Multiple paternity, testes mass and body mass data were collected from the studies listed below. In addition, unpublished data was contributed by Geremy Cliff, Sheldon Dudley, Edward Farrell, Wally Bubley and the National Oceanographic and Atmospheric Association (NOAA). For further details, please see Supporting Information Table S2.

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Baremore, I.E. & Hale, L.F. 2012. Reproduction of the Sandbar Shark in the Western North Atlantic Ocean and Gulf of Mexico. Mar. Coast. Fish. 4: 560�572. 
Blais, J., Rico, C. & Bernatchez, L. 2004. Nonlinear effects of female mate choice in wild threespine sticklebacks. Evolution. 58: 2498�510.
Boomer, J.J., Harcourt, R.G., Francis, M.P., Walker, T.I., Braccini, J.M. & Stow, A.J. 2013. Frequency of multiple paternity in gummy shark, Mustelus antarcticus, and rig, Mustelus lenticulatus, and the implications of mate encounter rate, postcopulatory influences, and reproductive mode. J. Hered. 60: 371�9.
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Cogliati, K.M., Neff, B.D. & Balshine, S. 2013. High degree of paternity loss in a species with alternative reproductive tactics. Behav. Ecol. Sociobiol. 67: 399�408. 
de Bruyn, P., Dudley, S., Cliff, G. & Smale, M. 2005. Sharks caught in the protective gill nets off KwaZulu-Natal, South Africa. 11. The scalloped hammerhead shark Sphyrna lewini (Griffith and Smith). African J. Mar. Sci. 27: 517�528.  
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Egger, B., Oberm�ller, B., Phiri, H., Sturmbauer, C. & Sefc, K.M. 2006. Monogamy in the maternally mouthbrooding Lake Tanganyika cichlid fish Tropheus moorii. Proceedings. Biol. Sci. 273: 1797�802. 
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Farrell, E.D., O�Sullivan, N., Sacchi, C. & Mariani, S. 2014. Multiple paternity in the starry smooth-hound shark Mustelus asterias (Carcharhiniformes: Triakidae). Biol. J. Linn. Soc. 111: 119�125. 
Fitzpatrick, J.L., Desjardins, J.K., Stiver, K.A., Montgomerie, R. & Balshine, S. 2006. Male reproductive suppression in the cooperatively breeding fish Neolamprologus pulcher. Behav. Ecol. 17: 25�33. 
Fitzpatrick, J.L., Earn, D.J.D., Bucking, C., Craig, P.M., Nadella, S., Wood, C.M., et al. 2016. Postcopulatory consequences of female mate choice in a fish with alternative reproductive tactics. Behav. Ecol. 27: 312�320. 
Fitzpatrick, J.L., Kempster, R.M., Daly-Engel, T.S., Collin, S.P. & Evans, J.P. 2012. Assessing the potential for post-copulatory sexual selection in elasmobranchs. J. Fish Biol. 80: 1141�1158. 
Fitzpatrick, J.L., Montgomerie, R., Desjardins, J.K., Stiver, K. a, Kolm, N. & Balshine, S. 2009. Female promiscuity promotes the evolution of faster sperm in cichlid fishes. Proc. Natl. Acad. Sci. U. S. A. 106: 1128�32.
Griffiths, A.M., Jacoby, D.M.P., Casane, D., McHugh, M., Croft, D.P., Genner, M.J., et al. 2012. First analysis of multiple paternity in an oviparous shark, the small-spotted catshark (Scyliorhinus canicula). J. Hered. 103: 166�173.
Hain, T.J.A. & Neff, B.D. 2007. Multiple paternity and kin recognition mechanisms in a guppy population. Mol. Ecol. 16: 3938�3946. 
Hellmann, J.K., Ligocki, I.Y., O�Connor, C.M., Reddon, A.R., Garvy, K.A., Marsh-Rollo, S.E., et al. 2015a. Reproductive sharing in relation to group and colony-level attributes in a cooperative breeding fish. Proceedings. Biol. Sci. 282: 20150954. 
