A NEOTROPICAL FOREST BIRD CAN MEASURE THE SLIGHT CHANGES IN TROPICAL PHOTOPERIOD

Many tropical birds breed seasonally, but it is largely unknown which environmental cues they use to time reproduction. Changes in tropical photoperiod have been regarded as too small to be used as a proximate environmental cue. This hypothesis, however, has never been rigorously tested. Here, we report on experimental evidence that photoperiodic c hanges characteristic of tropical latitudes stimulate reproductive act ivity in a neotropical bird from the forest understory. In the central Republic of Panama (9 degrees N), photoperiod varies annually between 12 hours (December) and 13 hours (June). Free-living spotted antbirds (Hylophylax n. naevioides) had regressed gonads in December, but incr eased gonads ahead of the rainy (the breeding) season in May Captive s potted antbirds exposed to a 'long' photoperiod of 13 hours increased gonadal size eight-fold and song activity six-fold over that of contro l birds remaining on a simulated 'short' photoperiod of 12 hours of da ylight. Moreover, even a photoperiod of 12 hours 17 minutes was suffic ient to stimulate gonadal growth in photostimulated birds over that of controls. The dramatic changes in gonadal development were not accomp anied by similar changes in hormone titres such as luteinizing hormone and testosterone as expected from temperate zone birds. We propose a more general role of the tropical photoperiod in the regulation of sea sonal events in tropical organisms, or in temperate zone species migra ting to the tropics.