| Shark life history more closely resembles that of mammals than of other fish: they grow slowly, have low fecundity, and give live birth to well developed young. Reproductive behavior in sharks is largely cryptic, difficult to impossible to observe directly, and consequently little is known. In sharks, as in many vertebrates with conventional sex-roles, females bear the burden of reproductive investment and are thus expected to exercise monogamy (female mating with a single male). Recently, however, genetic studies of mating in species previously assumed to adhere to conventional sex roles have turned up surprising complexity. We now know polyandry (female mating with multiple males) to be a common strategy in reptiles, amphibians, crustaceans, insects, mammals, birds, and fish, including sharks.;Though the advent and application of DNA markers allows for the characterization of genetic mating systems, few studies to date have examined this in sharks because of difficulty in sampling. By examining the cryptic reproductive behavior of sharks in Hawaii using a variety of genetic techniques, I found that the sandbar shark (Carcharhinus plumbeus) the bignose shark ( Carcharhinus altimus), and the shortspine spurdog (Squalus mitsukurii) are capable of multiple paternity. Further, both the sandbar shark and spurdog in Hawaii showed a predominance of genetic monogamy in their mating system (40% and 11% multiple paternity, respectively). A comparison of maternal and biparentally-inherited DNA patterns in global populations of scalloped hammerhead sharks (Sphyrna lewini) showed clear male-biased dispersal patterns (global phiST = 0.749, global FST = 0.039), results which are consistent with two previously published studies investigating sex-biased dispersal in sharks.;From these results I conclude that polygynandry appears to be the rule for sharks, and that reproductive burden may constrain female dispersal relative to males since male-biased dispersal is the apparent trend in the three shark species investigated to date, including scalloped hammerhead sharks. Proximate causes influencing the frequency of multiple paternity likely include encounter rate and sexual conflict, with female mating avoidance to minimize trauma. Factors that may reduce encounter rate include spatial segregation of sexes, asynchronous reproductive cycle, and habitat. |
