The answer to this mysterious cat behaviour is both simple and complicated. It’s called ‘fabric softener.’ Now, before you think I’m as crazy as this cat behaviour appears to be, let me explain…
Sweaters, socks, towels, jumpers etc are washed in washing powder that contains an enzyme that has a similiar scent to the lactose in a female cats milk. This emzyne can be found in all brands of laundry powders and fabric softeners and is a derivative of wool called lanolin. Lanolin and lactose have similiar protiens and that is what attracts the cat. Strange but true.
Enzymes are proteins that catalyze (i.e., increase the rates of) chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. In the heating process during laundry washing, the enzyme in the washing power converts and gives off a strong scent which the cat is attracted to.
Lactose is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from lacte, the Latin word for milk, plus the -ose ending used to name sugars.
The item of clothing, be it socks, towels, jumpers simply attract the cat due to the animals highly developed sense of smell. The ‘humping’ behaviour is more prevelant in male cats, as opposed to female cats… however, female cats who display this behaviour have a dominate male gene in their genetic makeup. The dragging and, humping and howling are dominance signs… and stranger still, the kneeding that sometimes accompanies this behaviour is called a ‘softening.’ The softening is the cats instinct to soften its prey (ie: the towel) before it’s gifted to fellow pride members.
The behaviour is perfectly normal and often occurs at night because cats are nocturnal and in the wild, would usually hunt at night.