Hellmann, J.K., O�Connor, C.M., Ligocki, I.Y., Farmer, T.M., Arnold, T.J., Reddon, A.R., et al. 2015b. Evidence for alternative male morphs in a Tanganyikan cichlid fish. J. Zool. 296: 116�123.
Jones, A.G. & Avise, J.C. 1997a. Microsatellite analysis of maternity and the mating system in the Gulf pipefish Syngnathus scovelli, a species with male pregnancy and sex-role reversal. Mol. Ecol. 6: 203�213. 
Jones, A.G. & Avise, J.C. 1997b. Polygynandry in the dusky pipefish Syngnathus floridae revealed by microsatellite DNA markers. Evolution. 51: 1611�1622.
Jones, A.G., Kvarnemo, C., Moore, G.I., Simmons, L.W. & Avise, J.C. 1998. Microsatellite evidence for monogamy and sex-biased recombination in the Western Australian seahorse Hippocampus angustus. Mol. Ecol. 7: 1497�1505. 
Jones, A.G., Rosenqvist, G., Berglund, A. & Avise, J.C. 1999. The genetic mating system of a sex-role-reversed pipefish ( Syngnathus typhle ): a molecular inquiry. Behav. Ecol. Sociobiol. 46: 357�365.
Jones, A.G., Walker, D. & Avise, J.C. 2001. Genetic evidence for extreme polyandry and extraordinary sex-role reversal in a pipefish. Proceedings. Biol. Sci. 268: 2531�5. 
Jones, A.G., Walker, D., Kvarnemo, C., Lindstr�m, K. & Avise, J.C. 2001a. How cuckoldry can decrease the opportunity for sexual selection: data and theory from a genetic parentage analysis of the sand goby, Pomatoschistus minutus. Proc. Natl. Acad. Sci. U. S. A. 98: 9151�6. 
Jones, A.G., Walker, D., Lindstr�m, K., Kvarnemo, C. & Avise, J.C. 2001b. Surprising similarity of sneaking rates and genetic mating patterns in two populations of sand goby experiencing disparate sexual selection regimes. Mol. Ecol. 10: 461�469.
Katoh, R., Munehara, H. & Kohda, M. 2005. Alternative Male Mating Tactics of the Substrate Brooding Cichlid Telmatochromis temporalis in Lake Tanganyika. Zoolog. Sci. 22: 555�561.  Zoological Society of Japan .
Kelly, C.D., Godin, J.-G.J. & Wright, J.M. 1999. Geographic variation in multiple paternity within natural populations of the guppy (Poecilia reticulata). Proc. R. Soc. B Biol. Sci. 266: 2403�2408. 
Kvarnemo, C., Moore, G.I., Jones, A.G., Nelson, W.S. & Avise, J.C. 2000. Monogamous pair bonds and mate switching in the Western Australian seahorse Hippocampus subelongatus. J. Evol. Biol. 13: 882�888.
Kvarnemo, C. & Simmons, L.W. 2004. Testes investment and spawning mode in pipefishes and seahorses (Syngnathidae). Biol. J. Linn. Soc. 83: 369�376. 
Kvarnemo, C., Svensson, O. & Manson, W. 2010. Investment in testes, sperm-duct glands and lipid reserves differs between male morphs but not between early and late breeding season in Pomatoschistus minutus. J. Fish Biol. 76: 1609�1625. 
Lage, C.R., Petersen, C.W., Forest, D., Barnes, D., Kornfield, I. & Wray, C. 2008. Evidence of multiple paternity in spiny dogfish ( Squalus acanthias ) broods based on microsatellite analysis. J. Fish Biol. 73: 2068�2074. 
Largiad�r, C.R., Fries, V. & Bakker, T.C.M. 2001. Genetic analysis of sneaking and egg-thievery in a natural population of the three-spined stickleback (Gasterosteus aculeatus). Heredity. 86: 459-468. 
