When the pups are born in spring, they only weigh a few hundred grams. First they grow up in the cave, later they remain in the nursery and are fed there by the older wolves. That means, that the pack wolves always have to return to the den and later to the nursery after the hunt, to feed the puppies.
The wolves have to go on more and more migrations, to find loot. As long as there are still enough young game, does it work with the care of the puppies. But at some point the prey animals also migrate to safer areas and soon there is nothing left for the wolves to hunt in the wide area. By the beginning of autumn at the latest, they have to leave their summer camp and go hiking again.
Then the puppies must be big enough, to follow the pack. That means, that they grow up quickly and must be almost fully grown by the time they are just under six months old. They therefore need a lot of food and the older wolves not only have to take care of themselves, but also feed the hungry mouths of the puppies.
If that doesn't work, the weaker among the young die and only a few remain. But even if all puppies stay alive, no pack makes it, raise more than one litter per year.
This also explains the behavior of the alpha wolf, who takes care of it, that their puppies are raised without competition for food and care. Even for her rivals, the younger females, Does this birth restriction make biological sense?, because their puppies would hardly have a chance of surviving in a pack.
birth control
Why does only one female in a pack have cubs?? The answer lies with the puppies. If they are born in spring, they only weigh a few hundred grams. Half a year later, when they leave the nursery in autumn, do you have to 15 to 20 be heavy, otherwise they cannot follow the elders and must die.
To raise five or six such "gluttons"., requires a lot of hunting luck and work for the whole pack. Sometimes the older wolves don't make it, bring enough food. Then the weaker among the puppies die, and only a few remain. But even if everyone stays alive, no pack makes it, to feed more than one litter. So it would be completely nonsensical, to give birth to many puppies, who then almost all have to die. It makes a lot more sense, limit yourself to one throw from the start, and to take care of these puppies as well as possible.
Ultimately, this also serves to balance nature. Because it prevents the wolves from multiplying too much. And that works as a result: In areas with a lot of game, but for few wolves the wolf packs are small. As a result, almost all sexually mature females give birth to young, and the wolves multiply rapidly. This makes the packs bigger. As a result, there will soon be several sexually mature females in each pack, most of whom do not have young. This is "wolfish" birth control”.