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Unlike the northern subspecies, which builds nests of grass or uses abandoned birds' nests, the southern subspecies often do not make nests at all ( USFWS- Sacramento); rather, they may construct "loosely organized structures of dry grasses" (LSA 2004) Little genetic information is available ( Shellhammer, 1998), nor is there available information regarding weaning or the age of sexual maturity (Cal /EPA DPR online). Lifespan. Because of their small size, they are not long -lived; a span of nine months is exceptional ( Shellhammer, 1998). Behavior. Salt marsh harvest mice are "... much calmer - acting than their grassland cousins, the western harvest mice, and most other mice... ( Shellhammer, 1998). Shellhammer (2000) terms their behavior "placid," to the extent that their behavior is used as a secondary criterion for characterizing them to the species level. Salt marsh harvest mice are quite buoyant because their fur does not get saturated quickly, and they are good swimmers and climbers (Cal /EPA DPR online). During the highest water tides, they move into higher grasslands (Veloz 2003) They are primarily nocturnal, but under laboratory conditions, they have been observed to be active at twilight or before sunrise (Cal /EPA DPR online). R r. raviventris can become torpid, especially in the early morning; neither the northern subspecies nor the closely related western harvest mouse exhibits this behavior (CDFG on line) They are active all year round (Cal /EPA DPR online). Ecological threats. The 1984 USFWS Recovery Plan listed the principal reasons for the decline of the salt marsh harvest mouse: habitat loss, fragmentation of remaining marshes, widespread loss of the high marsh zone through backfilling, land subsidence, and changes in vegetation. Over the long term, the mice may be threatenedby a sea level rise as great as 1.2 meters predicted to occur within the next 100 years, which could result in a permanent decrease in marsh habitat. In addition, upstream water diversions and /or dams reduce sediment supply per habitat by 45 percent, according to one estimate (DWR IEP). Changes in water salinity, which impact the saline vegetation that is primary habitat for the mouse. Additionally predators contribute to the decline of the mouse. Dilution ofsalt water. San Jose's sewage treatment plant pumps about 120 million gallons per day of treated water into San Francisco Bay near Alviso, which can dilute the salt content of the marshes, ultimately changing marsh ecology. The salt water turns brackish, causing some plants and animals to die off, including pickleweed, a necessity for salt marsh harvest mice (Rendon 1999). Predators. With development around the Bay margin, access to salt marsh harvest mice by predators such as feral and house cats and red foxes has become easier. Native predators include hawks, owls, heron, and clapper rails ( Shellhammer, 1998). Other predators include egrets and night herons (Cal /EPA DPR online). Mammalian Predators Trapped at Redwood Shores Peninsula, May 22, 2000 — December 31, 2003 12 Number & Type of Disposition of Predators (following Inclusive Dates Predators Trapped transport to PHS FY 2000 October 1, 12 feral cats Felis 2 cats returned to owners, 3 cats ado red, 5 12