During reproduction, dung beetles of the genus Onthophagus typically bury portions of dung removed from the dung pad. Even though females are capable of successfully constructing and provisioning nests when unaccompanied by males, extensive levels of co-operation between sexes have been reported. In some species, males show a clear dimorphism in body plan and only large horned males are known to assist females. In O. taurus this form of paternal investment is facultative and depends on social context (namely the presence of other males). We studied relationships between body size and paternal investment in five Onthophagus species (i.e. O. coenobita, O. fracticornis, O. illyricus, O. taurus and O. vacca). Two types of tests were established. In test A, a large and a small male were placed with a single, medium-size female; in test B, a large and a small female were placed with a single large male. In controls one male was placed with one female. Individuals were maintained for 10 days. Then the enclosures were opened to assess the position of each beetle. When two mates were found in close vicinity or in the same tunnel provisioning dung, a co-operative behaviour was assumed. In competitive tests, small males and small females never co-operated with their potential mates whilst large and medium-size individuals often co-operated with mates inside the same tunnels. No differences in frequencies of co-operation were found between large and medium-size females. Small females and small males scarcely co-operated with large or medium-size mates in controls. These results suggest that large individuals outcompete the small ones. They also suggest that paternal investment depends on body size of both mates. Large males of all species assist medium-size and large females, whereas small males do not. However, large males do not invest much parental effort on small females that, in fact, are poorly assisted. We assume that large individuals of both sexes are reproductively superior to small ones. The propensity of both small females and small males to excavate was lower than that of larger individuals; accordingly, digging capacity could be a proximate factor explaining facultative paternal investment in these species.