EFFICIENCY OF UPHILL LOCOMOTION IN NOCTURNAL AND DIURNAL LIZARDS

Nocturnal geckos can walk on level ground more economically than diurn al lizards. One hypothesis for why nocturnal geckos have a low cost of locomotion is that they can perform mechanical work during locomotion more efficiently than other lizards. To test this hypothesis, we comp ared the efficiency of the nocturnal gecko Coleonyx variegatus (averag e body mass 4.2 g) and the diurnal skink Eumeces skiltonianus (average body mass 4.8 g) when they performed vertical work during uphill loco motion, We measured the rate of oxygen consumption when each species w alked on the level and up a 50 degrees slope over a range of speeds. F or Coleonyx variegatus, the energetic cost of traveling a unit distanc e (the minimum cost of transport, C-min) increased from 1.5 to 2.7 ml O-2 kg(-1) m(-1) between level and uphill locomotion, For Eumeces skil tonianus, C-min increased from 2.5 to 4.7 ml O-2 kg(-1) m(-1) between level and uphill locomotion. By taking the difference between C-min fo r level and uphill locomotion, we found that the efficiency of perform ing vertical work during locomotion was 37 % for Coleonyx variegatus a nd 19 % for Eumeces skiltonianus. The similarity between the 1.9-fold difference in vertical efficiency and the 1.7-fold difference in the c ost of transport on level ground is consistent with the hypothesis tha t nocturnal geckos have a lower cost of locomotion than other lizards because they can perform mechanical work during locomotion more effici ently.