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飼育下キタオットセイの長期モニタリングによる血液性状の周年変動と繁殖生理に関する研究

香山, 薫 岐阜大学

2022.03.14

概要

本研究の材料であるキタオットセイ Callorhinus ursinusは北部太平洋に生息する一属一種の海棲哺乳類であり,その上質な毛皮を得るための乱獲により 1900年代前半には絶滅の危機に瀕した。この絶滅の危機は国際条約に基づく保護により回避することができたが,2000年前後より原因のはっきりしない生息数の減少が認められている。またキタオットセイは1年の2/3を上隆することなく大洋上を索餌回游して過ごし,残る 1/3を繁殖島に集結して繁殖行動を行なうという独特の生活サイクルを持ち,出生した仔は生後約4か月で成と同様の回游に出た後,性成熟するまで回游を続け、繁殖島に回帰しないという特徴を持っている。

キタオットセイについては野生個体を捕獲・観察することによる生理学的及び生態学的研究,飼育下個体を用いた種々の研究が報告されているものの,飼育下個体の血液性状正常値や野生個体との異同については未だ知られていない。本研究ではこれまでに知られていなかった飼育下個体の血液正常値を求めて既報の野生個体の結果と比較し,その異同を確認することを目的として研究を行なった。

第1章では4頭の健康な飼育下個体の血液性状及び血液生化学性状を 2年間にわたり 1ヶ月に1回測定した結果から,これまで知られていなかった飼育下個体の血液正常値を明らかにした。また既報の野生個体血液正常値と比較した結果,赤血球数,ヘモグロビン量,ヘマトクリット値,アルカリフォスファターゼ値,尿酸値,コレステロール値では今回得られた結果の80%以上が既報の正常範囲外となることが示された。

第2章では,第1章の実験中に血液測定結果の季節に伴う変動が感じられたため,研究期間を延長して血液性状の周年変動についての研究が継続して行われた。その結果、キタオットセイの血液性状には赤血球数やヘマトクリット値等は秋冬期に高値となり、自血球数等は春夏期に高値となるという周年変動があることが示された。血液性状の周年変動の明らかな原因を示すことはできなかったが,キタオットセイの特徴的な生熊学的生活サイクルが影響を与えている可能性が示唆された。

第3章では研究期間中に春機発動に達した個体が観察されたことから,成長に伴う性ステロイドホルモンの消長と体重との関連及びステロイドホルモン観察による妊娠診断の可能性についての研究が継続された。その結果,春機発動には年齢だけでなく成長が影響しており,繁殖期前3ヶ月の体重の平均が20kgを超えるまで春機発動が起こらないこと,冬期から初春のプログステロン測定により妊娠診断が可能であることなどが示された。

以上の知見は,特徴的な生態から生理学的,生態学的情報が十分解明されていないキタオットセイの生理・生態を解明する貴重なデータとなり,飼育個体や漂着保護個体の疾病治療や健康管理,飼育下における繁殖の推進において大いに有益であると考えられる。

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