93 - Intraspecific Variation and Correlations with Nonfeeding Ecological Variables in the Mandibular Morphology of Monachine Seals (Pinnipedia:Phocidae)

Poster Board Number: 93
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Abstract Body : Introduction & Objective
Pinnipeds, a clade of secondarily aquatic carnivorans, have mandibular morphology which is derived from that of their fissiped relatives. It is unclear whether this morphology reflects functional adaptations to their piscivorous diet and aquatic feeding strategies or a release of selection of diet and increased selective pressures of non-feeding related ecological variables.
Methods
Here we used landmark based geometric morphometrics and phylogenetic generalized least squares (PGLS) to determine whether seven nonfeeding ecological, environmental, and life history variables correlate with mandibular shape in monachine seals. We used 14 three-dimensional, unilateral landmarks collected in Stratovan Checkpoint to capture mandibular morphology across all nine species of Monachine phocids: Hydrurga leptonyx (n=11), Leptonychotes weddellii (n=20), Mirounga leonina (n=6) and Mon. angustirostris (n=7), and Monachus tropicalis (n=3), Mon. monachus (n=1), and Mon. schauinslandi (n=19), Lobodon carcinophaga (n=18), and Ommatophoca rossii (n=5). After removing size, rotation, and translation from the landmarks using generalized Procrustes analysis in Checkpoint, we used PAST statistical software to perform principal component analysis to reduce the dimensionality of the original landmark coordinates dataset into major principal components (PC). We used variation in species-averaged PC scores to identify patterns of intraspecific variation. We then took the species-averaged PC (PC1-3) scores for all species but Mon. tropicalis and used PGLS to determine if shape correlated with nonfeeding ecological variables using the R package ape. We also averaged the Procrustes coordinates of each species but Mon. tropicalis and visualized them in a phylogenetic PCA using the R packages phytools and geomorph.
Results
In the non-phylogenetic PCA, the species with the largest amounts of intraspecific variation were the elephant seals, Mirounga leonina and Mir. angustirostris, likely due to their high degree of sexual dimorphism. The Weddell Seal, Leptonychotes weddellii, also had a higher amount of intraspecific variation relative to other lobodontines. This could be due to sexual dimorphism, which has not been noted in this species previously, released selection on mandibular morphology, or their larger sample size in this study relative to some other species. The only significant correlation between non-phylogenetic PC score and nonfeeding ecological factors was between PC2 and lactation duration (-0.82). In the phylogenetic PCA, we observed a separation of the Ross Seal (Ommatophoca rossii) and the Weddell Seal from other lobodontine seals across opposite sides of PC3. This suggests possible functional or behavioral differences in these species, possibly the fact they do not filter feed, although little is known about the ecology of the Ross Seal.
Conclusion
These findings suggest that correlations with monachine mandibular morphology outside of feeding behavior are limited to life history and no correlation with environment or social behavior. This could also mean that the variation observed in pinniped mandibular morphology is still influenced by aspects of dietary-related function or behavior, but further research quantifying pinniped feeding behavior is required to confidently assess this relationship.