ハチやアリなどの社会性昆虫はごく小さな脳しかもっていないが,おそろしく困難なナビゲーションの仕事を成し遂げている.サバクアリのさまざまなナビゲーション行動はそのよい例である.サバクアリは北アフリカの塩田,砂漠環境がもたらす高温と乾燥により死んだ昆虫を捜し回るため,1年のうちの,そして1日のうちの最も暑い時期に地下の巣を離れる.この際,アリは餌を探して曲がりくねった道のりを100m以上も巣穴から離れることもあるが,その後まっすぐ巣へ戻る.アリは自分が曲がった角度,歩いた距離をすべて記録すると同時に,その経路を積算し自分が巣からどの方向にどのくらい離れているかを知っている.つまりはベクトルナビゲーションを行っているわけである.このベクトルを計算するために,アリは方向(コンパス)と距離についての情報を得なければならない.さらに,サバクアリは半砂漠環境において低木や草の茂みなどランドマークが存在すると,ナビゲーションのためにランドマークを利用する.これまでに,神経生物科学的解析と行動学的解析により,アリのコンパスやランドマーク利用について解明されてきた.そしてその結果をもとにして自律的エージェント(autonomous agent, Sahabot, Sahabot2)が開発されてきている.


Further Reading

昆虫ミメティックス Insect Mimetics(2008),針山孝彦,下澤楯夫,pp.866-877


Visual navigation in the desert ant

responsibility for wording of article: Akira Takashima (OIST)

Social insects such as bees and ants have a very small brain; however, they are capable of complex navigation. Various navigation behaviors shown by the desert ant is an example considered here. The desert ant lives in salt plains and desert environments in North Africa. To forage for dead insects that these hot and dry environments provide, the ants move away from their underground nests at the hottest times of the year and the day. During these periods, the ants may wander around searching for food at distances more than 100 m from the nest. However they always return to the nest in a straight path. The ant registers the angles in which it has turned and the distance that it has walked and uses this information to calculate its distance and direction from the nest. In other words, it engages in vector navigation. To calculate vectors, the ant needs to obtain information on the compass direction faced and the distance traveled. For navigation in a semi-desert environment, the desert ant uses landmarks such as shrubs and grasses. The utilization of a compass and landmarks by the ant to determine its position has been elucidated through previous neurobiological and behavioral investigations. Autonomous agents (Sahabot and Sahabot2) have been developed on the basis of these findings.

The desert ant does not use a magnetic compass like human seafarers; however, it uses the sun’s position in the sky and the polarized-light gradient as a sun compass. The polarized-light compass is the most important navigation method used by the ant. Honey bees and ants use the pattern of polarized light (e-vector pattern) to find a specific point on the compass (for example, 30° left from the direction of the sun), even when it is cloudy and only part of the sky can be used. It is the e-vector pattern that is used as the compass. This pattern changes with the position of the sun, and expressing it physically would be extremely difficult. Detailed behavioral experiments have shown that the desert ant uses only the overall pattern of the e-vector. The compass system of the autonomous agent Sahabot was developed on the basis of these findings. Neurobiological and behavioral studies on desert ant navigation using landmarks such as shrubs and grasses have shown that the ant memorizes the landmarks around the goal (the nest entrance) and matches these with the current retinal image. The autonomous agent Sahabot2 was built on the basis of this knowledge, and it can find its way back to the nest with a high degree of accuracy. 

Further Reading

昆虫ミメティックス Insect Mimetics(2008),針山孝彦,下澤楯夫,pp.866-877

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