Growth rate and body size at maturity are important life-history traits of interest because they represent a potential source of fitness variance within a species and provide information for understanding the nutritional condition, fecundity, and dynamics of populations. My objective here was to examine the growth rate and body size at maturity of Florida black bears Ursus americanus floridanus using body length, chest girth, and body weight measurements fitted to the nonlinear von Bertalanffy, Gompertz, and logistic size-at-age growth functions. The von Bertalanffy model had the largest Akaike weight, indicating the best fit for all measurements of both sexes. Growth models showed that females grew more slowly, with a younger age at maximum growth, faster rate at which maturity was reached, and attained significantly smaller asymptotic body length, chest girth, and weight than males. A more conservative growth strategy by females to invest available energy resources to costs of reproduction, together with intrasexual selection among males for larger body size to enhance intimidating and fighting ability to increase reproductive and survival success, are implicated as determinants of the male-biased direction and degree of sexual size dimorphism. In both sexes, the presence of human food in the diet increased the asymptotic body weight from the estimate for bears consuming a natural diet, but differences were insignificant. Females consuming human food had a slightly younger age at maximum growth and continued growth in body weight for a somewhat longer duration than did conspecifics that consumed a natural diet. In contrast, males that consumed human food had a slightly older age at maximum growth and decreased body weight growth somewhat earlier than did conspecifics consuming a natural diet. Florida black bears exhibited a larger asymptotic body size, faster growth rate, and younger age at maximum growth and maturity when compared with conspecifics in other mainland populations. Recognition of Florida black bear growth rate and adult body size provides wildlife managers a foundation for implementing measurable criteria to assess trends in population health.

Growth rate and body size at maturity are important life-history traits invariably linked to characteristics of the environment and reproductive fitness (Stearns 2004). For example, regional differences in the quantity and quality of available food resources contribute to variation in a species' nutritional condition and growth patterns (Mahoney et al. 2001; Vaughan 2009; McLellan 2011). Duration of the immature stage is determined by juvenile growth rate and minimum body size at reproductive maturity, so individuals with faster maturing rates will be able to begin breeding sooner than those with slower development (Stearns and Koella 1986). Growth rate and body size at maturity also have a strong influence on reproductive fitness; a short juvenile period is beneficial because it reduces the risk of mortality before reproducing, whereas a larger body size is generally correlated with greater competitive ability and higher fecundity (Blueweis et al. 1978; Clutton-Brock and Harvey 1983; Stearns 2004). However, at some level a trade-off between growth rate and body size at maturity is expected since it will take longer and more energetic resources to grow to a larger body size (Stearns and Koella 1986). Variation in growth rates and adult body sizes will be determined by inevitable differences in the quality of the environment and individual health that in turn depends on factors such as food availability, population density, competition, and weather, which then may result in different optima in different populations (Clutton-Brock and Harvey 1983; Lindstedt and Boyce 1985; Stearns 2004). Determining the length of time required to grow from birth to reproductive maturity is important for wildlife managers because population dynamics are generally more sensitive to this life-history trait than to variation in any other (Williams et al. 2002). Therefore, growth rate and body size at maturity are of interest because they represent a potential source of fitness variance within a species and provide information for understanding the nutritional condition, fecundity, and dynamics of populations.

