1. In oviparous species providing maternal care, the choice of nest site is crucial for the survival of both the eggs and the mother. Most embryos only develop successfully within a narrow range of incubation conditions, which may differ from the mother’s own requirements.
2. How, then, do nest-attending mothers select sites that provide suitable conditions for embryonic development, without compromising their own viability?
3. We investigated nest-site selection in flat-rock spiders, Hemicloea major, a species that guards fixed egg sacs in a thermally challenging environment (under sun-exposed rocks). Females glue egg sacs beneath rocks during late spring and guard their eggs during summer, when temperatures beneath rocks often exceed 50 °C.
4. Our field surveys show that spiders laid eggs beneath rocks that were larger and thinner, and thus hotter, than were most available rocks. However, the egg sacs almost invariably were glued to the coolest sites on the substrate beneath a rock, rather than to the (hotter, by about 9 °C) underside of the rock.
5. By affixing their egg sacs to the coolest locations beneath the hottest rocks, females ensured that their developing offspring experienced moderate temperatures and avoided lethal extremes and, simultaneously, gave themselves access to much hotter areas (that enhance their feeding and growth rates) under the same rock. This strategy allows mobile adult spiders to actively select higher temperatures than can be tolerated by their embryos, while remaining close enough to their eggs for effective nest guarding.