• Question: Why do rats and some other pests get immune to poisons so quickly?

    Asked by chemicalexplosion3 to Anna, Chris, Jane, Iain, Nick on 11 Mar 2014.
    • Photo: Anna Middleton

      Anna Middleton answered on 11 Mar 2014:

      Hi @chemicalexplosion3 it will be to do with their genes. Genes control how we react to things, a slight change in the genes means we can react better/worse than before. Genes can change randomly and once they do they can get passed on to offspring. Rats breed quickly, so if you get one that is immune to poison this one can pass it on quickly, whereas the others will die from the poison

    • Photo: Iain Moal

      Iain Moal answered on 11 Mar 2014:

      Another great question chemicalexplosion3,
      If you look at a population of rats, there will be some variations between the individuals. Some will be better at dealing with the poisons than others. So, when you give the first dose, you might kill 99% of the rats, but the 1% which were capable of resisting it will then breed and pass the genes which give them resistance to the next generation. That way, the next generation will be much more resistant to the posion.

    • Photo: Jane Charlesworth

      Jane Charlesworth answered on 12 Mar 2014:

      Natural selection! This is what Darwin described. In a population of rats (or other pests) there is always genetic variation. For example, we use a poison called warfarin to kill rats. This poison works by preventing the rat’s blood from clotting, so they bleed to death if they get a cut (nasty).

      However there is a genetic mutation in the rats that can stop the poison working. Rats that carry that mutation will survive being poisoned and pass their mutant copy of the gene onto their offspring, so over time a population of resistant rats will build up, even if the mutant gene started in only 0.000001% of the rat population.

      In the case of the rats, though, there’s an added twist–that mutation is harmful to rats if they carry two copies of it and helpful if they have one. Let’s call the “normal” gene A and the mutant gene “a”. Animals have two copies of each gene (one from each parent), so for our gene a rat can be AA, Aa, aA or aa. Rats that are AA die if they are poisoned. Rats that are Aa are immune from poison and rats that are aa die from carrying the mutation. So while the mutant “a” gene increases in the population, you will never find it in every rat.

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