Female Cotton Rats Research Articles

Fluorosis risks to resident hispid cotton rats on land-treatment facilities for petrochemical wastes.

Land-treatment of petroleum wastes is a widely used industrial practice, yet there has been no comprehensive evaluation of the long-term risks to human or terrestrial ecosystems from such practices. We evaluated cotton rat (Sigmodon hispidus) populations on three sites in Oklahoma (USA) that historically used land-treatment for disposal of various petroleum wastes (July 1995-March 1997). Average concentrations of fluoride in soil from these sites ranged from 878 to 4317 mg/kg. A census of resident cotton rats on land-treatment sites revealed a high incidence (40% overall) of dental lesions compared to reference populations (<1% dental lesions). During winter there was a 34% to 65% increase compared to summer in frequency of dental lesions in cotton rats on two of the three land-treatment sites. Incidence of dental lesions on two land-treatment sites was greater (9-16%) in female cotton rats compared to males. Cotton rats from land-treatment sites had higher concentrations of fluoride in bone and greater severity of dental lesions compared to reference animals. Dental lesions were considered to be most consistent with dental fluorosis because of elevated fluoride in bone. Neither concentration of fluoride in soil nor level of fluoride in bone was a good predictor of severity of dental lesions in cotton rats on land-treatment sites.

Toxicity of 2,4,6-TrinitrotoIuene (TNT) in Hispid Cotton Rats (Sigmodon hispidus): Hematological, Biochemical, and Pathological Effects

The contamination of soil and water with munitions chemicals and their degradation products has been reported at certain munitions production waste disposal sites and at certain Army installations. The effects of 2,4,6-trinitrotoluene (TNT) on wild cotton rats ( Sigmodon hispidus) were evaluated to identify target organ toxicity that could be used to develop biomarkers for exposure assessment for ecological and health risks. The oral LD50 values for TNT in corn oil were 607 and 767 mg/kg body weights for male and female cotton rats respectively. Hematological, pathological, and biochemical effects of TNT were determined after daily oral gavage of TNT in corn oil at doses of 0, 75.9, 151.8, and 303.5 (males) or 0, 96, 192, and 384 mg/kg (females) for 7 days. Cotton rats treated with TNT showed an increase in spleen weights in males (303.5 mg/kg) and in females (192 and 384 mg/kg). Cotton rats of both sexes treated for 7 days with TNT had marked hemolytic anemia with reduced erythrocytes, hemoglobin, and hematocrit in high-dose groups; methemoglobin levels were elevated significantly in males at mid and high dose. Hepatic drug metabolizing enzyme analysis revealed that microsomal O-dealkylation of methoxy, ethoxy, and pentoxy resorufin were elevated in male (high dose) cotton rats. The activity of hepatic glutathione S-transferases (GST) was significantly elevated in male (mid and high dose) and female (all doses) cotton rats exposed to TNT. Histopathological analysis of spleen revealed mild to marked splenic congestion with mild extramedullary hematopoiesis, hemosiderosis, and lymphoid hyperplasia in male and female cotton rats treated with TNT (all doses). Liver weights were increased in males (mid and high dose) and in females (high-dose group). In the high-dose groups, histological changes in liver (mild to moderate hepatocellular hypertrophy, increased hemosiderin pigment in Kupffer cells) in both sexes, and in testis (premature exfoliation of spermatozoa from dilated seminiferous tubules) were observed (mid and high dose). These results suggest that hepatic GST and hemolytic anemia may be biomarkers in cotton rats of terrestrial contamination with TNT or other nitroaromatic explosive compounds.

Developmental Plasticity of Postweanling Cotton Rats (Sigmodon hispidus) as an Adaptation to Nutritionally Stochastic Environments

Elucidating interrelationships between rate of growth and sexual maturation in unpredictable or stochastic environments could increase our understanding of life-history strategies of small mammals. It has been hypothesized that species living in environments where food availability is unpredictable might become sexually mature at smaller sizes and channel excess energy into reproduction rather than into compensatory growth. We explored this hypothesis in female cotton rats (Sigmodon hispidus) by feeding variable levels of dietary protein during early postweanling development (14–45days of age) and monitoring compensatory growth and fitness after nutritional rehabilitation (45–100days of age). Growth was optimum in females fed diets containing 16% protein, with minimal requirements estimated to be 12%. Females fed diets containing <12% protein exhibited suppressed development, including delayed puberty. However, these nulliparous females demonstrated compensatory growth during the early period of nutritional rehabilitation, regardless of the severity of previous restrictions in protein. No long-lasting fitness consequences from postweanling nutritional restrictions were apparent as we observed no difference in date of conception, body mass of dams at parturition, litter size, or rate of growth of neonates. We offer a possible adaptive explanation for this observed plasticity in growth and development.

Spontaneous ECLomas in cotton rats (Sigmodon hispidus): tumours occurring in hypoacidic/hypergastrinaemic animals with normal parietal cells.

