Causes and consequences of body armour in the group-living lizard, Ouroborus cataphractus (Cordylidae)

Broeckhoven, Chris (2015-03)

Thesis (PhD)--Stellenbosch University, 2015.


ENGLISH ABSTRACT: Cordylidae is a family of predominantly rock-dwelling sit-and-wait foraging lizards endemic to southern Africa. The significant variation in spine length and extent of osteoderms among taxa makes the family an excellent model system for studying the evolution of body armour. Specifically, the Armadillo lizard (Ouroborus cataphractus) offers an ideal opportunity to investigate the causes and consequences of body armour. Previous studies have hypothesised that high terrestrial predation pressure, resulting from excursions to termite foraging ports away from the safety of the shelter, has led to the elaboration of body armour and a unique tail-biting behaviour. The reduction in running speed associated with heavy body armour, in turn, appears to have led to the evolution of group-living behaviour to lower the increased aerial predation risk. In this thesis, a comparative and integrative approach is used to provide more insight into the conditions under which body armour could have evolved in O. cataphractus and the consequences of body armour for life-history traits. Chapter 2 attempts to investigate how competitive and predatory pressures affect the activity patterns of O. cataphractus. Analysis of activity patterns, obtained via remote camera trapping techniques, show low levels of activity during summer in O. cataphractus, resulting from increased competition for food and high predation pressure. In contrast, a shift in activity to spring, when food availability is relatively high, appears to override the negative effects of body armour and group-living behaviour in O. cataphractus. Chapter 3 tests the hypothesis that body armour serves as protection against attacks from predators during foraging excursions away from the safety of the shelter. The relatively high skin toughness, due to the presence of thick osteoderms in the dermis, protects O. cataphractus against most terrestrial predators, while the skin toughness values for other cordylid lizards are well-below the bite forces of potential mammalian predators. The remaining chapters focus strongly on the feeding behaviour of O. cataphractus and how it is linked to body armour. Chapter 4 investigates the consequences of a reduction in running speed for the diet and tests for the presence of compensatory alternative performance capacities (i.e. increased bite force or jaw closing velocity). A comparative phylogenetic analysis shows that the possession of body armour affects the proportion of evasive prey items that can be included into the diet, thereby restricting the prey spectrum of heavily armoured taxa, such as O. cataphractus, to slow-moving prey (e.g. Coleoptera). Although the results indicate a relatively high bite force in O. cataphractus, the primary selection pressure seems to be tail-biting behaviour, rather than the consumption of hard-bodied Coleoptera (Chapter 5). Bite force, however, trades-off with jaw closing velocity in lizards. A novel lingual prehension mode, exclusive to O. cataphractus (Chapter 6) appears to have evolved in response to the force-velocity trade-off. Given the slow nature of lingual prehension, increased prey capture efficiency appears to be the main selection pressure (Chapter 7), rather than miminsing exposure to predators by reducing the time spent in the open.

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