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The ubiquitous furry feline, legendary nemesis to the family dog, may keep you up at night with its nocturnal antics but your cat is not the only type of pet who enjoys night owl status. Also on this list are pets such as rats, leopard geckos and hedgehogs. Hamster owners quickly discover that peace and quiet at night is achieved either by relocating the cage or purchasing a silent hamster wheel so their palm-sized pets can scurry along happily during their wakeful graveyard shift without disturbing the rest of the household.
Similarly instilled with an aversion for midday are the animals that are crepuscular, or active just after sunrise or just before sunset, such as the domesticated dog or rabbit. The crepuscular animals most active at the break of dawn are known as matutinal, and the ones who reach the peak of their energy late in the evening as the sun is setting are known as vespertine. Ferrets and guinea pigs are popular pets that fall into the crepuscular category.
The absence of daylight in the nocturnal animal's environment creates a need for enhancement in other areas of perception. It is for this reason that these animals generally have a well-developed olfactory sense. In fact the nocturnal rat's sense of smell ranks second overall, outclassed only by the African elephant. Rats possess 1,207 olfactory receptors, whereas dogs, often employed as scent trackers, have only 811 and finish in ninth place in the animal kingdom nose rivalry.
Also of benefit to animals who are out after dark is enhanced auditory ability. Cats have ears that they can move, which enables them to more accurately determine the source of a sound. Hamsters can hear ultrasonic frequencies and as a result can communicate without attracting the attention of other animals.
Nocturnal animals have adapted to the low light of night and have eyes that are more sensitive to light. Some animals such as cats have a slit pupil, which both open wide enough to allow in more light, as well as close quickly enough to protect the animal's sensitive eyes from light that is too bright. If you've ever seen the reflection of an animal's eyes at night, you've witnessed the action of the tapetum, a reflective eye membrane that amplifies light for perception by the nocturnal animal's retina. Rods and cones have adapted as well. The rods, which detect low light, are more developed in nocturnal animals. The cones, which are for bright light, are absent in some nocturnal creatures such as bats and nocturnal snakes and lizards.
Nocturnal and diurnal, or awake during daylight hours, behavior is regulated by an organism's circadian rhythm. Circadian rhythms operate on a 24-hour schedule and are dependent on light to be regulated. The effect light has on an organism's circadian rhythm makes it possible to alter its circadian clock with repeated light exposure at the wrong times. When the light hits the retina of the eye, signals are sent to the circadian pacemaker in the brain. In diurnal animals, light stimulates activity, whereas for those that are nocturnal, light stimulates sleep.