and B) of 4 gray wolves (Canis lupus) each during January 1980. The groups were fed for 9 days and fasted for 10 days in a cross-over design. Blood and urine samples and weight data were collected every 2-3 days during each trial. Hemoglobin (Hb) concentrations, red blood cell (RBC) counts, and hematocrits (HCT) were elevated in both groups during fasting. White blood cell (WBC) counts, serum urea nitrogen (SUN), triiodothyronine (T3), and insulin concentrations decreased during fasting in Groups A and B. Mean corpuscular hemoglobin concentration (MCHC), serum cholesterol, triglyceride, and iron (Fe) concentrations were diminished in fasted Group A wolves compared to fed Group B. Creatine phosphokinase (CPK) concentrations were elevated in fed Group A wolves. Serum creatinine (C) concentrations were reduced in both groups during feeding. Urinary urea:creatinine (U:C), potassium: creatinine (K:C), and sodium:creatinine (Na:C, pooled Group A and B data) ratios decreased in fasted wolves. Differences were not found between fed and fasted wolves for mean corpuscular volume (MCV), serum cortisol, glucose, calcium (Ca), bilirubin, serum glutamate-oxaloacetate transaminase (SGOT), serum glutamate-pyruvate transaminase (SGPT), alkaline phosphatase, and luteinizing hormone (LH) concentrations, total iron binding capacity (TIBC), and urinary calcium: creatinine (Ca:C) ratios. Analysis of multiple blood or urine samples collected from free-ranging wolves would be useful in enabling researchers and managers to identify the nutritional status and general health of wolves over time. J. WILDL. MANAGE. 51(1):1-10 Examination of the physiological and nutritional status of wild animals via blood and urine chemistries has become widespread. A common starting point for such investigations is to establish baseline values for each species. Year-round values for several blood characteristics have been described for a colony of captive adult gray wolves (Seal and Mech 1983) fed ad libitum. However, wild wolves often live a feast-or-famine existence, so conceivably, their blood values may vary considerably depending on how recently and how much they have eaten. The excreted quantities of metabolic products in urine also might vary. Thus, it is important to compare values of blood and urine constituents between fed and fasted wolves. The objective of this study was to determine the influence of feeding dynamics on blood and urine characteristics of these animals. We appreciate the financial support of the Res. Serv., Veterans Adm.; the U.S. Fish and Wildl. Serv.; the Special Proj. Found. Minneapolis Big Game Club; and the logistical support of the Minn. Dep. Nat. Resour. We also thank M. F. Seal for record keeping and sample preparation, M. D. Lewis, N. Manning, and M. K. Twite for laboratory assays, and L. Glaser and T. Pierzina for assisting with the animal care and handling.

What time is it? Choice of time origin and scale in extended proportional hazards models

The analysis of telemetry data offers many unique challenges due to both the observation process and the complexity of the underlying system (e.g., risk of mortality may be influenced by both age and a wide range of environmental variables). Although semi-parametric proportional hazards (SPPH) models have been proposed for analyzing ecological data, recent applications have failed to address the importance of choosing an appropriate time origin and scale for analysis. We compared models fit to a long-term deer (Odocoileus spp.) survival data set using three alternative survival timescales: age, time since start of study, and time since 6 June (with a seasonally recurrent timescale). Temporal variability in risk resulted from multiple sources (e.g., changes in hunting pressure, winter severity), and the risk of mortality varied nonlinearly with age (highest risk for young and older individuals). Age-varying hazards were represented well using regression splines, but temporal variability was more difficult to model using parametric assumptions. Annual survival estimates using the three timescales differed considerably. The model using a study-based timescale most closely tracked temporal patterns in risk. Given the difficulties in modeling temporal variability using parametric assumptions, we recommend this approach over an age-based or recurrent timescale when using SPPH models to evaluate the impact of large (naturally occurring or experimental) disturbances or to estimate annual age-specific survival rates. Lastly, we discuss the strengths and limitations of SPPH models relative to fully parametric approaches.

Survival, Birth Characteristics, and Cause-Specific Mortality of White-Tailed Deer Neonates

Abstract Understanding survival of and factors that may predispose newborn deer (Odocoileus spp.) to mortality contribute to improved understanding of population dynamics. We captured free-ranging white-tailed deer neonates (n = 66) of radiocollared females that survived severe (Winter Severity Index [WSI] = 153) and mild (WSI = 45) winters 2000–2001 and 2001–2002. Mean dates of birth (26 May ± 1.7 [SE] days and 26 May ± 1.3 days) and estimated birth-masses of 2.8 ± 0.1 kg and 3.0 ± 0.1 kg were similar between springs 2001 (n = 31) and 2002 (n = 35), respectively. Neonate survival was similar between years; pooled mortality rates of neonates were 0.14, 0.11, and 0.20 at 0–1 weeks, 2–4 weeks, and 5–12 weeks of age, respectively, and overall survival rate for neonates to 12 weeks of age was 0.47. Predation accounted for 86% of mortality; the remaining 14% of deaths were attributed to unknown causes. Black bears (Ursus americanus) were responsible for 57% and 38% of predation of neonates in springs 2001 and 2002, respectively, whereas bobcats (Felis rufus) accounted for 50% in 2002. Wolves (Canis lupus) accounted for only 5% of predator-related deaths. Low birth-mass, smaller body size, and elevated concentrations of serum urea nitrogen (26.1 ± 2.6 mg/dL vs 19.3 ± 0.8 mg/dL) and tumor necrosis factor-α (82.6 ± 78.6 pg/mL vs. 2.3 ± 0.5 pg/mL) were associated with neonates that died within 1 week of birth. Even though we did not detect a direct relation between winter severity and birth or blood characteristics of neonates, evidence suggests that birth-mass and key serum indices of neonate nutrition were associated with their early mortality. Thus, managers can make more informed predictions regarding survival and cause-specific mortality of fawns and adjust management strategies to better control deer population goals.

