Following on from the discussion further down about nest boxes, I thought I'd mention exceptions to the rule. In general, I agree with the broadly accepted idea that burrowing monitors should be provided with suitably deep substrate in their enclosures which can be used for both hide burrows and nests. Of course it doesn't hurt to have a nest box as an added option and I can see how having a nest box only, without suitable substrate elsewhere, is limiting the female's options and forcing her to use the nest box.



With the lace monitors, Varanus varius, that I breed, though, I think nest boxes are the preferred option. Having deep substrate as an added option is good (it's generally good to have deep substrate in any enclosure to retain humidity, anyway) but not offering a heated nest box is limiting the female's option and consequently forces her to nest in the ground/enclosure substrate, which is not natural. The reasoning behind this is that in the wild, in most of their range (actually, in all of their range until proven otherwise), lace monitors nest in termite mounds. There's never been much solid proof of them nesting anywhere else, although I could imagine how they could nest successfully in the ground in the tropical north of their range.



The reason lace monitors nest in termite mounds is simple. Like other monitors, lace monitor eggs do best when incubated at a temperature of around 30C and consequently females prefer to lay their eggs in spots which maintain that temperature. Although lace monitors are found in the tropics, the bulk of their range is outside of the tropics and they are found quite far south, into temperate areas. The ground temperature in much of their habitat would be too cool to incubate eggs for a long period, especially in a species whose incubation goes right through the cold winter months. Luckily, termites happen to be very good at controlling the temperature (and humidity) within their mounds such that it is usually in the 30C range and very humid. They also seal the mound and protect it, so it's a perfect spot to incubate their eggs. It's no coincidence that the only other Australian monitor whose range extends as far south as the lace monitor (and further), the heath monitor, Varanus rosenbergi, also nests in termite mounds.



In captivity, simulating the conditions within a termite mound by building a nest box is quite easy and effective. I've been using a nest box for years and have helped others set theirs up, with similar results. I know of one breeder whose lace monitors lay in dirt mounds on the ground, but he lives in the tropics (at the very far extreme of lace monitor range, almost as far as they go) and the ground temperature in his area is hovering around 30C during the breeding season.



I realise there aren't many lace monitor keepers on this site, but as there was a long discussion about nest boxes further down I thought I'd put this out there.

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