Taste-rejection by predators and the evolution of unpalatability in prey

Aposematic species advertise their unpalatability to potential predators using conspicuous warning colouration. The initial evolution of aposematism is thought to occur by warningly coloured mutants emerging in an already unpalatable cryptic species. However, possessing defence chemicals is often costly, and it is difficult to understand what the selective benefits might be for a mutation causing its bearer to be defended in a population of otherwise palatable cryptic prey. One solution to this problem is that chemically defended individuals are tasted and rejected by predators, and are, therefore, more likely to survive predatory attacks than undefended individuals. Using naïve domestic chicks Gallus gallus domesticus as predators and cryptic green chick crumbs as prey, we asked whether the accuracy with which birds discriminated between palatable and unpalatable prey was affected by the palatability of the unpalatable prey (moderately or highly defended), or their frequency in the population (10 or 25%). Birds could discriminate between green prey on the basis of their defences, and showed better discrimination between palatable and unpalatable prey when defended crumbs were highly unpalatable, compared to when they were moderately unpalatable. Although there was no detectable effect of the frequency of unpalatable prey in the population on predator taste-rejection behaviour in our main analysis, frequency did appear to affect the strategies that birds used in their foraging decisions when prey were only moderately unpalatable. How birds used taste to reject prey also suggests that birds may be able to monitor and regulate their chemical intake according to the frequency and defence levels of the unpalatable prey. Taken together, these results show that avian predators can generate selection for unpalatability in cryptic prey by sampling and taste-rejecting prey, but that a relatively large chemical difference between palatable and unpalatable prey may be necessary before unpalatable prey can enjoy a selective advantage. The exact nature of this evolutionary dynamic will depend on other environmental factors, such as defence costs and prey availability, but it provides a mechanism by which defences can evolve in a cryptic population.     

The megabenthic fauna in the deep sea south of New England,USA

Data from 105 benthic trawls made in an area south of New England, USA between 40 and 5000 m show the fauna to be zoned with depth, areas of rapid faunal change separating regions of relative faunal homogeneity. Distinct faunal assemblages with characteristic catch rates, diversity, and dominant species are found on the shelf (40 to 264 m), upper continental slope (283 to 650 m), middle continental slope (653 to 1290 m), lower continental slope (1380 to 1947 m), the transitional region from slope to rise (2116 to 2481 m), the upper continental (2504 to 3113 m), the middle continental rise (3244 to 3470 m), and lower continental rise to abyssal plain (3879 to 4986 m). Catch rates and diversity are greatest on the lower continental slope and transition to the upper rise, and are lowest at the greatest depths. Dominance, particularly by echinoderms, is an important aspect of community structure. The 3 major taxa represented (decapod crustaccean, echinoderms, and fishes) do not always display the same patterns within and between assemblages. Generalities derived from study of a single group need not apply to all segments of the deep-ocean community. Overall patterns in the megafauna are similar to those described in other groups and areas, but species assemblages are not the same everywhere and perhaps too much has been made of the horizontal extent of zones. Trophic level is related to degree of zonation, but where predators are generalists their ranges may be wide rather than restricted. Diversity patterns can be understood in terms of the interrelationships of predation, competition, environmental heterogeneity, and trophic level. Faunal zones are of importance as the geographical units within which evolution, community development, and diversification take place.This research was supported by the National Science Foundation under Grant OCE76-21878, the Office of Naval Research under Contract N00014-74-C0262, and, in part, by the United States Department of Energy under Contract No. EY-76-C-02-0016 and the Canadian Natural Sciences and Engineering Research Cancil under Grant A-7230     

Effect of surface incorporation of broiler litter applied to no-till cotton on runoff quality

Surface application of broiler litter to no-till cotton could lead to degradation of water quality. Incorporation of broiler litter into the top surface soil (0.05 m) could alleviate this risk. A 2-yr field study was conducted on a silt loam upland soil to determine the effect of incorporation of broiler litter into the soil surface on nutrient and bacterial transport in runoff. The experimental design was a randomized complete block with four treatments and three replications. Treatments were (i) unfertilized control; (ii) surface-appliedbroiler litter at 7.8 Mg ha(-1) without incorporation; (iii) surface-applied broiler litter at 7.8 Mg ha(-1) with immediate incorporation; and (iv) inorganic fertilizer N (urea ammonium nitrate, 32% N) and inorganic fertilizer P (triple superphosphate) at the recommended rate. Phosphorus was surface appliedat 25 kg ha(-1) and N was injected at 101 kg ha(-1) into the soil using a commercial liquid fertilizer applicator. Runoff was collected from small runoff plots (2.4 m by 1.6 m) established at the bottom side of main plots (13.7 m by 6.0 m). Incorporation of broiler litter reduced total N (TN), NO3-N, water soluble P (WSP), and total P (TP) concentrations in runoffby 35, 25, 61, and 64%, respectively, and litter-associated bacteria by two to three orders of magnitude compared with unincorporated treatment. No significant difference in total suspended solids (TSS) in runoffwas obtained between incorporated and unincorporated treatments. Incorporation of broiler litter into the surface soil in the no-till system immediately after application minimized the potential risk for surface nutrient losses and bacteria transport in runoff.     

Modelling leachate quality and quantity in municipal solid waste landfills.

