766 Comparative Animal Physiology 



post-nuptial molt immediately after the breeding period; the new regenerat- 

 ing plumage becomes the winter or eclipse plumage. A second, usually much 

 less extensive molt, the pre-nuptial, occurs in the spring. The regenerating 

 plumage constitutes the breeding or nuptial plumage. This latter molt is most 

 extensive in those species which exhibit conspicuous breeding coloration. 

 The times at which birds assume the nuptial and winter plumages are gov- 

 erned by a number of factors. In some species, such as the African weaver 

 finch, Pyromelanox, and the starling, the changes appear due to an inherent 

 cyclically rhythmic hypophyseal activity,^'^'^ although the occurrence of the 

 rhythm may depend in part on length of daily light periods.^^- ^^' ^^^ Some 

 other species, such as mallard ducks, Anas platyrhyncha, the white-throated 

 sparrow, and the bob-white, can be induced to molt and undergo a plumage- 

 type change at a time other than their regular season by subjection to arti- 

 ficially increasing or decreasing light periods. Therefore, the annual plumage 

 changes of most birds, like migrating and breeding activity, appear to be con- 

 trolled in good measure by the annual cycle of day lengths. 



Certain northern birds and mammals show a seasonal color change from 

 brown in summer to white in winter. The times of these changes in such 

 forms as the ptarmigan, Lagopus,^^ the ermine, Mustela,-^ and the varying 

 hare, Lepus,^^"^ have been shown to be determined by the seasonal changes 

 in day lengths; the animals could be caused to whiten out of season by ap- 

 propriate experimental alteration of the daily lighting. Experiments involv- 

 ing masking of the varying hare^^^ indicate that the eyes are the normal re- 

 ceptors. Both in the natural cycle of color change and in color changes in- 

 duced artificially by modification of illumination, the varying hare is phys- 

 iologically brown when large amounts of gonadotropic hormones are pres- 

 ent in the blood and is physiologically white when these hormones are low in 

 concentration. Molting in a physiologically brown animal is followed by 

 production of brown hair; in a physiologically white one, by production of 

 white hair. Extracts of whole pituitary containing gonadotropic hormones 

 will convert physiologically white animals into physiologically brown ones 

 and simultaneously induce shedding of the white hair. Hypophysectomy in 

 ferrets abolishes the cyclic molting.^^- ^^ No endocrine gland other than the 

 pituitary appears to be involved in these color changes. Castrated and thy- 

 roidectomized hares undergo the normal seasonal color changes.^"^ 



HORMONES AND BIRD MIGRATION 



Some years ago it was postulated that the migrational behavior of birds 

 was under the control of hormones from the gonads.^^^- ^^"' ^^^ Northward 

 migration of birds in the northern hemisphere was considered a result of the 

 recrudescence of the gonads in the spring; regression of the gonads after the 

 breeding season was correlated with migration southward. In the junco, the 

 northward migration appeared correlated with periods of greater interstitial 

 cell activity of the gonads. In the crow, southward migration appeared in- 

 dependent of gonadal regression, as indicated by castration. In the European 

 starling, northward migration appeared correlated with gonadal secretion and 

 southward migration with absence of secretion.'*^- ^^ 



Other evidence seems to indicate with moderate clarity that changes in 

 gonadal activity are not causally related to migrational behavior. Removal of 



