Sexual Behaviour and Seasonal Changes in the Gonads and Adrenals of the Mallard.
Authors: Höhn E. O. 
Published: August 2010

Summary.(1) The mallard has an elaborate pair‐forming display which never leads to coition and which occurs from September to February. The female exercises a definite choice of partner in this display. The bearing of these facts on sexual selection is discussed. The frequent occurrence of partial albinism in females only is suggested as offering circumstantial evidence that sexual selection does in fact take place in this species.(2) The simpler pre‐coitional display is described and it is shown, as had already been observed on the Continent, that mating occurs throughout autumn from September onwards. The fact that both these types of display are in evidence long before any general reactivation of the gonads takes place seems to be in disagreement with recent theories, which attribute to sexual display a gonad stimulating function.(3) Various forms of sexual pursuit of strange females by paired males are described as well as a new maternal defence reaction. The frequently hostile relationships between adults and young of other broods of the same species and occasionally their own young are described.A study of 29 adult male and 17 adult female mallards showed the following results:—(4) There is a pronounced fall in body‐weight during the breeding season, accompanied by a profound decrease of depot fat. Females less clearly show a similar trend. An inverse relationship between depot fat and gonadal function seems to exist in this as well as other species of birds.(5) Seasonal changes in weight and histological structure of the testes are described in detail. The weight of the organ varies directly with seminiferous tubular activity, which begins by an appearance of mitotic figures in January, reaching its maximum; spermatogenesis, in February to early March. A rapid decline in tubular activity occurs (though not simultaneously in different individuals) in June and July to a resting condition which persists until January. Interstitial cells, on the other hand, show a minimal development only during July and August, these cells become very prominent again in autumn. Autumnal mating is ascribed to the secretory activity of the interstitial cells.(6) In agreement with previous workers no correlation could be established between seasonal activity of the seminiferous epithelium and seasonal plumage changes. On the other hand, a more close relationship (in time) exists between interstitial cell development and plumage.(7) Ovaries were examined for weight and follicle diameter only and these show a single period of maximal development in the breeding season. The oviduct varies directly in size and weight with ovarian follicular development.(8) A comparison of testicular weight curves with the curve for number of daylight hours supports the experimental evidence of others that gonadal reactivation in spring is controlled by light increase. Temperature changes and atmospheric pressure changes even more so, cannot so readily be correlated with the testicular weight curve.(9) The adrenal glands of both sexes show no significant seasonal variations as regards weight, nor in males as regards histological structure. In females, however, a relative increase of cortical tissue was found at the height of the breeding season.