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Are Pinnipeds Functionally Different from Fissiped Carnivores? the Importance of Phylogenetic Comparative Analyses

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Are Pinnipeds Functionally Different from Fissiped Carnivores? the Importance of Phylogenetic Comparative Analyses Evolution, 54(3), 2000, pp. 1011±1023 ARE PINNIPEDS FUNCTIONALLY DIFFERENT FROM FISSIPED CARNIVORES? THE IMPORTANCE OF PHYLOGENETIC COMPARATIVE ANALYSES OLAF R. P. BININDA-EMONDS1,2 AND JOHN L. GITTLEMAN3,4 1Department of Zoology, University of Oxford, Oxford OX1 3PS, United Kingdom 3Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996-1610 Abstract. It is widely assumed that adaptations to an aquatic lifestyle are so profound as to produce only obvious differences between pinnipeds and the remaining, largely terrestrial carnivore species (``®ssipeds''). Thus, comparative studies of the order Carnivora routinely examine these groups independently. This approach is invalid for two reasons. First, ®ssipeds are a paraphyletic assemblage, which raises the general issue of when it is appropriate to ignore monophyly as a criterion for inclusion in comparative studies. Second, the claim that most functional characters (beyond a few undoubted characteristic features) are different in pinnipeds and ®ssipeds has never been quantitatively examined, nor with phylogenetic comparative methods. We test for possible differences between these two groups in relation to 20 morphological, life-history, physiological, and ecological variables. Comparisons employed the method of independent contrasts based on a complete and dated species-level phylogeny of the extant Carnivora. Pinnipeds differ from ®ssipeds only through evolutionary grade shifts in a limited number of life-history traits: litter weight (vs. gestation length), birth weight, and age of eyes opening (both vs. size). Otherwise, pinnipeds display the same rate of evolution as phylogenetically equivalent ®ssiped taxa for all variables. Overall functional differences between pinnipeds and ®ssipeds appear to have been overstated and may be no greater than those among major ®ssiped groups. Recognition of this fact should lead to a more complete understanding of carnivore biology as a whole through more uni®ed comparative tests. Comparative studies that do not include monophyletic groups for phylogenetically based comparative tests should be reconsidered. Key words. Aquatic adaptations, Carnivora, comparative biology, independent comparisons, life histories, monophyly, pinnipeds. Received June 1, 1999. Accepted November 2, 1999. Comparative studies in evolutionary biology have been 1991); that is, a group consisting of all and only those species revolutionized by phylogenies (Harvey and Pagel 1991; Har- descending from a common ancestor. In this paper, we con- vey et al. 1996). Phylogenetic trees permit rigorous tests of sider the adverse effects from one long-standing example adaptive hypotheses, correlated evolution among quantitative where this phylogenetic criterion has been routinely ignored traits, lability and rates of evolutionary change, and recon- due to supposed functional considerations: exclusion of the struction of ancestral conditions. Phylogenies are also a guide aquatic pinnipeds from comparative studies of carnivores. to basic methodological decisions that go into any compar- Other examples from the comparative literature, such as ex- ative study. Aside from issues about quality and quantity of cluding volant bats from mammal studies (Millar 1981; Hof- comparative data, a researcher must decide at the outset man 1993) or sea birds from avian comparisons (Blackburn which taxa to include in a study. This involves an upper and et al. 1996), suggest that this is not an isolated case. lower bound to taxonomic, or ideally phylogenetic, selection. Evolutionary studies of aquatic organisms have been con- Typically, the lower level is the species. Not only are most troversial and confusing (see Taylor 1914; McLaren 1960; comparative data collected at this level, but species are the Edwards 1985; Bateman 1996). For example, across amniotes fundamental taxonomic unit in biology and, as such, are the (``higher vertebrates'') many groups including sea snakes, focus of many ecological and evolutionary hypotheses. With cetaceans, sirenians, otters, pinnipeds, and penguins have in- species values in hand, comparative analyses are then con- dependently become secondarily aquatic, often confusing ducted among sister taxa that represent independent evolu- evolutionary relationships and patterns for these taxa (Wyss tionary units (Felsenstein 1985). Selecting an upper taxo- 1988; Coates 1991; Coates and Clack 1991). Among mam- nomic unit is more ambiguous, driven largely by what ques- mals, a dif®cult problem involves the origin and evolution tion is asked. If one is interested in evolutionary convergence, of