Temporal gait parameters in the alpaca and the evolution of pacing and trotting locomotion in the Camelidae

Alpacas are increasingly popular as domesticated companion and commercial animals. Their footfall patterns, however, are not well documented. It would be fascinating to know if artificial selection has changed alpacas' locomotor patterns from the ancestral condition in the vicuna. Some members of the Camelidae pace rather than trot, but the gaits of most species have not been studied quantitatively. Thus, investigating alpacas' gaits might contribute to understanding the factors influencing gait choice and evolution. We aimed to quantify the temporal footfall patterns of alpacas to determine observed gaits and to describe gait parameters as a function of speed. Kinematic data (3D motion capture) of locomotor patterns over a range of speeds were collected from four alpacas. We quantified the influence of speed on stance time, swing time, stride time (and frequency), duty factor and stride length, and created gait diagrams for symmetrical and asymmetrical gaits. Alpacas moved using lateral sequence walks, mainly lateral couplets walks and runs with some lateral sequence, singlefoot footfall patterns. Remarkably, the alpacas never truly paced or trotted. At faster speeds, they switched to asymmetrical gaits, predominantly transverse gallops. With increasing speed, stance and stride times decreased, and stride frequencies and stride length increased. Swing times decreased slightly with speed for symmetrical gaits, and a tendency towards an increasing swing time was evident in asymmetrical gaits. It is still uncertain why quadrupeds choose particular gaits and intriguingly, extant camelids (alpacas, dromedary camels) do not trot. The apparent absence of pacing in alpacas deserves further investigation in an experimental and comparative framework. Yet, this absence seems inherited from their vicuna ancestors, indicating that pacing gaits may not be ancestral or common for Camelidae. Future studies should include kinetic and anatomical data to provide insight into whole-body mechanics, and include other unstudied species such as guanacos and vicunas.