Aim. This study examines how climate shaped Microtus californicus (Rodentia: Arvicolinae) ecomorphology throughout the Quaternary. It tests three hypotheses: (1) climate corresponds with consistent shape variation in M. californicus dentition; (2) Quaternary warming and drying trends caused M. californicus morphotypes to predictably shift in range through time; (3) Quaternary warming and drying led to predictable changes in tooth morphological variation. Finally, we discuss how shifts in climate-linked morphological variation may affect the potential of M. californicus to react to future climate change.

Location. Western United States.

Taxon. Microtus californicus (Peale, 1848)

Methods. Geometric morphometrics and partial least squares (PLS) analyses were used to discern how climate contributes to consistent variation in the shapes of the M. californicus lower first molar (m1), validated for the full toothrow. We further corroborate this relationship, reconstructing precipitation at fossil localities using m1 morphology and comparing those values to paleoclimate-model-derived precipitations. Disparity analyses and a MANOVA were performed to examine changes in variation and whether a shift in tooth shape occurred through time.

Results. M. californicus m1 and toothrow shapes are narrower and more curved in cooler, wetter climates. Morphology-based paleoclimate reconstructions align with model-based paleoclimate estimations. When time averaging is accounted for, M. californicus demonstrates a 12% reduction in variation from fossil to present-day specimens, and these changes in tooth shape correspond with climate-related morphotypes.

Main conclusions. As California became drier and hotter since the late Pleistocene, M. californicus dental morphology generally tracked these changes by adapting to the consumption of rougher vegetation in drier environments. This resulted in the loss of some high-precipitation morphotypes, indicating that ecomorphology, often observed at the species and community levels, translates to intraspecific variation and dynamically changes in response to changing climates. The loss of climate-linked morphological variation since the late Pleistocene may limit the ability of M. californicus to respond to future changes in climate. These findings portend that other species may have experienced similar losses in adaptability.

We analyzed 101 fossil m1s, 307 modern m1s, and 361 modern upper toothrow specimens of M. californicus. Fossil specimens from the 7 localities are housed at the University of California Museum of Paleontology (UCMP; Berkeley, USA) or the Los Angeles County Museum of Natural History (LACM; Los Angeles, CA; Fig. 1; Supplementary Table 1; McGuire & Davis, 2013). Modern specimens are from the Museum of Vertebrate Zoology (MVZ; Berkeley, USA) and were evenly sampled from throughout the species’ range, with a male and female specimen selected from each county and subspecies designation (Fig. 1 & Supplementary Table 2). Specimens were photographed using a Nikon D70s and AF Micro-NIKKOR 60mm f/2.8D macro lens ensuring, and best practices for small-specimen photographing were followed (Fox, Veneracion, & Blois, 2020).

Left m1s and upper left toothrows were digitized on Tiff images using tpsDig 2.10 (Rohlf, 2006), designating 21 landmarks and 15 semilandmarks for the m1s (after McGuire, 2011 semilandmark placement) and 34 landmarks for the toothrows (Fig. 2). Semilandmark curves were drawn and initially subsampled by length using tpsDig 2.10 (Rohlf, 2006). They were slid using bending energy in tps Relative Warps 1.45 (Rohlf, 2007) and subsampled again in tps Utility 1.40 (Rohlf, 2008).

The final two points of the molar .tps file are scale points that are 5 mm apart.

The first two points of the toothrow .tps file are scale points that are 5 mm apart.

"IMAGE=" indicates the museum specimen number. This can be cross-referenced with the specimen table in the paper supplements to find the online museum record for each tooth or toothrow.