the PinnipediaÐseals (Phocidae), sea lions (Otariidae) and then the upper bound might encompass broad ordinal com- walruses (Odobenidae)Ðwithin the order Carnivora. Al- parisons (e.g., insectivores vs. rodents). If the question in- though the close relationship of pinnipeds to arctoid carni- volves coevolution of parasites and hosts, then the upper level vores has been largely unquestioned since the middle of the might be restricted to a lower taxonomic unit such as a family 19th century (e.g., Turner 1848; Flower 1869), their precise or genus. The essential element for inclusion at this level is sister group relations within arctoids remain controversial. monophyly (Brooks and McLennan 1991; Harvey and Pagel Until recently, the same was true of their now widely accepted monophyletic status as well (for a review, see Bininda- Emonds and Russell 1996). A division is thus often made 2 Present address: Section of Evolution and Ecology, University within carnivores between pinnipeds (literally, ``®n-foot'') of California, Davis, California 95616; E-mail: orbininda@ and the remaining, mostly terrestrial forms, the ®ssipeds (lit- ucdavis.edu. 4 Present address and address for correspondence: Department of erally, ``claw-foot''). This is re¯ected in many taxonomies, Biology, Gilmer Hall, University of Virginia, Charlottesville, Vir- which present pinnipeds as a separate suborder within car- ginia 22903; E-mail: [email protected]. nivores (e.g., Turner 1848; Flower 1869; Mivart 1885; Simp- 1011 q 2000 The Society for the Study of Evolution. All rights reserved. 1012 O. R. P. BININDA-EMONDS AND J. L. GITTLEMAN son 1945; King 1983) or even an order unto themselves (e.g., results (for a discussion of taxon sampling in comparative Scheffer 1958; Ewer 1973; Hall 1981; Corbet and Hill 1991; analyses, see Gittleman 1989b). Nowak 1991). In part, this confusion has resulted in the near- Empirical support for this division within carnivores has complete separation of pinnipeds and ®ssipeds in compara- not been tested. Here, we use phylogenetic comparative meth- tive studies of carnivores, an approach certainly unfounded ods to investigate: (1) possible functional differences be- phylogenetically and perhaps functionally. tween pinnipeds and the remaining ®ssiped carnivores; (2) The more driving justi®cation behind a nonphylogenetic what functional traits underlie observed differences between distinction is simply that pinnipeds are very different due to the two groups and the evolutionary consequences of these their aquatic specializations. Simpson (1944), for example, differences; and (3) the effects of accounting for phylogeny cited carnivores as one of the most fascinating illustrations versus using cross-species analysis. We focus on a wide va- of how numerous ``adaptive zones'' can be expressed in a riety of allometric, physiological, and life-history traits pro- single taxon, well-illustrated by the ``sharply discontinuous'' posed as general aquatic adaptations (following Bartholomew pattern between aquatic carnivores and the remaining taxa. 1970; Repenning 1976; Estes 1989) because these should Clearly, pinnipeds do not resemble another ®ssiped lineage reveal potential taxonomic differences. morphologically, behaviorally, or reproductively. Compara- tive studies of carnivores thus typically exclude pinnipeds MATERIALS AND METHODS for a variety of reasons. These include historical uncertainty Data over a possible dual origin for the group (for a review, see Wyss 1989), the impression that aquatic specializations will We collated data on 20 morphological, life-history, phys- mask functional patterns across terrestrial carnivores (Eisen- iological, and ecological variables that directly test putative berg 1981), and subjective decisions that pinnipeds must be aquatic adaptations hypothesized by Bartholomew (1970), different from ``true carnivores'' despite their phylogenetic Repenning (1976), and Estes (1989) (see Appendix 1; also history. A summary of the argument is provided by Gittleman Bininda-Emonds 1998; O. R. P. Bininda-Emonds, J. L. Git- (1989a, p. ix) in the preface to a book on carnivores: ``I have tleman, and C. K. Kelly, unpubl. ms.). Speci®cally, we ex- thus excluded the pinnipeds from discussion; the behavioral, amine traits that emphasize possible differences in size, phys- ecological, and evolutionary features of adaptations for iology, and life history in contrast to those strictly relating aquatic living set this group apart from the other terrestrial to locomotion and sensory modalities (e.g., vision, olfaction, carnivores. A further, more practical reason is that other vol- and audition). We do not test the latter traits because we umes nicely synthesize recent advances on research on the accept that some obvious, and characteristic, functional dif- pinnipeds.'' The ®nal sentence illustrates that the pinniped- ferences
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