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Prepulse inhibition was used with C57BL/6J (C57) mice to assess behavioral-perceptual correlates of previously demonstrated physiological changes in the central auditory system associated with age-related hearing loss. Normal-hearing CBA/CaJ (CBA) mice and DBA/2J (DBA) mice, which exhibit extremely rapid hearing loss, were also tested. Tone prepulse stimuli (S1s) were presented 100 ms prior to a startle-evoking noise stimulus (S2), and a decrease in startle amplitude served as the measure of startle modification. As high-frequency hearing declined in C57 mice between 1 and 12 months of age, the efficacy of lower-frequency S1s was significantly enhanced. CBA mice exhibited no age-related changes in startle modification. DBA mice exhibited changes similar to those observed in C57s but at an accelerated rate. The enhanced behavioral saliency of low and middle frequencies in C57 and DBA mice appears to be a consequence of neural plasticity in the central auditory system.

The fear-potentiated startle paradigm has been used with great success to examine conditioned fear in both rats and humans. The purpose of this study was to examine fear-potentiated startle in inbred mice. One-month-old C57BL/6J (C57) and DBA/2J (DBA) mice were given tone + foot shock training trials. The amplitude of the acoustic startle reflex was measured in the presence and absence of the tone both before and after training. Both strains showed fear-potentiated startle after training as evidenced by larger startle amplitudes in the presence of the tone than in its absence. However, the magnitude of fear-potentiated startle was greater in DBA mice than in C57 mice. These results not only demonstrate fear-potentiated startle in mice for the first time but also suggest that fear-potentiated startle can be influenced by characteristics of the mouse strain.The amplitude of the acoustic startle response is potentiated in the presence of a stimulus that elicits fear (Brown, Kalish, & Farber, 1951;McAllister & McAllister, 1971). This so-catted fear-potentiated startle (FPS) effect (Davis & Astrachan, 1978) has been used with great success to increase researchers' understanding of the neural systems involved in conditioned fear. Studies using traditional techniques of brain lesions, drug infusions, and electrical stimulation have revealed much about the neural circuits involved in FPS (Davis, Campeau, Kim, & Falls, 1995). However, virtually all of the neurobiological research on FPS thus far has been performed on rats, and to our knowledge, the literature is devoid of studies examining FPS in mice. The present study is the first in a series of studies that will begin to remedy this situation by examining FPS in mice.Laboratory mice offer unique advantages for neurobiological and behavioral research, and their use in research on FPS has great potential. Some of the most exciting possibilities lie in the use of transgenic, knock-out, or other genetically engineered mice in which specific neurobiological properties can be manipulated to affect FPS. To begin to examine FPS in mice, we used a more traditional, tried-and-true approach, measuring FPS in inbred strains of mice. Numerous inbred strains are available, and many have been shown to differ on dimensions that might influence FPS, such as emotionality (

Summary1. Ericameria nauseosus (Asteraceae) is a highly variable shrub species of western North America, whose achene mass varies sevenfold among subspecies and populations. We examined trade-offs between seedling fitness and dispersal ability by asking: does greater achene mass reduce dispersal ability, and is this potential disadvantage to large achenes compensated for by increased pappus investment? Does smaller achene mass reduce seedling fitness, and is this potential disadvantage to small achenes compensated for by increased relative growth rate (RGR)? 2. Dispersal ability was measured as fall rate in still air, while seedling fitness was measured as dry mass at two ages for 21 populations belonging to nine subspecies. 3. Fall rate was positively correlated with achene plume loading -1/2 , but because of variation in pappus investment and geometry, both plume loading -1/2 and fall rate were only weakly positively correlated with achene mass. Relative pappus investment did not show a consistent increase with achene mass. 4. Seedling dry mass was strongly positively correlated with achene mass for 2-week-old seedlings grown without added nutrients. This correlation was significant but weak with added nutrients at 2 weeks, and by 4 weeks it was no longer significant. Seedling RGR was strongly negatively correlated with achene mass. 5. Achene mass variation was related to both seedling fitness and dispersal ability, but compensation was evident mainly as faster RGR in populations with small achenes.

Regulation of seed germination timing is an important component of population persistence, especially for short-lived plants in seasonal environments with unpredictable year-to-year weather variation. Seed germination patterns were examined in the laboratory for seeds from 135 populations belonging to 38 species and 13 sections of the genus Pensternon (Scrophulariaceae), from habitats ranging from warm desert to alpine tundra. Seeds of most species were dormant at dispersal and required a period of moist chilling to become germinable. Response to chilling was related to probable chilling duration at the collection site. Populations from habitats with severe winters produced seeds with long chilling requirements, while those from habitats with mild winters produced seeds with short chilling requirements. Populations from midelevation habitats produced seeds with intermediate chilling requirements but with a sizeable fraction whose dormancy was not broken by chilling of any length. Another pattern observed mostly in warm desert populations was little primary dormancy at autumn temperatures combined with induction of a fraction into secondary dormancy by short chilling. Species with a wide habitat range included populations with contrasting germination patterns. Parallel habitat-correlated patterns were observed in different sections. Most species showed germination patterns that combined predictive mechanisms with potential for carryover of a persistent seed bank. Results of common garden experiments suggested that germination differences had a strong genetic basis both among populations and among plants in a population. Adaptive radiation in the genus Penstemon has thus included the evolution of habitat-specific germination timing strategies in multiple lineages within the genus.