We have identified cotton rats with a high female-predominant occurrence of spontaneous gastric carcinomas localized to the oxyntic mucosa, classified as malignant enterochromaffin-like (ECL) omas. The present study was made to further characterize these ECLomas and surrounding oxyntic mucosa, both morphologically using histochemical and immunohistochemical methods, and for gene expression by northern blot analysis. Among eight female cotton rats, three had an irregularly thickened oxyntic mucosa, increased stomach weight and a high serum gastrin level. Histopathological examination showed adenomatous hyperplasia of the thickened oxyntic mucosa with areas of an invasive neoplastic tumour. Immunohistochemistry, using the general neuroendocrine cell marker chromogranin A (CgA) and the specific ECL cell marker histidine decarboxylase (HDC), showed a considerably increased ECL cell density. These ECL cells displayed active proliferation, with hyperplasia, dysplasia and neoplasia. Parietal cells were not found in the tumour tissue. Parietal cell density was only slightly reduced in the surrounding oxyntic mucosa. The antral mucosa was histopathologically normal with a normal number of gastrin-immunoreactive cells. Likewise, somatostatin-immunoreactive cells did not show any differences in the antral and oxyntic mucosa between rats with pathological and normal oxyntic mucosa. Northern blot analysis revealed increased expression of CgA and HDC mRNA in the thickened oxyntic mucosa, whereas H(+)/K(+) ATPase mRNA was similar in the oxyntic mucosa of those with thickened and normal oxyntic mucosa. Gastrin mRNA in the antral mucosa was high in animals with thickened oxyntic mucosa. Somatostatin mRNA expression was similar in the antral mucosa of control animals and animals with a thickened oxyntic mucosa. We conclude that the spontaneous gastric carcinoma occurring in female cotton rats is an ECLoma developing secondary to hypergastrinaemia due to reduced intragastric pH. The mechanism for reduced acidity is not known, but is not gastric atrophy.

Transmission of Black Creek Canal virus between cotton rats.

Black Creek Canal (BCC) virus is a hantavirus associated with hantavirus pulmonary syndrome in southeastern North America. The virus was isolated from the spleen of a cotton rat (Sigmodon hispidus) trapped in southern Florida. Our previous studies have shown that we could consistently infect male cotton rats with BCC virus in the laboratory. These animals became persistently infected and virus could be detected in salivary glands, urine, and feces. In this report we show: (1) female and male cotton rats are equally susceptible to BCC virus infection, (2) susceptibility to infection was not influenced by age, (3) all inoculated rats transmitted the infection to uninoculated cage mates, and (4) offspring of infected rats became infected despite the presence of high maternal antibodies. The course of BCC virus infection, as determined by antibody response and the ability to isolate or detect virus, appeared to be similar regardless of whether the rats obtained their infection by inoculation or contact with inoculated rats. J. Med. Virol. 60:70-76, 2000. Published 2000 Wiley-Liss, Inc.

Rapid Communication ELEVATED OVARIAN AND THYMIC CELL APOPTOSIS IN WILD COTTON RATS INHABITING PETROCHEMICALCONTAMINATED TERRESTRIAL ECOSYSTEMS

The objective of this study was to determine the rates of apoptotic cell death in ovary and thymus collected from wild female cotton rats ( Sigmodon hispidus ) inhabiting five petrochemical-contaminated and five ecologically matched reference sites in Oklahoma. Overall comparison of reference and contaminated sites, using individual sites as replicates, revealed a significantly increased rate of ovarian cell apoptosis in cotton rats inhabiting contaminated sites. In comparison to rats from reference sites, the number of uterine scars was lower in rats collected from the contaminated sites. There were no significant differences in the percentage of atretic follicles among animals collected from reference and contaminated sites. The rate of thymocyte apoptosis was elevated at one of five contaminated sites, although the overall rate of thymocyte apoptosis was not significantly different when comparing all sites. To our knowledge, this is the first study documenting elevated rates of ovarian and thymic cell apoptosis in wild mammals exposed chronically to environmental toxicants.

Black Creek Canal Virus infection in Sigmodon hispidus in southern Florida.

A total of 1,500 small mammals were collected and tested for antibodies cross-reactive to Sin Nombre virus (Hantavirus: Bunyaviridae) at 89 sites in a 1,600 km2 study area of southern Florida. More than 95% of the 123 seropositive animals were cotton rats (Sigmodon hispidus), suggesting infection by Black Creek Canal Virus, although seroreactive Rattus rattus (5 of 294) and Peromyscus gossypinus (1 of 39) also were captured. Crude seroprevalence in S. hispidus was 11%. Seroprevalence increased with body size and was more common in male (18%; n=451) than in female (6%; n=593) cotton rats. Infection within S. hispidus populations was widespread throughout the study area. Prevalence ranged from 0% to 60% at sites where more than five cotton rats were sampled but was not only a function of sample size. Sites with seropositive cotton rats were geographically clustered compared with sites with no seropositive cotton rats. Clustering was not due to the spatial distribution of sites with few animals, season of collection, or sex bias of animals captured at these sites. However, sites with no seropositive animals had an excess of animals in the intermediate size class (60-99 g) and a deficit of the largest and smallest animals. These data suggest that population structure within the habitat mosaic may play a significant role in the spatial distribution of hantavirus infection in local populations of reservoir species.

