Frogs ogle up from water and down from trees: bulging eyes differ more than they look

Semi-aquatic frogs have dorsally protuberant eyes placed above the level of the skull. This allows the animals to hide in water while spotting any predator approaching on land or from the air.

Tree frogs – including those specialised to the degree of being able to glide from tree to tree – have similar placement of the eyes despite the fact that many of them hardly immerse themselves.

At first sight, this commonality seems puzzling, because it seems to suggest a failure to adapt.

Some readers might assume the following explanation:

• not everything is adaptive: the ancestral design of the head has been conserved as frogs evolved from semi-aquatic to arboreal, and
• some of the advantages remain: although the water level is irrelevant to tree frogs, it pays them to keep their eyes high as in semi-aquatic frogs.

However, can we think of an adaptive explanation?

Semi-aquatic frogs can see only at and above the level of their eyes. This is because their eyes are close together, their pupils are tilted upwards, and their wide jaws obstruct lines of sight.

In tree frogs, by contrast, the visual field extends below the head because their eyes are far apart and bulge beyond the width of the jaws.

Because their eyes face the sides and front more than those of semi-aquatic frogs, tree frogs can even see partway under the body and head. Any dorsal protrusion of the eyes in tree frogs is merely because the distance between the roof of the skull and the bottom jaw is less than in semi-aquatic frogs.

Far from mounting the eyes as high as possible for maximum vigilance, tree frogs orient them to see downwards rather than upwards.

In addition, the wide separation of the eyes in tree frogs gives them a relatively deep binocular field, allowing for the accurate assessment of distances when leaping to another perch. Whereas semi-aquatic frogs leap without aiming precisely, tree frogs often leap to a particular spot.

The crucial points, previously overlooked, are

• firstly, that tree frogs see downwards whereas semi-aquatic frogs see upwards, and
• secondly, that tree frogs have a more extensive field of binocular vision than that of semi-aquatic frogs.

Do the visual differences between these groups of frogs not show that each has adapted in its own way?

If so, this, in turn, would mean that phylogenetic constraint need not be invoked to explain any superficial resemblance in the positioning of the eyes between semi-aquatic frogs and tree frogs.

In summary:

• Tree frogs, like semi-aquatic frogs, have eyes that protrude from their heads. It is easy to assume that the position of their eyes helps them to locate predators approaching from above.
• However, tree frogs and semi-aquatic frogs see differently. Tree frogs can look down. Semi-aquatic frogs can only look up.

Posted on June 14, 2022 07:25 PM by milewski