A female Yellow-spotted monitor Varanus panoptes excavates what appears to be a shallow, decoy nest burrow after completing work at the spiralling burrow near her tail. Image courtesy of Colin McHenry, used with permission.

Regular readers might remember the various coverage that monitor lizards, or varanids, have had here over recent months (see the list of links below). We’re not done yet – there’s lots more I plan to say about monitors. Back in September 2014, we looked at the recent discovery of complex, spiralling nest burrows produced by the Yellow-spotted monitor Varanus panoptes. Members of this species dig descending burrows that have a straight, sloping upper section, a spiralling section formed of three descending convolutions, and a terminal nesting chamber located right at the bottom (Doody et al. 2014). After laying her eggs in the chamber, the female partially back-fills the upper parts of the burrow with soil, presumably to help maintain a moist, stable environment for the developing brood, or to help keep predators out. Or maybe both possible functions apply.

I finished the relevant bit of that 2014 article by saying that even more news on complex varanid burrows was due to be published in the near future. Well, that day is here.

Scaled diagram of V. panoptes burrow from Doody et al. (2015). Scale bar = 3 m.

Anatomy of a corkscrew. As described by J. Sean Doody and colleagues in Biological Journal of the Linnean Society, V. panoptes burrows are not just notable because of their complexity, but also because of the depths they can reach. Previous studies have described burrows where the nest chamber is 1.5 m beneath ground. This new study reports numerous burrows descending to 2.5 m, the deepest ending with its nest chamber an incredible 3.6 m beneath ground (Doody et al. 2015).

The burrows concerned all come from the Kimberley region of Western Australia where the lizards excavate them either late in the wet season or early in the dry season – between, that is, about February and June. The burrows aren’t especially rare: Doody et al. (2015) report finding 52 of them, 37 of which were old nests with hatched eggshells within. These burrows occur within complex warrens, some of which are inhabited by many animals and involve more than 30 closely associated burrows (Doody et al. 2015). Yes, warren-dwelling monitor lizards. If this is new to you, I hope you’re impressed. Lizards do neater, more complex things than we ever thought possible just a few decades ago.

These corkscrew V. panoptes burrows are the first helical burrows known for any reptile, and also the deepest nesting excavations produced by any tetrapod. Yes, there are other tetrapods that build much longer burrows, but these aren’t created specifically for nesting. Note that spiralling structures are also created by other tetrapods (the molerat Cryptomys makes helical structures round the tubers it eats: Doody et al. (2015) cite Lovegrove & Painting (1987) here).

Old, hatched eggs discovered within a nesting chamber. Photo by Prue Wright.

The V. panoptes burrows start with a short sub-horizontal section, then have a long curving, descending section, at the end of which is a descending vertical helix that can have as many as eight convolutions. Some of the helices are clockwise, others are anti-clockwise, and some switch from one direction to the other (Doody et al. 2015). A nesting chamber is located right at the bottom of the helix. Eggshells and intact eggs with deceased embryos were found in many of the excavated burrows, and one burrow even had a V. panoptes skeleton preserved within it, 1.93 m below the ground surface (Doody et al. 2015).

Are we sure that these burrows were constructed by V. panoptes? Well, yes. The fact that copious evidence for active nesting was found in many of the burrows is pretty compelling data, and there’s no evidence for the presence of any other animal species. Furthermore, lizards of this species were filmed and photographed excavating and exploring and descending into these various burrows.

One of many remote camera images from the project showing a female V. panoptes at a nest burrow. The lizards are highly alert while excavating and frequently stop to look around. Image courtesy of Colin McHenry, used with permission.

Over 250 million years of corkscrew burrows. It might not have been lost on you that these spirals are very reminiscent of certain burrows known from the fossil record. Vertical, spiralling burrows termed Daimonelix (or Daemonelix or Daimonhelix) are known from sediments more than 20 million years old (dating to the Late Oligocene and Early Miocene) and were created by a rodent: the small, grassland-dwelling beaver Palaeocastor. These burrows, first reported in 1892, were controversial until the 1970s when Martin & Bennett (1977) discovered Palaeocastor skeletons (including those of babies) inside them. Spiralling vertical burrows have also been discovered in the sediments of the Permian; these ones were made by the dicynodont Diictodon (Smith 1987).

Excellent example of Daimonelix from Agate Fossil Beds National Monument, Nebraska. Image in public domain.

