Current Biology Vol 16 No 17, 668-670

道を渡る野生チンパンジー:危険への対処法

Road crossing in chimpanzees: a risky business.

Kimberley J. Hockings, James R. Anderson, Tetsuro Matsuzawa

京都大学霊長類研究所 松沢哲郎所長・教授らの研究グループは、西アフリカ・ギニアのボッソウ村の近隣の森に暮らしている野生チンパンジーの暮らしについての長期継続調査のなかで、チンパンジーが道を渡るときには、偵察、先陣、見張り、しんがりの明確な役割分担のあることを明らかにしました。道を渡るという危険の程度に応じて、チンパンジーは互いに助け合いながら暮らしており、チンパンジーがもっている柔軟な知性の新たな側面がみつかりました。

研究の背景

西アフリカ・ギニアのボッソウ村の近隣の森に、1群の野生チンパンジーが暮らしている。1976年から30年間にわたって、京大霊長類研究所の調査隊が、彼らの暮らしについて長期継続調査をしている。ボッソウのチンパンジーは石器を使うチンパンジーとして有名だ。一組の石をハンマーと台にして硬いアブラヤシの種を叩き割って、中の核を取り出して食べる(光村図書・中学2年国語教科書、「文化を伝えるチンパンジー」参照)。人間の住む村に隣接した森をすみかにしているので、人間の暮らしと交差しながら共存している。

概要

ボッソウのチンパンジーの暮らす範囲は、村の周辺の約15平方キロの森だ。純粋の森(一次林)の面積はわずかで、人手の入った二次林や、焼畑の跡地、さらには人間の耕作地にも出没する。住処の森が、南北を貫通する大きな道路(道幅約12メートル)で東西に分断されている。この道は、通行人だけでなく、自動車やオートバイも通る。もう1本細い道(道幅約3メートル)があって、ここは通行人しかほぼ通らない。この2本の道を横断して、チンパンジーは東西の森を行き来して暮らしている。人間と共存しているとはいえ、チンパンジーにとって道路を横断するのは危険が伴う。2005年にこの2本の道路を渡るチンパンジーのようすを丹念に記録し、ビデオに収録し、道を渡る行列の構成について詳細な分析をおこなった。今回の調査の時点で、群れの構成は、おとなの男性3個体、おとなの女性5個体、子ども3個体、あかんぼう1個体、の合計12個体である。なお、チンパンジーは、いつもはだいたい数個体ずつのバラバラな小集団に分かれており、出会っては別れ、別れてはまた出会う。道を渡るときは、全員で渡るときもあるし、数個体で渡るときもある。道を渡るときは危険が伴うので、たくさんの個体が同時に渡ることが多い。

  今回の研究で、道を渡るときには、明確な役割分担のあることが明確にわかった。まず、道の端にでてきてようすをうかがう偵察者がいる。そのあと、実際に先陣をきって渡るものがいる。道の途中で立ち止まって他のものたちの通過を見守るものがいる。そして、しんがりをつとめるものがいる。偵察、先陣、見張り、しんがり、である。今回の調査の時点で、おとなの男性3人を含めて男女多数が一緒に渡った28例について分析してみた。おとなの男性たち(順位によって第1位、第2位、第3位と呼ぶ)が役割を分担して、女性や子どもたちを守るようすが行列の構成から見えてきた。主な4点を指摘する。
(1) 偵察し先陣をつとめるのは、群れの第2位の男性が多かった。しんがりは、群れの第1位の男性がつとめることが多かった。女性や子どもたち、とくにあかんぼうを抱える女性は、行例のなかほどに位置する。
(2) 広い道をわたるときには危険度が高いので、道の端にでてきてようすをうかがいはじめてから渡るまでの待ち時間が長かった(広い道だと約3分かかるが、狭い道だと24秒で渡る。つまり偵察して、すぐには渡らない)。
(3) 狭い道だと、最初に現れた偵察チンパンジー(第2位のチンパンジー)が、左右をうかがい、そのまま先陣として渡る(100%そうだった)。しかし広い道だと、その割合が70%にまでおちて、別の者が先陣をつとめることがある。つまり、偵察と先陣の役割分担が起こる。ではだれが先陣をきるかというと、後ろから来た年齢が40歳を過ぎた老練な男女(「第3位のおとなの男性」と「現在第1位の男性の母親」)が先陣をきることが多かった。
(4)狭い道では、必ずしも第1位の男性だけがしんがりをつとめるわけではない。しかし広い道では、第1位の男性がしんがりをつとめる割合がぐんと増える。

