Modern spotted hyenas and wolves are social hunters, and meals are shared by the clans and packs, respectively. Spotted hyenas consume the entire skeleton, bones included, usually in one feeding session (Kruuk, 1972). In contrast, wolves are often unable to crack large limb bones, such as those of European bison, and leave substantial parts of the skeleton intact; however, skeletons of smaller prey, such as red deer, suffer far more damage and fewer bones are left uneaten (Fosse et al., 2012). In this regard, the bone-processing abilities of wolves are closer to those of brown and striped hyenas than either group is to spotted hyenas. Brown and striped hyenas are solitary foragers and hunters in most observations, although they do have social structures associated with bone accumulations at dens (Watts and Holekamp, 2007). Bone-cracking borophagines, such as Borophagus, are equipped with far more robust teeth and sturdy jaws than those of extant grey wolves (Balisi et al., 2018), although as a clade they did not reach the degree of morphological specialization observed in hyaenids (Van Valkenburgh et al., 2003). It is thus reasonable to assume that Borophagus is capable of cracking larger bones than living wolves do, possibly comparable to hyaenids. Whether or not Borophagus would systematically consume an entire skeleton is still a matter of speculation, but this is likely to depend on the competitiveness of their group feeding.

Van Valkenburgh et al., 2003 considered large borophagine canids—such as Epicyon saevus, E. haydeni, Borophagus secundus, Aelurodon ferox, and A. taxoides (B. parvus was not included in their study)—to be hunters due to their craniodental morphometrics and abundance in the fossil record, as well as energetic considerations. In contrast to felids that commonly develop a sharp retractile claw as an effective weapon for prey capture (Gonyea and Ashworth, 1975), canids never developed a retractile claw (with the possible exception of their arboreal ancestors; [Wang, 1993]). Vanvalkenburgh and Hertel (1993) and Van Valkenburgh et al., 2003 thus argued that these large borophagines were likely social hunters in order to overcome their inability to capture large prey by a single individual. Furthermore, Carbone et al. (1999) demonstrated an empirical relationship between the body size of carnivorans and their prey size: extant predators of 21.5–25 kg or greater in body mass tend to prey on animals of their own body mass or greater, possibly due to energetic considerations. Our estimate of body mass for Mehrten B. parvus is 18.9 ± 1.6 kg based on lengths of the first lower molar or 24.3 ± 3.7 kg based on limb bone circumference and cortical area (see Materials and methods). The latter is generally considered to be more accurate because long bones, as direct weight-bearers, are proportional to body size (e.g., [Anyonge, 1993]). Mehrten B. parvus thus is comparable in body size to the modern maned wolf Chrysocyon brachyurus (23 kg) and African wild dog Lycaon pictus (24 kg).

Due to some dental and postcranial parallels between borophagines and modern hyenas, the ‘hyaenoid dogs,’ as borophagines were earlier known (VanderHoof and Gregory, 1940; Simpson, 1945), were frequently dismissed as mere scavengers (Munthe, 1989) and as such were not presumed to have been able to directly drive the evolution of their prey. Such misconceptions, however, are as much a popular myth about hyenas as a reflection of the fossil dogs. Up to 80% of food consumed by the modern spotted hyena is obtained by their own hunting efforts (Kruuk, 1972), in contrast to brown and striped hyenas that are primarily omnivorous scavengers of large prey with less than 5% of food consumed from fresh kills (Macdonald, 1978; Rieger, 1981; Mills, 1982). As active hunters not dependent on the availability of carrion, spotted hyenas typically have a far greater population density and wider distribution than their scavenging relatives. Some large borophagines, such as Borophagus secundus, have a continent-wide distribution and abundant fossil record that strongly suggest that they, too, were hunters (Wang et al., 1999; Wang et al., 2008). In contrast, brown and striped hyenas maintain both smaller species geographic ranges and lower population densities, both of which are likely associated with their solitary hunting of prey smaller than the preferred prey of spotted hyenas (Wagner, 2006). From the new coprolite evidence alone, it is unclear whether B. parvus from the Mehrten crossed the size threshold and became an obligate predator of large prey. Our rough body size estimates based on the largest rib fragment inside one of the coprolites (LACM 158708) suggest that the Turlock Lake Borophagus probably preyed on ungulates equivalent in size to a modern mule deer Odocoileus hemionus (45 to 150 kg), vicuña Vicugna vicugna (35 to 65 kg), and guanaco Lama guanicoe (90 to 140 kg): animals substantially larger than their own size (see Materials and methods). However, remains of similarly large prey are known from spotted, striped, and brown hyena scats and could represent either scavenged (more likely for striped and brown hyenas) or actively hunted (more likely for spotted hyena) sources (Kruuk, 1972; Wagner, 2006). Combined with other evidence presented in the preceding paragraphs, the presence of large prey is consistent with—although does not exclusively support—Borophagus as social hunters of large mammalian prey.