Biogeographic variation in the baboon: dissecting the cline

Area of Science:

• Paleoanthropology and Evolutionary Biology
• Primate Anatomy and Morphology
• Biogeography and Phylogenetics

Background:

• Intraspecific anatomical variation is common across species, with general rules like Bergman's and Allen's attempting to explain geographic structuring.
• Previous research indicated limited support for these rules in certain Old World monkeys, necessitating further investigation in other widely distributed species.
• Baboons (Papio spp.) exhibit extensive clinal variation across sub-Saharan Africa, making them an ideal model for studying geographic morphological structuring.

Purpose of the Study:

• To map and visualize the primary axes of baboon skull variation in size and shape using trend surface analysis.
• To differentiate between shape variation driven by allometry (size-related changes) and other factors.
• To assess the influence of environmental variables (rainfall, temperature, spatial position) and phylogenetic history on baboon morphology.

Main Methods:

• Trend surface analysis was employed to visualize patterns of skull size and shape variation across the baboon's geographic range.
• Size-controlled shape analyses were conducted to isolate shape variation not attributable to allometry.
• Partial regression analysis was used to disentangle the effects of environmental factors and spatial position on morphological variation.

Main Results:

• Skull size variation showed an east-west pattern, with smaller baboons at the extremes and larger ones in Central/Southern Africa.
• Shape variation mirrored the size pattern, with small individuals displaying paedomorphic morphology, but an additional north-south axis also emerged.
• Phylogenetic history, particularly the southern origin and northward colonization of baboons, best explains the observed north-south morphological axis, with environmental factors showing weak correlations.

Conclusions:

• The east-west component of baboon skull shape variation is largely driven by allometry (skull size).
• The significant north-south axis of variation is primarily explained by phylogenetic history and evolutionary divergence, not simple environmental associations.
• Phylogenetic history is a dominant factor shaping baboon morphological variation across their sub-Saharan range.