He succeeded under the principle of tanistry, and established this as the basis for succession to the Scots throne, which lasted for the duration of the House of Alpin, until the reign of Malcolm II. Under tanistry, the successor to a king would be chosen during the king's lifetime from amongst the eldest and worthiest of his kin (subject to being of sound body and mind and of noble birth), whether or not his direct descendant. This generally led to succession by cousins rather than sons, but kept the descent within a kin-line (in this instance that of Alpin). The custom was also then carried through to the lesser Scots kinship lines, such as chieftains of clans and septs.
Donald succeeded Kenneth in 858CE, and ruled for 5 years. His insitution of tanistry was as part of the more general settlement of the Scots laws, which became known as the laws of Aedh, but little more is known of his reign, at this remove. So far as is known, he never married, and left no children (although the later king Giric has been proposed as his son). Equally it is not now known if he died at peace or in battle. He was suceeded, under tanistry, by his nephew, Kenneth's son, Constantine.
Some sources say that he was known as the wanton son of the foreign woman. Foreign may, one supposes, refer to the fact that Alpin, his father, and Kenneth his brother had imposed Dalriadic rule on the rest of Scotland, but why the word wanton should be included is also a mystery.
Kenneth I MacAlpin