Reproduction and Survival of the Desert Tortoise (Scaptochelys agassizii) in Ivanpah Valley, California

Abstract

Eighty-seven adult desert tortoises (58 females, 29 males) in Ivanpah Valley, California, were fitted with radiotransmitters in 1980 and 1981. All were marked, weighed, and measured and released. Tortoises were then recaptured, weighed and released about every two weeks in 1980 and weekly in 1981. The experiment concluded in the spring of 1982, when 62 tortoises were recovered. Survival of males and females did not differ, but the observed death rate in 1981-82 (18.4%) was significantly greater than that in 1980-81 (4.4%). Females > 200 mm in plastron length exhibited two periods of distinct loss of mass in 1980 (15-28 May, 12-25 June) when masses of males were essentially unchanged. Losses of > 100 g were interpreted as evidence of egg-laying. Eleven of 15 females for which we had season-long records in 1980 laid two clutches, two laid one clutch and two did not lay eggs. The mean number of clutches laid was 1.60. Of 40 females considered in 1981, 20 laid one clutch, 12 laid two clutches and eight did not lay. The mean number of clutches was 1.10. The estimated dry standing crop of forbs in 1980 was about 100 times that in 1981. Estimated biomass of grasses in 1980 was about 25 times that measured in 1981. Analyses of tortoise feces showed that grasses were eaten with equal frequency until mid-May in both years, but that after this time grasses essentially disappeared from 1981 diets. Annuals and seeds were much better represented in early spring diets in 1980 than in 1981. Cacti were more frequent in scats collected in 1981, particularly after mid-May-when about 87% of material in feces was from such plants. Cacti may serve as a reserve supply of energy, but more importantly as a potential source of water. This may partially explain the egg production in 1981-one of the most unfavorable years on record in the Mojave Desert.