given that: Hawaiian (H) Maori (M) Samoan (S) Tongan (T) /l/ in H S T = /r/ in M /t/ in M S T = /k/ in H
why do we find words with /l/ /r/ /n/ alternations instead of the common attested /l/ /r/ alternation, such as: H lima M rima S lima T nima - why /n/ instead of /l/?
a similar case can be found in the alternation between /t/ /k/ /s/ instead of common attestes /t/ /k/ alternation, such as: H kila M tira S tila T sila - why /s/ instead of /t/?
Tongan /s/ seems to be the regular reflex of *t before /i/. Wikipedia says
Tongan has retained the original proto-Polynesian *h, but has merged it with the original *s as /h/. (The /s/ found in modern Tongan derives from *t before high front vowels).
That article also says that as a general rule, Tongan retained a Proto-Polynesian phoneme reconstructed as *l but lost a different Proto-Polynesian phoneme reconstructed as *r. In fact, Tongan /l/ vs. ∅ is apparently the main criterion for reconstructing PP *l vs. *r, as they have merged reflexes in other Polynesian languages. So Tongan nima seems to be a real outlier, which is in agreement with the note on the Wiktionary page Reconstruction:Proto-Polynesian/rima:
If it were *rima, Tongan would be expected to have *ima, and if it were *lima, Tongan would be expected also to have *lima. No other Polynesian language fully distinguishes /ɾ/ and /l/. [The reconstruction *rima] assumes that nima evolved out of *ima, which may have been modified to avoid confusion with a common homophone.
Alternation between /n/ and /l/ or /n/ and /r/ (either sporadic, or as part of regular sound changes) is not that uncommon. There was a previous question about /n/~/l/ alternation in Asian languages.
Well, even in Niuean, closely related to Tongan, the word for 'five' is lima. In Fijian, which is somewhat closely related to Polynesian languages, the word is lima. In Fijian *l remains l, while *r remains r. So we can reconstruct *lima for Proto-Polynesian. Proto-Polynesian is ultimately descended from Proto-Austronesian, where the word is *lima too. So that is further confirmation that the Proto-Polynesian word was *lima, so that means that Tongan had an irregular change of *l to n in this word. The word for 'arm, hand' is nima in Tongan too, this is because hand has five fingers.In Samoan the word is lima, it is the same for example in Hawaiian. So that means that also the word for 'arm, hand' was *lima in Proto-Polynesian, due to the fact that a hand has five fingers.
