Evolution and Fat: Internal versus External Storage

Nature All Around Us: A Guide to Urban Ecology
by Beatrix Beisner , Christian Messier , Luc-Alain Giraldeau (Editors)
Publisher: University Of Chicago Press; First Edition edition (September 21, 2012)

Another book I chanced upon at the local library is titled, "Nature All Around Us: A Guide to Urban Ecology", edited by Beatrix Beisner, Christian Messier and  Luc-Alain Giraldeau. It is a short book of around 160 pages which contains little snippets about the nature one sees in the urban environment around us. It has a focus on the living things one is likely to see in North America. I would recommend this book to anyone who wants to find out fascinating information about animals and plants which we see around us; and pause to wonder why they look the way they are, and if we can find out what the evolutionary and social strategies favoured by the creature. 

I could write a blog based on each chapter in the book. I wish when I was a kid, I was taught from something like this- to help build a greater appreciation in me about nature that I saw around me. If we stop and think about it, even common creatures  like crows, sparrows, squirrels and pigeons have an evolutionary advantage for living in around an urban human environment.

One fascinating chapter is called a "A Life of Extremes", and discusses an important difference between a squirrel and its cousin, the marmot. I saw both these creatures while doing the "Highline Trail" in Glacier National Park, Montana on a sunny day in late July 2013. It did not enter my mind to think why these creatures are the way they are; a relatively thin chipmunk, the size of a squirrel, (I do know which was the exact sub-species I saw in the picture below), and a pudgier slightly larger marmot.

Chipmunk: External Storage of energy in fats in nuts. Picture taken at Glacier National Park, Montana, USA, in July 2013

The reason for the difference in their shape turns out to be related to the diet of the two creatures. Chipmunks eat a diet rich in energy dense nuts, which are not bulky and easy to store as far as volume is concerned. During winter they sleep in their burrows- but do not sleep all the time. They sleep for up to six days, wake up, gorge on stored nuts and sleep again, continuing the cycle throughout winter.

Marmot: Internal Storage of energy in body fat: Picture taken at Glacier National Park, Montana, USA, in July 2013

Marmots in contrast have a diet of grass and other plants like clovers, buttercups and dandelions. It would take around 50 cubic feet to store all the energy needs in grass for a winter for a single marmot- which can be quite problematic for a small creature the size of a human infant. It is not simply a problem of finding storage space, but grass does not store as readily as nuts, and can get infested by bacteria and fungi. The strategy chanced upon by evolution for them is to store their energy reserves for winter in brown body fat- which is the same kind of fat that newborn human infants have! The presence of a lot of brown fat in human infants helps to keep them warm, and the brown fat for the marmot serves the dual purpose of keeping them warm and providing a source of fuel for the body when nothing grows in winter.

For some creatures storing fat internally is the only option. An extreme case is that of the ruby-throated hummingbird. Some of them fly across the Gulf of Mexico, a distance of 800 km from the Yucatan to Texas in a single non-stop flight. This is possible because the birds weighing about 3 grams double their weight by storing the extra energy in fat for the journey.

It is amazing to think how evolution shapes things in creatures like chipmunks, marmots, humans and hummingbirds. On the surface, we as humans are quick to jump to a hasty conclusion calling an animal cute, fat or slim. In reality the reason for the animal's shape has a lot to do with survival in the external environment, and nothing to do with our perceived aesthetic tastes.