The entepicondylar foramen is absent, as in all members of the Sigmodontinae; if present, as in some other rodents, this foramen perforates the distal end of the humerus. In poor conditions, the weight of the adrenal gland may increase up to 200%, and rice rats are unable to conserve water well when dehydrated, and in water contaminated with oil, they swim less and their mortality increases. The median amount of radiation needed to kill a marsh rice rat is 5.25 Gy and the lethal dose of potassium cyanide is 7.20 mg/kg; both values are relatively low for cricetid rodents. In one study, wild rice rats in radioactively contaminated areas did not show signs of disease. Exposure to more daylight and higher food availability cause increased development of the gonads in both adult and juvenile rice rats. When the pineal gland is removed or melatonin is administered in male rice rats, the testes are reduced and tend to regress into the body. The marsh rice rat currently occurs in much of the eastern and southern United States, northeast to southern New Jersey, and south to southeastern Texas and far northeastern Tamaulipas, Mexico. The northernmost records in the interior United States are in eastern Oklahoma, southeastern Kansas, southern Missouri and Illinois, and the southern half of Kentucky, but the species is absent in much of the Appalachians. Fossils of the marsh rice rat are known from Rancholabrean (late Pleistocene, less than 300,000 years ago) deposits in Florida and Georgia and remains referred to the extinct subspecies "O. p. fossilis" are from the Wisconsinan and Sangamonian of Texas and Illinoian and Sangamonian of Kansas. In the Florida Keys, rice rats occur on most of the Lower Keys, but are absent from the Upper Keys, which are of a different geological origin and were probably never connected to the mainland. The western and eastern "Cytb" clades within the marsh rice rat may represent expansions from different glacial refugia which the species was restricted to during a glacial period. Cave and archeological remains indicate that the range of the marsh rice rat has extended substantially further north and west earlier in the Holocene, into central Texas, eastern Nebraska, southwestern Iowa, central Illinois, southern Indiana, southern Ohio, West Virginia, and southwestern Pennsylvania. Most northern archeological sites date from about 1000 CE and are associated with corn cultivation, but in some older cave sites the rice rat is found with the extinct giant armadillo "Dasypus bellus", suggesting warm climatic conditions.