Morphological Divergence in Mountain Ungulates

Abstract

Mountain ungulates inhabiting contrasting altitudinal and climatic gradients within and across mountain ranges provide exceptional model systems for studying the interplay of natural selection, phenotypic plasticity, and genetic drift in generating morphological divergence. This study quantified morphological variation in four mountain ungulate species - Alpine ibex (Capra ibex), chamois (Rupicapra rupicapra), mouflon (Ovis gmelini musimon), and Carpathian red deer (Cervus elaphus hippelaphus) -- across 18 mountain populations spanning elevational gradients from 800 to 3,200 m in the Alps, Carpathians, Pyrenees, and Apennines, using 24 standardised skeletal and morphometric measurements from 1,284 museum specimens and 284 live-captured individuals. Body mass, horn/antler dimensions, limb proportions, and skull morphometrics showed significant population-level divergence in all four species (MANOVA Wilks' lambda p < 0.001 in all cases). Bergmann's rule conformity (positive body mass-elevation correlation) was confirmed in chamois (r = +0.72, p < 0.001) and Alpine ibex (r = +0.68, p < 0.001) but reversed in mouflon (r = -0.42, p = 0.008), the latter attributed to thermoregulatory challenges at high altitude in a species of Mediterranean origin. Horn allometry showed significant population-level divergence in Capra ibex, with high-elevation populations showing 18.4% shorter but 12.4% wider horns than low-elevation conspecifics. Geometric morphometric analysis of skull shape identified altitude-correlated cranial shape changes in all species consistent with dietary shifts from browse-dominated to grass-dominated forage. Molecular variance analysis (AMOVA; microsatellite loci) confirmed that morphological divergence substantially exceeded neutral genetic differentiation (QST > FST in 3 of 4 species), indicating directional natural selection rather than genetic drift as the primary driver of morphological divergence across altitudinal gradients.