1 December 2012 Effects of Population Density On Patterns of Movement and Behavior of Gopher Tortoises (Gopherus polyphemus)
Craig Guyer, Valerie M. Johnson, Sharon M. Hermann
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Patterns of burrow size, tortoise size, home range size and overlap, movement distances, and mating rates were compared among six sites that differed in density of Gopher Tortoises. Burrow sizes differed among sites because tortoise size distributions differed among sites, but this was due principally to the unusually small size of animals on the Conecuh National Forest. A linear relationship between tortoise density and burrow density was documented from the six sites, suggesting that tortoises, on average, created 2.5 burrows per site or that the burrow-to-tortoise conversion factor for our sites was 0.40. The average distance from a burrow to its nearest three neighbors was greater for low-density sites than for high-density sites, indicating that animals probably were more isolated from each other on sites with low tortoise densities. Tortoise home ranges were larger in males than females, a feature documented in other studies of tortoise movements. Home range sizes were greatest for densities of approximately 0.4 tortoises/ha and decreased in size above and below this density. This suggests that animals moved to visit close neighbors in areas of high density, expanded movements to maintain contact with neighbors that became more widely dispersed as density decreased, and then restricted movements to a few close neighbors as density reached extremely low levels. Home range overlap increased linearly with increasing density, suggesting that opportunities for social interactions decreased with decreasing density. When tortoises moved between burrows, males moved longer distances than females and tortoises of both sexes moved shorter distances on high-density sites than did tortoises on low-density sites, suggesting greater movement costs for males than females and for tortoises on low-density sites. Males traveled up to 500 m to visit female burrows, but most movements were < 80 m. Median movement distances of males to visit females were negatively correlated with burrow density, suggesting that cost of male movements to find mates increased as population density decreased. Based upon patterns of 95% confidence limits, rates of mountings of female tortoises approached zero when females occupied burrows approximately 200 m from neighboring burrows. If burrows were uniformly distributed 200 m apart, then reproductive failure would be a statistically supportable outcome at a density of 0.3 burrows/ha (0.12 tortoises/ha). These values are similar to the values of 0.4 tortoises/ha (1.0 burrows/ha) that our data suggest is the density at which social structure associated with movements within home ranges are altered.

The Herpetologists' League, Inc.
Craig Guyer, Valerie M. Johnson, and Sharon M. Hermann "Effects of Population Density On Patterns of Movement and Behavior of Gopher Tortoises (Gopherus polyphemus)," Herpetological Monographs 26(1), 122-134, (1 December 2012). https://doi.org/10.1655/HERPMONOGRAPHS-D-10-00004.1
Accepted: 1 February 2012; Published: 1 December 2012
density dependence
gopher tortoise
Gopherus polyphemus
home range
Home range overlap
reproductive costs
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