The American black bear Ursus americanus Pallus is a medium-size bear native to North America, and is widely distributed in forested areas throughout subarctic, temperate, and subtropical regions from Alaska and Canada to central Mexico (Scheick and McCown 2014). Food habits, growth, and reproduction characteristics of black bears have been studied in several different geographic areas. Specifically, the black bear has a low-protein diet concentrated on vegetation, and food habits vary depending on geographic location and season (Maehr and Brady 1984; Maehr 1997; Roof 1997; Vaughan 2009; McLellan 2011). Sexual size dimorphism is pronounced, with adult males growing larger in body dimensions and weight than females (McRoberts et al. 1998; Mahoney el al. 2001; Bartareau 2011; Bartareau et al. 2012). Individuals consuming abundant dietary protein (Mahoney et al. 2001) and high-calorie human-related foods (McLean and Pelton 1990; Johnson et al. 2017) are generally heavier than conspecifics utilizing mainly natural and vegetation diet (Beckmann and Berger 2003; McLellan 2011). Seasonality in precipitation and temperature also leads to changes in food resource availability and thermoregulation energy demands that influence growth patterns (Lindstedt and Boyce 1985; Kennedy et al. 2002). In subarctic and temperate climates, where winter temperatures are extremely low and individuals undergo an extended period of food stress, body weights can fluctuate widely between seasons because fat deposited during spring to fall is then metabolized for energy during the winter dormancy period (Noyce and Garshelis 1994; Samson and Huot 1995; Vaughan 2009; McLellan 2011; Bartareau et al. 2012). In subtropical climates, where winter temperatures are mild with a shorter period of food stress, body weights may be larger and less variable because bears actively forage for a longer period and thermoregulation energy demands are lower (Maehr et al. 2001). The rate of growth and adult body weight is a particularly important measure of an individual's health (Noyce and Garshelis 1994), and serves as an indicator of spatial and temporal differences in food habits (McLean and Pelton 1990; Mahoney et al. 2001; Vaughan 2009; McLellan 2011), nutritional condition (Cattet 1990; Cattet et al 2002; McLellan 2011; Bartareau et al. 2012), and has a pivotal role is assessing fecundity (Beecham 1980; Elowe and Dodge 1989; Stringham 1990; Samson and Huot 1995; Kovach and Powell 2003; Costello et al. 2009). Quantifying variation in black bear growth patterns under natural conditions, therefore, has implications for assessing individual and population health. Intraspecific variation in growth rate and body size at maturity indicates that it is necessary to obtain specific estimates for distinct populations or even among different food resources frequented by the same population.

A practical method for estimating the growth rate and adult body size of a species under natural conditions is the use of body size-at-age measurements fitted to algebraic models that describe how individuals grow as a function of age (Zullinger et al. 1984). This approach gives one the opportunity to interpolate nonobserved measurement intervals and describe consistent changes in the underlying growth processes. Growth models are also useful in wildlife research because they summarize growth data with uniform model parameters that provide quantitative indices for relative growth rate and asymptotic body size. These variables can be used to reveal variation caused by the environment and facilitate comparison between sexes and among populations, or variation within the same population through time. Historically, the most common practice in black bear growth studies is to select a priori a single growth model and fit it to the data (McRoberts et al. 1998; Mahoney el al. 2001; Costello et al. 2009; Bartareau et al. 2012). However, when a particular model is chosen independently of the data and used to approximate the growth curve as a basis for inference, uncertainty in the model's selection is assumed to be zero (Burnham and Anderson 2002). If model selection uncertainty is overlooked, then precision can be overestimated and estimation of confidence intervals of the parameters below the nominal level (Burnham and Anderson 2002). Another approach is to fit more than one model to the data and then use a suitable criterion for model selection and the estimation of parameters and their precision (Burnham and Anderson 2002). Despite the importance of testing concordance among different models fitted to the same data set, few black bear growth studies have done so (Zullinger et al. 1984; Bartareau 2011).

Studies addressing black bear growth have been conducted primarily in temperate geographic areas at the northernmost limit of its distribution (McRoberts et al. 1998; Mahoney et al. 2001; Bartareau et al. 2012). Few published data exist regarding aspects of body size growth at the subtropical southern geographic limit of its distribution (Maehr et al. 2001; Costello et al. 2009; Bartareau 2011) and comparable studies are not available. My objective here was to examine the growth rate and body size at maturity of wild Florida black bears Ursus americanus floridanus, hereafter referred to as Florida bear and black bear when referring to the species as a whole, using body length, chest girth, and weight measurements fitted to the nonlinear vonBertalanffy, Gompertz, and logistic size-at-age growth functions. Quantifying growth rate and body size at maturity will make available the foundation necessary for comparison with other black bear populations and assessing trends in individual and population health.

