O'Brien G M
Department of Physiology and Pharmacology, University of Queensland, Australia.
Reprod Fertil Dev. 1993;5(5):499-521. doi: 10.1071/rd9930499.
Megachiroptera inhabit the Old World tropics and most are seasonal breeders having defined times of testis growth, mating and parturition. In Pteropus scapulatus, the little red flying fox, the robust rhythm of testis cycles is resistant to modification by photoperiod. P. poliocephalus, the greyheaded flying fox, can be manipulated by photoperiod but responds slowly and incompletely. Most mammals live in the tropics, many in seasonally harsh climates, and many breed seasonally. However, few long-lived tropical mammals have been investigated for photoperiodic entrainment of annual reproductive cycles, and only animals from the edge of the tropics have responded. Thus, in long-lived tropical mammals, factors that regulate seasonal breeding have not yet been identified. Endogenous oscillators may generate circannual rhythms centrally. Downstream pathways (reproduction, metabolism, antlers, etc.) may derive their rhythm directly from the oscillator or may be modified by environmental cues. Plasticity of the circannual oscillator resolves confusion from previous contrasts between circannual rhythms and environmentally cued patterns. Plasticity may continue throughout life (species responsive to zeitgebers), but the oscillator may be 'set' in utero in some tropical species. Feedback effects from temperature, nutrition, hormones, etc. can be readily tested in this model of an oscillator generating an endogenous circannual rhythm.
大蝙蝠亚目动物栖息于东半球热带地区，大多数为季节性繁殖者，有特定的睾丸生长、交配和分娩时间。在小红狐蝠（Pteropus scapulatus）中，睾丸周期的强烈节律不受光周期改变的影响。灰头狐蝠（P. poliocephalus）则可受光周期调控，但反应缓慢且不完全。大多数哺乳动物生活在热带地区，许多处于季节性恶劣气候中，且许多进行季节性繁殖。然而，很少有长寿的热带哺乳动物被研究其年度繁殖周期的光周期同步情况，只有来自热带边缘的动物有反应。因此，在长寿的热带哺乳动物中，调节季节性繁殖的因素尚未被确定。内源性振荡器可能在中枢产生年节律。下游途径（繁殖、新陈代谢、鹿角等）的节律可能直接源自振荡器，也可能受环境线索影响。年节律振荡器的可塑性解决了先前年节律与环境线索模式之间对比产生的困惑。可塑性可能在整个生命过程中持续存在（对授时因子有反应的物种），但在一些热带物种中，振荡器可能在子宫内就已“设定”。在这个产生内源性年节律的振荡器模型中，可以很容易地测试温度、营养、激素等的反馈效应。
