Kaj, fast breeding does not just incur a cost on the cubs, but on the mothers developing the cub fetuses. No matter the dearth of rabbits/food, as long as it’s less than the amount needed to sustain the current fox population, the less energy and time spent by a fox mother developing unnecessary fetuses, the less likely she will die before child birth. You can’t just calculate the raw probability of cubs surviving by saying “Each cub has X% chance of surviving, therefore the more cubs, the greater total chance that some will survive”. A cub is taken care of primarily by it’s mother, by nature of non-group selected genes. If each cub has roughly equal capability of aquiring food from the mother, that leaves the same amount of food for a larger number of cubs than the restrained breeding fox families. If 6 cubs have to share the amount of food that can only sustain 1 cub, it’s likely no cubs will survive. Even if some cubs manage to horde more food than others, there still can only be so many surviving cubs based on the amount of food available, which will be the same amount as can survive from the restrained-breeder families. That means that the unrestrained breeder-mothers just spent much more energy and time producing the same amount of viable cub offspring as the restrained breeders, leading to a worse long term outlook for the survival of that unrestrained breeding family.
Kaj, fast breeding does not just incur a cost on the cubs, but on the mothers developing the cub fetuses. No matter the dearth of rabbits/food, as long as it’s less than the amount needed to sustain the current fox population, the less energy and time spent by a fox mother developing unnecessary fetuses, the less likely she will die before child birth. You can’t just calculate the raw probability of cubs surviving by saying “Each cub has X% chance of surviving, therefore the more cubs, the greater total chance that some will survive”. A cub is taken care of primarily by it’s mother, by nature of non-group selected genes. If each cub has roughly equal capability of aquiring food from the mother, that leaves the same amount of food for a larger number of cubs than the restrained breeding fox families. If 6 cubs have to share the amount of food that can only sustain 1 cub, it’s likely no cubs will survive. Even if some cubs manage to horde more food than others, there still can only be so many surviving cubs based on the amount of food available, which will be the same amount as can survive from the restrained-breeder families. That means that the unrestrained breeder-mothers just spent much more energy and time producing the same amount of viable cub offspring as the restrained breeders, leading to a worse long term outlook for the survival of that unrestrained breeding family.