EMBRYONIC ENERGETICS IN EGGS OF TWO SPECIES OF AUSTRALIAN SKINK, MORETHIA BOULENGERI AND MORETHIA ADELAIDENSIS

-We used eggs of two species of Australian skinks, Morethia boulengeri and M. adelaidensis, to: (1) estimate the amount of energy consumed during embryonic development, and (2) to assess the assumption that lipid provides most of the energy to fuel development. We estimated energy consumption in two ways, using standard closed-system respirometry and by estimating energy loss during development by comparing energy contained in fresh eggs and in hatchlings. Eggs of M. boulengeri are smaller (173 mg ? 4) than those of M. adelaidensis (217 mg ? 8), as are the hatchlings (36.4 mg ? 1.4, and 46.2 mg ? 1.7 dry mass, respectively). Incubation period is virtually identical (M. boulengeri: mean = 57.6, range 54-63; M. adelaidensis: mean = 57.7, range 55-60; P = 0.942). The estimate of energy consumption during development was similar using both methods for M. adelaidensis (46.8 0.8 ml 02 or 898 + 15 J from respirometry and 807 + 57 J from bomb calorimetry) and M. boulengeri (33.3 ? 0.8 ml or 640 16 J and 576 ? 36 J). Commensurate with their larger size, embryos of M. adelaidensis consumed more energy during development than M. boulengeri, but there was no significant difference in the dry mass-specific metabolic cost for M. boulengeri (18.2 kJ g-1 ? 0.7 kJ) and M. adelaidensis (19.9 ? 0.8 kJ g-1, P = 0.1123). The respiratory exchange ratio was 0.76 ? 0.01 in both species, indicating that approximately equal quantities of energy are derived from protein and lipid during development At the time of oviposition, lecithal eggs of amniotes contain all the nutrient required to synthesize and fuel development of the embryo and thus provide ideal models for the study of energetics of embryonic development (Vleck and Hoyt, 1991). Data for oviparous crocodilians, turtles, squamates, and birds suggest differences in the ways that yolk is utilized during development among taxa (Thompson and Stewart, 1997), but scant data for some taxa prevent detailed comparisons that would enable interpretation of the basis for the differences. Among the most poorly characterized taxa are the lizards (Thompson and Russell, 1998), despite the large diversity of lizard species. The usual method of estimating the total en291 This content downloaded from 157.55. 39.136 on Thu, 19 May 2016 06:21:49 UTC All use subject to http://about.jstor. org/terms M. B. THOMPSON AND K. J. RUSSELL ergetic cost of development is to make repeated measures of rates of oxygen consumption (Vo2) throughout incubation, calculate the total amount of oxygen consumed (V02(TOT)) during development, and convert it to an energy equivalent by making assumptions about the metabolic substrates (Vleck and Hoyt, 1991). Usually lipid, which has a respiratory quotient (RQ) of 0.71, is assumed to provide most of the energy to fuel embryonic development (Rahn et al., 1974). However, that assumption is tested rarely, even though the additional measurement of carbon dioxide production (Vco) provides an easy test. Additionally, the assumption that Vo,(TOT) is a robust measure of energy consumption during development can be tested by direct measurement of the energy content of fresh eggs and hatchlings. The difference between the energy content of fresh eggs and hatchlings should equal metabolic losses during development and be the same as the total cost of development estimated using respirometry. We selected two species of Australian skinks, Morethia boulengeri and M. adelaidensis, as models for description of energy consumption during embryonic development with the aim of quantifying differences in energetics of development within lizards. In addition to measuring V0,, we measured Vco2 to test the assumption that lipid is the predominant source of energy, and we made direct measures of the energy content of components of fresh eggs and hatchlings as an independent check on the results of our respirometry. MATERIALS AND METHODS Female M. boulengeri and M. adelaidensis were collected within 100 km of Adelaide in South Australia in November, 1993. All lizards were housed individually in plastic boxes 130 mm x 220 mm x 75 mm high in a room at the University of Adelaide. Litter was provided in each box (5-10 mm organic potting mix) and one end of the box was heated to approximately 34 C to provide a thermal gradient for 12 h per day. The room was maintained at 20-25 C with a natural day length. Dried leaves and pieces of bark were provide as shelter for the lizards. Drinking water and mealworms Tenebrio molitor were provided ad libitum. Every day, the whole box was sprayed with a fine mist of water until the potting mix was moist to prevent desiccation of