The demography and physiology of Melomys sp. (Rodentia: Muridae) in the Mitchell Plateau area, Kimberley, Western Australia

Abstract

Populations of a mosaic‐tailed rat (Melomys sp.) were studied for one year in vine‐thicket and mangrove habitats in the Mitchell Plateau area of the tropical Kimberley district of Western Australia. Demographic and physiological (haematological values, androgens, corticosteroids and their binding proteins) parameters were measured together with parasite prevalence and electrophoretic protein variation. Seasonal variation occurred in most parameters measured. Higher values were found in the vine‐thicket for number of sympatric mammal species, growth rate of immatures and androgens. Higher values were found in the mangroves for population density, survival, size at sexual maturity, body weight, home range overlap, albumins, total and free‐corticosteroids and parasite prevalence. The phenotypic frequencies of two protein polymorphisms (albumin and 6–phosphogluconate dehydrogenase) differed between sites, while a third (adenosine deaminase) showed no between‐site differences; these suggest differential selection between the adjacent habitats which are separated by 10 km. The overall physiological response of Melomys was more entrained to seasonal variation in the vine‐thicket than in the mangroves, which is in accord with the relative variation in the vegetation and temperature in the two areas. It is hypothesized that the greater population density in the mangroves results in behavioural feedback through the hypothalamo‐pituitary‐adrenal axis. This results in increased stress (higher free‐corticosteroid levels) which slows the growth rate and the onset of sexual maturity. The study is the first to indicate that such a mechanism, commonly proposed for temperate small mammals, may operate in tropical areas.

Rainfall and rats: Climatically‐driven dynamics of a tropical rodent population

“…For most of the year pythons are far more abundant on the backswamp than on the drier floodplain (Madsen & Shine 1996), so that rats living on the backswamp experience a higher risk of predation, as well as a higher food availability and thus a potential for a higher reproductive output. Similar differences in growth rates and demography between adjacent study sites have been documented in other rodent species in tropical Australia (Kemper et al 1987).…”

Haematology and serum biochemistry in captive Australian native murids: black-footed tree-rat (Mesembriomys gouldii) and greater stick-nest rat (Leporillus conditor)

“…Differential leukocyte counts and leukocyte morphology was consistent with previous descriptions in other murids and between individuals. Blood biochemistry values were unremarkable except for the high level of globulin in stick-nest rats when compared to previous murid research (Bradley et al 1988 ; Kemper et al 1987 ; Monamy 1995 ; Old et al 2005 , 2007 ; Thrall et al 2012 ).…”

“…Serum biochemistry values for the black-footed tree-rat (Table 4 ) and greater stick-nest rat (Table 5 ) were similar to previously reported murids for most parameters analysed. Stick-nest rats however had a high globulin concentration (Table 5 ) in comparison to other murids (Bradley et al 1988 ; Kemper et al 1987 ; Monamy 1995 ; Old et al 2005 , 2007 ; Thrall et al 2012 ). All other values are reported in Table 5 .…”

“…Healthy specimens of both species had elevated total WBC counts in comparison to the other murids (Bradley et al 1988 ; Kemper et al 1987 ; Monamy 1995 ; Old et al 2005 , 2007 ; Thrall et al 2012 ). Tree-rats had a mean WBC count that was almost double that reported previously for murids, while the stick-nest rats were within the expected range for murids, but at the higher end.…”

“…In addition, the causes and consequences of disease in native murid species are generally poorly understood, particularly for Australian species, and long-term data sets are required for health and disease studies. For example, there is generally a paucity of information regarding the haematology, serum biochemistry and leukocyte morphology for most Australian native murids, with only a few having previously been documented for captive specimens (Bradley et al 1988 ; Kemper et al 1987 ; Monamy 1995 ; Old et al 2005 , 2007 ). Haematology and serum biochemistry values for the black-footed tree-rat and greater stick-nest rat are needed as differences in physiology and life history traits compared to other murids, means values for other species are not directly comparable.…”