Seasonality of reproduction in a neotropical rain forest bird

Tropical wet forests are commonly perceived as stable and constant environm ents. However, many rain forest organisms reproduce seasonally. To understa nd the proximate regulation of life history events in tropical organisms, w e asked three questions: (1) How predictable are seasonal changes in the tr opical rain forest? (2) Can tropical organisms anticipate environmental sea sonality, despite the presumed lack of long-term environmental cues in near -equatorial areas? (3) What environmental cues can tropical organisms use? We studied Spotted Antbirds, monogamous understory insectivores, which star ted breeding in Panama (9 degrees N) in May (wet season) and continued unti l September/October. Breeding patterns were consistent between years, indic ating that tropical seasons were as predictable for Spotted Antbirds (predi ctability 70%) as they are for many north temperate birds. Individual Spott ed Antbirds shut down reproductive capacity (i.e., decreased gonad size) fr om October until February. In March, during the height of the dry season an d about six weeks ahead of the wet season, gonads started to grow again. Th e growth rate of gonads was influenced by the amount of rainfall, which has been shown to correlate with food abundance. Gonad growth was paralleled b y changes in luteinizing hormone, but not in testosterone, which remained a t very low plasma levels year-round. The latter contrasts with the pattern for most migratory temperate-zone birds. Seasonal changes in reproductive a ctivity correlated strongly with changes in the tropical photoperiod, but w eakly with light intensity and rainfall, and nor with temperature. Thus, Sp otted Antbirds likely anticipated rain forest seasonality using long-term c ues (tropical photoperiod) and fine-tuned their reproductive activities usi ng short-term cues (food/rainfall). The use of long-term environmental info rmation could apply to most vertebrate species that live in the tropics.