Study site

Individual Florida bears were captured in public and private lands spanning the full geographic range of the species in Florida. Habitat types included several varieties of hydric and mesic forests together with open-canopy marshes, prairies, and both agricultural and urban lands. These habitats provided common food sources, shelter, and den sites (Maehr 1997). The climate in Florida (Obeysekera et al. 1999) is typical of the subtropical region, characterized by hot and wet spring (March, April, May) and summer (June, July, August) with mild and dry fall (September, October, November) and winter (December, January, February). Average winter lows range from 18°C in southern Florida to near 5°C in northern areas, whereas daytime highs range from 18°C to 25°C. Average summer lows range from near 21°C in northern Florida to near 27°C in southern areas, whereas daytime highs range from mid- to lower 30s°C statewide. Rainfall averages about 1,600 mm annually throughout the state.

Data collection

Personnel from the Florida Fish and Wildlife Conservation Commission live-captured and handled Florida bears for biological research and nuisance-bear management purposes throughout a year during 2000–2012 (Table S1, Supplemental Material). The capture and handling procedures followed a standard data collection protocol that was approved by the Florida Fish and Wildlife Conservation Commission and is consistent with that of Kreeger (1996) and Gannon and Sikes (2007). Cubs were captured in dens by hand and examined while conscious (Garrison et al. 2007). Juveniles and adults were captured using physical restraint (e.g., Aldrich spring-activated foot snares, cage traps), and a pole or CO2-charged projectile syringe delivery system was used to remotely deliver immobilizing drugs. Each bear was immobilized with either a mixture of tiletamine hydrochloride and zolazepam hydrochloride (Telazol®) or ketamine hydrochloride (Ketaset®) and xylazine hydrochloride (Rompun®). While immobilized, the scale weight, chest girth, and body length were measured and a first premolar tooth was extracted. Body weight was measured to the nearest pound using a calibrated hanging spring scale. As the bear was lying on its side, chest girth was measured to the nearest centimeter using a no-stretchable tape measure pulled tight and allowed to relax at the largest circumference of the thorax. Body length was measured to the nearest centimeter as the distance along the contour of the spine between the distal hairline on the snout and tail. Age was determined by Matson's Laboratory LLC (Milltown, Montana) from counts of cementum annuli in the extracted premolar tooth (Willey 1974). The age of bears was adjusted, except cubs of known birth date, to the month of sampling assuming that average birth date was February 1 (Garrison et al. 2007).

Data analysis and model building

Data analyses were conducted following the techniques in Zar (1999) using Microsoft Excel (Microsoft Corporation, Redmond, WA) and Statistix 9.0 (Analytical Software, Tallahassee, FL). All values are presented as mean ± standard error. I used P < 0.05 to identify significant effects. Body length-, chest girth-, and body weight-at-age for individuals (i.e., recaptured bears excluded) were fitted to the three-parameter 1) von Bertalanffy, 2) Gompertz, and 3) logistic growth models in the form:

