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 Tromso, Norway (69°40'N), Goteborg, Sweden (57°42'N), and Milano, Italy (45°26'N) were compared under the same lighting conditions. Male great tits, collected during midwinter, were transferred to Goteborg, 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 (≃1.6 mm) and plasma levels of LH (≃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 Tromso, when day length was greater than 12 hr: and in great tits from Goteborg, 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 (≃6.8 mm) in all three populations. It is concluded that selection pressure has favored individuals with a higher photoperiodic threshold in the north and that because of this and the gonadal growth rate, the onset of breeding in free-living populations cannot occur much earlier than the observed dates for the laying of the very first eggs in the populations (Milano, 22 March; Goteborg, 26 April; Tromso, 3 May). The differences between the three populations were much more pronounced when looking at the photoperiodically induced LH responses. In the Milano birds LH levels increased dramatically when day length had reached 10 hr. The corresponding day length was 11 hr for the Goteborg birds and 11.5 hr for the Tromso birds. For all three populations LH values reached a plateau of about 1 ng/ml. Although separated from Tromso and Milano by the same distance (12- 13 latitudinal degrees) the photoperiodic responses in the birds from Goteborg resembled more those in great tits from Tromso than those from Milano. It is possible that these differences reflect the fact that the Goteborg and the Milano populations have been separated for some 1000 years, whereas the Tromso population was established less than 50 years ago and originated from a more southern population. © 1993 by Academic Press, Inc.