Are five cryptic species of the European lesser blind mole rat, Nannospalax leucodon (Rodentia: Spalacidae), sexually dimorphic based on cranial variation?

Assessing sexual dimorphism (SD) is an important step in evaluating divergence among cryptic species of Nannospalax leucodon superspecies, although evolution of SD remains poorly understood. We analysed sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) in dorsal and ventral cranial views of four cryptic species from Serbia and one from North Macedonia. Significant SSD and SShD were found in both cranial views in all cryptic species except N. l. syrmiensis, where the absence of SD may be linked to urbanisation. Males and females of each cryptic species shared common allometric trajectories, and allometry significantly influenced cranial shape variation. After correcting for allometry, SSD in upper incisors width and SShD in the ventral cranium remained significant only in N. l. montanoserbicus. Shape differences between sexes were consistent across cryptic species and closely reflected allometric changes. The observed pattern of SD may represent an ancestrally inherited condition, as the most sexually dimorphic cryptic species, N. l. montanoserbicus, is also among the evolutionarily oldest. Because SSD and SShD were detected in most examined cryptic species, male and female size and overall shape data should not be pooled in future studies. Larger and more robust male crania, with broader upper incisors, may indicate greater bite force, while selection favouring SSD likely contributes to SShD. Although occupying similar ecological niches, males dig longer burrows, especially during mating, provide no parental care, and show higher aggression. These behavioural differences between sexes indicate that sexual, rather than natural, selection primarily drives SD in N. leucodon superspecies.