formula
formula
formula

where S(t) is the body size (cm or kg) at age t (y), S is the asymptotic body size (cm or kg) of the sampled population, K is the growth rate constant (y−1), I is the age at inflection point (y), and e is random error (Kaufman 1981). The S is defined as the average size individuals in the population would reach if they were to grow for an infinitely long time. The K describes the average rate at which the S is reached. The I is the theoretical age at maximum growth. The size-at-age data were fitted to candidate models with the Levenberg–Marquardt–Nash algorithm. The corrected-Akaike's information criterion (AICc) and Akaike weight (wi) were used to rank the suitability of candidate models in the set by order of decreasing fit and relative likelihood on the basis of a balance of model fit and the accuracy of estimates (Burnham and Anderson 2002). The coefficient of determination (R2) was used to evaluate the goodness of fit of the models to the observed pattern of growth. Analysis of variance was used to evaluate the effects of sex on model proportional residuals (i.e., [observed body size − predicted body size]/predicted body size). Pearson correlation (r) was used to assess the association between model proportional residuals and age. The 95% confidence interval on the bias of the estimated model parameters was calculated using a bootstrapping method. Overlap in the 95% confidence interval estimate was visually compared to test the null hypothesis of no differences in the growth parameters between sexes. To evaluate potential effects of differences in available food resources on body weight growth patterns, I used the best-fitted growth model to compare the sex-specific body weight-at-age growth pattern for individuals in different food habit categories. Individuals captured for research purposes consuming a natural diet were categorized as “natural” food habit bears. Individuals captured for nuisance-bear management purposes consuming high-calorie and protein-rich human-related foods (e.g., apiaries, garbage, bird and stock feed) were categorized as “human” food habit bears. My food habit category assignment may be biased to some degree because the complete diet of all individuals over their lifetime in the sample is not known. Complete assessment of diet in wild individuals is practically impossible because of the large home range, cryptic behavior, and food items and scats being difficult to locate for diagnostic examination. Consumption of human-related foods, however, was known for individuals captured in residential and agricultural areas for nuisance-bear management and the food habit in this subsample was biased relative to those individuals handled in natural areas for research purposes. Truncated data showed that the exclusion of dependent cub and juvenile size-at-age classes biased the estimates of K, whereas removing adults reduced the S (Bartareau 2011). To eliminate this bias and “anchor” each model during the early growth phase, I included the dependent cubs (i.e., t ≤ 0.83 y; n = 39 females and 32 males) in each data set because no body weight-at-age data was available for these age classes consuming human food.

According to the AICc the von Bertalanffy model best described the body length (Table 1), chest girth (Table 2), and body weight (Table 3) size-at-age data. The evidence supporting the von Bertalanffy growth model (wi) ranged from 0.782 to 0.799 in females and 0.601 to 0.995 in males. The proportional residuals did not differ significantly between sexes in von Bertalanffy models for body length (F1,505 = 0.52, P = 0.473), chest girth (F1,505 = 0.16, P = 0.694), and body weight (F1,505 = 0.49, P = 0.485). The proportional residuals also showed no significant pattern with age in von Bertalanffy models for body length (r2 = 0.095 [P = 0.179] in females and 0.055 [P = 0.336] in males), chest girth (r2 = 0.086 [P = 0.224] in females and 0.038 [P = 0.504] in males), and body weight (r2 = 0.007 [P = 0.918] in females and 0.002 [P = 0.976] in males). These results indicate constancy of variances in the sex-specific modeled relationships.

Table 1

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body length-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S = asymptotic size, K = growth rate constant, and I = age at the inflection point.

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body length-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.
Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body length-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.
Table 2

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic chest girth-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected-Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S = asymptotic size, K = growth rate constant, and I = age at the inflection point.

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic chest girth-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected-Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.
Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic chest girth-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected-Akaike's information criterion, wi = Akaike weight, R2 = coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.
Table 3

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body weight-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = the coefficient of determination, S = asymptotic size, K = growth rate constant, and I = age at the inflection point.

Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body weight-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = the coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.
Goodness-of-fit and parameter estimates (mean ± SE [95% CI]) for von Bertalanffy, Gompertz, and logistic body weight-at-age growth models of 203 female and 304 male Florida black bears Ursus americanus floridanus captured during 2000–2012. AICc = corrected Akaike's information criterion, wi = Akaike weight, R2 = the coefficient of determination, S∞ = asymptotic size, K = growth rate constant, and I = age at the inflection point.

