Plasticity of the Avian Song Control System in Response to Localized Environmental Cues in an Equatorial Songbird

Abstract

A striking feature of the vertebrate brain is its plasticity. In high-latitude vertebrates, seasonal plasticity of the brain is driven by ubiquitous photoperiod cues and therefore is highly predictable and synchronous across extensive geographic ranges. A pronounced example of seasonal brain plasticity occurs in the nuclei that regulate song behavior in songbirds. These nuclei are larger in breeding than in nonbreeding birds. In the tropics, photoperiod varies little annually, and other environmental cues important for breeding can show considerable local geographic variability. We investigated whether localized patterns of seasonal breeding in tropical birds are associated with brain plasticity. We studied two populations of rufous-collared sparrows (Zonotrichia capensis) that breed, only 25 km apart, on the equator but out of phase with each other. We measured gonadal activity and the size of song nuclei (high vocal center, robust nucleus of the arcopallium, and area X) during each population's breeding and nonbreeding periods. Breeding males had larger song nuclei and greater gonadal activity than did nonbreeding birds. This plasticity was associated with local environmental cues, such that the two populations exhibit asynchronous changes in brain structure. These results demonstrate the sensitivity of the brain and its ability to use a variety of environmental cues to coordinate seasonal plasticity and reproduction.

Photoperiod-Independent Changes in Immunoreactive Brain Gonadotropin-Releasing Hormone (GnRH) in a Free-Living, Tropical Bird

“…This is most similar to the opportunistically breeding crossbills [MacDougall-Shackleton et al, 2001]. Interestingly, both species also exhibit seasonal changes in the song control system [MacDougall-Shackleton et al, 2001;Moore et al, 2004b] and fl exible breeding schedules [Hahn, 1998;Moore et al, 2005]. The similarities in GnRH-ir among the current tropical species, opportunistically breeding species [MacDougall-Shackleton et al, 2001;Pereyra et al, 2005], and species that exhibit relative photorefractoriness [Foster et al, 1988] suggest that more robust plasticity of the GnRH system evolved with obligate photoperiodicity.…”

“…At that time, the brains were cryoprotected in 30% sucrose until they sank, then frozen on dry ice and stored at -80 ° C. All work was approved by the University of Washington IACUC. Hormone data and attributes of the neural song control system were previously reported from these same birds [Moore et al, 2004b].…”

“…The Papallacta population generally breeds from mid-August to December, and the Pintag population breeds from mid-November through April. Males of this species only sing during the breeding season, and do not sing during molt or during the non-breeding period [Moore et al, 2004b]. The telencephalic song control system shows seasonal growth and regression paralleling seasonal changes in song behavior.…”

“…Thus this seasonal neuroplasticity is also out of phase between the populations [Moore et al, 2004b]. During the breeding period males have elevated plasma levels of testosterone that are as high as those observed in closely related northern species [Moore et al, 2002], with seasonal peaks that are also asynchronous between the two populations [Moore et al, 2004b] but not related to territorial aggression [Moore et al, 2004a]. The purpose of our study was to determine if asynchronous seasonal changes in reproductive hormones and the song control system in these two populations are correlated with neuroplasticity in the GnRH system by measuring the number and size of GnRH-ir neurons during the breeding and non-breeding seasons.…”

“…The telencephalic song control system shows seasonal growth and regression paralleling seasonal changes in song behavior. Thus this seasonal neuroplasticity is also out of phase between the populations [Moore et al, 2004b]. During the breeding period males have elevated plasma levels of testosterone that are as high as those observed in closely related northern species [Moore et al, 2002], with seasonal peaks that are also asynchronous between the two populations [Moore et al, 2004b] but not related to territorial aggression [Moore et al, 2004a].…”