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Northern populations of the White-crowned Sparrow, Zonotrichia leucophrys, show varying degrees of social modulation of testosterone. In general, males of midlatitude breeding and multiple-brooded populations show transient increases in secretions of luteinizing hormone and testosterone when challenged by another male for a territory, or when exposed to sexually receptive females. These surges of testosterone do not appear to activate aggression associated with territory defense or mate-guarding, but appear to enhance persistence of aggression during and after the behavioral interaction. Males may continue to sing spontaneously and patrol the territory for many hours even after the behavioral interaction is over. However, males of high latitude and altitude populations do not socially modulate testosterone levels. Several hypotheses, not necessarily mutually exclusive, have been proposed to explain why males in some populations do socially modulate testosterone and others do not. Males will not socially modulate testosterone if: (1) the breeding season is so restricted in time (e.g., high latitude and altitude) there are few social interactions; (2) populations in which males are essential for parental care; and (3) populations in which extra-pair copulations are low and male-male interactions are minimal. Tropical populations of the Rufous-collared Sparrow, Z. capensis costaricensis, have extended breeding seasons and they are multiple-brooded, but do not socially modulate testosterone. This is unlike Z. leucophrys. Whether austral populations of the Rufous-collared Sparrow socially modulate testosterone under presumably similar constraints of mid- to high-latitude seasonality are currently under investigation.Much of the research cited in this review was supported by grant numbers OPP-9911333 and IBN-0317141 from the National Science Foundation to J.C. Wingfield. All research conducted by the senior author on these grants was approved by the University of Washington Institutional Animal Care and Use Committee. R.A.V. acknowledges support from IEB-grant P05-002-ICM

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