Might the superficial similarity between these burrow designs tell us something about the selection pressures that led to the evolution of this burrow-building behaviour in the first place? And do these similar burrows show that the species that made them were somewhat similar in ecological or behavioural respects?

A nice representation of how deep the burrows are: this photo was taken during the excavation of one of the structures. If you're wondering, viable eggs were saved, returned to their nest chambers, and the burrows were repaired (Doody et al. 2015). Photo by Prue Wright.

Doody et al. (2015) collected abundant data on temperature and moisture within the burrows and their nest chambers. While the temperature of those burrows close to the surface fluctuated somewhat according to seasonal surface conditions, there was scarcely any change in the temperature of nest chambers 1-3 m beneath the surface (Doody et al. 2015). Moisture increases at depth, being 37% greater at 3 m below ground than it is at 1 m below ground. Perhaps the reason for the depth of these nests is that soil moisture sufficient for embryo development is only present at depth, with the stable, constant temperatures present in nest chambers deeper than 1 m below the surface also giving embryos at this depth a survival advantage relative to those closer to the surface.

The biggest question about these burrows is why the animal construct them in helical fashion. One popular hypothesis is that they’re anti-predator structures, their width and tight coiling nature preventing larger animals from accessing the nest chamber. Doody et al. (2015) note that now extinct predators like thylacines and the giant V. priscus could have predated upon V. panoptes nests in the past (V. komodoensis – a former inhabitant of Australia – can be added to the list as well). Males of V. panoptes might also be among the animals that burrow-building V. panoptes want to have excluded from their nests. An anti-predator function has also been suggested for the Palaeocastor burrows.

It’s also conceivable that helical nest burrows are advantageous as goes drainage during flooding events, since the helical shape increases the burrow’s surface area. Or perhaps helix construction increases the chances that the burrow-builder will encounter a region of sediment more favourable for excavation of a nest chamber than will excavation of a straight burrow.

Wouldn't it be great if this monitor-themed illustration was available on a t-shirt? Oh, waitaminute...

While it isn’t yet possible to provide firm conclusions, Doody et al. (2015) imply that corkscrew burrows most likely evolved because the animals that build them need to burrow so deep to avoid hot, dry surface conditions unconducive to the survival of their eggs and babies. Are corkscrew, Daimonelix-type burrows the best solution? And is it that they can only be constructed by certain kinds of animals? Questions, questions...

One final thing. Prior to these new papers on goanna burrows, it was assumed that mammals (and other synapsids) were the only tetrapods building corkscrew burrows. But these new finds have shown that living reptiles – dumb, unsophisticated, simple, cold-blooded reptiles* – are doing some incredibly complex things. Co-operatively built family burrows (McAlpin et al. 2011), warrens formed of numerous individual burrows, and – now – deep, deep, spiralling corkscrew-like burrows. The expectation that complex structures of this sort can only be attributed to mammals.... ha, it’s dead.

* That was meant to be ironic if you didn’t get it.

Refs - -

Doody, J. S., James, H., Colyvas, K., McHenry, C. R. & Clulow, S. 2015. Deep nesting in a lizard, déjà vu devil’s corkscrews: first helical reptile burrow and deepest vertebrate nest. Biological Journal of the Linnean Society doi: 10.1111/bij.12589

- ., James H., Ellis, R., Gibson, N., Raven, M., Mahony, S., Hamilton, D. G., Rhind, D., Clulow, S. & McHenry, C. R. 2014. Cryptic and complex nesting in the Yellow-spotted monitor, Varanus panoptes. Journal of Herpetology 48, 363-370.

Lovegrove, B. G & Painting, S. 1987. Variations in the foraging behavior and burrow structures of the Damara molerat Cryptomys damerensis in the Kalahari Gemsbok National Park. Koedoe 30, 149-163.

Martin, L. D. & Bennett, D. K. 1977. The burrows of the Miocene beaver Palaeocastor, western Nebraska, USA. Palaeogeography, Palaeoclimatology and Palaeoecology 22, 173-193.

McAlpin, S., Duckett, P. & Stow, A. 2011. Lizards cooperatively tunnel to construct a long-term home for family members. PLoS ONE 6 (5): e19041

Smith, R. M. H. 1987. Helical burrow casts of therapsid origin from the Beaufort Group (Permian) of South Africa. Palaeogeography, Palaeoclimatology and Palaeoecology 60, 155-170.