Lorenzoni, M., Martin D�rr, A.J., Erra, R., Giovinazzo, G., Mearelli, M. & Selvi, S. 2002. Growth and reproduction of largemouth bass (Micropterus salmoides Lac�p�de, 1802) in Lake Trasimeno (Umbria, Italy). Fish. Res. 56: 89�95. 
Marino, I.A.M., Riginella, E., Gristina, M., Rasotto, M.B., Zane, L. & Mazzoldi, C. 2015. Multiple paternity and hybridization in two smooth-hound sharks. Sci. Rep. 5: 12919. 
Massey, B.R. & Francis, M.P. 1989. Commercial catch composition and reproductive biology of rig (Mustelus lenticulatus) from Pegasus bay, Canterbury, New Zealand. New Zeal. J. Mar. Freshw. Res. 23: 113�120. 
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Mobley, K.B., Amundsen, T., Forsgren, E., Svensson, P.A. & Jones, A.G. 2009. Multiple mating and a low incidence of cuckoldry for nest-holding males in the two-spotted goby, Gobiusculus flavescens. BMC Evol. Biol. 9: 6.
Mobley, K.B. & Jones, A.G. 2009. Environmental, demographic, and genetic mating system variation among five geographically distinct dusky pipefish (Syngnathus floridae) populations. Mol. Ecol. 18: 1476�1490. 
Mobley, K.B. & Jones, A.G. 2007. Geographical variation in the mating system of the dusky pipefish (Syngnathus floridae). Mol. Ecol. 16: 2596�2606. 
Neff, B.D. 2001. Genetic Paternity Analysis and Breeding Success in Bluegill Sunfish (Lepomis macrochirus). J. Hered. 92: 111�119. 
Neff, B.D. & Clare, E.L. 2008. Temporal variation in cuckoldry and paternity in two sunfish species (Lepomis spp.) with alternative reproductive tactics. Can. J. Zool. 86: 92�98.
Neff, B.D., Fu, P. & Gross, M.R. 2003. Sperm investment and alternative mating tactics in bluegill sunfish (Lepomis macrochirus). Behav. Ecol. 14: 634�641. 
Neff, B.D., Pitcher, T.E. & Ramnarine, I.W. 2008. Inter-population variation in multiple paternity and reproductive skew in the guppy. Mol. Ecol. 17: 2975�2984. 
Orlando, E.F., Bass, D.E., Caltabiano, L.M., Davis, W.P., Gray, L.E. & Guillette, L.J. 2007. Altered development and reproduction in mosquitofish exposed to pulp and paper mill effluent in the Fenholloway River, Florida USA. Aquat. Toxicol. 84: 399�405. 
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Sa�di, B., Brada�, M.N. & Boua�n, A. 2009. Reproductive biology and diet of Mustelus punctulatus (Risso, 1826) (Chondrichthyes: Triakidae) from the Gulf of Gab�s, central Mediterranean Sea. Sci. Mar. 73: 249�258.
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Schaedelin, F.C., van Dongen, W.F.D. & Wagner, R.H. 2015. Mate choice and genetic monogamy in a biparental, colonial fish. Behav. Ecol. 26: 782�788.
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Sefc, K.M., Mattersdorfer, K., Sturmbauer, C. & Koblm�ller, S. 2008. High frequency of multiple paternity in broods of a socially monogamous cichlid fish with biparental nest defence. Mol. Ecol. 17: 2531�2543. 
Skinner, A.M.J. & Watt, P.J. 2007. Phenotypic correlates of spermatozoon quality in the guppy, Poecilia reticulata. Behav. Ecol. 18: 47�52. 
Soucy, S. & Travis, J. 2003. Multiple paternity and population genetic structure in natural populations of the poeciliid fish, Heterandria formosa. J. Evol. Biol. 16: 1328�1336. 
Stiver, K.A., Fitzpatrick, J.L., Desjardins, J.K. & Balshine, S. 2009. Mixed parentage in Neolamprologus pulcher groups. J. Fish Biol. 74: 1129�1135. 
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