Bioenergetic components of reproductive effort in viviparous snakes: Costs of vitellogenesis exceed costs of pregnancy James U. Van Dyke ⁎, Steven J. Beaupre 1 601 SCEN, Department of Biological Sciences, 1 University of Arkansas, Fayetteville, AR, USA abstract article info Article history: Received 26 June 2011 Received in revised form 17 August 2011 Accepted 17 August 2011 Available online 23 August 2011 Keywords: Embryogenesis Gestation Metabolism Phylogenetic Method Reproductive Physiology Respirometry Ultrasonagraphy Yolk Reproductive effort has been defined as the proportion of an organism's energy budget that is allocated to re- production over a biologically meaningful time period. Historically, studies of reproductive bioenergetics considered energy content of gametes, but not costs of gamete production. Although metabolic costs of vitel- logenesis (MCV) fundamentally reflect the primary bioenergetic cost of reproductive allocation in female reptiles, the few investigations that have considered costs of reproductive allocation have focused on meta- bolic costs of pregnancy (MCP) in viviparous species. We define MCP as energetic costs incurred by pregnant females, including all costs of maintaining gestation conditions necessary for embryogenesis. MCP by our def- inition do not include fetal costs of embryogenesis. We measured metabolic rates in five species of viviparous snakes (Agkistrodon contortrix, Boa constrictor, Eryx colubrinus, Nerodia sipedon, and Thamnophis sirtalis) dur- ing vitellogenesis and pregnancy in order to estimate MCV and MCP. Across all species, MCV were responsible for 30% increases in maternal metabolism. Phylogenetically-independent contrasts showed that MCV were significantly greater in B. constrictor than in other species, likely because B. constrictor yolk energy content was greater than that of other species. Estimates of MCP were not significantly different from zero in any spe- cies. In viviparous snakes, MCV appear to represent significant bioenergetic expenditures, while MCP do not. We suggest that MCV, together with yolk energy content, represent the most significant component of rep- tilian reproductive effort, and therefore deserve greater attention than MCP in studies of reptilian reproduc- tive bioenergetics. © 2011 Elsevier Inc. All rights reserved. 1. Introduction Reproductive effort is a central concept of life history theory (Fisher, 1930; Williams, 1966; Gadgil and Bossert, 1970; Schaffer, 1974; Charlesworth and Leon, 1976; Zera and Harshman, 2001), and has been defined as the proportion of energy, procured over a biologically meaningful time period, that is allocated to reproduction (Hirshfield and Tinkle, 1975). Because energy available for allocation is limited, in- creased energy allocations to reproduction result in trade-offs with competing functions (e.g. activity, growth, maintenance; Dunham et al., 1989), that may reduce parental survival and/or future fecundity, and thus reduce fitness (Williams, 1966; Schaffer, 1974; Charlesworth and Leon, 1976; Niewiarowski and Dunham, 1994; Harshman and Zera, 2006). While reproductive effort describes the energetic invest- ments of reproduction, the fitness consequences of resulting time and energy trade-offs define costs of reproduction (Williams, 1966; Bell, 1980; Tuomi et al., 1983; Reznick, 1985; Niewiarowski and Dunham, 1998; Harshman and Zera, 2006). Historically, empirical examinations of the bioenergetic demands of reproductive effort have focused on the material and energetic contents of gametes and/or offspring, usually as a ratio to maternal body mass or energy content (Relative Clutch Mass, RCM; Ballinger, 1977; Bazzaz et al., 1979; Seigel and Fitch, 1984; Seigel et al., 1986; Vitt, 1981; Vitt and Congdon, 1978; Vitt and Price, 1982). However, size-specific ratios of physiological variables have been shown to vio- late the assumption of isometry inherent to ratio analysis (Packard and Boardman, 1988, 1999). Estimates of reproductive effort made using RCM methods would therefore be expected to also violate the assumption of isometry. More recent comparisons of RCM have uti- lized analysis of covariance (ANCOVA) to correctly standardize for body size (Ford and Seigel, 1989a; Seigel and Ford, 1991; Reznick and Yang, 1993). However, even an ANCOVA-based approach to RCM has limited ability to estimate reproductive effort, for two rea- sons. First, maternal mass or energy content is used as a proxy of total maternal energy budget, because of the difficulty in estimating all of the components of a complete energy budget (Hirshfield and Tinkle, 1975; Tinkle and Hadley, 1975; Congdon et al., 1982; Cuellar, 1984). Second, RCM only accounts for the energy contents of gametes or offspring, and thus ignores other potential parental energetic Comparative Biochemistry and Physiology, Part A 160 (2011) 504–515 ⁎ Corresponding author at: Department of Fisheries and Wildlife Science, Virginia Polytechnic Institute and State University, 1880 Pratt Dr, Blacksburg, VA, USA. Tel.: +1 479 200 1200; fax: +1 540 231 7580. E-mail addresses: vandykeu@vt.edu (J.U. Van Dyke), sbeaupre@uark.edu (S.J. Beaupre). 1 Tel.: +1 479 575 7561; fax: +1 479 575 4010. 1095-6433/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2011.08.011 Contents lists available at SciVerse ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa