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2. Garrett, S. L., Adeff, J. A. & Hofler, T. J. J. Thermophys. Heat

Trans. 7, 595–599 (1993).

3. Garrett, S. L. US Pat. No. 5,647,216 (1997).

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Pat. No. 4,114,388 (1979).

6. Swift, G. W. J. Acoust. Soc. Am. 92, 1551–1563 (1992).

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105, 711–724 (1999).

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9. Smith, R. W. M., Keolian, R. M., Garrett, S. L. & Corey, J. C.

J. Acoust. Soc. Am. 105, 1072 (1999).

10.Chen, R.-L. & Garrett, S. L. in Proc. 16th Int. Congr. Acoust.

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Am., Woodbury, New York, 1998).

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3021 (1997).

12.Yarr, G. A. & Corey, J. C. US Pat. No. 5,389, 844 (1995).

can, and do, strike at a target using visual

information from just one eye.

The stronger refraction provided by the

cornea (as opposed to the lens) means that,

in each animal, the nodal point — the point

within the eye at which the lines connecting

points in the scene and corresponding points

in the image intersect — lies well in front of

the centre about which the eye rotates

1,6

. As a

result, when the eye rotates, the images of

objects at different distances move by differ-

ent amounts on the retina (Fig. 2). This

means that rotation of a single eye can pro-

vide information about the relative distances

of different objects. The animal can even

estimate absolute distances if the amount of

eye rotation is known. By gauging the dis-

tances of objects by rotating just one eye,

without using binocular stereopsis, and

without having to move the whole head as

some insects do

7,8

, the chameleon and the

sandlance minimize their chances of being

detected by potential prey

1,3

. At present,

however, such a means of distance estima-

tion by these animals is only a theoretical

possibility.

Despite belonging to such different

families, the chameleon (a reptile) and the

sandlance (a fish) seem to have converged on

a common set of design principles for their

visual systems. So, in these two animals, eye

design cannot be predicted by evolutionary

origin — rather, it has been crafted almost

exclusively by environmental constraints.

But the visual system of the sandlance seems

to differ from that of most other vertebrates

(including the chameleon) in that, after the

eye has moved to a particular position, it

does not stay there. Instead, Fritsches and

Marshall

2

find that the gaze drifts slowly back

towards the central viewing direction, pre-

sumably because of restoring forces exerted

by the muscles that rotate the eye. This drift

challenges the accepted idea

3,9

that animals

strive to keep a still image of the world on

news and views

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volution has generated a bewildering

variety of life forms on this planet.

Curiously, it has also occasionally pro-

duced animals from entirely different fami-

lies that seem to have converged on a similar

design plan — apparently as a result of simi-

lar lifestyles and environmental pressures.

One instance of this, reported by Pettigrew et

al.

1

and Fritsches and Marshall

2

in Current

Biology, is vision in the chameleon and in the

sandlance Limnychthyes fasciatus, a small

teleost fish

1–4

.

The chameleon catches insects with a

quick flick of its long tongue

5

. The sandlance

lies concealed in underwater sand beds with

only its eyes protruding above the surface,

then lunges at tiny, unsuspecting copepods

(aquatic crustaceans) that stray too near

4

.

Pettigrew et al. and Fritsches and Marshall

now show that there are extraordinary paral-

lels between these two behaviours, and

between the visual systems that mediate

them.

To a casual observer, the dart of the

chameleon’s tongue is very similar in its

trajectory to the lunge of the sandlance. The

sandlance is, in effect, a chameleon’s tongue

with eyes at the tip

1

. Moreover, both the

chameleon and the sandlance move the two

eyes independently and alternately (Fig. 1)

1,2

— while one eye remains motionless, the

other scans the environment in a saccadic

(jerky) fashion, presumably seeking prey.

The two eyes alternate when looking around

the environment, and each covers about half

the panorama. Scanning alternately with the

two eyes reduces the chance of alerting the

prey by tell-tale eye movements. Plus, given

that the two eyes are not yoked together to

look in the same direction, it would be

sensible not to move them simultaneously.

Otherwise, it could be difficult for the ani-

mal to decide whether the two eyes were

viewing the same target.

In both animals, the cornea of the eye —

rather than the lens — provides the bulk of

the refracting power. This extends the effec-

tive focal length of the eye, giving it a higher

angular resolution

1

. Moreover, the cornea

(not the lens) enables the eye to change its

focus and view objects at different distances.

This property makes the sandlance very

different from most other fish, which focus

mainly through the lens

3

. Pettigrew et al.

1

show that, in both the sandlance and the

chameleon, the refracting power of the

cornea is adjusted by a fast-acting, striated

‘cornealis’ muscle. There is also evidence that

the chameleon perceives depth by monitor-

ing how much one eye is focused, rather than

by combining the images from both eyes

(stereopsis)

5

. The sandlance probably adopts

a similar tactic

1

, meaning that both animals

Ecology

When one eye is better than two

Mandyam V. Srinivasan

Figure 2 How the sandlance and the chameleon could gauge depth by rotating a single eye. a, When

the eye faces straight ahead, objects A and B are imaged at the same place on the retina. b, When the

eye rotates, however, the image of A moves by a larger amount than that of B, revealing that A is closer

than B. This method of estimating distance is possible only if the nodal point of the eye (N) is located

well in front of the centre of rotation of the eye (C), as is the case in these animals.

Figure 1 Keeping an eye on things —

independent eye movements in the chameleon

and sandlance. The activity of the two eyes

alternates, each covering about half the

panorama. Pettigrew et al.

1

and Fritsches and

Marshall

2

show that there are many other

parallels between the visual systems of these two

animals, which are surprising given that they

come from such evolutionarily distinct families.

J

. D

. P

ET

TIGRE

W

ET AL.

C

N

C

N

A

B

A

B

a

b

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© 1999 Macmillan Magazines Ltd

their retina most of the time, to minimize

blur and to detect stationary or moving

objects more easily.

So why does the sandlance’s gaze drift?

One possibility is that there is simply not

enough visual texture in the murky, under-

water world in which it lives to help stabilize

the eye

2,3

. Another intriguing idea is that the

slow drift is exploited by the sandlance to

infer the distances to objects (Fig. 2). But, as

Fritsches and Marshall point out, another

advantage in drifting back to the central

position is that the eye is then best positioned

to move to the next target of interest, which

may appear anywhere in the visual field. This

strategy is akin to that used by tennis players,

who return to mid-court after dispatching

each ball. Indeed, the sandlance eye may

be a drifter with a purpose, and not just an

aimless wanderer.

Mandyam V. Srinivasan is at the Centre for Visual

Sciences, Research School of Biological Sciences,

Australian National University, PO Box 475,

Canberra, Australian Capital Territory 2601,

Australia.

e-mail: M.Srinivasan@anu.edu.au

1. Pettigrew, J. D., Collin, S. P. & Ott, M. Curr. Biol. 9, 421–424

(1999).

2. Fritsches, K. A. & Marshall, N. J. Curr. Biol. 9, R272–R273

(1999).

3. Land, M. F. Curr. Biol. 9, R286–R288 (1999).

4. Pettigrew, J. D. & Collin, S. P. J. Comp. Physiol. A 177, 397–408

(1995).

5. Harkness, L. Nature 267, 346–349 (1977).

6. Land, M. F. Nature 373, 658–659 (1995).

7. Wallace, G. K. J. Exp. Biol. 36, 512–525 (1959).

8. Kral, K. Behav. Processes 43, 71–77 (1998).

9. Walls, G. L. Vision Res. 2, 69–80 (1962).