[paul.fr]

I know that rats can come resistant to some poisons, and that periodically the poisons have to be changed. but how do they become resistant in the first place. Surely the rats that eat the poison die, so how do the other rats develop their resistance?

[another_someone]

I know that rats can come resistant to some poisons, and that periodically the poisons have to be changed. but how do they become resistant in the first place. Surely the rats that eat the poison die, so how do the other rats develop their resistance?

It is that old saying, that which does not kill you makes you stronger.

Firstly, in some cases, the rats do not necessarily become resistant to the poison, but simply learn to avoid it (maybe learn to smell it), but in other cases, they will actually develop direct resistance to it.

If a rat eats enough of the poison, it will die; but not all rats will eat enough poison to kill them (they may just nibble a very small bit of the bait, and become sick without dying).  Those rats that can nibble more of the bait without dying will survive better and will have more offspring, who will all be a little more tolerant of the poison.  They too will eat the bait, and some will die, but will not have eaten enough to die (but the number who die will be fewer than in their parents generation, and the amount of bait they can eat before becoming seriously sick will be more than in their parents generation).

[lgic]

There's a really cool age-related element to this, and I use this story to emphasize to my students that developmental changes are important.

Strychnine is a common rodenticide. Baby mice and rats are unaffected by it - only the adults are killed. If you read the package directions, they will tell you treat again 3 weeks after the initial treatment. This gives the babies who were spared a chance to grow up, and then die from the new batch of poison.

The reason for this is that the babies express a type of glycine receptor in their spinal cord neurons and brains that is unaffected by strychnine. As the animals grow up, they replace the baby type with an adult form that happens to be efficiently blocked by strychnine, causing the adults to have seizures and die.

[Bored chemist]

Last I heard the use of strychnine as a rodenticide was banned (here in the UK) by the "cruel poisons act"
http://www.webtribe.net/~shg/Animals%20(Cruel%20Poisons)%20Act%201962%20(1962%20c%2026).htm

except for moles because their death throes take place underground so no squeamish people see them.
Also, there's a strange parallel with people, we are all born with the enzymes required to digest the lactose in milk. Many adults lose the ability and develope lactose intolerance. In North West Europe the incidence of lactose intolerance is low because, over the centuries, many of us have learned not to switch off the enzyme. This allows us to continue to eat dairy products in adult life- very useful if you live in an area where grass grows better than most other crops. You can feed cattle or sheep with the grass and eat meat and cheese; a clear evolutionary benefit.
The rats would be a great evolutionary pressure to keep the "infant" type of glycine receptor. Strychnine looks like a rat poison that's almost bound to produce resistance.
BTW, does the fact that baby rats and mice live on milk complicate the issue? If their mothers get poisoned the babies die anyway.