From what I gather, single yu become more independent, but more prone to danger, thus shorter lifespan.
Larger families, the koyukkuris tend to stay in the nest longer, but they live longer because of learning from the mistakes of their sisters. Because their sisters did not survive, they learn how to avoid danger. Wisdom built on the backs of the dead.
Kind of like why roaches drop large clutches of eggs: in the hopes that one of them will be able to continue the breeding cycle.
the Large families offspring tend to go nest-independent sooner, fewer nest-clinging and have better post-independent survival rate.
Where as Single child offspring 'better' on 'survival till nest-independent rate'. only.
So, comparatively more single child survive TO leave the nest with worse survival rate AFTER leaving the nest.
Large families offspring survives on the wisdom gained off their siblings' multitude failures.
The average life span of post nest-independence favours those raised of plenty-children family.the Average time for nest-independent are faster for multi-children raised family. While there are more single child who cling to parents keeping from 'nest-independent'.Researchers think this is due to infant stage where they witness many death around them. The yu naturally gain the ability to manage danger.OtherAlthough the number has shown that only-child yukkuri have higher survival rate to reach the nest-independent ageMultiple sisters9 month to 1 year4-6 monthRefuse leaving the nest6-9 monthNot even 3 month3-4 monthFactory investigated wild herds and found difference between single child and mountain of children families.