Kondoh Daisuke, Watanabe Kenichi, Nishihara Kaori, Ono Yurie S, Nakamura Kentaro G, Yuhara Kazutoshi, Tomikawa Sohei, Sugimoto Miki, Kobayashi Saori, Horiuchi Noriyuki, Kobayashi Yoshiyasu, Sasaki Motoki, Kitamura Nobuo
Laboratory of Veterinary Anatomy, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Japan.
Brain Behav Evol. 2017;90(3):224-231. doi: 10.1159/000479180. Epub 2017 Aug 30.
The olfactory system of mammals comprises a main olfactory system that detects hundreds of odorants and a vomeronasal system that detects specific chemicals such as pheromones. The main (MOB) and accessory (AOB) olfactory bulbs are the respective primary centers of the main olfactory and vomeronasal systems. Most mammals including artiodactyls possess a large MOB and a comparatively small AOB, whereas most cetaceans lack olfactory bulbs. The common hippopotamus (Hippopotamus amphibius) is semiaquatic and belongs to the order Cetartiodactyla, family Hippopotamidae, which seems to be the closest extant family to cetaceans. The present study evaluates the significance of the olfactory system in the hippopotamus by histologically analyzing the MOB and AOB of a male common hippopotamus. The MOB comprised six layers (olfactory nerve, glomerular, external plexiform, mitral cell, internal plexiform, and granule cell), and the AOB comprised vomeronasal nerve, glomerular, plexiform, and granule cell layers. The MOB contained mitral cells and tufted cells, and the AOB possessed mitral/tufted cells. These histological features of the MOB and the AOB were similar to those in most artiodactyls. All glomeruli in the AOB were positive for anti-Gαi2, but weakly positive for anti-Gαo, suggesting that the hippopotamus vomeronasal system expresses vomeronasal type 1 receptors with a high affinity for volatile compounds. These findings suggest that the olfactory system of the hippopotamus is as well developed as that of other artiodactyl species and that the hippopotamus might depend on its olfactory system for terrestrial social communication.
哺乳动物的嗅觉系统包括一个能检测数百种气味的主嗅觉系统和一个能检测特定化学物质（如信息素）的犁鼻系统。主嗅球（MOB）和副嗅球（AOB）分别是主嗅觉系统和犁鼻系统的主要中枢。包括偶蹄目动物在内的大多数哺乳动物都有一个大的主嗅球和一个相对较小的副嗅球，而大多数鲸类动物则没有嗅球。普通河马（Hippopotamus amphibius）是半水生动物，属于鲸偶蹄目、河马科，这似乎是现存与鲸类关系最近的科。本研究通过对一只雄性普通河马的主嗅球和副嗅球进行组织学分析，评估了河马嗅觉系统的重要性。主嗅球由六层组成（嗅神经层、球状层、外丛状层、僧帽细胞层、内丛状层和颗粒细胞层），副嗅球由犁鼻神经层、球状层、丛状层和颗粒细胞层组成。主嗅球包含僧帽细胞和簇状细胞，副嗅球拥有僧帽/簇状细胞。主嗅球和副嗅球的这些组织学特征与大多数偶蹄目动物相似。副嗅球中的所有肾小球对抗Gαi2呈阳性，但对抗Gαo呈弱阳性，这表明河马的犁鼻系统表达对挥发性化合物具有高亲和力的犁鼻1型受体。这些发现表明，河马的嗅觉系统与其他偶蹄目物种一样发达，并且河马可能在陆地社交交流中依赖其嗅觉系统。
