Dear All, We are pleased to announce publication of our latest paper on genital morphometrics of northern fur seal males: Yukino Hirakawa, Takanori Horimoto, Ippei Suzuki, and Yoko Mitani "Estimation of Sexual Maturity Based on Morphometrics of Genital Organs in Male Northern Fur Seals, Callorhinus ursinus," Mammal Study 46(1), 1-11, (21 December 2020). https://doi.org/10.3106/ms2020-0004 Abstract It's not always possible to obtain samples from dead male pinnipeds that are fresh enough to determine the histological characteristics of spermatogenesis and their stage of sexual maturity. We propose that the morphometrics of genital organs of northern fur seals, Callorhinus ursinus, can be used as a new indicator of sexual maturity. We analyzed data from collected records of male fur seals around Hokkaido during 2011–2017 to examine morphometrics of genital organs and their stage in histological characteristics of spermatogenesis. First, we investigated ontogenetic and seasonal variation in three genital organs: testes, epididymides, and bacula. Second, we classified the stages of histological characteristics of spermatogenesis into three groups (Stage A–C) using non-linear discriminant analysis, based on the stage in histological characteristics of spermatogenesis determined by histological observation of seminiferous tubules. Testicular size, testicular mass, and epididymal mass showed seasonal variation associated with spermatogenesis. From our analysis, the stage in histological characteristics of spermatogenesis could be estimated using testicular mass with high accuracy of predicting spermatogenesis. And measurement of baculum was suggested to be an indicator of predicting sexual maturity. We conclude that this simple estimation may be used to determine sexual maturity based on individual growth patterns. Kind regards, Yoko -- 三谷曜子　Yoko Mitani 北海道大学北方生物圏フィールド科学センター 海獣班 HokkaidOcean Mammal Team 〒040-0051 北海道函館市弁天町20番5号 函館市国際水産・海洋総合研究センター内219号室 Tel: 0138-85-6558; Fax: 0138-85-6625 Email: yo_mitani@fsc.hokudai.ac.jp Yoko MITANI HokkaidOcean Mammal Team Field Science Center for Northern Biosphere, Hokkaido University Hakodate Research Center for Fisheries and Oceans 20-5 Benten-cho, Hakodate, Hokkaido 040-0051, JAPAN TEL: +81-138-85-6558; FAX: +81-138-85-6625 E-mail: yo_mitani@fsc.hokudai.ac.jp