Philip A. Medica

Reproduction and survival of the desert tortoise (Scaptochelys agassizii) in Ivanpah Valley, California

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.

Frequencies of broken tails among Uta stansburiana in southern Nevada and a test of the predation hypothesis

Frequencies of broken tails among hatchling (1-4 months), yearling (7-12 months) and 2-year old Uta stansburiana (19-24 months) in southern Nevada were examined for the years 1966-1973. Frequencies among males and females in the two older age groups did not differ significantly. The frequency of broken tails in hatchlings (0.06) was lower than that in yearlings (0.30), which was, in turn, lower than that in 2-year old Uta (0.51). Break frequencies among yearlings varied from 0.26 to 0.37 in different years, while frequencies among 2-year olds varied from 0.33 to 0.64. These differences were not statistically significant. Among hatchling Uta, tail break frequencies ranged from as low as 0.029 (1971) to as high as 0.099 (1966), and differences between years were highly significant. Tail break frequencies in all age groups were analyzed in terms of spring densities of Uta and of an important predator (Crotaphytus wislizenii). Tail breaks in yearling and 2-year old Uta were not correlated with either density variable. Tail-break frequencies of hatchling Uta were significantly correlated with predator density (r = 0.88), and multiple regression analysis (with densities of both lizards as independent variables) yielded an R2 of 0.80. This is the first test of the tail break

HORMONAL INDUCTION OF COLOR CHANGE IN FEMALE LEOPARD LIZARDS (CROTAPHYTUS WISLIZENII).

Follicle stimulating hormone (FSH) will induce red-orange pigmentation in non-ovariectomized female leopard lizards (Crotaphytus wislizenii) in 7-21 days depending upon the season. Progesterone is judged to be an important hormone in the process of color change, and will bring about pigmentation in the absence of ovarian tissue. Estrogen alone has no apparent effect on color change, but when administered before progesterone it appears to have a priming effect.

The Distribution and Abundance of the Flat-tailed Horned Lizard (Phrynosoma mcallii)

The occurrence of the flat-tailed horned lizard (Phrynosoma mcallii) was investigated in 458 sections (each 2.59 km2) in Riverside, San Diego and Imperial counties, California, in 1979. In all sections, 40 P. mcallii (0.09/man-hour) and 2,191 horned lizard scats (4.8/man-hour) were counted. When sections were grouped in 12 general areas, mean numbers of horned lizards ranged from 0.40/ hr (north of Benson Dry Lake) to 0.00/hr (northern East Mesa, Coyote Wells area). The density of P. mcallii in a plot in the Yuha Basin (40 km west of El Centro) was 6-8/ha. The flat-tailed horned lizard is similar to P. platyrhinos in terms of sex ratio (1:1), age at maturity (20 months), and ability to la more than one clutch of eggs in favorable years. Mean home ranges for five male an m-ale P. mcallii (estimated as convex polygons) were 0.13 ha and 0.05 ha, respectively. P. mcallii feeds almost exclusively (>95% of items) on ants. The most commonly consumed species are harvester ants (Veromessor pergandei, Pogonomyrmex californicus and P. magnacanthus). In 1980 the relative abundance of P. mcallii (L), density (P), diversity (A,) of perennial plants and relative abundance of harvester ants (A) were estimated in ten plots in southeastern California. The abundance of the lizard was positively correlated (r = 0.93) with perennial density, and a clear positive association between lizard and ant densities was exhibited in nine of the ten plots. A regression model accounted for 91% of observed variation in lizard density, and predicted relative abundance of P. mcallii as:

Effects of Radiation on a Fenced Population of Horned Lizards (Phrynosoma platyrhinos) in Southern Nevada

