No abstract

Pea aphids have several alternative responses to the detection of alarm pheromone produced by conspecifics. One of these, dropping from the feeding site to the ground, is potentially costly owing to the risk of desiccation-induced mortality on the ground before another host plant can be reached. Both dropping and walking from the feeding site incur a cost due to lost feeding opportunity. The aphids' decision as to which anti-predator tactic to use should be sensitive to the costs of their behaviour. Consequently, aphids should be less likely to drop when the risk of desiccation is higher, and less likely to drop or walk when the lost opportunity cost is higher. We tested these predictions by manipulating climatic severity (temperature and humidity) and host quality, respectively. As predicted, aphids are less likely to drop or walk in response to pheromone when feeding on high quality than on low quality hosts, and less likely to drop when the environment is hot and dry than when it is more benign. The latter is true whether the aphids are feeding on real or simulated leaves. Since all aphids were of the same clone, these results show that individual aphid genotypes possess the ability to adaptively modify their escape behaviour with changes in prevailing conditions. A number of other behavioural observations in the aphid literature may be interpreted in an economic or cost-benefit framework. The approach holds considerable promise for understanding many aspects of the anti-predator behaviour of aphids and other animals.

Feeding territory size and potential food abundance were measured simultaneously in a field population of juvenile (40–50 mm) coho salmon. Territory size was inversely related to the density of benthic food on the territory, as predicted from an energy-based model of territoriality. The relationship between the abundance of drift food and territory size was in the predicted direction, but was not significant. Territories were also smaller where intruder pressure was higher, but intrusion rate and food abundance were not directly correlated. Therefore, the effect of food abundance on territory size was not caused indirectly by attraction of nonterritorial fish to areas where food was abundant. In the laboratory, the distance from which a resident coho attacked an approaching model intruder increased asymptotically with hunger. The fish therefore appear to possess an appropriate behavioural mechanism (tactic) to adjust territory size to local food abundance.

No abstract

ABSTRACT. Body condition indices were assessed for pelagic juvenile cod (14 to 31 mm standard length, SL) over 2 cruises in spring 1987 in Canadian Atlantic waters on the southern Scotian Shelf. Three different indices determined from the residuals of univariate regressions were (1) triacylglycerol (TAG) content on standard length (SL), (2) dry body weight on SL, and (3) back-calculated change in SL from the peripheral 14 daily growth increments of the otolith on SL; hereafter referred to as TAGSL, DWTSL, and OTOSL respectively. Correlations between indices declined from TAGSL:DITSL (0.69, 0.64, cruises 87-1 and 87-2 respectively), to TAGSL:OTOSL (0.38. -0.05), to DWTSL:OTOSL (0.05, 0.03), a surprisingly low correlation considering that the indices were derived from the same individual. This pattern was also demonstrated by a principal component analysis (PCA), showing that after the first (size) component, otolith-based growth alone loaded on PC 2, then TAG contrasted with SL loaded on PC 3, and dry body weight showed the highest correlation with the starvation-independent SL. Supplementary analyses from 4 other cruises during 1985 and 1986 confirmed the weak correlation between DWTSL, OTOSL and other morphometric measures. All indlces were positively correlated with zooplankton biomass, but of the 16 separate cruise by index comparisons, only 6 were significantly correlated with zooplankton, of which 3 were the OTOSL. The rapld somatic/otollth growth of pelagic juvenile cod may respond rapidly to prey abundance, and rapid growth may preclude accumulation of Lipids (TAG), indicating that the utility of a con&tion lndex will depend on the species and life stage examined. Indices of recent otolith growth appear good measures of pelagic juvenile cod condition.