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USGS conducted population surveys for desert tortoises at two sites in the western Sonoran Desert of California, USA, from 2015-2018, during the driest ongoing 22-year period (2000-2021) in the southwestern USA in over 1200 years.

Gopherus agassizii
Photo of a threatened desert tortoise (Gopherus agassizii). 
X-radiograph of a desert tortoise showing four eggs and the radio transmitter
An X-radiograph of a desert tortoise showing the outlines of four shelled eggs. A radio transmitter and antenna are clearly visible. A penny is shown in the upper left hand corner for scale. Previous published research by the principal investigator determined that this technique poses minimal risk to females and embryos.

During the historic extended drought, SBSC researchers wondered if more female than male desert tortoises (Gopherus agassizii) would die, since females lose water and energy during egg production which could put them more at risk, especially during protracted periods of resource limitation with little water and food.

Over a four-year period from 2015-2018 when there was almost no rain, in two long-term study areas near each other in the Sonoran Desert of California, the researchers found that in the cooler, higher-elevation site, there were more live adult male tortoises, and even numbers of male and female tortoises had died.

And at the lower-elevation, warmer, drier site, they also found more live male tortoises, but more females than males had died.

High female mortality at the lower elevation site may have resulted from less water and annual plants and grasses that tortoises eat than at the higher elevation site, as well as from the bet-hedging reproductive strategy of tortoises in which they continue to produce clutches of eggs in drought years. Annual reproductive output results in an estimated loss of up to 13.5% of female tortoise body mass.

Male desert tortoise eating beavertail prickly pear due to drought, Santa Rosa Mts, CA
A radioed male tortoise eating a beavertail prickly pear cactus. During extreme drought conditions it may be the only food available for them with any moisture. Video provided by Dr. Bill Hoese at California State University, Fullerton provided to Jeff Lovich, SBSC, USGS with permission to use. 

Combined with dehydration during severe droughts, these losses may compromise their ability to survive droughts lasting more than 2 years.

The low tortoise density and high mortality of females the researchers observed may reflect reduced survival of tortoises near the southern edge of their range due to climate change, including protracted and intensified droughts.


Read the paper: 

Lovich, J.E., Puffer, S.R., Cummings, K., Arundel, T.R., Vamstad, M.S., and Brundige, K.D., 2023, High female desert tortoise mortality in the western Sonoran Desert during California’s epic 2012–2016 drought: Endangered Species Research, v. 50, p. 1-16,

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