Big cat biogeography, morphology and climate change

Abstract

This thesis investigates the long term biogeographic factors which may have contributed towards subspecific variation within modern populations of the lion (Panthera leo) and tiger (Panthera tigris). Present day morphological variation in both species is examined in light of this biogeographical modelling, existing genetic research, and further analysis of the effects of phenotypic plasticity. This research provides a spatially explicit view of the recent evolutionary history of the lion and tiger, and examines their morphology using an unprecedentedly large sample of specimens, from both captive and wild populations, and with known geographic origins. These strands of investigation are important, as it can corroborate, or challenge existing phylogenetic and morphological studies, thereby lending support to or challenging the validity of subpecific taxonomy, management units and clines. This thesis is organised around two main research themes: 1) Biogeographical range shifts in the tiger and lion: Species distribution modelling techniques, incorporating palaeoclimatic models form the basis of understanding the changing ranges of the lion and tiger through glacial/interglacial and pluvial/interpluvial cycles of the Late Pleistocene and Holocene. Models are augmented with palaeoenvironmental evidence including fossil records, rock art and historical records and evidence of large scale stochastic events. Standard species distribution models are built upon by creating Global Environmental Zones and Strata, which aids in their descriptive power. 2) Phenotypic variation within modern populations: Morphology is examined using an unprecedentedly large sample of linear craniometric measurements of lion and tiger specimens. The effect of phenotypic plasticity is examined by comparing the vastly different environmental conditions found between captive and wild lion and tigers. This preliminary analysis supports the interpretation of wild variation by separating the influences of life history, from evolutionary history. Geo-referenced specimens of wild lions and tigers are used to examine geographical patterns and potential environmental influences on modern day variation, in light of the biogeographical modelling and phenotypic analysis. Core Findings: Whilst interglacial conditions have largely benefitted the population extent of the tiger, allowing northward and westward dispersal, the opposite is true in the lion where combined interglacial and interpluvial conditions have decreased their potential range. The biogeographical modelling broadly supports contiguous potential populations of both species through changing climatic cycles, except where sea level change has isolated the Sunda populations of the tiger. Such broad scale analysis may not fully account for narrow, yet enduring barriers and conduits to dispersal such as rivers. Significant phenotypic plasticity is found within lion and tiger skull morphology, which is likely related to differences in the mechanical properties of diet. Morphological variation between wild populations is largely determined by clinal size differences in the tiger, although the similarly sized Amur and Indian populations are separable on multiple skull parameters which likely relate to phenotypic plasticity. Clinal size variation occurs less strongly in the lion with no step change in size or shape between the northern and southern subspecies. The Asian lion population shows shape differences from populations in Africa, likely due to a combination of environmental effects on skull plasticity, and the recent population bottleneck of the Asian population.