Photoperiodic adaptation to breeding at different latitudes in great tits.

The photoperiodic responses in testicular growth and plasma levels of LH in nonmigratory great tits from Tromsö, Norway (69 degrees 40'N), Göteborg, Sweden (57 degrees 42'N), and Milano, Italy (45 degrees 26'N) were compared under the same lighting conditions. Male great tits, collected during midwinter, were transferred to Göteborg, and shifted from an 8-hr day in early January to a lighting regime in which day length was increased by 1/2 hr per week for 16 weeks. Control birds (only from Sweden) were kept on a 8L:16D light regime during the entire study. Blood samples were collected weekly, and laparotomies were performed every other week. Initially testicular size (approximately 1.6 mm) and plasma levels of LH (approximately 0.45 ng/ml) were similar among the four groups. Between 4 and 8 weeks after the onset of the experiment testes gradually, but significantly, increased to about 2 mm in length in the control group. This slow growth was also observed in the experimental groups. The control bird testes did not increase in size any further. The onset of the rapid testicular growth phase differed among the three experimental groups. In great tits from Milano it began when day length exceeded 11 hr; in birds from Tromsö, when day length was greater than 12 hr; and in great tits from Göteborg, when day length was between 11 and 12 hr. Once the rapid gonadal growth phase had begun it took about 6 weeks for the testes to reach maximal size (approximately 6.8 mm) in all three populations.(ABSTRACT TRUNCATED AT 250 WORDS)