Learn Icelandic: Taugakerfið hans er mjög ólíkt öðrum dýrum. - His nervous system is very different from other animals.

The adjective ólíkur (different) dictates the case of the noun that follows it. It requires the thing being compared to be in the dative case, so önnur dýr becomes the dative plural öðrum dýrum.

Why do we use the form ólíkt instead of ólíkur?

Adjectives must agree with the noun they describe in gender, number, and case. The subject is taugakerfið (the nervous system), which is a neuter, singular noun in the nominative case. Therefore, ólíkur takes its neuter singular nominative form, ólíkt.

The sentence literally says his nervous system is different from 'other animals' rather than from their nervous systems. Is this natural in Icelandic?

Yes, this is a very common shortcut in everyday Icelandic, much like in conversational English. While you could technically say ólíkt taugakerfum annarra dýra (different from the nervous systems of other animals), comparing the system directly to öðrum dýrum is perfectly natural and widely used.

Why does the sentence use taugakerfið hans with the definite article instead of just hans taugakerfi?

In Icelandic, the standard way to show possession is to attach the definite article to the noun and follow it with the possessive pronoun, as in taugakerfið hans. Placing the pronoun first, like hans taugakerfi, is grammatically correct but sounds quite formal or heavily emphasizes that it is his system specifically.