Nesting in bushes and burrows: potential climate responses in the gopher tortoise

Abstract

As the climate continues to warm at an unprecedented rate, much research attention has been focused on climate responses in organisms. Climate responses in the egg stage are restricted to the mother’s choice of nest site in oviparous species without parental care, but can mothers choose nest sites that compensate for climate change effects on developing embryos? We studied nest site choice in the gopher tortoise, Gopherus polyphemus , near the hot end of the species range. We found 25 nests, of which 16 (64%) were located outside burrows (in or near the aprons, the mound of excavated sand at the burrow entrance), as is usually the case for the species. Surprisingly, however, we found nine nests (36%) inside burrows, a novel finding for the species. There was marked variation in the canopy openness of selected nest sites (mean = 42.0%; range 24–62%); openness values of nest sites generally fell between those of potential nest sites in the sunniest (mean =61%) and shadiest (15%) locations. Canopy openness and incident radiation intensity (IRI) were highly, significantly positively related for nest sites and potential nest sites and changed little over the incubation period. Collectively, our data reveal that mothers can reduce mean nest temperatures by ∼2.5–3 °C by nesting 1.5 m down the burrow, or by nesting in the shadiest locations within 50 m of their burrows. Wide variation in openness and IRI and their effects on nest temperatures suggests that, among generations, tortoise mothers have some scope to buffer their developing embryos against climate warming, provided that nest site choice behaviour is heritable and can evolve fast enough to keep pace with future warming. Our novel finding of nests inside burrows suggests that gopher tortoises may have a greater capacity to offset climate warming than most turtles, which do not excavate and inhabit burrows.