From experiences in Porto Rico, Creel, of the U. S. Public Health Service, concludes that M. norvegicus (decumanus), while essentially a burrowing animal and not addicted to climbing or swimming, is nevertheless quite capable of doing either. He was found to burrow in the hardest earth to a depth of two and one-half feet and to pass through all kinds of wood, soft brick and lime mortar, probably by gnawing.
The black rat and Alexandrine rat (M. rattus) in Porto Rico, according to the same observer, do not burrow at all, but can climb and jump in expert manner, and are the species found in the rural districts, remote from houses. He found that all varieties of rats may swim, from ships to the shore, distances of from one-fourth to one-half mile, but that they lack the sense of direction and probably do not land from ships naturally in this manner (Public Health Reports, No. 9, February 28, 1913).
The female decumanus is a prolific breeder and brings forth larger litters than the Mus rattus female.
Mus decumanus is generally conceded to be larger and more ferocious than Mus rattus. For this reason he drives the smaller rats to the upper floors, the decumanus species generally living near the ground. He is a burrower and is rarely found in the upper stories of buildings. Decumanus is known as a wharf rat, but is rarely trapped on ships on the Pacific Coast, according to the observations of Surgeon Simpson of the U. S. Public Health Service (Public Health Reports, April 11, 1913). According to the same observer, Mus rattus is the commonest ship-borne rat. He also states that the black rat and the roof rat (Alexandrinus), both varieties of M. rattus, differ chiefly in color. They live in upper floors, between ceilings, in walls and roofs and are remarkable climbers as well as being expert rope-walkers and wire-walkers. On account of their natural wariness and caution it is not always easy to induce them to enter or approach traps.
The photographs introduced were taken under my direction in Manila in 1912, 1913 and 1914. Some of them show the character of the house construction in Tondo District, Manila, where plague flourished in 1913. Others illustrate methods of rat-proofing bamboo timbers in houses of light material. These end openings were either closed by introducing cement or by placing tight-fitting tin cans over the ends of the bamboo rafters.
There are many interesting memoranda, gathered and made in connection with our antiplague work in Manila, especially concerning the location and construction of rat nests found by our laborers; the materials used and the fabrication of the nests. Memoranda giving details of rat catching and rat-proofing are also presented and notes showing the location of dead rats found in relation to dead human bodies of plague victims.
Notes concerning cases of multiple house infection are also presented as being of possible interest.
The Javan studies in 1911 and 1912 establish the fact that it is possible to form a fair judgment as to the length of time a rat has been dead, up to ten or twelve days, from the condition and appearance of the rat cadaver, both as to decomposition and drying. A series of 50 rats was studied. It is to be understood that the conditions under which these observations were made were tropical conditions. They would be fairly comparable with summer conditions in America, but should not be followed too closely at other seasons of the year. In my own experience I have observed that ants are likely to attack the cadaver early and to obscure the deductions by their destruction of the body.
PROGRESSIVE POST-MORTEM CHANGES IN RAT CADAVERS. THE NUMBERS INDICATE THE NUMBER OF DAYS AFTER DEATH