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Where do you think tens of billions of old mice/rats that are to die yearly are supposed to go, especially in developed areas where there are not many other predators?

In predation even without any specific choice by the predator, the resulting prey is naturally skewed toward weak/sick/old, ie the ones who is slower, has lower attention/senses, etc. Until some age the young are naturally on the weakest side and not surprisingly may fell prey a lot. Such filtration results in more healthy/agile/stronger adult population.




Relative size makes a big difference. The most extreme version of this is krill, as a blue whale really does not care about the fitness of the millions it needs to eat.

What your missing is most mice don't reproduce. The average litter is 6-8 and mice can have multiple litters per year. So, mice basically never reach old age.

The selective pressure has more to do with caution because a completely heathy mouse can't out run a cat / owl etc.

Put another way cats rub at up to 48 Km/h mice 13 Km/h. Thus, 'fitness' has little to do with athletic prowess.




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