"These are their mothers"

Translation: Hawa ni mama zao

[DouglasMcF5]

They why don't we say "Hizi ni mama zao"?

[ElDoctr]

How can "zao" be the appropriate noun class for mama? Should be m/wa, shouldn't it? And therefore "wao"?

[juryrigging]

From the tips and notes:

Note: The exceptions to this rule are certain kinship nouns that come from Arabic: baba, mama, bibi, babu, shangazi, binamu, rafiki, dada, kaka, etc. These nouns take the y- agreement in the singular and the z- agreement in the plural.

[AGreatUserName]

It's basically animate nouns in the N class. Otherwise you'd use wao for both plural and singular and since the nouns of this class don't show number, there'd be more cases where number cannot be determined.