The S, K, and I values for candidate growth models are shown in Tables 13. The S values for body length, chest girth, and body weight were larger in males than in females and differences were significant (P < 0.05). The K values for body length and body weight were larger in females than in males and the differences were significant in body length (P < 0.05) but not body weight (P > 0.05). The K values for chest girth were nearly identical in females and males. The I values for body length and chest girth were nearly identical in females and males. The I values for body weight were larger in males than in females and the differences were significant (P < 0.05).

The size-at-age data for younger adults are more scattered compared with data for older individuals (Figure 1). In general, the models showed the greatest size-at-age increments before 4 y old. According to the von Bertalanffy model, individuals have reached their asymptotic body length at age 6 y in females and 9 y in males, asymptotic chest girth at age 8 y in females and 12 y in males, and asymptotic body weight at age 12 y in females and 18 y in males.

Figure 1

Body length-, chest girth-, and body weight-at-age growth curves for von Bertalanffy size-at-age model of 203 female (□) and 304 male (▪) Florida black bears Ursus americanus floridanus captured during 2000–2012.

Figure 1

Body length-, chest girth-, and body weight-at-age growth curves for von Bertalanffy size-at-age model of 203 female (□) and 304 male (▪) Florida black bears Ursus americanus floridanus captured during 2000–2012.

Close modal

The S, K, and I values for body weight-at-age models in bears consuming human food and a natural diet are shown in Table 4. In both sexes, the presence of human food in the diet slightly increased the S from the estimate for bears consuming a natural diet (ΔS = 9.6-kg females and 8-kg males), but the differences were not significant (P > 0.05). The presence of human food in the female diet slightly decreased the KK = −0.5 y−1) and increased the II = 0.2 y) from the estimate for bears consuming a natural diet but the differences were insignificant (P > 0.05). In contrast, the presence of human food in the male diet slightly increased the KK = 0.1 y−1) and decreased the II = −0.4 y) from the estimate for bears consuming a natural diet, but the differences were insignificant (P > 0.05).

Table 4

Parameter estimates (mean ± SE [95% CI]) for von Bertalanffy growth models of Florida black bears Ursus americanus floridanus captured consuming a natural diet and human food during 2000–2012. S = asymptotic weight of the sampled population, K = the growth rate constant, and I = age at the inflection point.

Parameter estimates (mean ± SE [95% CI]) for von Bertalanffy growth models of Florida black bears Ursus americanus floridanus captured consuming a natural diet and human food during 2000–2012. S∞ = asymptotic weight of the sampled population, K = the growth rate constant, and I = age at the inflection point.
Parameter estimates (mean ± SE [95% CI]) for von Bertalanffy growth models of Florida black bears Ursus americanus floridanus captured consuming a natural diet and human food during 2000–2012. S∞ = asymptotic weight of the sampled population, K = the growth rate constant, and I = age at the inflection point.

The AICc revealed that von Bertalanffy was the best candidate growth model, corresponding with comparative studies of polar bears Ursus maritimus (Derocher and Wiig 2002) and brown bears Ursus arctos (Bartareau et al. 2011). Consistent with the size-at-age growth models of black bears in other populations (McRoberts et al. 1998; Mahoney et al. 2001; Bartareau et al. 2012), females grew more slowly, for a shorter duration, and attained a significantly (P < 0.05) smaller asymptotic body length, chest girth, and body weight than males. In both sexes the presence of human food in the diet increased the asymptotic weight from the estimate for bears consuming a natural diet, but differences were not significant, although the lack of complete diet data for individuals likely influenced my findings. These results agree with prior studies observing larger average body weight in human food-habituated black bears (McLean and Pelton 1990). In females and males, the presence of human food in the diet slightly (P > 0.05) decreased and increased the average rate at which the asymptote was reached and increased and decreased age at maximum growth, respectively. Variation in growth rates and adult body weight observed in this study indicates that differences in growth patterns depend upon intrinsic sex-specific physiological development processes and priorities in energy allocation and an individual's response to differences in food habits.

