Yellow-bellied marmots are rodents rather similar to squirrels, only bigger. Their closest relatives besides other marmots are the woodchucks that are so common in the eastern U.S. They are generally just shy of two feet long, varying in size from 45 to 57 centimeters long, with tails of 13 to 22 centimeters. Males are generally heavier than females. Weights vary from two to five kilograms. As their name implies, they generally have yellow or yellowish-brown straight fur on their undersides, with white tips. Lastly, the typical life span of these marmots is 13 to 15 years.
Yellow-bellied marmots are found in high rocky areas in many parts of the Sierra Nevada and Cascade mountain ranges, from Tulare County in California north to Oregon. In addition, they are found in parts of the White Mountains where they cross the border between California and Nevada. Finally, they also range in parts of the Rocky Mountains. While small populations can be found in the high desert hills of Nevada, they do not live below about 6000 feet in elevation, and do not thrive at the low end of their range of altitude, because they are better suited to higher-elevation living.
Yellow-bellied marmots usually occupy open territory, such as steppe, alpine meadows, pastures, or fields. They use rocky areas and talus for cover, and will usually build their burrows under the rocky part of their territory, if possible. This makes them much more difficult for their common predators to get at or dig out. They are found at altitudes of 2000 meters and up, with a wide range of suitable elevation. This is especially true in the Rockies, where it is common to find them at altitudes approaching 4000 meters or more.
Yellow-bellied marmots build burrows that are more than one meter in depth, with several entrances. They choose the best drained soils in their territory in which to build. Burrows built for hibernation can be as much as five to seven meters deep. Burrow tunnels can be ten to seventy meters in length. For reproduction, they build grass nests inside the burrows.
While they most often choose open grasslands, wet meadows, and fields in which to settle, they can also sometimes be found around the edge of stands of forest, especially subalpine conifer, as well as within open stands of lodgepole pines. They can be found additionally in the grassy understories of many types of forested areas, but these conditions are less ideal for them.
Yellow-bellied marmots eat the leaves and blossoms of many different plants and grasses. They also eat some fruits, legumes, grains, and even occasionally insects. In the late summer, they forage for seeds in preparation for winter hibernation. They confine themselves to food found on or near the ground.
Yellow-bellied marmots use the rocky areas near their burrows for both sunning and observation for predators. Their natural predators include badgers, coyotes, eagles, owls, and wolverines. They are a diurnal species, with food-gathering usually performed in two periods of activity mid-morning and late afternoon. They hibernate from late September and early October until late April or early May, depending on climatic conditions and snowpack.
They have a single breeding season each year, which begins shortly after they emerge from hibernation in the spring. The gestation period is roughly one month, so that most of the young are born in May and June. Litters can range in size up to nine pups. The average size, however, is three to five. For the first three weeks the young stay underground. After two weeks above ground, they are weaned. At the age of two, both males and females are sexually mature. However, it is unlikely for yellow-bellied marmots to reproduce before the age of three, especially at higher elevations. Since survival through the winter depends on fat accumulation in the months prior to hibernation, the young stand a better chance of survival the sooner they wean from their mothers. It is common for females to reproduce only every other year at high elevations. These marmots are territorial, with each male having a harem of females with whom to mate. They are antagonistic towards other males. Male offspring are allowed to remain with their mothers until their second summer, and then are driven out. About half of the female offspring remain with their mothers permanently. Since the reign of a male over a harem and territory lasts only about three years before being replaced, inbreeding is not a problem, because the female offspring are not sexually mature until at least their second year, and very rarely reproduce before the age of three. As the family group of a single male becomes too large (typically over twenty), the group splits, and a solitary male takes over the new pack. Marmots without a pack, both male and female, are highly susceptible to predation. Each male occupies a territory ranging in size from one half to five acres, with the average being about one and a half acres. Each harem consists of two to three females. The offspring are normally all raised jointly by the females of the harem. The males mark their territory with their scent. They do this by wiping a cheek scent gland over the rocky areas in their territory, and it has developed into a method for avoiding physical conflict with other males.