SIZE VARIATION OF EAST AFRICAN MOLE-RAT (TACHYORYCTES SPLENDENS) AND ITS IMPLICATIONS FOR LATE QUATERNARY CLIMATE CHANGE

This study develops a new proxy of Quaternary climate change in tropical Africa through analysis of size variation in modern and fossil East African mole-rats (Tachyoryctes splendens). Across a modern sample of 203 individuals from 23 localities in East and Central Africa, mole-rat tooth-row length is unrelated to annual precipitation, precipitation seasonality, temperature seasonality, or primary productivity. However, it is inversely correlated with annual mean temperature, in agreement with Bergmann’s rule, which (broadly defined) suggests that populations from cooler regions are larger than conspecifics from warmer regions. This relationship is observed at annual mean temperatures below 17-18°C, but tooth-row length is stable at higher temperatures. We apply these observations to Late Pleistocene mole-rat samples from Wakondo (~100 ka, n = 29) and Kisaaka (~50 ka, n = 28) in the Lake Victoria region of western Kenya and Holocene samples (7.2 to 3.2 ka, n = 69) from Enkapune ya Muto (EYM) in Kenya’s Central Rift. Contemporary annual mean temperature at Wakondo and Kisaaka is ~22°C, but tooth-row lengths in the Late Pleistocene fossil samples are larger than expected for mole-rats from areas with annual mean temperatures >17°C. Although this could indicate cooler temperatures than at present, the magnitude of temperature decline needed to drive this change (>5°C) exceeds the magnitude of temperature change between the Last Glacial Maximum and Holocene (~3.5 to 5°C). Such a decline is probably unreasonable for 100 ka and 50 ka. A more suitable explanation for the large size of the Lake Victoria mole-rats may be reduced atmospheric CO₂ concentrations, which translate to higher plant nutrient content, thereby driving an increase in body mass. The EYM sample provides evidence for a decline in body size after 6.1 ka, consistent with a warming trend observed in other tropical African lake cores. The reduction in size suggests an increase in temperatures that parallels a decline in nearby Lake Naivasha and local expansion of grasslands. While such changes are typically linked to orbitally-controlled precipitation dynamics, rising temperatures through the middle Holocene could have played a contributory role in driving ecosystem change by increasing evaporation rates and wildfire frequency.