All species of monitor lizard Varanus spp. are are considered threatened or endangered. No attempt has yet been made to incubating eggs in captivity with subsequent release of young (i.e. head-starting) to bolster wild populations. This is in part due to variability in hatch success and lack of incubation protocols. This study investigated the effects of incubation temperature and moisture regimes on development of white-throated monitor V.albigularis eggs and compared these to natural nests in Etosha National Park, Namibia.
Nine adult females were fitted with radio-transmitters in March 1990 and tracked in their arid savanna habitat. After mating activities ceased in mid-September, five were caught and placed in a 50 m² outdoor enclosure (containing nine nest sites) at the Etosha Ecology Institute.
Monitoring of the four other females continued, nest locations were marked (three excavated in loose soil in old ground squirrel burrows, one in an accumulation of leaf litter) and nests were examined after hatchling emergence. Incubation time, clutch size (number of eggshells plus unhatched eggs), and weight and hatchling snout-vent length (SVL) were recorded. Incubation temperatures were estimated.
All captured females laid eggs within 20 days. They were released after egg-laying at the site of capture. A total of 148 eggs were laid (clutch size 26-32 eggs). Eggs were maintained in containers with a vermiculite (Terralite, grade 2) substrate, incubated at a combination of each of three temperatures (27, 29, 31°C) and moisture regimes (−150, −550, −1,100kPa), and weighed every 10 days.
Natural incubation: Hatching success of the natural nests was 47% (50 of 107). Of 18 hatchlings captured at emergence, average SVL was 114.4 (±1.2) mm and weight 20.6 (± 0.7) g.The leaf nest maintained a temperature of 30-33°C; hatchlings emerged after <120 days incubation. Incubation was around 135-150 days in burrow nests (depths 88, 95 and 102 cm); temperatures were probably between 27-29°C.
Artificial incubation: Hatching success was > 80% in all treatments. Average hatchling SVL (115.1 mm) was similar to that of natural nests. Incubation time and hatchling size were positively correlated with substrate moisture and negatively correlated with temperature (the interaction between moisture and temperature had only a minor influence on incubation length). Incubation was about 35 days longer at 27°C than at 31°C (eggs at 29°C intermediate; overall similar to natural nests). Embryos took longest to incubate on the wettest substrate and least on the driest substrate.
Temperature had a marked effect on hatchling morphology: lower temperature produced long, heavier hatchlings with elongate and wide heads; higher temperature produced shorter, lighter hatchlings with shorter, narrower heads. The effects of moisture were less obvious but wetter conditions tended to produce larger individuals. Such variation in physical characteristics may have conservation implications.
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