Sea fog creeps into the desert. Water. Winds blow detritus, scraps of dry grass and seeds, into the dune fields. Food. Thus the basic requirements are satisfied for animal life at the bottom of the food chain in the Namib.
It is here that the humble beetle finds its niche. Not just one beetle, dark and round, but a horde of them. Commonly called tok-tokkies for a sound they make to attract a mate, the tenebrionid family consists of some 200 species.
They inhabit slipfaces and interdune valleys on the sheltered side of dunes where windblown detritus accumulates. Slipfaces occupy less than 1% of dune surfaces, but are alive with insects and reptiles, above all species endemic to the Namib. The attraction for all of them is a food supply. Basically, beetles and fishmoths eat detritus; lizards and geckos eat beetles and fishmoths; and snakes eat lizards and geckos.
Although they once had wings, tenebrionid beetles are flightless. In the distant past their wing cases, made to move aside to expose their wings for flight, came to be fused together. A cavity with one small opening was left at the place where they once folded their wings. As their spiracles or respiratory pores now opened into this humid cavity, whereas previously they had been exposed to the dry air outside, water loss due to evaporation was much reduced.
It was a case of evolutionary backtracking as wings are an advanced adaptation in insects. In an environment that was becoming hyper-arid tenebrionid beetles had more to gain from water retention than from the ability to fly away from their natural enemies. They now take their chances on the ground.
The "fog-basking" beetle (Onymacris unguicularis) taps the fog for drink. Although it is ordinarily diurnal, it emerges from the sand on foggy nights and climbs to the dune crest, where water condensation is greatest. Head lowered and posterior raised in a kind of handstand, it faces into the fog-bearing wind, to let moisture condense on its back and trickle down to its mouthparts.
The "fog-trapping" beetle (Lepidochora discoidalis) does it differently. After it has foraged during the first part of the night, it normally digs itself back into the sand. It only returns to the surface when fog comes, late at night or early in the morning, so as to excavate a narrow trench in the sand across the path of the fog. Somehow it knows that ridges alongside the trench will absorb all the moisture it needs to drink.
Reptile with a water tank
The shovel-snouted lizard (Meroles anchietae), one of five lizard species endemic to the Namib, stores water in a large bladder-like diverticulum -- a blind tube off its lower intestine -- where the reserve remains available for weeks on end.
It drinks as much as 12% of its own body weight in one go, equivalent to 7,5 litres in three minutes for a 75kg man.
Its only source of water is fog. The trouble is that fog condenses on the dunes in the middle of the night when the weather is cool. Lizards like heat. They spend the day in the sun and burrow into the sand to keep warm when night falls.
Between a rock and a hard place, the shovel-snouted lizard goes out into the night air, drinks its fill and goes back to bed. The creature that basks in the sun the next day is decidedly plumper than the one that first crawled in the night before.
Lizards that swim in sand
The shovel-snouted lizard performs a "thermal dance" when the sand becomes too hot for its feet. It props itself up on its tail, lifts a front foot and back foot, holds them up for a while and finally exchanges them with the opposite pair.
The dance is repeated over and over again until the sand cools down a bit. When all else fails the lizard dives into the loose sand and "swims" down to a cooler level.
Lizard species with the right stuff are called "sand-diving" or "sand-swimming" lizards. Typically their snout is streamlined to ease entry into the sand and minimise resistance as they swim through it; their nostrils are on top of the snout to keep sand out; they are splay-legged and their toes are elongated.
When a lizard wants to cool down or feels threatened, it dives head-first into the sand and is gone in an instant. With air trapped between the sand grains, the lizard can remain buried for 24 hours or longer -- half a metre down -- without any ill effects.
If it finds itself on the surface when rain falls, it is in trouble, because the sand becomes too firm for it to penetrate. Stranded above ground, it is easy prey for predators.
The camel bird ........