Cotton Rat

Sigmodon hispidus

cotton rat

Cotton rat, photographed at Green Cay Nature Center, Boynton Beach, Palm Beach County, in May 2014.

The cotton rat, Sigmodon hispidus, is a reproduction machine. A single female, given optimum conditions and assuming all offspring survive and reproduce at the same maximum rate, could be a great-great-great grandmother and the ancestor of 15,500 cotton rats. All within the space of a year.

Fortunately, things happen, like predation and disease, to keep the cotton rat population in check. And their lives are very short: chances are our theoretical great-great-great grandma will be long dead before her theoretical 15,500th descendant is born.

The cotton rat is found throughout Florida and much of the Southeast and South-Central United States and parts of the Southwest. Its range extends through Mexico into Central America and northern South America.

It's believed the cotton rat evolved in South America, and made it's way north after the Panamanian land bridge emerged from the sea. Even now, the population continues to move northward, into Nebraska and Virginia.

Cotton rats can approach a foot in length, from the tip of its nose to the tip of its tail. They weigh about a third of a pound. It has black or dark brown hairs mixed with some gray; the underside is white. The tail, which is shorter than the length of the head and body, has few hairs.

The lifespan of a cotton rat is extraordinarily short, on average about six months. About 13 percent of cotton rats live long enough to celebrate a first birthday. In captivity, however, they can live as long as five years.

It inhabits places with tall grass that offer concealment, freedom of movement and sources of food. It will burrow in some areas, nest in thick clumps of grass or other places that afford protection. It is mostly a vegetarian but there is some evidence that it will eat bird eggs.

Conversely, it is on the menu for coyotes, bobcats, hawks, owls and snakes. However, in parts of its range, the cotton rat population has been known to inexplicably explode at times. The biggest check on the cotton rate population is disease. As the population grows, the rats are forced to live in closer promity with each other and eventually conditions become ripe for disease that brings the population back down.

On the other hand, they are carriers of disease, including the Everglades virus, which can infect both humans and horses. Mosquitos are the intermediate vector when they draw blood from an infected rat, then bite a human. Another factor, believe it or not, is the presence of Burmese pythons in South Florida. As the invasive snakes deplete the mammal populations in the wild, mosquitos become more dependent on the blood of cotton rats and more likely to spread the Everglades virus to humans and horses.

Cotton rats are also known to carry the hantavirus, which is spread through their urine, saliva and feces. Humans can contract hantavirus pulmonary syndrome, a deadly disease, when they breathe in aerosolized virus.

One might think the name, cotton rat, some how comes from its looks, but it's evident that's not the case. It's not "cottony" in any sense. It does stem from the damage these rats are known to do to cotton and other crops, including sugarcane and rice. The scientific name hispidus, refers to its bristly hair. Sigmodon comes from the rat's S-shaped molars.

A typical female has five litters of two to 10 offspring each year, but can have as many as nine litters. The gestation period is about four weeks; the newborns develop quickly, opening their eyes within 36 hours, teeth emerging in five or six days and weaning within two to three weeks. They are sexually mature within 40 days and indistinguishable from their parents within six months. Females start breeding again within hours after giving birth.

Cotton rats are members of Muridae, the largest family of rodents and also the largest family of mammals, on the planet. They are also known as hispid cotton rats, distinguishing them from related rats that are also commonly called cotton rats — there are more than a dozen.

Green Cay Nature Center

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Published by Wild South Florida, PO Box 7241, Delray Beach, FL 33482.

Photographs by David Sedore. Photographs are property of the publishers and may not be used without permission.