The Better To Eat You With: The Comparative Feeding Morphology And Evolution Of Feeding Strategies In Phocid Seals (Pinnipedia, Phocidae)

OF THE THESIS The Better to Eat You With: The Comparative Feeding Morphology and Evolution of Feeding Strategies in Phocid Seals (Pinnipedia, Phocidae) by Sarah Elizabeth Stachura Kienle Master of Science in Biology San Diego State University, 2014 During the transition from land to water, phocids (seals) evolved adaptations that allowed them to efficiently obtain prey in an aquatic habitat. Phocids are the most diverse pinniped (seals, sea lions, and walruses) lineage with eighteen extant species and a rich fossil record. Previous qualitative research hypothesized the existence of four feeding strategies in phocids: filter, grip and tear, pierce, and suction feeding. Most species are classified as generalists, employing pierce or suction feeding. Grip and tear and filter feeding are specialized strategies used only by Hydrurga leptonyx and Lobodon carcinophaga, respectively. No quantitative morphological study of feeding strategies in extant and fossil phocids has been conducted to date. The first objective of this study was to determine whether the original hypothesized feeding strategies were supported by qualitative and quantitative data and then determine the feeding strategies used by extant phocid species. Three dimensional landmark data were collected for all extant taxa. Principal Component Analysis and Canonical Variate Analysis were performed to summarize cranial and mandibular variation. Prey type data and data from facial musculature dissections were included to provide a comprehensive description of each feeding strategy. The results of this study found evidence for three of the four previously hypothesized feeding strategies: filter, grip and tear, and suction feeding. There was no evidence for pierce feeding being a distinct feeding category. Instead, most phocid species are generalist feeders, using biting and suction to opportunistically capture prey. Grip and tear and filter feeding are derived strategies that require specific morphological adaptations; these unique adaptations have allowed Hydrurga leptonyx and Lobodon carcinophaga to exploit novel niches and prey types. This is the first study to provide evidence for suction feeding in Cystophora cristata, Mirounga angustirostris, and Mirounga leonina. These results provide a framework for understanding the evolution and adaptability of feeding strategies employed by extant phocid species. The second objective of this study was to calculate standardized diet compositions for extant phocid species with a focus on endangered and data deficient species and investigate the relationship between prey quality and quantity. Standardized diet compositions were calculated for all extant phocid species based on published quantitative dietary studies. Prey items were divided into nineteen categories based on taxonomic class. Average energy densities (kilojoules per gram) were calculated for each species’ diet. Standardized diet composition results revealed that most phocids are prey specialists, consuming actinopterygians, cephalopods, or malacostracans. The energy densities of actinopterygians, v cephalopods, and malacostracans were positively correlated with phocid diet composition. The average relative energy density of the phocid diet was 4.76 kJ/g. The findings of this study confirm that phocids are apex predators in the marine ecosystem with trophic levels similar to other marine predators, including other marine mammals, rays, and sharks. These results provide information for future studies of interspecific variation in diet, as well as indicated the need for more in-depth investigations of diet, especially for phocids with limited quantitative diet data. The third objective of this study was to determine the feeding strategies used by fossil pinnipedimorphs, test evolutionary hypotheses to understand the evolution of each strategy, and conduct comparative phylogenetic analyses on the evolution of diet and feeding strategies in Phocidae. Three-dimensional landmark data were collected from fossil pinnipedimorphs and extant phocids. Principal Component Analysis was performed to describe the major axes of cranial and mandibular variation in the dataset. The OUCH method was used test six evolutionary hypotheses. The ancestral state prey type and feeding strategy were reconstructed using maximum parsimony and Bayesian methods. This study found evidence for the existence of filter, generalist, and grip and tear feeding in fossil pinnipeds. Stem pinnipedimorphs used grip and tear feeding to capture and consume prey, suggesting that grip and tear feeding may be the ancestral feeding strategy in pinnipeds. The evolution of cranial and mandibular shape in extant phocid species is driven by multiple factors, specifically feeding strategy, phylogeny, and prey type. The ancestral prey type of phocids was likely actinopterygians, and the ancestor of phocids used a generalist feeding strategy. This was the first study to provide quantitative and qualitative evidence for feeding strategies in extant and extinct pinnipedimorphs. Knowledge of feeding strategies in phocids is fundamental to understanding the adaptations that allowed pinnipeds to reinvade the marine ecosystem.