By contrast, the success of wolves hunting bison increased across pack sizes over which elk capture success was constant (4-11 wolves) and leveled off at a group size over 3 times larger than that of wolves hunting elk (13 wolves; MacNulty et al. 2014). Wolves were probably more cooperative hunting bison than elk because a single wolf has practically no chance of killing an adult bison by itself; whereas, a single wolf has about a 2% chance of killing an adult elk by itself. Low solo capture success is expected to foster cooperation because it leaves ample scope for an additional hunter to improve the outcome enough to outweigh its costs of active participation (Packer and Ruttan 1988).