Monthly Archives: July 2012

Why do mangoes have such huge seeds?

 

North Indian summers can be terribly hot and a miserable time for the residents. Every year people hope that the summer gets delayed. And when it starts people wait eagerly for the monsoons to come and provide some relief from the terribly hot days. However there is one aspect of Indian summers that somehow makes it worthwhile- mangoes. Mangoes are the most delicious fruit in the world and are rightfully given the title of the “king of fruits”. If there were a competition of deliciousness in fruits, mango would win it by a huge margin. Of course, papaya would come last- maybe it would have to pass the test of being a fruit with “grace marks”.

Have you ever wondered why mango seeds are so huge? Compare them say with the seeds of an apple. Apple seeds are so small compared to the fruit but a mango pit can be several inches long. In this VERITAS we will understand the reason for the huge pit in mangoes and will understand some interesting facts about evolution.

First, let’s understand why the seed exists. Simple. The seed exists to create the next generation. A more interesting thing to understand is why the fruit exists. The fruit exists and is sweet so that animals may eat it and disperse the seeds. This is just one of the many ways in which a plant disperses its seeds to give life to its offspring.

There are other methods too. Some plants have seeds that get entangled in the fur of animals and reach far away before falling off. Some other seeds like acorn and some nuts are foods in themselves for some animals. These seeds rely on animals like squirrels or mice trying to store them and forgetting where they were stored or not being able to eat the entire collection.

It is evolutionarily beneficial for seeds to be dispersed to great distances from the parent plant and also be dispersed in varied locations. If all seeds fall in one location and that location becomes unfavorable for the growth of the plant, then the entire next generation is gone. So plants “want” to disperse their seeds far away and in different places. However plants cannot do it by themselves. Animals provide an excellent way to do that. Other ways are to use the wind or moving water to reach distant places.

Now lets look at how fruit plants/trees use animals to disperse seeds. First they make delicious fruits and advertise the deliciousness by bright colors and delightful smells. Animals are attracted to the colour, smell and taste of the fruits. You may have noticed that most seeds have really hard coverings. Fruit trees/plants rely on animals eating the whole fruit and then passing the seeds out in their stools. The seeds have hard coverings to be able to stand the digestive juices in the stomachs of the animals and come out intact. Through this method seeds can travel huge distances- they use the digestive systems of animals as their transportation mechanisms! Now don’t be squeamish about it. This is how nature works. For an excellent discussion of how this works read Jared Diamond’s “Guns, Germs and Steel”. I have recommended this book in an earlier VERITAS article also.

The dispersal of seeds in animal feces is a hugely developed and important method for a lot of fruit plants and trees. To give you an example, there are some seeds that MUST mass through the digestive system of some animals to be able to germinate. There is a species of cucumber in Africa which is only found where an animal named  aardvark exists. It has no way to propagate  except by its fruits being eaten by aardvark and the seeds coming out of its feces. Guess what the cucumber is called. Aardvark cucumber. The fruit is so thick and the seed is so deep inside that without the aardvark’s help, the seed cannot come out. And the aardvark is equally “sincere” to the cucumber- the only fruit that the aardvark eats is this cucumber.

Another important thing is the timing. When a seed is not ready to be planted, the fruit is of a less attractive colour and does not have  good taste or smell. The fruit changes colour, smell and taste to become attractive/tasty for animals only when its seeds are ready to be planted. We call it ripe at that stage.

The interesting thing in all this is that it seems that plant evolution is acting in an intelligent fashion- using animals to disperse seeds far away. However a single plan does not even think. The “intelligence” emerges over millions on years and is caused by natural selection. Plants which succeed in dispersing their seeds far away using animals survive, the others just die. So using the animals is not a conscious intelligent choice- it just happens because other options die out.

Now, lets talk about mango. You may think that the above theory of propagation of seeds by animal’s feces would not apply to mango. You may say that the seed/pit of the mango is so huge that there is no animal that exists which can eat it whole and pass it out in its feces. You are right. There is no animal that exists TODAY which can eat a whole mango and pass out the seed in its feces. But there were animals at an earlier time in Earth’s history which could do this and mango evolved to be able to take advantage of these animals. I am talking about two extinct creatures: the giant sloth and the Gomphothere. A giant sloth could reach the height of almost 17 feet and weighed about 5 tonnes. The Gomphothere was an elephant like creature. These animals became extinct quite recently( in terms of geological time scale)- giant sloths became extinct about 5000 years ago and Gomphotheres became extinct about 10000 years ago, Mangoes used these animals for the dispersion of their seeds. These giant creatures ate mangoes whole and the seeds would come out in their feces.

Mango is not the only fruit to witness the extinction of its evolutionary “partner”. Avocado also suffered a similar fate. So mango and avocado are examples of plants who evolved to be able to take advantage of certain animals which have now become extinct. There is a scientific name for this phenomenon- evolutionary anachronism.

There are examples like this in the animal world also. The pronghorn antelope which lives in north America is built for speed. It can run at speeds of over a 100 km per hour. It is the second fastest animal in the world after the african cheetah. But why does it run so fast? There is not a single animal in America which can catch it. So how did it evolve to run so fast? There once existed a species of cheetah in North America. The pronghorn evolved to run so fast so as to save itself from the American cheetah.The American cheetah became extinct almost 11000 years back leaving the pronghorn to run superfast without a real reason!

A book that describes evolutionary anachronism in detail is: The Ghosts Of Evolution: Nonsensical Fruit, Missing Partners, And Other Ecological Anachronisms by Connie Barlow.

Let me now tell you an interesting thing about almonds. Fruits are tasty because they want to be eaten. Their seeds have a hard covering protecting them from digestive juices. However if a seed does not have a protective covering then it does not want to be tasty. It does not want to be eaten. Almonds are like that. Wild almonds are bitter and if eaten you could die from them because their bitter chemicals produce hydrogen cyanide- a deadly poison! This happened because almonds did not want to be eaten by birds. However thousands of years ago humans found a mutation of almonds which were not bitter. They started planting them and the almonds that we eat today are descended from this mutant type. So almonds evolved into their “sweet” form because of human interaction and preference.

Friends, no species in the world, animal or plant has evolved on its own. It has evolved in response to something else- living or non living. So we cannot understand any species in itself – it has to be understood as just one variable in a giant equation that involves every other creature also. And we should fear any variable of this equation becoming much bigger than others or causing other variables to vanish- the equation may become completely unbalanced and then the result may be disastrous. For all of us!

 

Kanwar

 

========================================================================

Go wondrous creature, mount where science guides

go measure earth, weigh air, state the tides,

instruct the planets in what orbs to run

correct old time, regulate the sun

======================================================================