Androgen in high concentrations in the blood of female western gulls, Larus occidentalis.

the sun's arc is always exactly adapted to the local situation where the birds live, which may differ markedly within the dis- tribution range of a bird species. And it may even include the possibility of their adjusting the sun compass to the seasonal changes in the sun's arc. Calculations have indicated that the pigeons' sun compass pays regard to the different rates of change of the sun's azimuth during the day [8]; our findings here offer a simple explana- tion how this is achieved. Supported by the Deutsche Forschungsge- meinschaft; all calculations were carried out at the Hochschulrechenzentrum of the Universitfit Frankfurt a.M. We gratefully acknowledge the help of A. Doss-Scholz, W. Mann and U. Kotzur, and we thank W.T. Keeton for critically reading an early draft of this manuscript. that some females are capable of establish- ing territories, and that there are few dif- ferences in sexual behavior between the sexes. Males tend to mount and copu- late more than females although the latter have been observed to mount other fe- males and occasionally males [5]. Further- more, males and females have identical plumage and there is some overlap in body size. Given the slight sexual dimorphism in be- havior, identical plumage and similarity in body size in western gulls, it is perhaps not surprising that plasma levels of andro- gens are so similar in the two sexes. In females plasma levels of estrogens increase only in spring at the time of development of precopulatory displays [5]. The similarity in levels of the two andro- gens measured in male and female western gulls, does not, of course, preclude the pos- sibility of differences in the secretion of other androgens, such as androstenedione. Furthermore, it is possible, indeed very probable, that there are differences be- tween the sexes in numbers and sites of androgen receptors. Also not to be pre- cluded are possible differences in aromati- zation of testosterone to estradiol in non- ovarian tissues [6]. Received June 23, 1980 1. Schmidt-Koenig, K.: Naturwissenschaften 2. Wiltschko, W., Wiltschko, R., Keeton, W.T. : Behav. Ecol. Sociobiol. 1, 229 (1976) 3. Batschelet, E.: Statistical Methods for the Analysis of Problems in Animal Orientation and certain Biological Rhythms. Washington, D.C.: Amer. Inst. Biol. Sci. 1965 4. Lindauer, M.: Z. vergl. Physiol. 42, 43 5. Keeton, W.T. : Science 165, 922 (1969) 6. Walcott, Ch., Green, R.P.: ibid. 184, 180 7. Hoffmann, K. : Naturwissenschaften 40, 148 8. Wiltschko, R. : in preparation Androgen in High Concentrations in the Blood of Female Western Gulls, Larus occidentalis J.C. Wingfield, A. Newman, G.L. Hunt, Jr. and D.S. Farner Department of Zoology, University of Washington, Seattle 98195 and, Department of Ecology and Evolutionary Biology, University of California, Irvine 92717 On Santa Barbara Island, California, breeding colonies of western gulls contain significant numbers of female-female pairs which produce and incubate clutches that are often larger than the normal three eggs [t]. By radioimmunoassay of the plasma concentration of androgens [2] we tested the hypothesis that the formation of a fe- male-female pair is based on an endocrine masculinization of one of its members. Since we found no significant differences in the plasma levels of testosterone during the breeding season among homosexual fe- males, heterosexual females and breeding males (Table 1), and also no differences in levels of dihydrotestosterone (DHT), the hypothesis was rejected. It appears that breeding plumage and most aspects of reproductive behavior of both sexes in at least several species of gulls are under the control of androgens, with estrogens apparently involved primarily in the functional development of the repro- ductive apparatus and uniquely female be- havior [3]. Although testosterone has been reported from the plasma of breeding fe- males of several avian species, and very likely is of general occurrence in female For species of birds in which the male and female are equally involved in defense of territory, in which plumages are similar and in which there are few sex-specific traits, we now hypothesize that the circu- lating concentrations of androgens may also be similar. Furthermore, we suggest that androgens may play a more significant role in the reproductive function and be- havior of female birds than has thus far been recognized. birds, the female-male ratio in plasma le- vels in breeding western gulls is approxi- mately an order of magnitude greater than in other species thus far studied [2, 4]. The apparently equal levels of androgen in the sexes can be rationalized, at least in part, by quantified observations of be- havior that show that male and female western gulls defend territory equally well, Table 1. Plasma concentrations of testosterone (pg/ml plasma) in male and female western gulls at different phases in a reproductive cycle (means + S.E.). An analysis of the plasma concentration of testosterone as a function of time of day yielded no indication that diurnal cycles contribute to the variance of the means Phase Courtship Ovulation Parental N Testosterone N Testosterone N Testosterone II Male Female Homosexual female Male Female Homosexual female Naturwissenschaften 67 (1980) 9 by Springer-Verlag 1980