ホーム   出版物   Chimpanzee mothers at Bossou, Guinea carry the mummified remains of their dead infants.
Biro D, Humle T, Koops K, Sousa C, Hayashi M, Matsuzawa T (2010) Chimpanzee mothers at Bossou, Guinea carry the mummified remains of their dead infants. Current Biology Volume 20, Issue 8, R351-R352
死んだ子どものミイラを運びつづけるギニア・ボッソウにすむチンパンジーの母親たち
野生チンパンジーとその子どもの死についての事例報告

Dora Biro, Tatyana Humle, Kathelijne Koops, Claudia Sousa, Misato Hayashi, Tetsuro Matsuzawa

Chimpanzee mothers at Bossou, Guinea carry the mummified remains of their dead infants.

Current Biology, Volume 20, Issue 8, R351-R352, doi: 10.1016/j.cub.2010.02.031


 西アフリカのギニアにあるボッソウ村周辺では、30年以上にわたって野生チンパンジーの調査が継続されている。村に近接した山にすむボッソウのチンパンジーの群れは、世界遺産のニンバ山からサバンナで隔てられ、20人前後という少数のメンバーで構成されてきた。本論文は、ボッソウの小さな群れで観察された、子どもの死に対するチンパンジーの母親の行動について、3つの事例を報告している。

1992年、2歳半の子ども(ジョクロ)が呼吸器系の病気で死亡した。数週間で死体は完全にミイラ化し、母親のジレは27日以上も子どもの死体を持ち運んだ。

2003年末の乾季には、ボッソウで呼吸器系の伝染病が流行した。5人のチンパンジーが死亡して、群れのメ ンバーは19人から14人に激減した。死亡したチンパンジーの中には、1歳のジマトと、2歳半のベベという2人の子どもが含まれていた。母親のジレとブアブアは、それぞれ68日と19日にわたって死んだ子どもの体を運びつづけた。

3例とも、子どもの死体は完全なミイラになった。死亡から数日で体はふくらんだあと、次第に乾燥していった。毛はすべて抜けてしまうが、乾燥した丈夫な皮膚に覆われて、体は原形を保っていた。回収されたベベの体(図3)は、歯が数本抜けただけのほぼ完全な状態だった。

3例とも母親は、つねに子どもの体を持ち運び、子どもの毛づくろいをし、体にたかるハエを追い払った。ハエを追うのに道具を使った例も2回観察された。このような母親の行動は、地面の上に放置した場合では数日内におこるであろう死体の腐乱を防ぎ、ミイラ化を促進した可能性がある。

 群れの他のメンバーは年齢や性別に関係なく全員が、子どもの死体に触ったり、手足を持ち上げたり、においをかいだりという行動をみせた。日数がたつと、母親から離れた場所まで、子どもや若いチンパンジーが遊びの中で死体を持ち運ぶようにもなった。ミイラを引きずって遊んでいる子どもが近くを通るときに、その母親が体を動かしてミイラに触れないようにするという動画内の1例をのぞいて、
死体への忌避的な行動は観察されなかった。特に死亡後数日は強い腐敗臭がただよい、見かけも生きているときとはまったく違ってしまうにもかかわらず、群れの他のメンバーは攻撃的な行動もせず、非常に寛容だった。

 母親がいつ子どもの体を運ぶのをやめるのかについては、偶然の亡失以外では、生理的な変化が考えられる。通常、出産後に授乳をしている期間は生理周期が止まっている。子どもが死んだことで授乳が止まり、生理周期が再開して繁殖可能な状態になる。次の子どもの繁殖に備えるために、前の子どもの体を運ぶのをやめるという生理的・心理的なメカニズムが働いている可能性がある。それにもかかわらず、ジレという母親が長期間子どもの体を運びつづけたのには、ジレにとってジマトが第8子で子育ての経験が豊富だったことと、ジマトが死んだ年齢が幼かったことも関係しているだろう。

 母親のジレやブアブアは、どこまで子どもが死んだことを「理解」していたのだろう。特に死亡直後では、死体がまだ生きた子どもであるかのように毛づくろいをする行動がみられた。だが、体がもう動かないということも母親は十分にわかっていたようだ。子どもの手足をつかんで引きずるように運んだり、肩と首の間に子どもの手足をはさんで背中にのせて運んだりという、子どもが生きているときには観察されなかった行動がみられた。

