Niacin is an essential vitamin for the cat; dietary deficiency can lead to anorexia, weight loss and an increase in body temperature. Biosynthesis of niacin occurs by metabolism of tryptophan via the kynurenine pathway to quinolinic acid, the niacin precursor. However, cats have a high activity of picolinic acid carboxylase, which converts one of the intermediates to picolinic acid instead of quinolinic acid. As a result, niacin can become deficient and require supplementation.
The "Growltiger's Last Stand" sequence has been changed multiple times over the course of the show's history. In the original London production, the "last duet" for Growltiger and Griddlebone was a setting for an unpublished Eliot poem, "The Ballad of Billy M'Caw". For the original Broadway production, the Ballad was replaced with "In Una Tepida Notte", a parody of Italian opera. The mock aria was introduced so as to add more slapstick humour and its lyrics are based on the Italian translation of the "Growltiger's Last Stand" poem. This new version was eventually incorporated into all other productions of Cats.
Then, as Rum Tum Tugger's song fades, a shabby old grey cat stumbles out wanting to be reconciled; it is Grizabella. All the cats back away from her in fear and disgust and explain her unfortunate state ("Grizabella: The Glamour Cat"). Grizabella leaves and the music changes to a cheerful upbeat number as Bustopher Jones, a fat cat in "a coat of fastidious black", is brought to the stage ("Bustopher Jones: The Cat About Town"). Bustopher Jones is among the elite of the cats, and visits prestigious gentlemen's clubs. Suddenly, a loud crash startles the tribe and the cats run offstage in fright. Hushed giggling sounds signal the entrance of Mungojerrie and Rumpleteazer, a pair of near-identical cats. They are mischievous petty burglars who enjoy causing trouble around their human neighbourhood ("Mungojerrie and Rumpleteazer"). After they finish, they are caught off-guard and confronted by the rest of the cats.
After the Jellicle Ball, Old Deuteronomy contemplates "what happiness is", referring to Grizabella. However, the cats do not understand him, so he has Jemima (or Sillabub, depending on the production), the youngest of all Jellicles, sing it in simpler terms ("The Moments of Happiness"). Gus – short for Asparagus – shuffles forward as the next cat to be introduced ("Gus: The Theatre Cat"). He was once a famous actor but is now old and "suffers from palsy which makes his paws shake". He is accompanied by Jellylorum, his caretaker, who tells of his exploits. Gus then remembers how he once played the infamous pirate captain, Growltiger a.k.a. the Terror of the Thames ("Growltiger's Last Stand"). Gus tells the story about the pirate captain's romance with Lady Griddlebone, and how Growltiger was overtaken by the Siamese and forced to walk the plank to his death.
Cats are similar in anatomy to the other felid species, with a strong flexible body, quick reflexes, sharp teeth and retractable claws adapted to killing small prey. They are predators who are most active at dawn and dusk (crepuscular). Cats can hear sounds too faint or too high in frequency for human ears, such as those made by mice and other small animals. Compared to humans, they see better in the dark (they see in near total darkness) and have a better sense of smell, but poorer color vision. Cats, despite being solitary hunters, are a social species. Cat communication includes the use of vocalizations including meowing, purring, trilling, hissing, growling and grunting as well as cat-specific body language. Cats also communicate by secreting and perceiving pheromones.