Florida bears show the same general determinate size-at-age growth pattern as other mammal species (Zullinger et al. 1984), with a rapid juvenile development phase followed by reduced growing among adults to asymptotic size. Growth models show that the average age of maximum growth was significantly younger in females than in males. The estimated I values suggest that body length, chest girth, and weight growth rates in both sexes peaked before weaning (Farley and Robbins 1995; Maehr 1997) and family dissolution (Seibert et al. 1997). This result is logical because suckling and maternal care is expected to provide increased availability of high-quality food resources (Farley and Robbins 1995) and consequent enhanced growth rate. The rate at which the body length asymptote is reached was significantly faster by females than by males, but not significantly so for chest girth and body weight. Both sexes reach sexual maturity near 2 y and males may not mate until establishing a home territory at around 3 y (Maehr 1997). Like other mammals (Stearns 2004), both sexes reached age at sexual maturity before attaining asymptotic body size. Growth then slowed considerably, with asymptotic size attained by females earlier than males. Even if age at maturity and all growth model parameters except S were equal between the sexes, to maintain the same K, members of the larger sex must have a faster growth rate (Stamps and Krishnan 1997). The raw data indicate that male Florida bears were larger than a female of the same age, which therefore requires that males must have a faster growth rate than females. This inference therefore necessitates that a male must then harvest and metabolize more energy resources for growth than a female because there are associated nutritional and physiological costs to growing faster and maintaining a larger body size. Variation in circulating testosterone (Wingfield et al. 1990) may facilitate the faster growth rate in males than in females. Sex differences in growth rates evidently develop at birth, or soon thereafter, and continue after weaning through to family dissolution and reproductive maturity. The sex differences in growth patterns appear to involve divergences in how females and males allocate available energy resources between increasing body weight, maintenance costs, and investment in reproduction and survival.

Reproduction is energetically costly (May and Rubenstein 1985; Gittleman and Thompson 1988), and female growth rate peaked directly after age at reproductive maturity, indicating that energy resources were directed away from increased body size and maintenance costs toward reproduction. Changes in pregnant females involve an investment in uterine, placental, and mammary tissues, as well as an increase in the maintenance costs associated with these new tissues (Gittleman and Thompson 1988). After birth, female black bears then invest available energy resources into lactation and the care of offspring to family dissolution. Costs of egg production, gestation, lactation, and rearing offspring apparently preclude female Florida bears from growing fast enough to reach the same body size as same-age males. In some mammal species, the early allocation of available energy resources to reproduction indicates a physiological preparation for gestation, lactation, and rearing offspring (May and Rubestein 1985; Gittleman and Thompson 1988). This appears to be true for female black bears because reproductive maturity and giving birth depend on reaching a minimum threshold body weight at conception rather than a specific age (Beecham 1980; Noyce and Garshelis 1994; Samson and Huot 1995). In female black bears, both primiparous and multiparous females undergo delayed implantation, with failure to gain adequate body weight leading to premature termination of pregnancy (Noyce and Garshelis 1994, Samson and Huot 1995). My growth model predicted that the body weight of females at the average age at sexual maturity was 48.9–54.1 kg (95% CI). This result is consistent with estimates of previous studies for female black bears in the temperate parts of this species' range. In those studies, longitudinal data and behavioral observations were used to estimate that a female black bear must attain a spring body weight > 41 kg (Noyce and Garshelis 1994), > 50 kg (Beecham 1980), or ≥ 56 kg by summer (Samson and Huot 1995) to subsequently produce cubs that winter. Females appear to have a more conservative growth strategy than males; increased growth is traded for reproduction with mothers attaining critical minimum body weight, and additional input of energy beyond maintenance of that size is allocated to parturition and rearing offspring.

