Ovarian Growth in Tree Sparrows (spizella Arborea)

T•E gonadal responses of several species of birds exposed to stimulatory daily photoperiods of constant duration conform in that an initial rectilinear relationship exists between the logarithm of testicular weight and time on long days. They conform further in that the rate of photoinduced testicular growth, as determined from the rectilinear relationship, is a function of daylength. These and other observations support the thesis that increased aylength plays an indispensable role in initiating vernal testicular growth and determining its rate in many species. The current consensus i , however, that long days are gonadostimulatory not because they exceed a critical length, but because they illunfinate a photoinducible phase of a circadian oscillation in photosensitivity. (For reviews see Farner, 1959, 1961, 1964a, 1964b, 1970; Hamner, 1966; Lofts et al., 1970.) Quantification of photoperiodic gonadal responses has been limited essentially to males. To our knowledge, only four definitive studies-one on White-crowned Sparrows (Farner et al., 1966) and three on Japanese Quail (Follett and Farner, 1966; Sayler and Wolfson, 1967; Follett and Sharp, 1969)--describe the temporal course and/or rate of ovarian growth induced by constant daylengths, and only one suggests that rate of ovarian growth varies with daylength (Follett and Sharp, 1969). Moreover, although a circadian oscillation in photosensitivity apparently is involved in the control of ovarian growth in Japanese Quail (Follett and Sharp, 1969), there is no evidence for or against involvement of a similar oscillation in the control of photoperiodic ovarian growth in species that breed seasonally. In this report, we describe the temporal course and rate of ovarian growth in Tree Sparrows (Spizella arborea) exposed to daylengths of 8, 11, 15, or 20 hours and examine rate of photoinduced ovarian growth as a function of daylength. In addition, we report results of a preliminary experiment that suggest a circadian oscillation operates in the photoperiodic ovarian response of the Tree Sparrow. The pineal body is exanfined as the possible seat of the circadian oscillator.