Friday, September 24, 2010

When life is shi#

A skipper on bird dropping
People using the phrase need to watch out for they may very well be describing a life of heavenly bliss ! There are a whole lot of animals that depend entirely on the excretions of others. Scatology must be one of those subjects that universities are not advertising about but there are lots of amazing questions out there waiting to be researched. In some habitats there are very few sources of nutrition available, particularly for organisms with limited locomotion. A cave salamander has been found to be dependent on bat guano. A 2008 article discusses the antiquity of coprophagy in the beetles of the family Scarabaeidae - the dung beetles. It seems like they only took to this diet after the rise of the mammals , making it a relatively recent trend, evolutionarily speaking. The paper notes that herbivorous dinosaur dung is unlikely to have supported dung beetles due to the high nitrogen content - the result of the urinary and excretory tract merging into a cloaca. The feature is present in the birds and a study of beetle fauna in an oil bird cave found just one Scarabaeid, but that one fed on decaying plant matter. Coprophagy is however shown by many other insect groups and interestingly it is rather rare among the lepidoptera (as a primary diet)- two pyralids are known to live on mammalian droppings. Interestingly many adult butterflies will visit animal faeces, but this is related to special functions and do not appear to be part of their regular diet. The uninitiated are usually shocked when they see some of the prettiest butterflies they have ever seen perched on poop. Entomologists  however use the attraction to get closer to some of the rarer butterflies by baiting them. Skipper butterflies (Hesperiidae) have a particular fancy for bird droppings and some South American species are known to follow army ants which in turn are followed by ant-birds to get to the bird droppings !

Painting by J B Fraser with Greater Adjutants
An exhibition of old illustrations of India at the National Gallery of Modern Art caught my attention recently. An old (1819) watercolour by James Baillie Fraser showed Adjutant storks sitting over buildings in Calcutta, While researching the subject, I decided to improve the Wikipedia article on the Greater Adjutant Stork and realized in the process that this rather ugly-looking bird is essentially endangered by improved sanitation. At one time, there were thousands of this bird all over northern India, particularly in the city of Calcutta. They were busy feeding on the dead bodies disposed into the Ganges and disposing off offal, animal and human excreta with such efficiency that the Calcutta Municipal corporation of that time decided to make them their mascot, two birds facing each other became a part of the corporation logo. Their populations however rapidly declined during the Nineteenth Century and kept dropping to become the endangered bird that it is today. The best places to see them in India now are apparently garbage dumps in Assam and the only major breeding areas are in the Brahmaputra valley.

Other scavengers have declined sharply too- the most famous being the vultures - once found along the avenues of Delhi and floating in thermals in large numbers, they are almost absent today and the cause  is usually attributed to the non-steroidal anti-inflammatory drug Diclofenac. Interestingly these hit the vultures of the genus Gyps the most. There is however a smaller vulture - the Egyptian Vulture (Neophron percnopterus) which has also declined across its range and some Spanish scientists have suggested that they might have been affected by antibiotics - which apparently depress their innate immune system. Now this black-and-white vulture has a bright yellow bare facial skin and it seems that the birds derive the colour from carotenoids derived from a diet of mammal excreta. The usual ideas is that the fitter males have a brighter face and females choose them as their mates and it seems like this might have the cost of exposing their immune systems to the onslaught of bacteria. And having a weak immunity could be bad under those circumstances - so it seems like antibiotics, pain-killers and sanitation are not without ill-effects for some.

As another aside one should note that the ancient Egyptians who revered both the dung beetles and vultures  introduced some innovations in sanitation and it is comforting to see that there are scholars looking at these aspects of daily human life.

Further reading
  • Arillo, Antonio & Ortuño, Vicente M. (2008) Did dinosaurs have any relation with dung-beetles? (The origin of coprophagy). Journal of Natural History 42(19): 1405-1408
  • Another blog post on an allied theme 
  • Campos-Arceiz, A., 2009. Shit happens (to be Useful)! Use of elephant dung as habitat by amphibians Biotropica. 41(4):406-407.

1 comment:

  1. Very interesting article, Shyamal! On a trip to Assam in March, we saw a huge colony of adjutants in the Guwahati garbage dump - if I remember correctly, I think Adesh said that this was probably the largest colony in India....