Adaptive strategy of post-prime adult male chimpanzees

2023 Fiscal Year Final Research Report

• Project/Area Number: 19K06867
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 45050:Physical anthropology-related
• Research Institution: Kamakura Women's University
• Principal Investigator: Hosaka Kazuhiko 鎌倉女子大学, 児童学部, 教授 (10360215)
• Project Period (FY): 2019-04-01 – 2024-03-31
• Keywords: チンパンジー / 生活史 / 加齢 / 優劣 / 狩猟 / 肉食 / 食物分配 / アロマザリング

Outline of Final Research Achievements

Twelve adult male chimpanzees were observed at Mahale, Tanzania, in 2019 and 2023. I focused particularly on behavioral changes in two males of the same age that had transitioned from 28-year-old prime males to 32-year-old post-prime males. Both males avoided struggles for dominance after downfall from high ranks, such as alpha and beta, to low ranks. On the other hand, both were observed frequently directing allomothering toward a 3-year-old orphaned male. This is a rare case of paternal care observed among adult male chimpanzees. Analysis of long-term data on frequency in which adult male chimpanzees captured red colobus revealed that the two males that experienced the alpha status captured prey at a high frequency during the peak of their dominance ranks, whereas the two males that did not experience the alpha status showed no age-related changes in capture frequency. This is interesting in relation to the hypothesis that alpha males use meat sharing for political purposes.

Free Research Field

生物人類学

Academic Significance and Societal Importance of the Research Achievements

女性の生殖終了後生存期間（PRLS）の適応的進化を説明するおばあさん仮説は有名であるが、男性のPRLSの生物学的意味については議論がわかれている。ヒトに最も近縁の現生種チンパンジーの雄は資源保持能力及び繁殖成功のピークでもある25―29歳の壮年期を終えた後もしばしば50歳前後まで生存し続けることがあり、その適応的意味を考えることはヒトの男性のPRLSをめぐる議論に1つのヒントを与えることが期待できる。今回、幸運にも研究期間中に失脚したアルファ雄を含む「最盛期を終えた雄たち」が3歳の孤児雄にアロマザリングをする様子が観察されたことは、父性の起源を考察する材料の1つを提供することにもなるであろう。