The role of prolactin in parental care in a monogamous and a polyandrous shorebird.

We compared circulating prolactin levels in two species of shorebirds which have very different social systems, and which breed sympatrically at La Perouse Bay, 40 km east of Churchill, Manitoba. Semipalmated Sandpipers (Calidris pusilla) are monogamous and share incubation equally, although females normally desert broods earlier than males. Rednecked Phalaropes (Phalaropus lobatus) are facultatively polyandrous, and only males care for eggs and young. High prolactin values were correlated with persistent incubation behavior in male Rednecked Phalaropes, and male and female Semipalmated Sandpipers. Prolactin levels in Semipalmated Sandpipers increased dramatically at the onset of incubation, and were not different between the sexes. Incubating male phalaropes had greater prolactin values than the non-incubating males and females. Changes in prolactin levels, however, did not explain the early brood desertion of female Semipalmated Sandpipers. Prolactin levels did not decline with age of brood in either sex of this species. Received 16 October 1989, accepted 29 April 1990

Sex-role reversal of a monogamous pipefish without higher potential reproductive rate in females

In monogamous animals, males are usually the predominant competitors for mates. However, a strictly monogamous pipefish Corythoichthys haematopterus exceptionally exhibits a reversed sex role. To understand why its sex role is reversed, we measured the adult sex ratio and the potential reproductive rate (PRR), two principal factors influencing the operational sex ratio (OSR), in a natural population of southern Japan. The adult sex ratio was biased towards females throughout the breeding season, but the PRR, which increased with water temperature, did not show sexual difference. We found that an alternative index of the OSR (Sf/Sm: sex ratio of ‘time in’) calculated from the monthly data was consistently biased towards females. The female-biased OSR associated with sex-role reversal has been reported in some polyandrous or promiscuous pipefish, but factors biasing the OSR differed between these pipefish and C. haematopterus. We concluded that the similar PRR between the sexes in C. haematopterus does not confer reproductive benefit of polygamous mating on either sex, resulting in strict monogamous mating, and its female-biased adult sex ratio promotes female–female competition for a mate, resulting in sex-role reversal

Directional changes in sexual size dimorphism in shorebirds, gulls and alcids

sexual selection; size dimorphism; waders. The Charadrii (shorebirds, gulls and alcids) are one of the most diverse avian groups from the point of view of sexual size dimorphism, exhibiting extremes in both male-biased and female-biased dimorphism, as well as monomor-phism. In this study we use phylogenetic comparative analyses to investigate how size dimorphism has changed over evolutionary time, distinguishing between changes that have occurred in females and in males. Independent contrasts analyses show that both body mass and wing length have been more variable in males than in females. Directional analyses show that male-biased dimorphism has increased after inferred transitions towards more polygynous mating systems. There have been analogous increases in female-biased dimorphism after transitions towards more socially polyandrous mating systems. Changes in dimorphism in both directions are attributable to male body size changing more than female body size. We suggest that this might be because females are under stronger natural selection constraints related to fecundity. Taken together, our results suggest that the observed variation in dimorphism of Charadrii can be best explained by male body size responding more sensitively to variable sexual selection than female body size