Egg laying: Water Dragons being a reptile, lay eggs (oviparous). The clutch of eggs range between 6 to 18 eggs, with a mean of 9 eggs. The mean mass of an egg is between 4 to 5 grams.
Nesting: The female Water Dragons dig shallow nests in a sunny open position generally in a sandy soil. The nests are dug at a slight angle to the vertical to a depth of 10 to 16 cm and have a chamber at the end. The female Water Dragon lies above the burrow and deposits the eggs into the chamber below. If an egg misses, the Water Dragon will roll the egg into the chamber with her snout, or pick it up in her mouth and deposit it into the nest. The female then back-fills the burrow by scraping the soil with her back legs and then packing it down with her snout. Loose debris is then scattered over the nest area. The Water Dragon will check the nest for several days after laying, and will redo the nest if it is disturbed, but after that the nest and subsequent hatchlings are on their own.
The nests are normally located not far from water either near rivers, streams or ponds. In the Australian National Botanic Gardens they are located near the ponds, especially in the Tasmanian Garden area.
Nest temperatures: A study of the nesting behaviour of Water Dragons at five sites across Australia (Doody et al), found the Water Dragons at the Australian National Botanic Gardens dig their nests significantly deeper than at other sites. The temperature of all the nest sites across Australia did not vary significantly at any of the sites, so the slightly deeper nests at the ANBG could be due to the friable nature of the well-mulched soils. The nest temperatures are kept relatively constant, despite fluctuations in external temperatures. The nest temperatures are usually in the range of 22 to 32oC.
Temperature dependant sex determination: The determination of the sex of the hatchlings is related not to sex chromosomes in sperm, but to the pivotal temperature (those that divide the sexes) of the nests during incubation. Turtles, crocodiles and some lizards and fish have this temperature dependant sex determination (TSD). There are three known patterns of TSD. The Water Dragons comply to TSD II, with females produced at hot and cold temperatures and males at the intermediate temperatures. Basically in Water Dragons, eggs incubated at 28oC and above mainly produce females. Males are predominantly produced in the range of 26 to 28oC, and below 26oC females are again mainly produced. It is not 100% of only one sex at these ranges, but the ratio of sexes is significantly altered at each of these ranges. In the study by Doody et al, at greater than 30oC 100% of females are produced, but the low-end temperatures did not produce 100% females.
ANBG Nesting Period: Water Dragons in the Australian National Botanic Gardens start laying eggs in early to mid November through to the end of December. Normally two clutches of eggs are produced in this period, roughly a month apart. The hatchlings appear from late summer to early autumn.
Sexual Maturity: Water Dragons reach sexual maturity at about five years. The size of the male when they become sexually mature needs to be at least 210 mm from snout to vent and have a mass of 400 grams or greater. Below this size their testosterone levels are too low. Sexually mature males exhibit brighter throat colouring — which is blotched or striped with orange, yellow or blue.
Dominance: Dominant males will attempt to control a harem of many females and maintain a territory. They will exhibit territorial behaviour towards other males in spring, with head-bobs (see below), push-ups, arm waving, tail flicks and fighting. They use their tails like a whip when fighting to unbalance their opponent, as well biting and scratching with their teeth and claws. They can draw blood and as a result of fighting dragons can lose their tail tips, toes or claws.