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Many life-history traits co-vary across species, even when body size differences are controlled for. This phenomenon has led to the concept of a "fast-slow continuum," which has been influential in both empirical and theoretical studies of life-history evolution. We present a comparative analysis of mammalian life histories showing that, for mammals at least, there is not a single fast-slow continuum. Rather, both across and within mammalian clades, the speed of life varies along at least two largely independent axes when body size effects are removed. One axis reflects how species balance offspring size against offspring number, while the other describes the timing of reproductive bouts.

Pinnipeds (seals, sea lions and walruses) are secondarily marine carnivorans that exhibit a wide range of feeding and reproductive specializations. Extant pinnipeds are split into three families: Phocidae (seals), Otariidae (sea lions) and Odobenidae (walruses). Morphometric analyses were used to examine cranial morphology in otariid and phocid pinnipeds. Phocids are more ecologically and taxonomically diverse than otariids, and this study quantitatively assessed the effects of life history, phylogeny and ecology on cranial morphology in these closely related clades of aquatic carnivorans. Fifty-three to 58 three-dimensional landmarks were gathered from 138 specimens, representing 31 of the 33 extant species of otariids and phocids. Principal components analysis was used to identify major axes of variation, and principal component scores were compared with phylogenetic distances and ecological variables to test for significant correlates of skull morphology. Results showed that phocids exhibit a much greater diversity of adult skull morphology than otariids. Shape differences within adult otariids were dominated by males of only one species, Otaria flavescens. In contrast, several species of phocids deviated markedly from the mean phocid morphology. These atypical morphologies were consistently associated with specializations of either feeding or mating strategies. Ontogenetic shape changes are greater, relative to interspecific differences, in otariids than in phocids, and shape dimorphism was observed in only one otariid and two phocid species. Unexpectedly, neither otariids nor phocids showed strong correlations between phylogenetic relationship and cranial morphology. Both clades show strong correlations between cranial shape and some life history and some environmental variables, but phocids show stronger correlations with life-history variables, perhaps reflecting the broad range of reproductive strategies observed in phocids. [ABSTRACT FROM AUTHOR]

The mammalian os penis or baculum is an extremely diverse bone but our understanding of the selective forces behind this diversity remains poor. While genital morphology is generally assumed to be sexually selected, there have been relatively few attempts to test this hypothesis. Here we investigate associations between baculum length and two measures of sexual selection intensity (mating system and testis mass) across bats. Analyses were conducted using both species-level data and phylogenetic comparative techniques. Controlling for the effect of male body mass, baculum length was positively associated with relative testis mass in all cross-species analyses. Additionally, in an analysis that also included mating system, there was an interaction between testis mass, body mass and mating system that significantly influenced baculum length. However, after using independent contrasts to control for phylogenetic inertia in these data, baculum length was not significantly associated with mating system, testis mass or body mass. Thus, increased length of the baculum in bats appears not to be associated with increased sperm competition risk or sexual selection intensity. [ABSTRACT FROM AUTHOR]