 ボッソウで子どもが死亡した3事例すべてで、母親が子どもの死体を運びつづけたということは、この行動がボッソウの群れの中では稀なものではなく、観察学習などで受け継がれている可能性もある。だが、絶滅に瀕したボッソウのチンパンジーのためには、このような悲しい出来事がこれ以上おこらないように祈るばかりだ。

 本論文の補助データと動画は、雑誌のホームページ(http://www.cell.com/current-biology/home)からみることができる。また、霊長類研究所のホームページでも1992年の事例についての動画とパンフレットをみることができる。論文の補助データとして、子どもが死亡にいたるまでの経緯について記載されている。ベベという子どもは、2003年12月30 日の死亡前、12月2日から13日まで母親と離れて単独でいるところを10日に研究者に発見された。なぜベベが生きたまま母親に見放されたのかは不明のままだ。発見時には衰弱していたベベだが、母親と再会して体調も回復傾向にあったが、25 日以降徐々に食欲がなくなり、衰弱が進んで死亡に至った。

 本論文とともに、飼育下で老衰のため亡くなったチンパンジーの死亡前後に、他のチンパンジーどのような行動をみせたかについての報告も掲載される。ヒトにもっとも近い生物で あるチンパンジーは、死についてどのように理解しているのか。死と直面した際の彼らの行動を詳細に記述して知見をつみかさね、ヒト以外の霊長類で初の科学的な死生学(Thanatology)への第一歩となる画期的な報告といえるだろう。


図1: 死後1日たったベベの死体を運ぶ
母親のブアブア
死後17日たった子供のミイラを運ぶ母親チンパンジー
図2: 死後17日たったジマトのミイラ
を運ぶ母親のジレ
チンパンジーのミイラ
図3: 回収されたベベのミイラ

アブストラクト

The forests surrounding Bossou, Guinea, are home to a small, semi-isolated chimpanzee community studied for over three decades [1]. In 1992, Matsuzawa [2] reported the death of a 2.5-year-old chimpanzee (Jokro) at Bossou from a respiratory illness. The infant's mother (Jire) carried the corpse, mummified in the weeks following death, for at least 27 days. She exhibited extensive care of the body, grooming it regularly, sharing her day- and night-nests with it, and showing distress whenever they became separated. The carrying of infants' corpses has been reported from a number of primate species, both in captivity and the wild [3], [4], [5], [6] and [7] — albeit usually lasting a few days only — suggesting a phylogenetic continuity for a behavior that is poignant testament to the close mother-infant bond which extends across different primate taxa. In this report we recount two further infant deaths at Bossou, observed over a decade after the original episode but with striking similarities.



Full Text

During the 2003 dry season, a respiratory epidemic broke out at Bossou, claiming the lives of five chimpanzees (reducing their number from 19 to 14) [8]. Among the dead were two infants: 1.2-year-old Jimato and 2.6-year-old Veve (see Supplemental Information available on-line with this issue). The mothers of both infants (Jire and Vuavua) continued to carry their offspring's lifeless bodies for 68 and 19 days after death, respectively (Figure 1A; Table S1). Thus, for the first three weeks of January 2004, Bossou was home to two mothers carrying dead infants.

Mummified infant chimpanzees during and after prolonged carrying by their mother
Figure 1. Mummified infant chimpanzees during and after prolonged carrying by their mother.(A) An adult female chimpanzee, Jire, carries the mummified remains of her infant, Jimato, who died in a respiratory disease epidemic at Bossou, Guinea, 17 days earlier. The body is carried dorsally, with Jimato's arm gripped between Jire's shoulder and neck. Jire continued to carry the corpse for a further 51 days, before abandoning it (see also Table S1). (Image by Dora Biro.) (B) Head of an infant chimpanzee's (Veve) mummified remains after being carried for 19 days post-death by the mother (Vuavua). Photo was taken immediately after Vuavua had abandoned (dropped, and did not retrieve within 24 hours) the corpse. (Image by Claudia Sousa.)