Male Florida bear growth rate did not decrease as rapidly as it did for females proximate to onset of reproductive maturity. Males grew faster than females before and after the age at female reproductive maturity, suggesting that both pre- and postmaturational selective forces affect sex differences in growth rates. This also suggests that males do not experience the same energetic trade-off between increased body size growth and costs of reproduction as females. Copulation is the extent of a males' parental investment, and only a female rears offspring, so sex differences in reproductive roles and costs of reproduction are logical. The reproductive success of males advances with the opportunity to inseminate females, whereas in a female it increases with the ability to conceive, gestate, and then raise offspring to family dissolution, creating a divergence in the context of natural and sexual selection between the sexes. However, males carry out more competitive behaviors for possible mates (Kovach and Powell 2003; Costello et al. 2009) and maintain larger home range than females (Maehr 1997), which would incur additional costs to growth and maintenance in the form of increased metabolic expenditure and decreased energy acquisition. Consequently, each sex experiences different selective pressures during ontogeny when juveniles compete with conspecifics and later as adults when they engage in their respective reproductive activities. Male black bears fight frequently and would benefit by growing as fast, large, and muscular as possible to enhance intimidation and fighting success, thereby allowing more successful competition with other males for territory and access to females (Kovach and Powell 2003; Costello et al. 2009). The pervasive male-biased intraspecific competition suggests that sex differences in growth rates is a correlated response to divergent selection and early onset of functional importance of increased body size for juvenile and adult male survival and reproductive success.

Intraspecific variation in body weight-at-age growth patterns among different black bear populations appears to reflect differences in the quantity and quality of forage. Mahoney et al (2001) distinguished food habit effects on body weight-at-age growth patterns in black bears and concluded that larger body weight resulted from differences in the availability of protein-rich food resources experienced by individuals within each population that would affect growth. Similarly, I found that Florida bears that consumed high-calorie human foods had a somewhat different growth pattern compared with conspecifics consuming a natural diet. The human food habit individuals were typical nuisance black bears known from other populations in urban-interface communities that had wandered into the yards of homes attracted to garbage cans (e.g., McLean and Pelton 1990; Beckman and Berger 2003; Johnson et al. 2017). When natural food is scarce, or when black bears are traveling to new territory, they may visit human-populated areas in search of food. Human food sources are very attractive to Florida bears, as they are a source of large amounts of easily obtainable calories and protein and are regularly replenished. In both sexes, Florida bears consuming high-calorie human food attained a slightly (P > 0.05) larger body weight at age than conspecifics that consumed the natural diet. Although previous studies of black bears in other populations demonstrated the effect of human food in the diet on increasing average body weight (McLean and Pelton 1990), mine is the first to explicitly elucidate the potential food habit effects on sex-specific body weight-at-age growth patterns. My results indicate that females that consumed human food had a somewhat (P > 0.05) younger age at maximum growth and continued growth in body weight for a longer duration than did conspecifics consuming the natural diet. In contrast, males consuming human food had a somewhat (P > 0.05) older age at maximum growth and decreased body weight growth earlier than did conspecifics that consumed the natural diet. If the Florida bears accustomed to feeding on the human food are residents who use the high-calorie and protein-rich foods as their primary nutritional source rather than as a supplement to natural foods, they may grow to a body weight that cannot be supported by a natural diet. Because these human foods are so rewarding, individual bears would be more likely to repeat the nuisance feeding behavior in search of additional rewards, contributing to ongoing human–bear conflict. These results therefore have implications for understanding the causal factors affecting spatial and temporal variations in a Florida bear's nutritional condition, growth patterns, fecundity, movements among foraging grounds, home range sizes, population density, and human–bear conflicts.

