Data from: Female MHC type affects male testosterone levels and sperm number in the horse (Equus caballus)

Odours of vertebrates often contain information about the major histocompatibility complex (MHC), and are used in kin recognition, mate choice or female investment in pregnancy. It is, however, still unclear whether MHC-linked signals can also affect male reproductive strategies. We used horses (Equus caballus) to study this question under experimental conditions. Twelve stallions were individually exposed either to an unfamiliar MHC-similar mare and then to an unfamiliar MHC-dissimilar mare, or vice versa. Each exposure lasted over a period of four weeks. Peripheral blood testosterone levels were determined weekly. Three ejaculates each were collected in the week after exposure to both mares (i.e. in the ninth week) to determine mean sperm number and sperm velocity. We found high testosterone levels when stallions were kept close to MHC-dissimilar mares and significantly lower ones when kept close to MHC-similar mares. Mean sperm number per ejaculate (but not sperm velocity) was positively correlated to mean testosterone levels and also affected by the order of presentation of mares: sperm numbers were higher if MHC-dissimilar mares were presented last than if MHC-similar mares were presented last. We conclude that MHC-linked signals influence testosterone secretion and semen characteristics, two indicators of male reproductive strategies.

脊椎动物的气味通常携带与主要组织相容性复合体（major histocompatibility complex, MHC）相关的信息，此类气味可应用于亲缘识别、配偶选择以及雌性孕期的亲代投资行为。然而目前仍未明确，与MHC相关的信号是否同样会影响雄性的繁殖策略。
我们以家马（Equus caballus）为实验对象，在可控实验条件下探究该问题。本研究将12匹种公马进行单独暴露实验，分为两种暴露序列：先接触1匹陌生的MHC相似型母马，随后接触另一匹陌生的MHC相异型母马；或反之。单次暴露时长为四周。每周采集外周血样本，检测睾酮（testosterone）水平。在完成两种母马暴露后的第一周（即第9周），为每匹公马采集三次射精样本，用以测定平均精子数量与精子运动速度。
研究结果显示，当种公马与MHC相异型母马近距离共处时，其睾酮水平显著升高；而与MHC相似型母马共处时，睾酮水平则显著降低。单次射精的平均精子数量（而非精子运动速度）与平均睾酮水平呈正相关，同时还会受到母马暴露顺序的影响：若最后接触的为MHC相异型母马，精子数量显著高于最后接触MHC相似型母马的组别。
综上，与MHC相关的信号可影响睾酮分泌与精液特性，而这二者均为雄性繁殖策略的重要评价指标。