Geographic variation and environmental determinants of reproductive output in the desert tortoise

Geographic variation and environmental determinants of reproductive output in the desert tortoise

Principal Investigator: Dr. Jeff Lovich
Co-Principal Investigators: Hal Avery, Phil Medica, Todd Esque

The desert tortoise is listed as a threatened species under federal and California versions of the Endangered Species Act. Most of its remaining habitat is on land managed by the Bureau of Land Management, the National Park Service, and other federal land management agencies. The recovery plan for the desert tortoise recommended that additional data be collected on the reproductive ecology of the species throughout the range. Annual meetings between USGS, Biological Resources Division researchers and the Technical Advisory Committee to the Management Oversight Group for the desert tortoise, an interagency management and research council, have consistently ranked this research issue as a high priority. This study is the first to examine geographic variation in egg production of tortoises and relate that variation to differences in resource availability. Biomass of winter annual plants, upon which tortoises feed, varies appreciably from year-to-year and site-to-site due to rainfall and other climatic variables, and has been demonstrated to be highly correlated with clutch frequency.

Three study sites were established in the spring of 1997: Joshua Tree National Park, Mojave National Preserve, and an area of land administered by the Bureau of Land Management (BLM) near Palm Springs, CA. Thirty six female tortoises were equipped with radio transmitters, located at weekly or bi-weekly intervals from April-July, and x-rayed to determine the presence of shelled eggs. Studies in the Mojave National Preserve are complemented with use of ultrasonography to determine the presence and size of follicles prior to their detectability using x-radiography. At the BLM-managed site, 9 out of 10 females produced a total of 72 eggs. Of these females, 6 produced second clutches and one produced a third clutch. The earliest date of egg-laying occurred between April 18-23, about one month earlier than previously reported in the literature. In contrast, at sites nearby in Joshua Tree National Park, only 1 of 8 females produced a clutch (5 eggs), and she occupied the wettest site in the Park in 1997. Most of the other monitored tortoises in the Park occupied areas that were in the second year of a drought with no production of annual food plants. Modest germination at the Mojave National Preserve allowed 12 of 18 monitored tortoises to produce single clutches (there were no subsequent clutches) in 1997.

Adult male desert tortoises, like this one near Palm Springs,
are slightly larger than females. Males compete vigorously with each other for
opportunities to mate. New sites were added in Piute Valley, Nevada and St. George, Utah in 1998. The results for 1998, an El Niño year, were remarkably different. At Palm Springs 12 of 13 tortoises laid eggs, all 12 that produced eggs laid second clutches, and about one third produced triple clutches. At Joshua Tree National Park 7 out of 7 females laid eggs, and 5 produced second clutches. These differences with 1997 data seem to reflect the wet and highly productive conditions fostered by El Niño's rains.

Graph showing the relationship between clutch size and body size

Adult male desert tortoises, like this one near Palm Springs, are slightly larger than females. Males compete vigorously with each other for opportunities to mate.

PRODUCTS

Lovich, J.E. and R. Daniels. 2000. Environmental characteristics of desert tortoise (Gopherus agassizii) burrow locations in an altered industrial landscape. Chelonian Conservation and Biology 3(4):714-721.
Hinton, T. G., P. Fledderman, J. Lovich, J. Congdon, and J. W. Gibbons. 1997. Radiographic determination of fecundity: is the technique safe for developing turtle embryos? Chelonian Conservation and Biology 2:409-414.
Lovich, J. E., P. Medica, H. Avery, K. Meyer, G. Bowser, and A. Brown. 1999. Studies of reproductive output of the desert tortoise at Joshua Tree National Park, the Mojave National Preserve, and comparative sites. Park Science 19:22-24. This reprint is available in pdf format.
Kathie Meyer holds a desert tortoise at the study site
near Palm Springs. Females at this site produced as many as three clutches per year.

Kathie Meyer holds a desert tortoise at the study site near Palm Springs. Females at this site produced as many as three clutches per year.

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Three study sites were established in the spring of 1997: Joshua Tree National Park, Mojave National Preserve, and an area of land administered by the Bureau of Land Management (BLM) near Palm Springs, CA. Thirty six female tortoises were equipped with radio transmitters, located at weekly or bi-weekly intervals from April-July, and x-rayed to determine the presence of shelled eggs. Studies in the Mojave National Preserve are complemented with use of ultrasonography to determine the presence and size of follicles prior to their detectability using x-radiography. At the BLM-managed site, 9 out of 10 females produced a total of 72 eggs. Of these females, 6 produced second clutches and one produced a third clutch. The earliest date of egg-laying occurred between April 18-23, about one month earlier than previously reported in the literature. In contrast, at sites nearby in Joshua Tree National Park, only 1 of 8 females produced a clutch (5 eggs), and she occupied the wettest site in the Park in 1997. Most of the other monitored tortoises in the Park occupied areas that were in the second year of a drought with no production of annual food plants. Modest germination at the Mojave National Preserve allowed 12 of 18 monitored tortoises to produce single clutches (there were no subsequent clutches) in 1997.

		New sites were added in Piute Valley, Nevada and St. George, Utah in 1998. The results for 1998, an El Niño year, were remarkably different. At Palm Springs 12 of 13 tortoises laid eggs, all 12 that produced eggs laid second clutches, and about one third produced triple clutches. At Joshua Tree National Park 7 out of 7 females laid eggs, and 5 produced second clutches. These differences with 1997 data seem to reflect the wet and highly productive conditions fostered by El Niño's rains. Graph showing the relationship between clutch size and body size
Adult male desert tortoises, like this one near Palm Springs, are slightly larger than females. Males compete vigorously with each other for opportunities to mate.