The body size-at-age models in this study present the first growth rate and asymptotic body weight estimates reported for the black bear in subtropical population. My results indicate that the asymptotic body size, growth rate, and age at maximum growth and maturity in both sexes of Florida bears were significantly larger, greater, and younger when compared with conspecifics in other mainland populations at northern latitudes (Mahoney et al. 2001, Bartareau et al. 2012). Although the largest asymptotic body weight in both sexes of black bears was reported for a northern-latitude Newfoundland island population, Florida bears have a significantly faster growth rate and younger age at maximum growth and maturity. This result is opposite that predicted by Bergmann's rule (McNab 1971) and consistent with geographic variation in the dimensions of skulls from adult black bears known to be largest among the more southern localities; the smallest individuals are found in the more northern locations (Kennedy et al. 2002). It appears that the potential thermoregulatory advantage of a larger body weight (Lindstedt and Boyce 1985) is not the primary factor influencing growth in the body size of black bears. In the more temperate and boreal parts of the species' range, black bears remain inactive in their dens for most of the winter months and body weight growth ceases during this dormancy period. Black bears that have ample food available during fall and winter tend to shorten their dormancy period, particularly where they could feed on human foods year round (Beckman and Berger 2003; Johnson et al. 2017). The slower and reduced body weight growth of black bears in the temperate and boreal populations is more likely attributed to the cooler thermal environment and destruction of surface vegetation that results in shorter foraging period that decreases the opportunity for feeding and physiological development. I attribute the larger asymptotic body size, more rapid growth rate, and younger age at maximum growth and maturity of Florida bears to the much warmer temperatures during fall and winter months, leading to lower thermoregulation energy demands, a longer growing season, and almost year-round foraging activity than the temperate and boreal populations examined by Mahoney et al (2001), McLellan (2011), and Bartareau et al. (2012).

In conclusion, the effects of sex and availability of food resources on Florida bear growth patterns were evident in differences in the growth rates and body sizes at maturity between sexes and food habits. Information gleaned from body weight-at-age growth models has direct application to management of black bears because of the strong correlation of body weight with nutritional condition, fecundity, and ecological covariates. Further comparisons are needed at a finer regional scale to determine whether or not the growth patterns are similar among subpopulations with different habitat quality, food habits, and densities. A better understanding of the circumstances causing differences in growth rates (e.g., variation in habitat quality, proportion of bears that feed on human food, food competition) would help to identify ecological interactions that give rise to variations in nutritional condition, fecundity, space requirements, population density, and human–bear conflicts. A Florida bear population under nutritional stress is expected to respond by individuals exhibiting decreased growth rate, adult body weight, fecundity, and increased home range size or a shift in food habits and spatial extent corresponding with associated changes in food availability. If human food habit Florida bears do grow slightly more rapidly and heavier, as indicated in my study, the associated potential for higher productivity could have a substantial effect on population structure and dynamics, carrying capacity, and managing bear–human conflicts. This understanding is critical for the development and evaluation of strategic plans and management options for Florida bears. Recognition of Florida bear growth rate and body size at maturity provides wildlife managers a foundation to implement measurable criteria to assess trends in population health.

Please note: The Journal of Fish and Wildlife Management is not responsible for the content or functionality of any supplemental material. Queries should be directed to the corresponding author for the article.

Table S1. Data file (.xls) of date captured, food habit, sex, age (y), body length (cm), chest girth (cm), and body weight (kg) measurements from live captures of 203 female and 304 male Florida black bears Ursus americanus floridanus during 2000–2012.

Found at DOI: https://doi.org/10.3996/082018-JFWM-076.S1 (47 KB XLS).

This study was made possible thanks to the Florida Fish and Wildlife Conservation Commission and many people who assisted with data collection. Thanks to Dr Walter Meshaka, editors, and anonymous reviewers for constructive comments on the manuscript, and L. Pulliam for diligence in sourcing literature.

Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Author notes

Citation: Bartareau TM. 2019. Growth rate and body size at maturity of Florida black bears. Journal of Fish and Wildlife Management 10(2):458–467; e1944-687X. https://doi.org/10.3996/082018-JFWM-076

The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.

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