Graham J. Alexander

Xenopus laevis embryos and tadpoles as models for testing for pollution by zinc, copper, lead and cadmium

The toxicity of bio available Zn, Cu, Pb, and Cd on the life stages of Xenopus laevis embryos and tadpoles was investigated. Cu and Cd were found to affect the hatching success of the embryos, with a strong negative relationship existing between the increase in Cu concentrations and the hatching of the embryos. Only concentrations above 0.6 ppm Cd affected the hatching of the embryos. All metals affected the survival of tadpoles over the seven days of exposure, with Zn and Cu showing a steady linear toxicity and Pb and Cd a threshold toxicity effect. Metals affected the growth of the tadpoles by reducing body length with increasing concentrations. An increase in the concentration of each metal resulted in an increase in the frequency and severity of malformations among the tadpoles. The percentage of malformed tadpoles was dependent on the metal and concentration to which they were exposed. Our data describe the relationships among hatching success, survival, growth rate and the frequency of malforma tions of tadpoles in relation to different concentrations of the four metals. Xenopus laevis embryos and tadpoles can be used successfully in bioassays to biologically quantify the severity of Zn, Cu, Pb and Cd pollution.

RESPONSES OF STRIPED MICE, RHABDOMYS PUMILIO, TO FAECES OF A PREDATORY SNAKE

We investigated the behavioural responses of captive wild-caught striped mice ( Rhabdomys pumilio ) to the faeces of the elapid snake, the rinkhals ( Hemachatus haemachatus ), a striped mouse predator. The rinkhals apparently conceals its presence from potential prey by defecating away from their retreats, and we predicted that R. pumilio recognises this snakes faeces and displays anti-predator responses. We also tested whether R. pumilio is capable of diet-related predator labelling by observing their reactions to faeces of rinkhals fed different meals. Both sexes showed a significant anti-predator response to the presence of rinkhals faeces compared to that of house mouse ( Mus musculus ) bedding, by decreasing activity and investigations of test samples. In addition, R. pumilio showed diet-related predator labelling since its anti-predator response was greater when presented with H. haemachatus faeces containing remains of conspecifics than with those containing remains of house mice. The increased response to faeces with stimuli from conspecifics may function to alert R. pumilio to the presence of a predator. However, the defecation behaviour of H. haemachatus suggests that encounters between R. pumilio and H. haemachatus faeces would not necessarily lead to a predator-prey encounter.

Thermal Dependence of Appetite and Digestive Rate in the Flat Lizard, Platysaurus intermedius wilhelmi

We investigated the thermal dependence of appetite and digestive rate in the flat lizard, Platysaurus intermedius wilhelmi, over a wide thermal range (12?-34?C). Because Platysaurus seldom venture off rock outcrops, their body temperatures are determined to a large extent by the thermal characteristics of this microhabitat. Our results demonstrate that appetite, measured as the number of food items eaten in a given time, is clearly temperature-dependent, increasing to a maximum that coincides with the thermal preference of 32?C for the species. Our measures for digestive rate, measured as the time to the first passing of feces after a meal, are temperature-dependent below 22?C but are temperature-independent between 22? and 34?C. Furthermore, our data indicate a critical body temperature of 12?C, below which all feeding ceases. We predict that lizards of this species with TbS below 20?C are effectively excluded from nutritive energy gain under natural conditions.

Circannual Rhythms of Appetite and Ecdysis in the Elapid Snake, Hemachatus haemachatus, Appear to Be Endogenous

We demonstrate that appetite and the probability of ecdysis vary seasonally in the temperate African elapid Hemachatus haemachatus. The periods of lowest food acceptance, and lowest probability of shedding, coincided with the late autumn and winter months of May through July, and we interpret these patterns as adaptations to avoid the costs of arrested digestion and of ecdysis during the coldest part of the year. The seasonal patterns we document have persisted in our animals without measurable attenuation for seven years to date, despite the snakes being kept at constant temperature (25 C) and photoperiod (12:12 h) for the duration of the study. Because these are the most likely cues for exogenous control of the seasonal patterns observed, we argue that we have provided good evidence that these rhythms are, at least partly, under endogenous control.

THE USE OF A MULTIVARIATE METHOD OF CLASSIFICATION (TWO-WAY INDICATOR SPECIES ANALYSIS) IN BIOGEOGRAPHY

ABSTRACT The use of a classification procedure to establish common patterns in distributions of groups of species is discussed with the aid of an example relating to the herpetofauna of Municipal Durban. This technique was found to be extremely useful and has advantages over other techniques currently in use, particularly that it is more objective. The most important limitation to its use is the requirement of an adequate data set. Although there are many types of numerical procedures for the recognition of pattern available, few appear to have been used in the field of biogeography.