Effects of population density on corticosterone levels of prairie voles in the field

Abstract

High population density is often associated with increased levels of stress-related hormones, such as corticosterone (CORT). Prairie voles (Microtus ochrogaster) are a socially monogamous species known for their large population density fluctuations in the wild. Although CORT influences the social behavior of prairie voles in the lab, the effect of population density on CORT has not previously been quantified in this species in the field. We validated a non-invasive hormone assay for measuring CORT metabolites in prairie vole feces. We then used semi-natural enclosures to experimentally manipulate population density, and measured density effects on male space use and fecal CORT levels. Our enclosures generated patterns of space use and social interaction that were consistent with previous prairie vole field studies. Contrary to the positive relationship between CORT and density typical of other taxa, we found that lower population densities (80 animals/ha) produced higher fecal CORT than higher densities (240/ha). Combined with prior work in the lab and field, the data suggest that high prairie vole population densities indicate favorable environments, perhaps through reduced predation risk. Lastly, we found that field animals had lower fecal CORT levels than laboratory-living animals. The data emphasize the usefulness of prairie voles as models for integrating ecological, evolutionary, and mechanistic questions in social behavior.

Keywords: Corticosterone; Fecal hormone assay; Microtus ochrogaster; Population density; Prairie vole; Stress.

Figure 1

Semi-natural enclosure facilities, demonstrating density treatments. High Density treatment location changed in different trials.

Figure 2

CORT sample-mass effect. A) Effect of sample mass on corticosterone concentration, showing both simple linear regression (p<0.0001, R² = 0.28) and 2^nd order polynomial (p<0.0001, R² = 0.34. B) Effect of sample mass on mass-corrected corticosterone, simple linear regression (ns).

Figure 2

CORT sample-mass effect. A) Effect of sample mass on corticosterone concentration, showing both simple linear regression (p<0.0001, R² = 0.28) and 2^nd order polynomial (p<0.0001, R² = 0.34. B) Effect of sample mass on mass-corrected corticosterone, simple linear regression (ns).

Figure 3

Log-logit transformed curves of serially diluted fecal extracts compared with log-logit transformed standard curves. Replicates (1:4, 1:10, 1:20, 1:40, 1:100, 1:200) are staggered for ease of viewing, but lie directly on the trendline of standard curve. Standard curve: y = −2.080x + 4.676, r² = 0.996; Total fecal extracts: y = −2.101x + 4.710, r² = 0.993.

Figure 4

Effects of swim challenge on fecal CORT metabolites. Mean fecal CORT before and after the swim challenge, with each data point representing the midpoint of each sample collection time interval. Data presented as Mean ± SE. “Time zero” indicates the beginning of the swim challenge. Sample sizes are reported below each time window.

Figure 5

Home ranges (75% kernel contours) for the high-density quadrant and for one low-density quadrant from Trial 2 Dotted lines indicate males, solid lines indicate females. Kernel center (and likely nest site) is indicated by + for males, o for females. Where these would otherwise be overlapping, they have been staggered for visibility. Animals of the same shade have been designated pairs (“residents”) and unique shades are unpaired (“wanderers”).

Figure 6

Effect of density on space use and CORT. A) Effect of density on 75% kernel home range area, male residents only. B) Effect of density on number of other males overlapping male resident home ranges. C) Effect of density on number of females overlapping male resident’s home range. D) Effect of density on average field fecal CORT metabolites. * p < 0.05, ** p < 0.01. Mean ±SE indicated, sample sizes indicated on figure.