Female reproductive cycle and sexual conflict over precopulatory mate-guarding in Thermosphaeroma (Crustacea, Isopoda)

In species with time-limited opportunities for insemination, precopulatory mate-guarding is expected to coevolve with the duration of female reproduct ive cycles. Despite this adaptation to female characteristics, it may also be advantageous for males to adjust the duration of guarding with respect t o sex ratio because the benefits of guarding are dependent on the availabil ity of females. If female fitness is reduced because of guarding, male guar ding behavior leads to intersexual conflict. We studied these aspects of ma le mate-guarding behavior in two closely related, thermal-spring isopods (T hermosphaeroma). First, guarding duration showed species specificity which was related to the duration of reproductive cycle; cycle length for females and duration of guarding by males in T. milleri were twice as long as in T . thermophilum. Second, males in both species adjusted their guarding durat ion with sex ratio, guarding longer when a competing male was present. Thir d, in T. thermophilum, ovarian development began immediately after the birt h of the previous brood and continued through guarding, sexual molt and pos t-molt periods until oviposition, whereas in T. miller i, ovarian developme nt was largely postponed until the post-molt period. Because guarding durin g ovary provisioning periods may be costly for females, we tested the exist ence of intersexual conflict over guarding duration in T. thermophilum. We compared the duration of guarding of control pairs with those of pairs in w hich either male guarding ability or female ability to resist guarding was reduced experimentally. Guarding durations for manipulated and control male s were equal, but manipulated females were guarded longer, suggesting that conflict exists and that females can effectively shorten guarding duration by their behavior. Moreover, we suggest that selection in the context of in tersexual conflict may play an important role in the evolution of delayed o viposition and sperm-storage organs in mate-guarding crustaceans.