Macrocephaly in the wild: uncovering a cranial anomaly in Peromyscus (Rodentia: Cricetidae)

Anatomical anomalies are uncommon deviations in bodily structures, and individuals exhibiting them in the wild are extremely rare because they usually do not reach adulthood, either due to embryonic abortion or survival limitations. We report a macrocephalic specimen of Peromyscus labecula with an enlarged neurocranium, while the splanchnocranium, skin, and coloration remain normal. The specimen is an adult female with morphological traits indicating an apparently healthy condition, suggesting that this cranial anomaly is not lethal during embryonic development or early postnatal life. Macrocephaly is rare in humans and domestic mammals and had not been documented in wild mammals.