Sex differences in testosterone and hematocrit levels reflect mating system differences of two Arctic-breeding shorebird species

Sex steroids, such as testosterone, are critical for the development of secondary sexual characteristics and shape traits beneficial for competition over mates and resources. Testosterone profiles may thus differ depending on variation in female and male mating strategies. Sex and mating system differences may also be found in hematocrit profiles, given elevated hematocrit levels during energetically demanding life stages such as migration or during sexual competition. Thus, males of polygynous species should maintain higher testosterone and hematocrit throughout the breeding season compared to monogamous or polyandrous males. Less is known about how mating systems affect testosterone and hematocrit in females: a recent study found higher testosterone in females of classically polyandrous species with reversed sex roles compared to females with typical sex roles. Here we compare baseline and peak plasma testosterone levels (induced by injecting gonadotropin releasing hormone GnRH) and hematocrit values in polygynous pectoral sandpipers and in classically polyandrous red phalaropes. In males, baseline testosterone concentrations were higher in the polygynous than in the classically polyandrous species, whereas in females, this pattern was reversed, with testosterone concentrations tending to be higher in the classically polyandrous species than in the polygynous one. In both sexes, the magnitude of the GnRH-induced increase in testosterone did not differ between species. Hematocrit was higher in the sex with higher competition for mates: in pectoral sandpipers, males had higher hematocrit than females; in red phalaropes, females had higher hematocrit than males. In conclusion, our results show that physiological parameters partially reflect differences in mating strategies.