Microclimate Buffering Across a 650 m Afro-Alpine Gradient: Thermoregulation at the Nest Level by Grauer’s Gorillas in the Kahuzi-Biega National Park

Nighttime temperatures in the Afro-alpine zone (>2050 m) of Kahuzi-Biega National Park frequently fall below 5 °C. However, the thermal advantages provided by night nests of Grauer’s gorilla, Gorilla beringei graueri along this elevation gradient have yet to be quantified. From 3 January to 7 January 2025, 80 night nests were located along the Mt. Kahuzi–Biega ridge (2000–2650 m above sea level); 66 with complete data were analyzed. Nest-interior and ambient temperatures were measured using calibrated mercury thermometers, while canopy openness was assessed through sky-facing photographs analyzed with ImageJ. Canopy openness ranged from 18% at 2050 m (dense bamboo) to 83% at 2625 m (open ericaceous scrub), with a mean of 50.5 ± 18.8%. The interiors of the nests consistently exhibited warmer temperatures than the humid ambient air, with a mean temperature difference of 2.03 ± 0.37 °C, ranging from 1.39 to 2.68 °C. Linear mixed-model analysis (n = 66) indicated a significant reduction in thermal buffering correlated with increasing elevation (β = −7.4 × 10−4 °C m−1, 95% CI −8.9 × 10−4 to −5.9 × 10−4, p < 0.001) and greater canopy openness (β = −0.020 °C per %, p < 0.001); fog density and precipitation from the previous night did not exhibit a significant effect. The model explained 78% of the variance in ΔT (marginal R2). Over a 650 m Afro-alpine gradient, Grauer’s gorillas create a 2.0 °C thermal refuge, which decreases by approximately 30% near the summit. This study represents the first quantitative evidence that canopy density can mitigate the elevation penalty for any African great ape. Canopy retention is the only terrestrial mechanism that can mitigate accelerated warming at high altitudes, which is occurring at a rate of +0.45 °C per decade. Without canopy retention, national conservation strategies for the Democratic Republic of Congo must allocate funds for extended energy subsidies at elevations exceeding 2500 m.