Although camelids and taxonomic ruminants share fundamental convergent properties, such as microbial fermentation and a sorting mechanism in the forestomach with a resulting high degree of digesta particle size reduction, there are important differences in the capacity to process high amounts of food. We describe qualitative and quantitative characteristics of the gastrointestinal anatomy of alpacas (Vicugna pacos, 44.4 ± 5.5 kg body mass), supplemented with some observations in dromedaries (Camelus dromedarius). The long large intestine of alpacas (610 ± 128 cm) corresponds to the adaptation of camelids to arid environments. An important difference to taxonomic ruminants is the position of the second forestomach compartment (C2 vs. reticulum), which is not in line of the major forestomach axis in camelids, and the size of the corresponding orifice (C1-C2 vs. Ostium rumino-reticulare), which is distinctively smaller in camelids (alpaca: 3.2 ± 0.3 cm width × 4.2 ± 0.3 cm height, dromedary: 8.5 ± 0.7 cm diameter) as compared to literature data for similar-sized ruminants. The opening between the second and third forestomach compartment (C2-C3 vs. Ostium reticulo-omasale) is of similar magnitude in camelids (alpaca: 2.2 ± 0.7 cm diameter, dromedary: 3.1 ± 0.5 cm diameter) and ruminants, but is opposite to the C1-C2 opening in camelids in a dorsal position. The generally smaller opening might represent a reason for the generally lower food processing capacity in camelids, and together with the arrangement of the openings might result in conditions during contractions of the C2 that are more favourable to (a low proportion of) larger particle escape into the C3. In contrast, the different arrangement of the openings and the larger connection to the preceding chamber might make such an escape of (a low proportion of) large particles from the reticulum to the omasum less likely in ruminants, giving their sorting mechanism a clear-cut anatomical threshold in the Ostium reticulo-omasale.

Anatomy, Camelidae, intake, intestines, mesentery, stomach