Fog and fauna of the Namib Desert: past and future

The future of fog-dependent habitats under climate change is unknown but likely precarious; many have experienced recent declines in fog. Fog-dependent deserts particularly will be threatened, because, there, fog can be the main water source for biota. We review the interactions between fog and fauna of the Namib Desert, about which there is 50 yr of research. We resynthesize the data, seeking patterns and mechanisms that could provide a framework for predicting outcomes of changes in fog regime in other fog-dependent deserts. In the Namib, fog constitutes the most-predictable form of free water. At least 48 Namib animal species consume free water from fog, or are likely to do so, employing both liquid and vapor phase. Fog also sustains plants that form the base for metabolic water production and wets the diet to provide pre-formed water. So fog provides or underpins all the water intake of Namib fauna. Only a few species are active fog-harvesters, though. Among Namib beetles, two species are unique in that they fog-bask; they assume stereotyped postures in wind-driven fog and droplets deposit on their carapaces. Some Namib beetle species construct surface ridges that trap fog water, which they consume. Some arthropods emerge from their subsurface habitats, or occupy its wet top layers, to access fog water, at times and in conditions outside their usual surface activity. Many more taxa, including vertebrates, use fog water opportunistically. They do not actively seek it out but use it when available. Acquiring fog water from droplets requires overcoming spherical surface tension so is possible only for animals heavier than similar to 100 mg. Smaller animals extract water from films or acquire it in the vapor phase. Some Namib animals use hygroscopic surfaces to extract vapor from unsaturated air, at ambient humidities attained in fog or sometimes between fogs. Rapid acquisition of water during episodic fog events creates problems for storage and osmoregulation, which some Namib animals have solved in enterprising ways, including long-term internal storage of water and sequestering of osmolytes. Although not yet comprehensive, the body of research reviewed, and the principles that we have elucidated underlying fog usage, should inform future research on fauna throughout fog-dependent deserts.