• Diagonal sequence;
  • duty factor;
  • locomotionprimates;
  • running;
  • walking

The symmetrical gaits of quadrupedal mammals are oftendescribed in terms of two variables: duty factor (S = the stanceperiod of one foot, as a percentage of the gait cycle) and diagonality(D = the percentage of the cycle period by whichthe left hind footfall precedes the left fore footfall). We showthat support polygons are optimized during walking  (i.e.the percentage of the locomotor cycle spent standing on only twofeet is minimized) for: (1) the diagonal-sequence, diagonal-coupletswalks characteristic of primates (50 < D < 75)when D = [hindlimb S]; (2) lateral-sequence,lateral-couplets walks (0 < D < 25)when D = [hindlimb S]− 50;(3) lateral-sequence, diagonal-couplets walks (25 < D < 50)when D = 100 −[forelimb S].To determine whether animal behaviour is optimal in this sense,we examined 346 symmetrical gait cycles in 45 mammal species. Ourempirical data show that mammalian locomotor behaviour approximatesthe theoretical optima. We suggest that diagonal-sequence walkingmay be adopted by ­primates as a means of ensuring thata grasping hindfoot is placed in a protracted position on a testedsupport at the moment when the contralateral forefoot strikes downon an untested support. © 2002 The Linnean Societyof London, Zoological Journal of the Linnean Society, 2002, 136,401−420