As in Jokro's case, the bodies of Jimato and Veve underwent complete mummification. Over the days following death, the bodies swelled, then gradually dried out. All hair was lost, but body parts remained largely intact, encased in dry leathery skin. When it was finally abandoned, Veve's skeleton was still remarkably intact, missing only some teeth in the upper jaw (Figure 1B). Because of the wearing effects of prolonged carrying, by the time Jire abandoned Jimato's body, much of the bony cranial structure had been destroyed, making most facial features unrecognizable. Nevertheless, fingers, toes, and even genitals were preserved within the layer of tough dry skin.

In all three cases, group members' responses to the corpses were highly similar. Mothers carried the bodies during all travel (typically by gripping a limb in hand, foot, or between shoulder and neck), groomed them regularly, and chased away flies that circled the corpses (twice with the aid of a tool; Supplemental Movie S1). These are all behaviours that may have facilitated mummification. Related and unrelated individuals from all age groups and both sexes attempted to touch, poke or handle the bodies, lifted and dropped their limbs, and sniffed them. In later stages, juveniles and infants were occasionally allowed to carry the bodies some distance from the mother in bouts of play (Supplemental Movie S2). With only one exception (Movie S2), we never observed a response that could be interpreted as aversion, despite the bodies' intense smell of decay and highly unusual appearance. Similarly, we observed no aggressive acts towards the infants' corpses during the entire period of carrying. Elsewhere, chimpanzees have been reported to treat violently and even cannibalise the corpses of dead infants after snatching them away from the mother shortly after death (for example [4] and [5]). No such incidents were recorded in the cases of Veve and Jimato. In general, therefore, all members of the community demonstrated high levels of tolerance towards the corpses.

What factors were responsible for the mothers finally abandoning the corpses? Besides accidental loss of the bodies and subsequent failure to recover, physiological changes in the mothers associated with infant-death may also have played a role. Postpartum amenorrhoea in chimpanzees lasts around four years, but is much shortened after an infant's death [9]. Because lactation ceased once the infants died, the mothers' reproductive cycle returned; such hormonal changes, which prepare the mother for the arrival of a new infant (normally around weaning), may have contributed to a gradual ‘letting go’ of the previous infant's remains. Intriguing parallels may exist with physiological and psychological changes experienced by human mothers, in whom the absence or cessation of breastfeeding may cause exaggerated desires to hold their infant [10]. The fact that Jire nonetheless carried her infant for considerably longer than Vuavua may have been rooted in several factors, including Jire's extensive experience as a mother (Jimato was her eighth infant; Veve was Vuavua's first) and Jimato's younger age at death.

An obvious and fascinating question concerns the extent to which Jire and Vuavua “understood” that their offspring were dead. In many ways they treated the corpses as live infants, particularly in the initial phase following death. Nevertheless they may well have been aware that the bodies were inanimate, consequently adopting carrying techniques never normally employed with healthy young (although mothers of handicapped young have also been known to respond appropriately). The fact that all three documented cases of infant deaths at Bossou were followed by extended carrying of the infants' remains suggests that this behaviour may not be a rare occurrence in this small community, and raises questions about the potential role of observational learning in promoting chimpanzee mothers' prolonged transport of deceased young. Nonetheless we hope that further data from this already threatened community will not be quick in coming.

Acknowledgments

We are grateful to the Direction National de la Recherche Scientifique et Technique, République de Guinée, for permission to conduct field work at Bossou, to Akino Kato-Watanabe and to local guides Guanou Goumy, Tino Zogbila, Paquilé Cherif, Pascal Goumy, Marcel Doré, Boniface Zogbila, Jules Doré, and Henry Camara for assistance in the field. The research was supported by Grants-in-Aid for scientific research from the Ministry of Education, Science, Sports, and Culture of Japan (grants 07102010, 12002009, 10CE2005, and the 21COE program). D.B. was supported by a Royal Society University Research Fellowship and by Somerville College, Oxford. K.K. was supported by a grant from the Lucie Burgers Foundation for Comparative Behavioural Research (Netherlands) and T.H. by an NIH Kirschstein-NRSA Postdoctoral Fellowship (MH068906-01).

References

Supplemental Information

Document S1. Supplemental Results, Supplemental References, and One Table


supplemental video-1
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