Thermal refuges as extended phenotypes: lodge construction by bush Karoo rats

Abstract Animal architecture has received considerable attention as an extended phenotype that buffers organisms against environmental harshness. However, few studies have described animal architecture in a representative sample, assessed and then tested its function experimentally. We adopted such an approach for stick lodges built by the bush Karoo rat ( Otomys unisulcatus ), a solitary rodent inhabiting semi-arid regions of southern Africa. These lodges are among the largest animal-built structures by solitary mammals. We conducted the first population-level survey of all lodges to assess external and internal lodge structure, assessed their effects on microclimate, and tested experimentally whether lodge size affected microclimate. Lodges were several hundred times larger than their builders and featured complex structure, including basking platforms, nesting, food and latrine chambers. Inside lodges, temperatures were closer to the bush Karoo rat´s thermoneutral zone and exhibited higher humidity than ambient conditions, which could reduce costs of thermoregulation and reduce water loss. These benefits correlated with initial lodge size and decreased when lodge size was experimentally reduced. Tunnels were often blocked off with thorny branches, possibly to deter predators. Platforms typically faced eastwards and were regularly used for basking. Bush Karoo rat lodges are complex, costly to build and maintain. They improve thermoregulation and provide predator protection and food storage. Bush Karoo rat lodges (1) are external structures, adding to the builder’s body (2) are functional, (3) probably increasing fitness, (4) result from behavioral activity, and (5) are likely genetically encoded, representing an extended phenotype.