以上のことから、道を渡るという危険の程度に応じて、チンパンジーが互いに助け合いながら暮らしているようすがあきらかになり、チンパンジーがもっている柔軟な知性の新たな側面がみつかった。

当記事は、京都大学WEBサイトの記事「道を渡る野生チンパンジー:危険への対処法(2006年9月5日掲載)」の内容を転載したものです。
Full Text

During group movements, monkeys may cooperate to reduce the risk of predatory attacks through adaptive spatial patterning. For example, adult males move toward the front of the group when travelling towards potentially unsafe areas such as waterholes, and bring up the rear when retreating [1,2,3,4]. Comparable data on progression orders in moving groups of great apes are lacking.

We hypothesised that chimpanzees evaluate risk when crossing roads, and draw on a phylogenetically-old principle of protective socio-spatial organization to produce flexible, adaptive and cooperative responses to risk. Progression orders were studied in the small community of chimpanzees (Pan troglodytes verus) at Bossou, Guinea, as they crossed two roads, one large and busy with traffic, the other smaller and frequented mostly by pedestrians. We found evidence that the degree of risk, estimated in terms of the width of roads and the amount and type of traffic they carried, influenced the waiting time before crossing the roads and the order in which the chimpanzees crossed.

The home range of the 12-strong chimpanzee community at Bossou (7' 39"N; 8' 30" W), covers about 15 km2 of mixed forests surrounded by abandoned and cultivated fields. It is dissected by a narrow road (3 m wide) which is used by pedestrians, and a recently widened larger road (12 m wide at the crossing point), which carries trucks, cars, motorbikes and pedestrians. The Bossou chimpanzees have to cross both roads regularly to reach foraging sites in their fragmented home-range and typically cross at specific points that the group has used for decades. There is forest up to the edge of the roads, the latter being separated by secondary forest and plantations. Two observers recorded progression orders in both directions (Figure 1 and Supplemental data ); the first chimpanzee to scan the road was termed first individual to scan. The latency between arrival of the first individual to scan and the last group-member to cross the forest-road edge was termed the waiting time. During the study (January - April 2005), the group contained three adult males, five adult females, three juveniles and one infant [5]. The social rank of the Bossou males has varied over the years, but during this study the relative statuses of the alpha male (Yolo), the second male (Foaf) and the third male (Tua) were stable [5].


Figure 1. How do chimpanzees cross roads?
Dominant individuals act cooperatively with a high degree of flexibility to maximise group protection.

Waiting time was analysed for 19 combined road-crossings in which the same group members crossed the two roads. The analysis of road-crossing progressions used data from 28 mixed-group progressions (17 small and 11 large road crossings), with mean group size 10.6 individuals (SD 2.1). Chimpanzees waited longer before crossing the large road than the small road (means: 180 versus 24 seconds; Wilcoxon test: T = 5, N = 19, p < 0.01). Presence or absence (either auditory or visual) of people had no effect on waiting time on the small road (Mann-Whitney test: Z = -0.168, N = 19, p = 0.905), but people (Z = -2.059, N = 19, p = 0.043) and vehicles (Z = -2.043, N = 19, p < 0.01) increased waiting time on the large road. A significant effect of direction of travel emerged concerning the large road (Z = -2.083, N = 19, p = 0.041); the same effect for the small road approached significance (Z = -1.915, N = 19, p = 0.062): the chimpanzees took longer to move from forest to more open areas than vice-versa.

