Light-induced Prolongation of Spermatogenesis in the European Starling, Sturnus Vulgaris

The annual testicular cycle of the European Starling, Sturnus uulgaris, follows the general pattern of many photoperiodic avian species of the north temperate zone. That is, the onset of testicular growth is associated with increasing daily photoperiods following the winter solstice. Captive starlings maintained under natural daylengths at 38” N achieve testicular maturation by mid-April, with the condition persisting until about mid-June. Thereafter, the testes undergo involution and remain quiescent until near the winter solstice ( Schwab, unpubl. data). Bissonnette ( 1931) believed that seasonally increasing daylengths were responsible for the onset of testicular maturation in the European Starling. Later, an extensive series of experiments on the photoperiodic control of starling testis cycles by Bissonnette and coinvestigators showed that the intensity and wave-length of visible light also affected the rate or degree of stimulation of testis activity ( for review, see Bissonnette 1937). Essentially, the amount of testis stimulation increases with increased intensity of illumination and exposure to wave-lengths near the red range of the visible light spectrum. Bissonnette and Wadlund (1932) state that “the duration of any phase of the sexual cycle bore an inverse relation to the effectiveness of the stimulating exposure to light, which, in turn, depended on the daily period, intensity, wave-length and rate of change of illumination.” These findings suggested the possibility that manipulation of some characteristic of the daily photophase could alter the duration or occurrence of certain phases of the testicular cycle. Thus, it was decided to examine the function of daily light durations with respect to induction of testicular involution in the European Starling. The specific factor(s) responsible for gonadal involution are not presently known. A number of hypotheses have been proposed, however, including: heat exhaustion (Cowles and Nordstrom 1946; Vaugien 1951); antagonism between the gonadotrophic and thyrotrophic functions of the pituitary (Davis and Davis 1954); an inherent need for a period of gonadal quiescence (Miller 1954); and a variety of other possibilities. Regardless, testicular involution can be induced by manipulation of the daily photoperiod. For example, testicular involution can be induced precociously by exposing the birds to very short daylengths (Burger 1947; Wolfson 1959; Hamner 1968). It seems, however, that this involution is not necessarily followed by insensitivity of the gonads to long daylengths (photorefractoriness), although such insensitivity follows spontaneous involution in most photoperiodic avian species. It appears likely that natural testicular involution is a function of long daily photoperiods, as suggested by a number of authorities (Famer 1964; Wolfson 1964; Famer and Follett 1966; Hamner 1966). The duration of time that testes remain in a spermatogenic condition can also be influenced by photoperiodic manipulation. Threadgold ( 1960) documented spermatogenesis for 200 days in English Sparrows, Passer domesticus, exposed to a chronic lL23D photoperiod. More realistic as to the photoperiods which animals might experience under natural conditions (e.g., equatorial species) is the 12L-12D regimen employed by Wolfson ( 1959) to prolong spermatogenesis for “almost one year” in Slate-colored Juncos, Junco hyemalis. This important finding seems never to have been extended to other avian species. Extensive prolongation of testicular activity has not been achieved in starlings under 12L12D (Burger 1953; Schwab and Lott 1969). The results reported here, however, document that certain other chronic daily photoperiods do indeed prolong spermatogenesis in starlings for at least 15 months.