The relationship between morphology and ecological performance in Malaysian insectivorous bats
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Relationships among morphological structure, function and performance capability of an organism are known as ecological morphology or ecomorphology. Ecomorphology hypothesizes that differences in morphology translate into differences in performance capability, which then results in differences in ecology or behavior that can ultimately facilitate resource partitioning. I tested this hypothesis using insectivorous bats of Krau Wildlife Reserve (KWR), Malaysia. Within paleotropical insectivorous bats, several families have experienced a rapid radiation, but have retained their ancestral feeding specialization as insectivores and show minimal gross changes in morphology. These radiations generated species-rich assemblages made up of species that broadly occupy the same trophic niche. However, the role of ecomorphological differences among species in facilitating coexistence in these assemblages is poorly known. I measured morphological and performance variables of species-rich insectivorous bats assemblages from KWR. My investigations focused on morphological variation in the craniodental structure of bats and bite performance, and variation in wing morphology and flight performance. Measures of morphological size explained most of the variation in morphology and bite force performance of 35 species studied. However, size is not the only factor in determining the maximum bite force. The size-independent mechanical advantage of the lever mandible was positively and significantly correlated with size-independent maximum bite force. Bats with shorter out-lever arms generate greater bite force, whereas bats with longer out-lever arms produce a lower bite force. However, the law of equilibrium as applied to levers states that the speed at which a load moves increases with the length of the out-lever arm. Although differences in size may explain differences in bite force capacity, the trade-off between force and speed of the mandible lever further explain the separation among species in the assemblage. To further improve my knowledge of the influence of functional morphology on the ecology of bite force in bats, I investigated the crucial functional features of biting mechanisms in bats. I used a two-dimensional model of the skull from digital images to estimate masticatory muscle stress across species. Muscle stress is the intrinsic force capacity exerted by muscle fibrils per unit cross-section area of a muscle. It is recognized as a key biomechanical property in the study of animal motion. However, it is impossible to determine in vivo stress of mastication muscles of bats without significant ethical implications and calibration problems. The masticatory muscle stress of the bats in this study was estimated by rearranging the bite force estimation equation by Thomason (1990). The masticatory muscle stress within 29 insectivorous bats studied ranged from 71.38 to 469.74 kPa. I found that measures of size did not fully explain the masticatory muscle stress differences among species. However, the masticatory muscles stress is linked more to the mechanical advantage of the mandible lever. To understand the relationship between wing morphology and flight performance, I tested 15 syntopic bat species that using an obstacle course with 11 different inter-string distances. Instead of using inferential statistical analyses to quantify maneuverability, I used a statistical approach from the human sciences, item response theory (IRT), to assess the ability of bats to negotiate obstacle courses of differing difficulties. I found that flight ability correlated with body mass and all wing variables tested; however, body mass and wing loading were the only significant predictors of flight performance. Body mass was directly correlated with wing loading, and in this experiment I found differences in wing loading that may facilitate niche partitioning within these 15 highly maneuverable bat species. In conclusion, differences in their performances (i.e. bite force and maneuverability) able to explain the patterns in resource partitioning within this species-rich insectivorous bats assemblage.