25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Reproduction in Sonoran Desert Tortoises

Roy C. Averill-Murray¹ and Christopher M. Klug²
Arizona Game and Fish Department, 2221 W Greenway Road, Phoenix, AZ 85023

We studied reproduction in desert tortoises for four years (1993, 1997-99) at a site in the northeastern Sonoran Desert. Environmental conditions varied greatly between years. Summer rainfall (July - October) in 1992 and 1996-1998 ranged from 6.9 to 22.1 cm, and winter rainfall (November - February) ranged from 7.2 - 36.7 cm; long-term means for the area are 12.8 and 13.8 cm, respectively. Tortoises experienced particularly favorable conditions for reproduction in 1993 (during which 80% of the females laid eggs) with maximum seasonal rainfall observed during the study occurring during the previous summer and winter. With the exception of winter 1997-1998, seasonal rainfall was below average during the remainder of the study, with 36-67% of the females laying eggs. Tortoises laid a maximum of one clutch each year. Oviposition occurred later on average during each year of the study, from 27 June in 1993 to 25 July in 1999, not including two probable cases of egg retention through hibernation. Preliminary analyses indicate that the proportion of females reproducing was positively correlated with winter rainfall. Mean annual clutch size ranged from 3.8 to 5.7 eggs/reproductive female and was not correlated with seasonal rainfall, but variation in clutch size (SD ranged from 1.26 to 2.43) was positively correlated with summer rainfall. The smallest tortoise observed to lay eggs was a 220-mm MCL female in 1993; minimum size reproducing each year was negatively correlated with winter rainfall.