On a Harsh Scottish Isle, Sheep Trade Strong Immunity for Lots of Lambs

By Andrew Moseman | October 30, 2010 5:44 pm

Soay_eweThe snowy, wind-blown Scottish archipelago of St. Kilda may be inhospitable, but because it is inhospitable,  it is an ideal natural laboratory. The last people left this place behind nearly a century ago, but the sheep stayed. And the in absence of human interference in their breeding, the sheep of St. Kilda have shown scientists something peculiar.

It has to do with the relationship between the immune system and reproduction. Andrea Graham and colleagues have studied the islands’ Soay sheep for years and years, and found the average lifespan of the ewes to be about 6 years. However, there’s great variation in there: Some lived just a few years, and some as many as 15.

The short-lived ewes had lower concentrations of antibodies than the longer-lived ones, which suggested why their lives were so short. But why was natural selection not weeding them out? Dr. Graham said the researchers found this to be a puzzle: “What are all these sheep doing with low antibody concentrations?” [The New York Times]

It’s a good question. If you just looked at the conditions on St. Kilda, you’d think that weaker sheep eventually would be wiped out.

The Soay sheep may not have any natural predators, but they do have the elements to contend with: a lack of food, brutal weather that can kill off half the population at a time and parasitic worms that live inside their bodies and suppress their immune systems. [MSNBC]

The answer, they found, was that the sheep with the weakest immune systems were the most fertile, reproducing like crazy during their short lives. Meanwhile, the sheep with strong immunity had less reproductive success, but they also lived a lot longer, so the reproduction stats just about even out. Therefore the archipelago hosts a population where neither the strong immunity/weak fertility sheep nor the weak immunity/strong fertility sheep have died out.

Graham’s study is out in Science.

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Image: Wikimedia Commons

  • megan

    What’s so surprising? It happens in all animal populations. Shorter lived with strong natural predators/disease, survive by breeding like crazy. Live longer and hardy defenses have lower birthrates. Humanity is no different, and being liberal/conservative/pro-anti abortion baby loving PC doesn’t change facts. Places where humans are poor, struggle and starving and dying like rats, respond with cultures and behavior to have lots of babies/sons to survive or help survive. Rich populations with means to defend (better food, medicine, jobs) reduce their breeding naturally, create logic/ideology to spend more time raising healthy few and having longterm success rates. It’s when the old survival biological response continues into a more surplus successful environment that things get imbalanced.

  • fatkid

    Megan, you are soo hot. You mentioned sons, but with women cycling together when in a group, it just takes one son to replentish the whole tribe. One very weary son..

  • peter beater

    fatkid, i like you because im fat as well and i would like to be that weary son and carry on because when i lay my head to reast i want to be sure to get poon tang. also im going to call on godwins law and say didnt hitler try something like this when he tried to create a “master” race?

  • Darius

    Lol 3 posts and goodwin prevails.

  • Dante The Canadian


    Good conclusion and very well thought out. Many different species have the same response. Seems natural to me too.

  • John

    This is a little different that what Megan discusses where we behave differently in different environments. In this case the environment is the same but nature came up with two solutions.

  • http://forums.cheapstuff.com/member.php?11049-coachoutlet3lud Johnson Guecho

    stunning techniques you’ve provided above. This has been quite


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