Moore and University of Washington, Seattle, researchers John C. Wingfield in biology and Eliot A. Brenowitz in psychology, looked at seasonal changes in the brains of birds that account for their singing, which is a part of the male mating behavior. In the Nov. 10 issue of The Journal of Neuroscience (www.jneurosci.org/), the official journal of the Society for Neuroscience, they report that birds in high latitudes are driven to sing by seasonal changes in the length of the day, which causes changes in the song-control nuclei of the brain. However, in the tropics, where the day length does not vary much by season, the propensity for birds to sing still changes, but is driven by environmental cues that vary by locale -- a fact that could mean those birds are more susceptible to global warming than birds in higher latitudes.
Correct timing of breeding is necessary for reproductive success, Moore said. Research by others has shown that testosterone is the main physiological cue regulating seasonal changes in the neural song-control system. Seasonal changes in the song-control system have been demonstrated by other scientists in all northern latitude species that have been investigated. But no one had researched whether seasonal changes occurred in the brains of birds in tropical areas where day-length changes are minimal. "We think it's probably still testosterone that causes tropical birds to sing, but that the environmental cue is different," Moore said. The scientists wanted to determine whether "seasonal changes in brain structure can be mediated by local environmental cues."
Moore and his colleagues looked at two populations of the rufous-collared sparrow only 25 km apart. The two populations are at the same latitude but are on the east and west slopes of the Andes, which have very different climate patterns. "At the time of year when birds in the Papallacta population were breeding (August to September), birds in Pintag were in nonbreeding state," the researchers wrote in The Journal of Neuroscience ("Plasticity of the Avian Song Control System in Response to Localized Environmental Cues in an Equatorial Songbird"). "Correspondingly, the song control nuclei were fully grown in the breeding Papallacta population when they were regressed in the nonbreeding Pintag population. Singing behavior also changed seasonally in both populations.
"Our observations of seasonal brain plasticity in these tropical birds demonstrate that the vertebrate brain is extremely flexible and sensitive to diverse environmental cues that can time seasonal reproductive physiology and behavior," they wrote. While it is not yet known what environmental cues signal breeding time, Moore hypothesizes that it could be rainfall, temperature, or food availability -- or all these cues.
Human beings are contributing to global warming, which affects factors such as temperature and rainfall, and thus food availability, but does not affect the seasonal day-length changes of higher latitudes. Therefore, Moore said, global warming could change the brain functions of tropical birds and cause problems with the timing of their mating seasons. "We're not going to change day length," he said. "We can change weather patterns. Studies show changes in the timing of breeding and migration in birds." Global warming could be the reason, he said, and, if the brain is truly sensitive to environmental cues, the changes due to global warming could have "effects we haven't thought of before."
While Moore's research was driven primarily by curiosity and not by conservation concerns, "you can't save things if you don't understand them," he said. "Every little bit of knowledge about how things works is useful." They will next try to determine which specific environmental cues are affecting tropical-bird mating processes. Moore's research is funded through fellowships and grants from the Society for Neuroscience and the National Science Foundation.