Between 1964 and 1966 minimum spring densities of horned lizards ranged from 1.0-2.5/hectare (ha) in 3 8 ha enclosures in Rock Valley, Nevada. Minimal survival of hatchling horned lizards to the age of 8 months was about 26-38 per cent. Annual minimal survival of older lizards was 50-60 per cent. Female horned lizards usually laid one clutch of eggs per year, but multiple clutches were observed in 1969. Conversely, no reproduction was observed in 1970. The maximum life span of horned lizards is at least 94 months. One of the enclosures was subjected to continuous gamma irradiation from a centrally located source of 137Cs. Between 1964 and 1966 numbers of horned lizards declined in all 3 plots. Between 1967 and 1970 numbers of horned lizards increased in the two control areas, but continued to decline in the irradiated plot. Female sterility owing to regression of ovaries is judged to be the cause of the population decline. Similar radiation effects have previously been observed among leopard lizards (Crotaphytus wislizenii), whiptail lizards (Cnemidophorus tigris), and side-blotched lizards (Uta stansburiana). * * *

Reproduction by Uta stansburiana (Reptilia, Lacertilia, Iguanidae) in Southern Nevada

Monthly mean clutch sizes of Uta stansburiana in southern Nevada were measured over a 10-year period. Overall mean clutch sizes of female Uta >12 months of age were 4.40 (March-April), 3.80 (May), 3.04 (June), and 2.65 (July). Overall means of yearling females (<12 months of age) were about 82-90% of those of older females between April and June. Reproduction by Uta in 1973 surpassed that observed in any of nine previous years. The mean size of the first clutch laid by older females was 5.35, and some females deposited up to eight eggs. The minimum number of clutches laid was four and the maximum seven. The net reproductive rate (R/sub 0/) estimated for 1973 was 2.34, exceeding an earlier estimate for 1966 (1.65).

Sterility among female lizards (Uta stansburiana) exposed to continuous gamma irradiation.

A natural population of the lizard Uta stansburiana occupying a fenced 9-ha area in southern Nevada was exposed to essentially continuous γ irradiation from an artificial source between February 1964 and September 1973. Tissue doses were estimated using implanted lithium fluoride microdosimeters. Females became sterile as early as 11 months of age, but many were still fertile at ages of 20 months and a very few may have reproduced at 32 months. Dosimetry showed some females to be sterile after accumulated doses of around 500 rad, while others may have required 1000 or more rad. One female estimated to have received over 1200 rad was still reproductive. Irradiated females may pass through a state of half sterility, during which time they possess one functional ovary. Female U. stansburiana are sterilized at lower doses than the sterilizing dose (1500 rad) previously suggested for the leopard lizard, Crotaphytus wislizenii.

Endocrine Function and Reproductive Impairment in an Irradiated Population of the Lizard Uta stansburiana

This study describes gonadal changes in lizards (Uta stansburiana) exposed to chronic low levels of γ radiation (1.5-10 R/day) under field conditions and attempts to evaluate endocrine involvement in these changes. Reproductive impairment in irradiated males is demonstrated during the breeding season by reduced testes weights and by seminiferous tubules devoid of germ cells. Ultrastructural morphology of Leydig cells, plasma testosterone levels, and developed accessory sex structures indicate normal steroidogenesis in such males; cytology of the pituitary gonadotropes and pituitary gonadotropin content indicate normal gonadotropin levels. These findings suggest that low levels of radiation affect the spermatogenic process directly, rather than through damage to the pituitary. Comparable irradiation causes complete resorption of ovarian tissue in some female Uta. The oviducts are atrophic and the pituitary gonadotropes hyperactive. Resemblance of gonadotropes to those of castrated animals and reduced pitui...

Altered energy metabolism in an irradiated population of lizards at the Nevada Test Site.

Field metabolic rates (via doubly labeled water), body compartmentalization of energy stores, and energy assimilation efficiencies were measured to assess all avenues of energy utilization in Uta stansburiana living in a low-level gamma-irradiated plot in Rock-Valley, Nevada. Comparison of energy budgets for radiation-sterilized females with those of nonirradiated control lizards revealed several substantial differences. Sterile females were heavier, mainly because they had extraordinarily large energy (fat) storage depots. Sterile females had much lower rates of energy expenditure via respiration and lower rates of energy intake by feeding. These differences are interpreted as indirect responses to radiation-induced sterility. Gastrointestinal tract function in sterile females was normal. There is little evidence of direct radiation effects on physiological functions other than reproduction.