Confused by a sentence from a article.
I read a article, and there is a sentence I'm so confused about.
Once the female lays her egg 40–150cm down, however, that’s it – her progeny is entirely on its own, effectively orphaned before its shell has cracked. She plays no part in tending the mound, with the burden of maintenance falling entirely to the male. Climate control is his obsession, and rotting compost his tool in sustaining an even incubation temperature.
What is the verb in the second clause of last sentence "and rotting compost his tool in sustaining an even incubation temperature. "?