PHOTOPERIODICALLY INDUCED TESTICULAR RECRUDESCENCE IN THE TRANSEQUATORIAL MIGRANT DOLICHONYX RELATIVE TO NATURAL PHOTOPERIODS

¹ Department of Zoology, University of North Carolina, Chapel Hill, North Carolina 27514

1.Dolichonyx oryzivorus, the bobolink, breeds in North America in June and July and spends the months of November through March in South America below the equator. Ninety-six males captured during migration were held in an outdoor aviary at Chapel Hill, North Carolina (Latitude 36° N) until start of artificial lighting.

2. Beginning in early October, they were exposed daily to 12.5-hour photoperiods for five weeks to terminate photorefractoriness. Thereafter, different groups were subjected to daily photoperiods of 14, 15, 16, 17 and 18 hours. These groups were sampled at intervals and progress of testicular recrudescence was determined by gonadal weights.

3. Evidence adduced from museum specimens indicates that testicular recrudesence begins about the end of March. In the experiments, recrudescence seemingly was initiated immediately, in November, on the longer photoperiods and proceeded at rates comparable to those shown by a temperate-zone migrant. Drastically different was the response to 14-hour photoperiods, initiation of recrudescence being delayed for ten or more weeks.

4. In South America bobolinks occur between Latitude 8° S and Latitude 30° to 35° S. Day-lengths at these latitudes never exceed 15.5 hours; so even at the December solstice males are not exposed to the longer photoperiods which brought about immediate recrudescence in the experiments. Because bobolinks do not come into breeding condition in the southern hemisphere, this response is viewed as a limiting factor in southward distribution. But since 15-hour photoperiods also induced early recrudescence, one would not expect the species to go as far south as Latitude 30° to 35°. This is the most puzzling lack of fit of the experimental results to nature.

5. Although 14-hour photoperiods greatly delay the onset of recrudescence, they do induce it earlier than its occurrence in nature, even though they are somewhat shorter than the average day-length occurring at the latitudes in the middle of the bobolink's winter range. Thus there is at best an imperfect fit of the experimental results to the normal timing.