27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Desert Tortoise Neonates: Weakest Link in Demographic Recruitment, Most Responsive Age-Class for Mitigation, or Both?

David J. Morafka
Department of Biology, California State University, Dominguez Hills, 1000 East Victoria Street, Carson, CA 90747-0005

For many decades neonatal and juvenile desert tortoises have been overlooked in both life history and conservation literature. They were rarely encountered in the field, active at times of year when few field surveys were conducted, and utilized burrows which lacked the easily recognized diagnostic characteristics of adult tortoise burrows. Furthermore, young reptiles have been traditionally viewed as simply miniatures of adults, simply growing in an isometric trajectory leading to reproductive adulthood. It has also been assumed that, taken individually, they were relatively unimportant to demographic recruitment.

None of these generalized assumptions are completely true, and most are false. Here we focus on neonatal tortoises in particular. Among their most distinguishing characteristics are the following: (1) Even in an isometric ontogeny, body surface area only squares as a function of linear growth while volume cubes. As a result neonatal tortoises have advantages in behavioral thermoregulation in the cool fall and winter seasons. (2) For the same reasons, they have lower water reserves and lower resistance to dehydration, and they have much greater vulnerability to predation. (3) They function as "post-natal lecithotrophs", capable of vigorous (> 1 km) dispersion and over-winter hibernation without eating. Nonetheless, residual yolk may only provide energy and re-hydration for movement and for some further morphogenesis of organs, not calcium for skeletal growth. (4) Conspecific coprophagy may play an important role inoculating the hindgut with cellulose digesting anaerobic mutualists. (5) Neonates utilize abandoned rodent burrows as their primary hibernacula, deferring construction of their burrows until soils are softened by winter rains in their spring or later. (6) Spring foraging may follow a complex sequence of selectivity in order to satisfy the needs of re-hydration, protein and calcium for growth, and the PEP (Potassium Excretion Potential) ratio to avoid intoxication. (7) Since the early 1980s demographic evidence has been accruing to indicate that egg nest success rates typically reach or exceed 50%, neonatal year survivorship > 50%, and estimates of survivorship from year one to sexual maturity range up to 89%, depending upon assumptions and evidence.

These assets and vulnerabilities of neonatal and juveniles are reviewed in the context of natural recruitment, and with regard to conservation interventions, including the possible role of hatchery-nurseries installed "in situ" at sites supporting distressed populations.