For what Vulpes --> Vulpecula, but Sorex never will be Soreculus

Question

Vulpes --> Vulpecula, all right, but:

Sorex --> Soriculus, not Soreculus

https://en.wikipedia.org/wiki/Soriculus

Answer 1

Latin has various diminutive suffixes. Although both words are spelled with "cul", the "c" in vulpēcula is part of the diminutive suffix, while the "c" in sō̆riculus is part of the root of the base word. We don't have *sō̆reculus because the orginal noun sō̆rex doesn't have an e after the r in forms like sō̆ricis, sō̆rices, sō̆ricum (i.e. we don't have forms like *sō̆recis, etc.)

The formation of Soriculus

The third declension noun sō̆rex has the genitive singular sō̆ricis (since the vowel alternation between short e in the nominative singular and short i in the other forms is usual for words whose nominative singular ends in -ex). If you take off the -is from the genitive singular form, you get sō̆ric-. Then add -ulus, and you get sō̆riculus.

I think Sō̆riculus may have first been coined as a taxonomic name (it doesn't seem to be in any Latin dictionary, although I was able to find an example of soriculorum being used outside of binomial nomenclature) so it isn't really the same kind of linguistic evidence as words that native Latin speakers formed themselves. However, it does seem to be regularly formed: The Formation of Latin Diminutives of Nouns and Adjectives, by Ian Andreas Miller, says that third-declension nouns with stems ending in -c- regularly form diminutives with -ul- (page 11).

The formation of vulpēcula

The third declension noun vulpēs has the genitive singular vulpis; take off the -is, you get vulp-.

Aside from the genitive singular, the nominative singular is also sometimes relevant to the formation of Latin diminutives, and it is in this case: third-declension nouns with nominatives ending in -ēs (which are mostly feminine) often form diminutives in -ēcula (Miller page 14). Miller notes that, just as some words with nominatives ending in -ēs have variants ending in -is (so in addition to vulpēs we can find vulpis used as a nominative singular) some of the derived diminutives of words of this kind end in -icula instead of -ēcula.

The form vulpiculus is listed by the Dictionary of Medieval Latin from British Sources as a synonym of vulpēcula. We can view this variant as being formed as vulp-iculus or vulpi-culus. Aside from showing -icul- instead of -ēcul-, this form has a different gender from vulpēcula. It's regular in Latin for a diminutive to match the gender of its base, and since vulpēs in Classical Latin is feminine, we expect its diminutive to be also. Variability in the gender of nouns for animals is not too uncommon, so it is not strange that either vulpēs or its diminutives were used as masculine nouns at some point in history, but for Classical Latin, it seems safest to stick to the feminine gender for vulpēs, and therefore to the ending -cula in its diminutive. (It is possible but not common for the diminutive gender-matching rule to be broken.)

If we were starting from a word *sō̆rēs, sō̆ris f., then a diminutive form *sō̆rēcula would not be unexpected.

Answer 2

The diminutive is formed from the genitive stem and the genitive of sorex is soricis.

The genitive of vulpes is vulpis so the diminutive can definitely be vulpiculus and it is so found in many manuscripts. For example, in the Codex Pithoeanus, the most important manuscript of Phaedrus' Fables, the spelling vulpiculus is used.

In general, formation of diminutives has a range of variation as has been discussed previously on this site.