Serum Leptin as an Indicator of Fat Levels in White-tailed Deer (Odocoileus virginianus) in the Southeastern USA

Leptin is an adipose hormone that plays a key role in regulating energy intake, appetite, and metabolism, and the concentrations have been shown to circulate at levels proportional to body fat in humans (Blum et al. 1997; Perry et al. 1997). Additionally, positive relationships between serum leptin concentration and fat reserves have been reported for various mammals, but most have been conducted on sheep (Ovis aires) or cattle (Bos taurus) (e.g., Delavaud et al. 2000; Ehrhardt et al. 2000; Thomas et al. 2001) or were based on captive individuals (European brown bears [Ursus arctos arctos]; Hissa et al. 1998). Suzuki et al. (2004) hypothesized the relationship of leptin to fat reserves in free-ranging, harvested Hokkaido sika deer (Cervus nippon yesoensis; from the island of Hokkaido, Japan) would allow for nonfatal evaluation of deer condition, and the authors reported a significant positive relationship between leptin and kidney fat mass (KFM), kidney fat index (KFI), and femur marrow fat index (MFI). Because conventional methods for evaluating wild ungulate physiologic condition necessitate euthanasia (e.g., Chitwood et al. 2013), understanding leptin-to-body fat relationships in free-ranging mammals could provide valuable body condition information without sacrificing the individual. Although work has been conducted on Hokkaido sika deer (Suzuki et al. 2004), no studies of leptin-to-fat relationships have been conducted on white-tailed deer (Odocoileus virginianus). We examined the relationship between leptin and fat levels in a free-ranging population of white-tailed deer from coastal North Carolina, USA. Specifically, we examined the relationship of leptin concentrations with KFM, MFI, and KFI and tested for differences in leptin concentrations across sampling seasons and by lactation and gestation status.

In July 2008 and March 2009, we head-shot deer at night using high-powered rifles. Within minutes of collapse, we collected whole blood via cardiac puncture and stored samples on ice until centrifuged (<6 hr after collection). After separation, we froze serum at −20 C until processing for assay. Deer collections were approved by the North Carolina Wildlife Resources Commission and the North Carolina State University Institutional Animal Care and Use Committee (08-082-O).

We determined KFI and KFM in the field and MFI in the laboratory following conventional protocols (Chitwood et al. 2013). We quantified serum leptin concentrations using the Linco Multi-Species Leptin Radioimmunoassay Kit (RIA; Linco Research Inc., St. Charles, Missouri, USA) in accordance with the manufacturer's protocol (Suzuki et al. 2004). We analyzed results using SPSS (IBM SPSS Statistics, Armonk, New York, USA). We used Spearman's rank-order correlation to examine correlations between leptin and KFI, MFI, and KFM. We used Mann–Whitney U-tests to compare the distribution of leptin concentrations between seasons and lactation status, and for all analyses we used α = 0.05. Because we only had two nongestating females, we descriptively compared leptin concentrations based on gestation status.

Average leptin concentration for 36 females was 1.354 ng/mL human equivalent (HE) units (SD = 1.042; range: 0.45–5.13 ng/mL HE). For 13 of 49 individuals (27%), we failed to detect leptin, indicating levels were below the assay's sensitivity (i.e.,<0.39 ng/mL HE). Leptin concentrations were not correlated with any of the body fat metrics (Table 1). Leptin concentrations were similar across sampling periods (U = 121.5, P = 0.456; Table 2). Lactating females (U = 110, P = 0.028) and gestating females had lower leptin concentrations than did nonlactating and nongestating females, respectively (Table 2).

Our leptin concentrations overlapped reported values in Hokkaido sika deer (range: 1.222–2.63 ng/mL HE; Suzuki et al. 2004). However, we failed to detect obvious trends between leptin concentrations and measured fat levels. Suzuki et al. (2004) were limited in detecting differences among months because of sample sizes, but they detected significant relationships between leptin and all three conventional fat indices. However, they reported four deer with leptin concentrations lower than the detectable levels of the RIA kit (Suzuki et al. 2004). Our failure to detect correlations between leptin and body fat metrics may be related to lower overall body size and lower amounts of fat compared to other ungulates (e.g., Hokkaido sika deer). Moreover, we sampled at times of lowest fat reserves for deer in the southeastern US and had a high number of individuals for which we could not determine a leptin concentration. Because deer of the southeastern US may not require large fat reserves (Finger et al. 1981; Chitwood et al. 2013) and because leptin tends to circulate at levels proportional to fat reserves, the utility of leptin as an indicator of fat reserves may depend upon the seasonal cycle of fat deposition and latitudinal range (i.e., leptin may be more effective in larger-bodied deer of higher latitudes). Regardless of this possible explanation, our results indicate that leptin may be a poor predictor of fat levels for white-tailed deer.

Leptin concentrations tend to increase in pregnant humans and rodents (Hardie et al. 1997; Tomimatsu et al. 1997), and the increases may be derived from leptin secreted from the placenta (Ashworth et al. 2000). However, our leptin concentrations appeared to be lower in gestating females than in those that were not gestating (though results should be interpreted with caution, as we had only two gestating females with measureable leptin concentrations). In fact, of eight females with leptin concentrations lower than the sensitivity of the assay, seven were pregnant with twins and the other was not pregnant. Suzuki et al. (2004) detected a similar trend in Hokkaido sika deer and suggested that placental leptin productivity may be limited in the species. Similarly, Thomas et al. (2001) determined that placental leptin was too low to contribute significantly to the maternal concentration. Thus, our data appear to support Suzuki et al. (2004), who stated that increased leptin levels in gestating individuals may not be common in all mammal species.

Our results indicate that serum leptin concentrations may be inadequate as a surrogate for the traditional lethal methods of determining body fat reserves in white-tailed deer. Regardless, further study is warranted because a nonlethal method for evaluating fat levels and physiologic condition in white-tailed deer will benefit researchers working in wild and captive settings, and additional research may allow the development of species-specific leptin assays that make the technique practical for wildlife population monitoring and management (Suzuki et al. 2004).

Funding was provided by the North Carolina State Natural Resources Foundation, the North Carolina State University (NCSU) Department of Forestry and Environmental Resources, and the NCSU Fisheries, Wildlife, and Conservation Biology Program. We thank the North Carolina Wildlife Resources Commission for help with deer collections. We thank J. H. Harrelson and A. Partin for help in the field. Additionally, we thank the undergraduate and graduate students of the NCSU Fisheries, Wildlife, and Conservation Biology Program for field assistance.