Singular patterns of skull shape and brain size change in the domestication of South American camelids

Patterns of selection in South American camelids (Lamini) and their unique demographic history establish the llama and alpaca as remarkable cases of domestication among large herd animals. Skull shape is implicated in many changes reported between wild and domestic taxa. We apply 3D geometric morphometric methods to describe skull shape, form, and size, differences among the four species of Lamini. In so doing, we test if domesticated Lamini exhibit changes similar to those in other domesticated groups: not only in the skull, but also in brain and body size. In contrast to other domesticated artiodactyls, very little change has occurred in domestic alpacas and llamas compared to their wild counterparts. Nevertheless, their differences are statistically significant and include a flatter cranium, inclined palate and increased airorhynchy in the domestics. Selection pressures that contrast with those on other herd animals, as well as recent population bottlenecks, likely have influenced the morphological patterns we note in Lamini. High-resolution 3D morphospace allows skull size, shape, and form (shape + size), to discriminate all four species, with form providing the greatest separation. These results help differentiate morphologically the Lamini, which in nature are distinguished mainly by body size, and provide an additional tool to archaeologists for distinction of wild and domestic remains. Most of our shape analyses suggest a marginally closer relationship between the alpaca and vicuna, to the exclusion of the guanaco, supporting the genetic relationships for this group. The expected brain size change between wild and domestic populations is lower than previously thought, with a 15.4% reduction in llama, and 6.8% reduction in alpaca. This is the lowest reduction in brain size thus far reported among domesticated Artiodactyla.

Automated summary

Patterns of selection in South American camelids (Lamini) and their unique demographic history establish the llama and alpaca as remarkable cases of domestication among large herd animals. Domesticated llamas and alpacas show minimal changes compared to their wild counterparts, with statistically significant differences in skull shape, brain size, and body size. This suggests that selection pressures and recent population bottlenecks have influenced the morphological patterns observed in Lamini. High-resolution 3D morphospace allows for the differentiation of all four species based on skull size, shape, and form, with form providing the greatest separation. The relationship between alpacas and vicunas appears to be closer than with guanacos, supporting genetic relationships within this group. Brain size reductions in domestic populations are lower than previously thought, with the smallest reduction in brain size reported among domesticated Artiodactyla.