Abstract: The role of differential sample responding in the differential outcomes effect was examined. In Experiment 1, we trained pigeons on a one-to-many matching task with differential sample responding required. Differential outcomes were associated with samples and comparisons, with comparisons only, or with neither samples nor comparisons. Slopes of delay functions for trials with pecked versus nonpecked samples suggested use of a single-code-default strategy in the nondifferential-outcomes group but not in the differential-outcomes groups. In Experiment 2, differential sample responding and differential outcomes were manipulated independently. Again, there were significant differences in the relative slopes of the delay functions. Results suggest that differential outcomes exert their effect independently of differential sample responding.

Abstract: Two experiments on the role of neophobia in poison-induced aversions to exteroceptive stimuli are reported. In Experiment 1, rats were given either 10 or 25 days of habituation to the test situation prior to conditioning. Those animals with the longer habituation period avoided a complex of novel exteroceptive stimuli while those with the shorter habituation period did not. In Experiment 2 rats initially avoided the more novel of two containers, but gradually came to eat equal amounts from both. A single pairing of toxicosis with consumption from either the novel or the familiar container reinstated the avoidance of the novel container in both cases. The results were discussed in terms of an interaction between habituation and conditioning procedures. It was suggested that previously reported differences between interoceptive and exteroceptive conditioning effects may have been influenced by the differential novelty of the two classes of stimuli in the test situation. It was further suggested that non-contingently poisoned control groups should routinely be included in poison avoidance conditioning studies.

Abstract: The present study evaluated the engorgement and drop-off rhythms of Anocentor nitens females feeding on horses. Drop-off rhythm was evaluated at 6h-intervals (06:00, 12:00, 18:00, and 00:00 h) on horses held in stalls or in a pasture. A new method of marking feeding female ticks (the bowknot technique) was developed to evaluate ticks on horses in pasture that attached to different parts of the horse's body. This technique was highly successful, indicating no significant interference on tick engorgement rate or final tick weight, length and reproductive capability. Horses held in the pasture during the summer produced only 28.2% of the tick detachment during the daylight period from 06:00 to 18:00 h. In contrast, 53.4% of the ticks detached during this same 12 h-period during the winter. This difference was probably related to the longer scotoperiod during the winter. Different drop-off rhythms were observed for females attached to different anatomical parts of the horse's body. For example, ticks attached to the ears, perineum, and tail showed similar drop-off patterns, but were different from ticks attached to mane, rump and other body parts. The idiosoma length of the feeding female ticks was individually measured every 6 h until the engorged female detached naturally. The engorgement rate (increase in millimeters of the body length per hour) was evaluated during the last 96 h of parasitism. The highest engorgement rates were observed during the last 24 h of parasitism (approximately 0.16 mm/h), which were four-fold higher than the engorgement rates of the previous 3 days ( approximately 0.04 mm/h), demonstrating that these lower and higher values corresponded to the slow and rapid feeding phases reported elsewhere. Based on these data, the 6 mm idiosoma length was estimated as the minimal length that would correspond to the time point (i.e. 24 h before detachment) during which ticks would undergo the rapid feeding phase and detach as fully engorged females. When this 6 mm length was tested to estimate the number of engorged females detaching from horses in a period of 24 h, the estimated accuracy varied from 58.5 to 97.7% (mean: 73.3%).

Abstract: The effects of grazing by feral horses on vegetation and dune topography at Assateague Island National Seashore were investigated using color-infrared imagery, lidar surveys, and field measurements. Five pairs of fenced and unfenced plots (300 m2) established in 1993 on sand flats and small dunes with similar elevation, topography, and vegetation cover were used for this study. Color-infrared imagery from 1998 and field measurements from 2001 indicated that there was a significant difference in vegetation cover between the fenced and unfenced plot-pairs over the study period. Fenced plots contained a higher percentage of vegetation cover that was dominated by American beachgrass (Ammophila breviligulata). Lidar surveys from 1997, 1999, and 2000 showed that there were significant differences in elevation and topography between fenced and unfenced plot-pairs. Fenced plots were, on average, 0.63 m higher than unfenced plots, whereas unfenced plots had generally decreased in elevation after establishment in 1993. Results demonstrate that feral horse grazing has had a significant impact on dune formation and has contributed to the erosion of dunes at Assateague Island. The findings suggest that unless the size of the feral horse population is reduced, grazing will continue to foster unnaturally high rates of dune erosion into the future. In order to maintain the natural processes that historically occurred on barrier islands, much larger fenced exclosures would be required to prevent horse grazing.