Litter effect

TABLE 8.3. Examples of Litter Effects in Control Litters... 

FIGURE 8.7. Litter effect with supernumerary rib in 1379 control litters. Between 1978 and 1988, fetal skeletons from 1379 litters of control rats were stained with alizarin red and examined for supernumerary rib in addition to other anomalies. The calculation of the expected number of fetuses with supernumerary rib in each litter was based on the assumption that each fetus had an equally likely chance of having supernumerary rib independent of the incidence among littermates (K. Soper, personal communication, 1990). 

Just as with an LD50 in adult animals where, by definition, one-half of the animals die and one-half live after acute chemical insult, and the reason(s) why one particular animal that is apparently identical to the other members of the group lives and another dies is not known. Some fetuses in a litter can be grossly malformed and the neighboring fetus can be normal. The litter effect is a term applied to the finding that, at some dose, different females treated in the same way vary in the degree and even type of response. 

A number of other possible confounding factors can enter into a bioassay unless design precludes them. These include (1) cage and litter effects, which can be avoided by proper prestudy randomization of animals and rotation of cage locations (2) vehicle (e.g., corn oil has been found to be a promoter for liver carcinogens) and (3) the use of the potential hazard route for man (e.g., dietary inclusion instead of gastric intubation). 

Effects on reproduction [size of litters/broods evidence of teratogenicity (physical defects) in foetuses]. 

The effects that changes in vegetation have on soil carbon pools and nutrient availability are also difficult to evaluate. However, several models have been successful in predicting vegetation-soil nutrient relationships because they assume that such changes occur as a result of different rates of decomposition and nutrient release from leaf litter of different taxa 50, 60), Such predictions could be tested and the models refined or parameterized for new taxa by measuring soil nutrient availability and respiration in stands of different species on the same soil type. For example, fifty years ago the U.S. Civilian Conservation Corps (CCC) established such stands as species trial plots measurements in some indicate large differences in soil nutrient availability (48), Further measurements in these stands would now occur at the same time-scale at which we expect the feedback between species replacement and soil processes to occur. 

Land use changes in the tropics have resulted in a landscape characterized as a mosaic of logged forests, cleared fields, and successional forests. This results in the transformation from extremely fire resistant rainforest ecosystems to anthropogenic landscapes in which fire is a common event (16, 17), Fires occur in disturbed tropical forests because deforestation has a dramatic effect on microclimate. Deforestation results in lower relative humidities, increased wind speeds, and increased air temperatures. In addition, deforestation results in increased quantities of biomass that are susceptible to fire. This biomass may be in the form of forest slash, leaf litter, grasses, lianas or herbaceous species (16, 18). 

The results of heat and water stress experiments (Table 12.3) show that these factors had no significant effects on nitrogen isotope ratios of bone collagen or hair. The mean collagen-diet difference (A Nco-d) values of the water-restricted litters ranged from 2.6 %o for group 10 on diet A (36°, water ad... 

Sydes, C. Grime, J.P. (1981a). Effects of tree leaf litter on herbaceous vegetation in deciduous woodland. I. Field investigations. Journal of Ecology, 69, 237-48. 

The progeny, mated within the experimental group, had a decreased number of pregnancies and reduced litter size at the 0.5 )u.g/kg while no effect was observed in 0.25 /xg/kg progeny. 

Other additional studies or pertinent information which lend support to this MRL Additional studies have reported developmental effects in rodents exposed to trichloroethylene. Following exposure of rats on gestation days 6-19, decreased litter size (Narotsky and Kavlock 1995), and increased micro- or anophthalmia (Narotsky and Kavlock 1995 Narotsky et al. 1995) were observed in the offspring at 1,125 mg/kg/day, but not at 844 mg/kg/day (Narotsky et al. 1995). 

No differences were noted in the litter sizes among those treated and the controls. No differences were noted in the number of stillborn pups or in pup weights. The study authors concluded that there was no evidence of adverse diisopropyl methylphosphonate-induced reproductive effects. However, as discussed in Section 2.2.2.1, there is some confusion regarding the actual doses to which the animals were exposed in the Hardisty et al. (1977) study. Therefore, results from this study are considered inappropriate for human health risk assessment. 

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