Understanding variation in autumn migration of northern white-tailed deer by long-term study

Abstract Much of our present knowledge of mixed migration strategies of northern populations of white-tailed deer (Odocoileus virginianus) comes from short-term studies, which limits the observed variation of winter severity and thus our understanding of its potential influence on the migration behavior of study cohorts. From 1991 to 2006, we assessed the incidence of autumn migratory versus nonmigratory behavior of 335 adult (>1.0 year old) females, what proportion were conditional versus obligate migrators, the importance of winter severity as a factor affecting the migratory response, and the effect of winter severity and study length on the classification of deer as conditional or obligate migrators and composition of study populations. Annual winter conditions ranged from historically mild to severe. The annual estimated proportion of deer migrating from spring–summer–autumn range to winter range was positively related to winter severity, and the cumulative probability of deer migrating tracked accumulating snow depths as winters progressed. However, the relationship was highly variable, largely attributable to the annual variation in migratory behavior of individuals radiomonitored for 2–7 years. Importantly, because of the variability of autumn–winter weather conditions, we noted that the proportion of deer we classified as obligate migrators was inversely related to the number of years individuals were monitored. Further, the composition (nonmigratory, conditional, and obligate migrators) of the study cohort was strongly influenced by the severity of winter conditions in the year of capture, as well as in subsequent winters of monitoring.

AGE-SPECIFIC FERTILITY AND FECUNDITY IN NORTHERN FREE-RANGING WHITE-TAILED DEER: EVIDENCE FOR REPRODUCTIVE SENESCENCE?

Abstract Population performance of white-tailed deer (Odocoileus virginianus) is driven largely by survival and reproduction. Knowledge of age-specific reproduction would enhance our understanding of population performance and dynamics relative to intrinsic factors, regulatory mechanisms, and their interaction with extrinsic factors. From 1991 to 2002, we examined serum progesterone as an indicator of pregnancy in free-ranging white-tailed deer (0.5–15.5 years old), age-specific fertility and fecundity, and the potential effect of reproductive senescence on population change. We did not detect relationships between serum progesterone concentrations and Julian date, age, or body mass at capture in 41 confirmed-pregnant, adult (≥1.0-year-old) does. Serum progesterone concentrations of 284 females ranging in age from 0.5 to 15.5 years were distributed bimodally with a narrow peak at 0.0–0.4 ng/ml (composed of samples from 46 of 50 fawns) and a broad peak centered at about 3.6 ng/ml. Only 1 (1.8%) of 55 fawns was pregnant, whereas pregnancy rates were 96.6% (112 of 116) for 2.5–7.5 year olds and 98.5% (64 of 65) for 8.5–15.5 year olds. Among adults, the lowest pregnancy rates occurred in yearlings (87.5%), not in the oldest does. Mean estimated fecundity was 1.3 fetuses per doe in yearlings and was 1.8 fetuses per doe in 2.5–15.5 year olds. We observed no evidence of senescence relative to fertility and fecundity in adult female white-tailed deer up to 15.5 years old. Because older does comprise a relatively small proportion of the population, fecundity rates of these females have little impact on population change (λ); however, their ultimate value to the population may be in their life-long reproductive success and associated genetic contribution.

Understanding margins of safe capture, chemical immobilization, and handling of free‐ranging white‐tailed deer

Abstract Improved understanding of the margins of safe capture, chemical immobilization, and handling of free-ranging animals for research and management relies on the documentation and examination of efforts involving various species, study designs, and environmental conditions. During 1991–2002 we had 984 captures and recaptures of white-tailed deer (Odocoileus virginianus), primarily by Clover trap, under a wide range of winter weather conditions and in an area saturated with wolves (Canis lupus). We captured and recaptured 337 radiocollared females (0.5–15.5 years old) 652 times. Total capture-related mortality was 5.4% (35 of 652). The incidence of capture accidents (e.g., trauma-induced paralysis, death) was 2.9%, and mortality that occurred within 14 days of release was 2.5%. Mean time to mortality for this latter group was 6 days (SE = 1.1 days, n = 16); wolf predation within 11 days was the proximate cause of 50% of these mortalities. A priori, we selected immobilization time for analysis by logistic regression to test for a potential effect of capture and handling on mortality while controlling for known risk factors (age, winter severity) but found no significant effects. Additionally, subsequent comparisons of means and standard errors (SEs) showed no differences among numerous aspects of the capture, immobilization, handling, or associated weather conditions. Success in capture and handling of free-ranging deer results in smaller sample sizes of capture-related deaths (i.e., events), which makes it difficult to infer causal relations between environmental variables, handling procedures, and capture-related mortality. The strength of such studies is that they may serve to demonstrate a range of conditions (environmental variables and handling procedures) over which capture-related mortality can be controlled at acceptably low levels.