incompatible mate choice results in this alternatively you are very unlucky and keep rolling the 10% unknown creature which has a chance for all babies but as far as non suitable suiters are concerned no animals will produce you offspring
max level is 99, max trait points is 104 with 5 rebirths level 99 and taking the boat on the 5th rebirth
you lose 20% of your total exp so rebirth by death is preferred only if you are low level which you are relatively speaking you should only lose a few levels at that level while for level 99 you would lose many levels so at such high levels as 99 taking boat is advised for rebirth since it costs you no exp and grants you a spare trait point for that rebirth only