Retention of solute and particle markers in the digestive tract of captive Somali wild asses (Equus africanus somaliensis)

Abstract

In contrast to the domestic horse, whose digestive physiology has been thoroughly investigated, knowledge on the digestive physiology of wild equids is scarce. Comparisons between the domestic horse and the domestic donkey suggest that wild asses might achieve higher digestibilities. This could derive from longer retention times or a greater difference in the mean retention time (MRT) of particles vs. fluid (the selectivity factor (SF)). Here, we measured MRT of a solute (fluid; MRTsolute) and a particle (<2 mm; MRTparticle) marker in five captive male Somali wild asses (Equus africanus somaliensis) fed a diet of 95% grass hay. At a mean dry matter intake of 94 ± 3 g kg−0.75 day−1, MRTsolute was 33.3 ± 5.4 h and MRTparticle 39.6 ± 3.9 h, resulting in a SF of 1.21 ± 0.14. For their food intake, Somali wild asses appeared to have slightly higher MRTparticle than expected based on domestic equid data, in contrast to Grevy zebras (Equus grevyi), potentially indicating higher capacities of the digestive tract. However, considering data on domestic horses, donkeys, and zebra, there was no evident difference in the SF of wild equids compared to domestic ones. Together with an absence of reported anatomical differences in the digestive tract of wild and domestic equids, the data suggest a general similarity in the digestive physiology of equid species that contrasts with the diversity in the digestive physiology of ruminants, and that might be one contributing factor to a lack of sympatric, niche-differentiated equid species.