Major body size change in an extinct tropical island mammal associated with glacial-interglacial environmental shifts

Patterns of late Quaternary body size change in small-bodied vertebrates remain poorly understood. The now-extinct Puerto Rican island-shrew Nesophontes edithae exhibits size variation across late Quaternary sites, but the chronology and correlates of this variation are unclear. Nesophontid material from two Pleistocene-Holocene sites demonstrates that a small morph was present for >30,000 years during the Last Glacial Period and was replaced by a large morph after the Last Glacial Maximum. Differing δ13C values indicate the small morph was associated with open savanna and the large morph was associated with tropical forest. This allochronic change does not match the Island Rule or Bergmann’s Rule, and may have been driven by an end-Pleistocene shift to high-productivity forest. Size morph replacement could reflect distributional shifts in niche-differentiated species or a rapid evolutionary response to climatic change. Our study demonstrates the resilience of some species to natural change but their vulnerability to anthropogenic pressures.