Not all heads are the same: species- and sex-specific differences in eye and postocular area morphology of burying beetles (Nicrophorus spp.)

Background Burying beetles (Coleoptera: Nicrophorus spp.) are necrophagous insects well known for their highly developed parental care, which in many respects resembles the reproductive behaviour of birds. As in many other taxa, to avoid intense competition, different sympatric Nicrophorus species occupy distinct habitats and show variation in both seasonal and daily activity. Although the influence of niche partitioning on eye and head morphology has been reported in numerous insect groups, this topic has been poorly explored in burying beetles. Additionally, because burying beetles exhibit sexual dimorphism in head size, we hypothesized that these differences may also be reflected in eye shape. Results Our results based on morphometric measurements from ~ 800 specimens clearly indicate species-specific differences in eye shape among the tested burying beetles. Nicrophorus vespillo is characterized by narrower eyes compared to the other species, whereas N. humator possesses notably wider eyes. The eye shapes of N. vespillo and N. interruptus are similar to each other and fall in between the other species. Further analyses revealed differences in eye scaling relative to body size. In N. humator , eye size changes in a manner close to isometry, while the remaining Nicrophorus species exhibit negative allometry. Moreover, the eyes of N. humator appear disproportionately large relative to body size compared to the other species. As hypothesized, sex-specific differences in eye morphology were also detected. Eye sexual dimorphism is lesser than other head measurements but was most pronounced in N. vespilloides and N. interruptus , species that also show clear variation in head size. This dimorphism is associated with an enlarged postocular area and a greater interocular distance in males. Conclusions In conclusion, our study reveals previously undocumented species- and sex-related differences in Nicrophorus eye morphology, including the first evidence of sexual dimorphism within the genus. These differences are likely linked to variation in the postocular region and interocular distance in N. vespilloides and N. interruptus . In contrast, these patterns are weak or absent in the larger species, N. humator and N. vespillo , suggesting that body size may constrain such morphological changes. Our findings open new avenues for investigating how visual system variation influences carcass detection, competition, and parental behaviour in burying beetles.