Microclimatic Conditions in Nests of the Cuban Iguana (Cyclura nubila)

Abstract

Although there is a growing body of literature concerning the physiology of lizard eggs, this information is largely derived from laboratory experiments, and there have been few measurements of the environmental conditions in natural lizard nests. Unlike the nests of most lizards, the nests of some tropical iguanas are obvious and can be observed and experimentally manipulated. Nests of the Cuban iguana Cyclura nubila were instrumented, and weekly measurements of soil temperature, soil water potential, and the concentrations of 02 and CO2 were recorded. Nest temperatures were similar among nests and over a 24 hr cyde with temperatures around 3 1?-32?C. However, there was more variability in soil water potential among the nests, as they dried at different rates after a soaking rain near the beginning of incubation. Although the gaseous environments of the nests were distinctly different from aboveground ambient conditions, the levels of 02 and CO2 were less extreme than in the nests of larger reptiles (sea turtles and crocodiles). The CO2 concentrations were found to be related to nest depth and, to a lesser extent, clutch size. LABORATORY STUDIES OF REPTILIAN EGGS have demonstrated the importance of water exchange between eggs and substrate (Andrews & Sexton 1981, Packard et al. 1977, Packard & Boardman 1982, Snell & Tracy 1985, Tracy 1980, Tracy et al. 1978, Tracy & Snell 1985) and incubation temperature (Joanen & Ferguson 1987, Licht & Moberly 1965, Muth 1980, Packard et al. 1977, Sexton & Marion 1974, Webb et al. 1987). In the laboratory temperature appears to affect survival and growth of embryonic lizards, and water uptake by lizard eggs from the soil appears to be required by embryos in order to survive (Muth 1980, Packard & Boardman 1982, Tracy 1980). Although the natural nests of some reptiles such as turtles (Ackerman 1977, 1980; Packard et al. 1985; Prange & Ackerman 1974; Ratterman & Ackerman 1989) and crocodiles (Lutz & DunbarCooper 1984, Whitehead 1987) have been examined, natural nests of most lizards (see Bustard 1969 for data for aboveground lizard nests) are generally difficult to find (Mautz 1982). However, the nesting sites of several species of tropical iguanas are well known due to their conspicuous nesting behavior (Carpenter 1966; Christian & Tracy 1982; Rand 1968; Snell & Tracy 1985; Werner 1982; Wiewandt 1977, 1982). Rand (1972) examined the temp ratures in nests of Iguana iguana, and Snell and Tracy (1985) measured soil moisture in the nests of Conolophus subcristatus, but otherwise the microenvironments of subterranean lizard nests are unknown. One of the unanswered questions concerning the physiology of lizard eggs is one of the most basic: What are the environmental conditions inside natural nests? The purpose of this study was to measure the microenvironments in nests of the Cuban iguana, Cyclura nubila. The measurements included temperature, water potential, and the concentrations of CO2 and 02 in the nest chamber. Soil moisture has been shown to be important to the hatching success of eggs and the size of hatchlings from natural nests (Snell & Tracy 1985) and important to the amount and form of energy reserves at hatching (Christian et al., 1991, Snell & Tracy 1985). Gas tensions have been shown to be important factors in the hatching success of sea turtle eggs (Ackerman 1977, 1980, 1981a, b), and they have been implicated as factors influencing metabolic rates of embryonic crocodiles (Lutz & Dunbar-Cooper 1984) and dutch size in other large reptiles (Seymour & Ackerman 1980).