When all three adult males were present, one led more often than expected (18 of 28 progressions; binomial test: p < 0.001) and was more likely to scan the road prior to crossing (binomial: p < 0.001). In contrast, an adult male did not occupy the rearmost position more frequently than expected. As the second- and third-ranking males frequently led, the frequency of the alpha male being last was tested; this was highly significant (Bonferroni correction included; binomial: p < 0.01).

Figure 2 summarizes the progression order data. The first individual to scan was the first to cross the small road in 100% of cases, compared to 70% for the large road. On the large road the second-ranking male sometimes continued scanning while the elderly third male and alpha female took up the lead on the large-road progressions. The alpha male increased his rearward presence on the large road, whereas the alpha female showed a dramatic reduction in frequency of being last; in other words when the degree of risk increased she took up a more forward position. Additionally, when the alpha male was present in mixed-group progressions containing one other adult male (N = 6, mean group size: 6.7), he was first to scan and cross in 50% of large road-crossings and last in only 33%. This suggests that his rearward position at other times was not due to fear.


Figure 2. Summary of progression order data.
The percentages of progressions in which the three males, the alpha female and the average non-alpha female and juvenile were: (A) first to cross; (B) first to scan the road; and (C) last in the progression on the two roads (infant excluded from analysis).

Modern Bossou chimpanzees encounter predators infrequently [6], and although humans themselves are not 'predators' of these chimpanzees, we propose that road-crossing, a human-created challenge, presents a new situation that calls for flexibility of responses by chimpanzees to variations in perceived risk.

Crossing the large road and leaving forest for open areas are potentially risky situations for chimpanzees, reflected in increased waiting time. During dangerous excursions certain positions may be more advantageous than others, depending upon age and sex [4]. Adult males, less fearful and more physically imposing than other group members, take up forward and rearward positions, with adult females and young occupying the more protected middle positions.

As hypothesised, the Bossou chimpanzees employ a phylogenetically-old mechanism to adapt to a more recent dangerous situation. However, the positioning of dominant and bolder individuals, in particular the alpha male, changed depending on both the degree of risk and number of adult males present; dominant individuals act cooperatively with a high level of flexibility to maximise group protection. At a proximate level each individual may have preferred and recognised positions; however, it is unknown whether positioning is individual- or rank-specific. Data on progression orders of other great ape populations are required, and would help shape hypotheses about emergence of this aspect of hominoid social organisation.

Acknowledgments
We are grateful to the Direction Nationale de la Recherche Scientifique et Technique, the Republic of Guinea, for granting us permission to carry out this research. We would like to thank all the guides who helped during this research period. This work was supported by a Stirling University studentship, MEXT grant #16002001 and JSPS-HOPE.

Supplemental data

Chimpanzees crossing a large road in front of on-looking villagers. The Bossou chimpanzees employ a phylogenetically old mechanism to adapt to a more recent dangerous situation.

References

  • Altmann, S.A. (1979). Baboon progressions: order or chaos? A study of one-dimensional group geometry. Anim. Behav. 27, 46-80.
  • DeVore, I., and Washburn, S.L. (1963). Baboon ecology and human evolution. In African ecology and Human evolution. Howell, F.C., Bourliere, F., eds. (Chicago: Aldine), pp. 335-367.
  • Rhine, R.J., and Westlund, B.J. (1981). Adult male positioning in baboon progressions: order and chaos revisited. Folia. Primatol. 35, 77-116.
  • Rhine, R.J., and Tilson, R. (1987). Reactions to fear as a proximate factor in the sociospatial organization of baboon progressions. Am. J. Primatol. 13, 119-128.
  • Matsuzawa, T. (2006). Sociocognitive development in chimpanzees: A synthesis of laboratory work and fieldwork. In Cognitive Development in Chimpanzees. Matsuzawa, T., Tomonaga, M., Tanaka, M., eds. (Tokyo: Springer), pp. 3-33.
  • Sugiyama, Y. (2003). Demographic parameters and life history of chimpanzees at Bossou, Guinea. Am. J. Phys. Anthropol. 124, 154-165.

Hockings KJ., Anderson JR. , Matsuzawa T (2006)Road crossing in chimpanzees: a risky business.

Current Biology, Vol 16 No 17, 668-670