Testing the Giles hypothesis using geometric morphometrics.

Area of Science:

• Primatology
• Comparative Anatomy
• Evolutionary Biology

Background:

• The Giles hypothesis proposes allometric scaling as the primary driver of cranial morphological differences between chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla).
• Previous research supporting this hypothesis relied on bivariate plots of linear measurements, necessitating re-evaluation with advanced methods.

Purpose of the Study:

• To retest the Giles hypothesis using geometric morphometric methods.
• To investigate whether extending the ontogenetic trajectory of a chimpanzee can produce an adult gorilla-like cranial morphology.

Main Methods:

• Collected 43 3D cranial landmarks from ontogenetic series of 76 chimpanzees and 58 gorillas.
• Computed ontogenetic trajectories of cranial shape change using multivariate regression against size and developmental vectors.
• Performed developmental simulations to extend chimpanzee ontogenetic trajectories and compared allometric trajectories using Procrustes ANOVA.

Main Results:

• Chimpanzees and gorillas exhibit significantly different allometric trajectories.
• Developmental simulations of chimpanzees did not yield adult gorilla-like cranial morphology, with increasing extension leading to greater divergence.
• Procrustes ANOVA revealed significant differences in allometric trajectories between the two species.

Conclusions:

• The Giles hypothesis is not supported; allometric scaling alone does not explain the cranial differences between chimpanzees and gorillas.
• Neither scaling a chimpanzee to gorilla size nor extending its developmental trajectory results in gorilla-like cranial morphology due to differing allometric patterns.