Doody, J. Sean
Department of Integrative Biology
University of South Florida
Tampa, Florida USA
Hasiotis, Stephen
Department of Geology
University of Kansas
Lawrence, Kansas USA
Shukla, Shivam
Department of Integrative Biology
University of South Florida
Tampa, Florida USA
Possibly unique to reptiles, two species of goannas (monitor lizards) construct extremely deep, helical nests. Both the yellow-spotted monitor (Varanus panoptes) and sand monitor (V. gouldii) construct helical burrows up to 4 m deep in the Australian wet-dry tropical savannah and desert. The burrows, which are soil filled, consist of an angled ramp, a turn and a second angled ramp, followed by 2-7 tight spirals straight downward, terminating in a slightly widened egg chamber. Hatchlings collectively excavate their own escape burrow through resistant soils, ignoring their mother’s nesting burrow. Deep nesting is likely a behavioral adaptation: large goannas lay large eggs that require 6-10 months to develop and must span a long dry season; our research showed that eggs deposited at shallower depths (~1 m deep) would likely desiccate during this time; conversely, nest temperatures were similar between 1 and 4 m deep, suggesting no thermal advantage of extreme deep nesting. Revealing the adaptive function of the helix has proven more difficult. The extended phenotype of helical burrowing behavior in animals has evolved independently many times since first appearing after the Cambrian explosion (~540 million years ago), but a general explanation has remained elusive. It occurs in many extinct taxa and also in some extant pocket gophers, prairie dogs and scorpions. We recently reviewed hypotheses proposed to explain the evolution of helical burrowing in certain taxa, including some of our own. Of 10 hypotheses, six were ‘post-construction’ hypotheses –– benefits to the creator or offspring, realized after burrow construction––and four were ‘construction’ hypotheses reflecting direct benefits to the creator during construction. Overall, the evidence does not support a general explanation for the evolution and maintenance of helical burrowing in animals. The function and evolution of the helix as an extended phenotype would seem, at least in some cases, to provide different advantages for different taxa. We briefly discuss the hypotheses and offer predictions for which other of the world’s lizards might nest at extreme depths with helical burrows.