Salt-Marsh Harvest Mouse
The Salt Marsh Harvest Mouse’s scientific name Reithrodontomys raviventris means “grooved-toothed mouse with a red belly". Two species of Reithrodontomys raviventris are recognized. The subspecies of study, R. raviventris halicoetes, the Northern Salt Marsh Harvest Mouse, is found in the marshes of northern and central San Francisco Bay, particularly in the marshes of San Pablo and Suisun bays. The southern subspecies, Reithrodontomys raviventris raviventris, are found in the marshes of Corte Madera, Richmond and South San Francisco Bay. Both subspecies have grooved upper front teeth, but, generally, only the southern subspecies have a cinnamon or rufous-colored belly. The northern subspecies has a long bicolored tail, large ears, grooves in the outer surface of its upper incisors and other body parts which are buff or brown. The backs and ears of the salt marsh harvest mice tend to be darker. The northern mouse has a combined head and body length of around 3 inches (118-175 mm) and an average weight of less than half an ounce (about 8-12 grams). The body of an adult mouse is about the size of one’s thumb and it weighs a bit less than a nickel. Therefore, the salt marsh harvest mice are among the smallest rodents in the U.S.
The life span of the salt marsh harvest mouse is about 8 to 12 months. This requires that the population renew itself every year in order to survive. While sexually active from March to November, females often bear only one of the three possible litters, and litters of only four offspring are typical. If there is a nest, it is only a loose ball of grasses on the surface of the ground. The salt marsh harvest mouse does not burrow. It is vulnerable to snakes, owls, hawks, and cats. Young salt marsh harvest mice can disperse a considerable distance, but not from fragmented habitat, but adults comprise the majority of the population. The mice are density-dependent species: when the populations are too high, breeding is suppressed further into the spring. If local densities are too high, populations can be reduced to the point of local extinctions. During summer, when salinities of water and vegetation increase, the mice gain a competitive edge, since they can drink and survive on pure salt water (they can withstand high salinities in food and water intake). Notably, most of the northern subspecies can survive on seawater, but prefer having fresh water.
Salt marsh harvest mice are cover-dependent species. That is, they only live under thick vegetation. They are dependent on thick cover of native halophytes (plants that thrive in salty environments) of the salt marsh environment, which is typified by salt marsh herbs, grasses and reeds. Salt marsh harvest mice use pickleweed as their primary and preferred habitat as long as they have non-submerged, salt-tolerant vegetation for escape during the highest tides. They eat leaves and stems of halophytes. The mice prefer the deepest (60-75 cm tall), most dense pickleweed. The mice are non-intra-aggressive; therefore, short durations of populations’ densities are sustainable (for the high tide period). The mice do not utilize marshlands with low salinities and sparse pickleweed.
Salt marshes are the optimal habitat for this species, in particular those that support dense stands of pickleweed and are adjacent to upland, salt-tolerant vegetation, for escape during high tides. The ability to tolerate high salinity in both food (grasses, forbs, seeds, and insects) and water, and the ability to swim and climb enable this mouse to take advantage of its unique habitat. Salt marsh harvest mice, like humans, other animals and plants, adapt over time to changes in their environment by variations in genetic make-up. Genetic variability allows animals with appropriate genes to respond to changes in the environment, like temperature fluctuations for example, by producing more surviving offspring. If an