When everything is upside down
In order to maintain its normal stance (also called “primary orientation”) a bird has to constantly balance its body on two legs: a dead bird falls over. Fish don’t balance on two fins, but most of them also “keel over” when dead. What this shows us is that the normal stance does not come automatically, but is an energy-demanding, active process, in which sense organs monitor the animal’s position and either directly, or via the brain, initiate correcting measures when required. We can identify dorsal and ventral sides in most animals without much difficulty and find that the dorsal side usually faces the sky, while then ventral side is directed towards earth. But what to do if you live in the water, your mouth is on the ventral side, but your food floats on the surface of the water? You turn upside down and that is exactly what the East African Synodontis and Rhinodoras catfish are doing all the time.
What if you live in water, your back is in the shape of a boat’s hull, your eyes sit at the top of your head and you need to swim swiftly around in the water, scanning what’s going on below you? Again, you’d turn upside down – and backswimming notonectid, aquatic bugs popularly known as “backswimmers” immediately spring to mind. Other animals, living in the water have also adopted a topsy-turvy lifestyle. Brine shrimps, for example, form back-stroking male/female tandems; the giant Antarctic slater Glyptonotus antarcticus (whose dorso-ventrally divided eyes I studied in Antarctica) swims short distances ventral side up, and marine snails of the genus Janthina sp. hang upside down under their bubble raft that floats on oceanic water’s surface.
In the terrestrial environment, there are also some inversion specialists: the famous sloth is but one example. When flamingos sift through the water for picking up floating algae and other small plankton, they hold their heads upside down and dip their beaks in an inverted orientation into the water. Bats and flying foxes see the world inverted whenever they open their eyes while resting or roosting; baldachin spiders and glowworm larvae lie in wait of prey ventral side up and a host of caterpillars have become so adapted to feeding exclusively from the underside of a leaf that even their body coloration has changed to the reverse from what is normal in caterpillars feeding from above the leaf: the larvae of the gum emperor moth are a good example. What has always puzzled me, however, is that there seems to exist no flying animal, whether bird, mammal, or insect that has evolved a way of gliding along or flying upside down in air (although sea gulls and frigate birds have been seen and filmed to perform 360° rolls while on the wing). Perhaps a theoretical zoologist with some knowledge of aerodynamics, bioenergetics, and anatomy could elaborate, for what sounds a simple question can often be a complex problem and turn out a real brain teaser.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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