Most of the lyrics in Cats were taken from Old Possum's Book of Practical Cats with very minor alterations. Supplementary verses from unpublished poems by Eliot were adapted for "Grizabella: The Glamour Cat" and "Journey to the Heaviside Layer", while the song "The Moments of Happiness" was taken from a passage in Eliot's The Dry Salvages. Cats director Trevor Nunn and lyricist Richard Stilgoe provided the remaining lyrics, namely for the opening number "Jellicle Songs for Jellicle Cats" and the most famous song from the musical "Memory". The former was written by Nunn and Stilgoe and was modelled after an unpublished poem by Eliot titled "Pollicle Dogs and Jellicle Cats", while the latter was written by Nunn based on another Eliot poem titled "Rhapsody on a Windy Night".
Vitamin A is required in kitten diets because cats cannot convert carotenes to retinol in the intestinal mucosa because they lack the enzyme so this vitamin must be supplement in the diet. Vitamin E is another required vitamin in kitten diets seeing as deficiency leads to steatitis, causing the depot fat to become firm and yellow-orange in colour, which is painful and leads to death. Also, vitamin D is an essential vitamin because cats cannot convert it from precursors in the skin.
In isolated landmasses, such as Australasia, there are often no other native, medium-sized quadrupedal predators (including other feline species); this tends to exacerbate the impact of feral cats on small native animals. Native species such as the New Zealand kakapo and the Australian bettong, for example, tend to be more ecologically vulnerable and behaviorally "naive", when faced with predation by cats. Feral cats have had a major impact on these native species and have played a leading role in the endangerment and extinction of many animals.
Within the first 2 days after birth, kittens acquire passive immunity from their mother’s milk. Milk within the first few days of parturition is called colostrum, and contains high concentrations of immunoglobulins. These include immunoglobulin A and immunoglobulin G which cross the intestinal barrier of the neonate. The immunoglobulins and growth factors found in the colostrum begin to establish and strengthen the weak immune system of the offspring. Kittens are able to chew solid food around 5–6 weeks after birth, and it is recommended that 30% of their diet should consist of solid food at this time. The kitten remains on the mother’s milk until around eight weeks of age when weaning is complete and a diet of solid food is the primary food source.
Feral cats are domestic cats that were born in or have reverted to a wild state. They are unfamiliar with and wary of humans and roam freely in urban and rural areas. The numbers of feral cats is not known, but estimates of the US feral population range from 25 to 60 million. Feral cats may live alone, but most are found in large colonies, which occupy a specific territory and are usually associated with a source of food. Famous feral cat colonies are found in Rome around the Colosseum and Forum Romanum, with cats at some of these sites being fed and given medical attention by volunteers.
Female domestic cats can have kittens from spring to late autumn, with litter sizes ranging from two to five kittens. Domestic cats can be bred and shown as registered pedigreed cats, a hobby known as cat fancy. Failure to control the breeding of pet cats by spaying and neutering, as well as abandonment of pets, has resulted in large numbers of feral cats worldwide, contributing to the extinction of entire bird species, and evoking population control.