The operational phase of landfills may last for 20 years or more. Significant changes in leachate quality and generation rate may occur during this operational period. A mathematical model has been developed to simulate the landfill leachate behaviour and distributions of moisture and leachate constituents through the landfill, taking into consideration the effects of time-dependent landfill development on the hydraulic characteristics of waste and composition of leachate. The model incorporates governing equations that describe processes influencing the leachate production and biochemical processes taking place during the stabilization of wastes, including leachate flow, dissolution, acidogenesis and methanogenesis. To model the hydraulic property changes occurring during the development stage of the landfills, a conceptual modelling approach was proposed. This approach considers the landfill to consist of cells or columns of cells, which are constructed at different times, and considers each cell in the landfill to consist of several layers. Each layer is assumed to be a completely mixed reactor containing uniformly distributed solid waste, moisture, gases and micro-organisms. The use of the proposed conceptual model enables the incorporation of the spatial changes in hydraulic properties of the landfill into the model and also makes it possible to predict the spatial and temporal distributions of moisture and leachate constituents. The model was calibrated and partially verified using leachate data from Keele Valley Landfill in Ontario, Canada and data obtained from the literature. Ranges of values were proposed for model parameters applicable for real landfill conditions.     

Antipredator responses of California ground squirrels to rattlesnakes and rattling sounds: the roles of sex,reproductive parity,and offspring age in assessment and decision-making rules

California ground squirrels (Spermophilus beecheyi) and northern Pacific rattlesnakes (Crotalus viridus oreganus) have an adversarial relationship. Adults are partially protected by venom resistance and harass rattlesnakes in part to defend their more vulnerable offspring. Larger, warmer snakes are more dangerous than smaller colder snakes, and in escalated conflict squirrels could benefit from risk assessment strategies. Rattlesnakes often rattle at harassing squirrels and rattling sounds produce cues related to body size and temperature. In study 1 we played back rattling sounds from snakes that varied in dangerousness and evaluated the roles of sex and parity in squirrel risk assessment strategies. In general, squirrels tail flagged and stood bipedally more, and were slower to reapproach the playback speaker following playbacks of rattling sounds from more dangerous snakes. In comparison with males and nonmothers, mothers were most responsive to rattling sounds and more sensitive to variation in snake dangerousness. Mothers tail flagged more than males and nonmothers, and this behavior tracked variation in snake dangerousness most closely, perhaps reflecting the effects of snake size and temperature on pup vulnerability. These findings suggest that many aspects of squirrel antisnake behavior are governed by their effects on descendant kin. In study 2 we tested the effects of offspring age on mothers responses to live rattlesnakes and rattling sounds. According to the offspring value hypothesis, mothers should take more risks in defense of older offspring because they are more likely to survive to reproductive age. By contrast, under the offspring vulnerability hypothesis, older offspring are less vulnerable to predators and thus mothers should take fewer risks. Risk-taking, as measured by behaviors that bring the squirrel close to the snakes strike range, was either unaffected by or negatively correlated with offspring age. Thus, our findings suggest that whereas offspring value is unimportant in squirrel antisnake behavior, offspring vulnerability may affect maternal defense. We suggest that offspring vulnerability in mammals, in comparison with birds, may play a larger role in parental defense against predators.Communicated by P.A. BednekoffAn erratum to this article can be found at     

Analysis of the causal components of assortative mating in water striders

Field studies demonstrate that natural populations of a group of water striders (Heteroptera: Gerridae) that share a common mating system are characterized by weak assortative mating by size and by large sizes of mating males and females relative to single individuals. This study presents an experimental assessment of the components of mating that may contribute to these mating patterns. The effects of male and female body size on each of three components of mating were studied in three water strider species in the laboratory. Large females of all three species mated more frequently, copulated for longer and were guarded longer than small females. Large males mated more frequently than small males in all three species, and also guarded females for longer in the two species where the average of mate guarding was long. However, we found an antagonistic effect of male size on copulation duration: small males copulated for longer than large males in all three species. We show that the combined effects of these size biases mimic the mating patterns found in the wild, e.g. weak and variable assortative mating, and stronger and less variable size ratios of mating versus non-mating females relative to males. We suggest that the antagonistic effects of male size on copulation and guarding duration may be a key source of interpopulational variation in assortative mating and sexual selection on male size. Further, neither spatial or temporal covariation in size, nor mechanical constraints, caused the assortative mating observed here in this group of water striders. Some combination of male and female choice (either active or passive forms) of large mates and male-male exploitation competition for mates play potentially important roles in producing population level assortative mating in water striders. Received: 17 March 1995/Accepted after revision: 28 October 1995     

An experiment on the economic value of visibility

This paper reports on the development and application of an iterative bidding technique to examine proposed visibility reductions in the Four Corners Region of the southwest United States. The iterative bidding technique is a method to elicit honest bids for “goods” not priced in formal markets. Four issues associated with direct revelation of preferences for environmental goods are explored: (1) the establishment of linkages between the physical parameters of visibility and representation of the “good” called visibility to individuals for valuation purposes; (2) a systematic test for both observed and induced biases associated with the interative bidding technique; (3) replication of the results of previous experiments; and (4) reporting of empirical divergences between compensating and equivalent surplus measures of consumer surplus.