The Mystery of the Shrinking Sheep

I have an interesting case study to share with you. This case provides a very valuable insight into how climate affect all organism, in this case, sheep. Hirta Island is an uninhabited island in St Kilda archipelago in the province of Scotland. There is only wild sheep, called Soay sheep and the grass in Hirta, creating a perfect natural laboratory. The Soay sheep has lived on the island for thousand of years. Scientists, led by Tim Coulson of Imperial College London had been studying Soay sheep since 1985, collecting massive data and detailed information for more than two decades now. This study is still ingoing.

In the year 2007, question arose when it was found out that Soay sheep had been decreasing in size and body weight since 1990's. The wild sheep had lose an average 5% or 81 grams of body weight annually. That is very strange, since classic evolutionary theory predict wild sheep would grow bigger as stronger, larger and fitter animals survive into adulthood and reproduce (the concept of natural selection).It was a mystery, and promptly investigated. What was the cause?

Now, some of you might have concluded that the reason is inbreeding, for the fact that they live in isolation.

IT IS NOT.

Read carefully, the reason the sheep of Hirta get smaller is too much grass!

This is fascinating and surprising. You see, in the 1990's, the usually harsh winter in Hirta Island change to mild. In 1980's, the harsh winter means there is no grass to graze. But 1990's mild winter, caused by warmer climate, allows grass to grow. Now there is grass all year round. This means, the weaker and smaller sheep can survive and reproduce. During 1980's, only the big, healthy and large sheep that had piled on the weight during summer can survive the harsh winters. Climate change in the 1990's brings milder winter so survival conditions are less challenging. Many individuals of weaker and smaller stature managed to survive, increasing their numbers in the population.

Apparently the mystery of the shrinking sheep of Hirta had been solved.

CONCLUSION: Goats that experience a period of scarce feed supply, like winter or droughts regularly, had to be larger in size. Their larger frame will allow a greater amount of fat to be stored in order to survive the lean period. That is why goats from temperate countries( European) where there is winter, or arid countries (South Africa, Australia, Arabs) where there is droughts, are bigger than our tropical goats, where there is feed all year round. For our goats, there is no need to be large framed. No need to store large amount of fat since lean period hardly exist.

This study also produced another important observation.

The Soay sheep rate of reproduction is affected by body size. When their body size is bigger, the scientists observed, they multiplies slower. But when body size is smaller, they multiply faster. Very interesting.

It is logical since bigger mammals grow slower in population numbers. For example, cows multiply slower than goats. Whales multiply even slower. But rabbits, smaller than goats, multiply faster. For rats, even faster.

Now dear readers, the conclusion we can make is the bigger the goats, the slower will be their rate of reproduction. We have to choose our priority in breeding goats, either go for bigger goats with slower reproduction rate or go for smaller goats with faster reproduction rate.

The third observation is also interesting.

Since Soay sheep in Hirta never have to deal with predators, we expect them to thrive and overpopulate the island. It does not happen. Another unseen predator, the gastro intestinal parasites took charge to control their population. Hmm.. the beauty of nature. The nature's way of preventing the ecosystem from being overran and destroyed.

Conclusion : We as breeders should never view parasites as enemy to be eliminated or killed to extinction. I realized that whenever we human begin practicing to kill any organism, it will breed resistance. If we can expel/repel or use diversion techniques or whatever method that do not involve killing, I believe we will not encounter resistance. Chemical ( modern dewormer, herbal, tannins ), physical ( copper oxide wire particle or COPW ), or genetic improvement ( natural, laboratory) are the usual paths to obtain effective parasite control. For me the best path is through genetic improvement, since it does not involve killing the worms