Monday, September 7, 2015

Desert Tales

Africa : NASA picture 
In the scorching sands of Saharan desert why doesn't the Cataglyphis bombycina get roasted like a peanut? These tiny creatures  go by the common name Saharan silver ants. As the name suggests these ants  are native to  the  hot Saharan desert and have a silvery glaze. Saharan desert, stretching across northern Africa,  skirted  by the Atlantic ocean on the west, the Red sea on the East and the Mediterranean sea on the north  is not at all a friendly terrain to pitch a hut.  Here ambient   temperature can soar to 54 degree Celsius and sand temperature could be as high as 80 deg.C. Most of the desert creatures are crepuscular, that is they come out either during dawn or dusk. But alas   poor silver ants can't afford to forage during  twilight hours  because of predators. So to avoid being caught and made into a meal these  ants forage when the predators are having  their siesta; that is  at noon, when the Sun is at the peak. Professor RĂ¼diger   Wehner and his team at the University of Zurich have been fascinated by these ants for quite some time.   They   have been looking into the   the neurophysiology and behavioral pattern of these ants with special emphasis on vision as  a strong cognitive capability. In the 17th July issue of Science magazine  Professor Wehner and his team provide  answers to the critical question : How do these tiny insects keep their cool ?   They could very well  have asked why are these ants so shiny.
Community dinner: courtesy wikipedia


  
There are  several interesting things about C. bombycina.   Their   unusually   long legs  lift the body 4-5 mm above the ground. Such high ground clearance means, the ellipsoidal body, not more than a cm in size is considerably cooler than the scorching sand below. Long legs also mean speed. These ants  can run  fast and far off in search of food. This has been captured in a excellent video. If it goes far off, how does it get back to its burrough? It seems smell and sights of the starting point stored in their brain  guide it safely back to home ground.

But how does it ward off the heat? Prof. Wehner and   his team provide the answers in their paper. The silvery sheen is not just a beauty enhancer. It is indeed a very effective thermoregulation feature. The shine  is due to a thick mop of fine silvery hair covering the dorsal and lateral sides of the body. The morphology of the hair has been studied by electron microscopy .  triangular cross section and taper off to the tip. These hairs perform  multiple tasks, strictly in accordance with the Laws of Physics. The triangular surfaces facilitate   total internal reflection and thus enhance reflectivity in the visible and near infra-red regions of the solar spectrum and      in the mid infrared region the hair layer demonstrates  very high emissivity. In other words exactly opposite of what  solar panels do. For excellent performance a typical solar heating panel must have zero reflectivity ( very high absorprtivity) at low wavelengths to capture maximum heat  and very low emissivity at high wavelengths to minimize radiation heat loss.

Desert creatures have evolved  several  adaptive strategies to encounter the twin threats of 
high heat and lack of water.  For example certain types of desert  rodents  generate water as a metabolic by-product.  At the cellular level, heat shock proteins  see to it that   heat and dryness don't impair the structure and hence the function of biological molecules.  



References:

1.  Keeping cool: Enhanced optical reflection and radiative heat dissipation in Saharan silver ants. Nan Shi et al Science 17 July 2015 vo. 349 pp 298-301