Environmental, age, and sex effects on cotton rat (Sigmodon hispidus) hematology.

We determined the effects of sex, age, and environment (inbred, captive-wild, and wild animals) on selected blood parameters of the cotton rat (Sigmodon hispidus) in central Oklahoma (USA) from 1990 to 1994. Male and female cotton rats had similar blood profiles. Age-related differences were confined to differential white blood cell counts where adults possessed greater numbers of neutrophils and lower numbers of lymphocytes compared to juveniles. Environment had a strong influence on many hematological parameters. Hematocrit, hemoglobin concentration, RBC count, and eosinophil number were generally greater for wild stocks compared to inbred animals, and differences were more pronounced for adults than juveniles.

Nutritional Requirements for Reproduction in the Hispid Cotton Rat, Sigmodon hispidus

To better understand resource allocation in the herbivore Sigmodon hispidus , nutritional requirements for reproduction were determined. Adult female cotton rats were fed Purina® #5001 Laboratory Chow ad lib in the laboratory. They and their progeny were sacrificed at either 10, 20, 24, or 26 days after breeding, at birth (27 days), or at weaning (39 days). Protein, lipid, and element concentrations in body tissues of pregnant females were constant. From birth to weaning, protein concentration in lactating females increased and lipid concentration decreased, whereas no concentration of elements changed significantly. In progeny, concentration of protein, K, Na, Fe, Zn, and Cu declined significantly, while concentration of lipids, Ca, P, Mg, Mn, and energy increased significantly from birth to weaning. An energy and mass-balance analysis indicated that laboratory chow easily met nutritional requirements for reproduction. This analysis was used to estimate minimum nutritional requirements for reproduction in S. hispidus . A second energy and mass-balance analysis using a natural diet indicated that protein and phosphorus ingestion by adult females was lower during autumn and winter than required for production of a litter. Thus, protein and phosphorus are critical dietary components that may limit reproduction in S. hispidus under field conditions.

The effect of maternal mass on litter size and offspring survival in the hispid cotton rat (Sigmodon hispidus)

We tested the proposition that maternal body mass affects litter production and recruitment in the cotton rat (Sigmodon hispidus) in Kansas. We also addressed hypotheses about why cotton rats in Kansas are larger and more fecund than conspecifics in Texas. We livetrapped cotton rats on both control and food-supplemented grids. Pregnant females were briefly confined until parturition; their pups were weighed and toe-clipped, then mother and young were returned to the site of capture. Subsequent trapping revealed survival of the offspring, i.e., ability of mothers to recruit pups into the population. Heavier mothers on both sets of grids gave birth to larger, heavier litters and had more pups survive to recruitment. The percentage of pups surviving was not related to maternal mass or to pup mass, so the increased recruitment resulted primarily from heavier mothers' having more rather than heavier pups. Mothers on the supplemented grids bore larger and heavier litters than their same-mass counterparts on the control grids. Despite their larger litters, lighter (ca. 100 g) mothers on the supplemented grids recruited fewer pups than their same-mass counterparts on control grids. Because heavier female cotton rats are more fecund, our results are consistent with the hypothesis that cotton rats in Kansas are larger than in those in Texas because of selection for increased litter size in Kansas. Seasonality in Kansas restricts the number of breeding opportunities and provides a nutrient pulse that supports rapid reproduction in the growing season.

Small mammal populations at hazardous waste disposal sites near Houston, Texas, USA

Small mammals were trapped, tagged and recaptured in 0·45 ha plots at six hazardous industrial waste disposal sites to determine if populations, body mass and age structures were different from paired control site plots. Low numbers of six species of small mammals were captured on industrial waste sites or control sites. Only populations of hispid cotton rats at industrial waste sites and control sites were large enough for comparisons. Overall population numbers, age structure, and body mass of adult male and female cotton rats were similar at industrial waste sites and control sites. Populations of small mammals (particularly hispid cotton rats) may not suffice as indicators of environments with hazardous industrial waste contamination.

Effects of social environment on sexual maturation in female cotton rats (Sigmodon hispidus).

Age at sexual maturation among female cotton rats was measured in a variety of intraspecific social environments. Two experiments were conducted. In Experiment I, female cotton rats attained vaginal perforation and first estrus at younger ages and lighter body masses when paired from weaning with a conspecific juvenile male than when caged alone. In Experiment II, these findings were replicated and extended. Females housed with juvenile males matured at the youngest ages, while those housed alone matured at the oldest ages. Females housed with adult males matured at intermediate ages. Presence of a second juvenile female during maturation was significantly associated with early vaginal opening but not with early first estrus. The results of this study are discussed in context of similar social environmental effects on female sexual maturation that have been identified in other rodent species.