Sherry D F, Forbes M R, Khurgel M, Ivy G O
Department of Psychology, University of Western Ontario, London, Canada.
Proc Natl Acad Sci U S A. 1993 Aug 15;90(16):7839-43. doi: 10.1073/pnas.90.16.7839.
Females of the brood-parasitic brown-headed cowbird (Molothrus ater) search for host nests in which to lay their eggs. Females normally return to lay a single egg from one to several days after first locating a potential host nest and lay up to 40 eggs in a breeding season. Male brown-headed cowbirds do not assist females in locating nests. We predicted that the spatial abilities required to locate and return accurately to host nests may have produced a sex difference in the size of the hippocampal complex in cowbirds, in favor of females. The size of the hippocampal complex, relative to size of the telencephalon, was found to be greater in female than in male cowbirds. No sex difference was found in two closely related nonparasitic icterines, the red-winged blackbird (Agelaius phoeniceus) and the common grackle (Quiscalus quiscula). Other differences among these species in parental care, migration, foraging, and diet are unlikely to have produced the sex difference attributed to search for host nests by female cowbirds. This is one of few indications, in any species, of greater specialization for spatial ability in females and confirms that use of space, rather than sex, breeding system, or foraging behavior per se, can influence the relative size of the hippocampus.
巢寄生的褐头牛鹂（Molothrus ater）雌鸟会寻找宿主巢穴来产卵。雌鸟通常在首次找到潜在宿主巢穴后的一到几天内返回产下一枚卵，并且在一个繁殖季节能产下多达40枚卵。雄性褐头牛鹂不会协助雌鸟寻找巢穴。我们预测，准确找到并返回宿主巢穴所需的空间能力可能导致牛鹂海马复合体大小出现性别差异，雌性更具优势。研究发现，相对于端脑大小，雌性牛鹂的海马复合体比雄性更大。在两种亲缘关系密切的非寄生性拟黄鹂——红翅黑鹂（Agelaius phoeniceus）和普通拟八哥（Quiscalus quiscula）中，未发现性别差异。这些物种在亲代抚育、迁徙、觅食和饮食方面的其他差异不太可能导致归因于雌性牛鹂寻找宿主巢穴的性别差异。这是在任何物种中为数不多的表明雌性在空间能力上具有更高专业化程度的迹象之一，并证实了空间利用而非性别、繁殖系统或觅食行为本身会影响海马体的相对大小。
