Circadian Rhythms Influence Animal Behavior
March 28, 2005
The morning songs of birds, the foraging behavior of the squirrels in autumn, and the human tendency to get a little blue in the winter all represent the natural rhythms of life. The presence of internal biological clocks, or circadian rhythms, is one of the most universal traits shared by all living things, from bacteria to fruit flies to humans.

Dr. Shelley Tischkau, a researcher at the University of Illinois College of Veterinary Medicine, explains, "Since the beginning of time on earth, these rhythms have been controlled by one consistent, essential force: the light/dark cycle." Circadian rhythms dictate the daily and seasonal timing of many behaviors, and are most heavily influenced by exposure to light, be it natural sunlight or artificial light from lamps.

This simple fact can pose a problem for pet owners who own nocturnal pets and pets whose breeding behavior is influenced by the light-dark cycle. Fortunately, pet owners can alter their animals' exposure to light or change the timing of other influencing factors (such as feeding and playtime) to help their pets adjust to living in the human world.

According to Dr. Tischkau, birds, mammals, and reptiles have a biological clock located in the hypothalamus, a part of the brain that controls basic biological functions such as respiratory rate, heart rate, and reproduction. Light is sensed by the eyes and the pineal gland in birds, located in the top of the head, and this sensory input is sent to the biological clock. The clock communicates to other centers in the brain, indicating the time of day and time of year.

Animals need to know when it's day or night so they can fill their niche in nature. Rodents, for example, have a strong sense of smell and forage for food at night; they also serve as food for nocturnal owls and felines. Animals that rely heavily on vision to find food and communicate, such as songbirds, are typically active during the day. Some animals, such as squirrels, are most active at dawn and dusk.

As presence of light signals day from night, day length indicates what season is approaching, so animals know when to migrate or hibernate. Animals also use their internal clocks to time their courting and mating. This ensures that their young are born during a season when weather is mild and food is plentiful, maximizing the chance for survival.

So what does this internal biological clock mean to the pet owner? In general, animals are much more reliant on their biological clocks than humans are, and if you own a nocturnal pet like a hamster, rat, or cat, your pet may be running around at night when you're trying to sleep, or napping during the day when you want to spend quality time with them.

An animal's response to light can be overridden and its circadian rhythms altered if the timing of essential activities is altered. For example, Dr. Tischkau makes sure her kittens have playtime in the early evening so that they are tired out by late night, and she feeds them at night so they don't bother her at 5 a.m. for food. This timing may not work the same for all pets, but Dr. Tischkau affirms that feeding and playtimes can have a huge impact on biological clocks.

Seasonal rhythms can pose health risks for birds and reptiles that may lay eggs as the days get longer. As the increasing day length signals breeding season, some animals may begin laying eggs. Excessive egg-laying can deplete calcium, resulting in a plethora of health problems.

One way owners can try to control egg-laying is by limiting the animal's exposure to light. Turning off artificial lights and/or covering the cage with a towel or blanket early in the evening can keep the internal clock from sensing the approach of spring.

For more information about seasonal behaviors such as egg-laying, consult your local exotics veterinarian.

For an archive of Pet Columns from Veterinary Extension/Office of Public Engagement at the University of Illinois College of Veterinary Medicine, please visit