A dvanda compound of words signifying those animals only among whom there is permanent enmity i.e. natural and eternal antipathy or quarrel, is singular.,
The word विरोध means enmity: and शाश्वतिकः means permanent.
Thus, मार्जारमूषकम् 'the cat and the rat'; श्वशृगालम् 'the dog and the jackal'; अहिनकुलं 'the snake and the mongoose.'
Why do we say 'natural and eternal'? Observe गौपालिशालंकायनाः कलहायन्ते 'Gaupali and Salankayana are quarrelling.'
The force of the word च in the aphorism is that of एव 'only.' Dvandva compounds of such animals only are invariably singular; no other rule, even if otherwise applicable, would apply to such compounds. Thus rule 1.4.12 says that Dvandva compounds of beasts and birds, is optionally singular. That rule (by (I. 4. 2.)) would have set aside the present rule. But च prevents that, and no optional plural number is allowed in case of beasts or birds that are naturally at war; as अश्वमहिषम् 'the horse and the buffalo'; काकोलूकम् 'the crow and the owl.',