Large size in an island-dwelling bird: intraspecific competition and the Dominance Hypothesis

Differences between island- and mainland-dwelling forms provide several classic ecological puzzles. Why, for instance, are island-dwelling passerine birds consistently larger than their mainland counterparts? We examine the 'Dominance hypothesis', based on intraspecific competition, which states that large size in island passerines evolves through selection for success in agonistic encounters. We use the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large-bodied island-dwelling race of white-eye (Zosteropidae), to test three assumptions of this hypothesis; that (i) large size is positively associated with high fitness, (ii) large size is associated with dominance, and (iii) the relationship between size and dominance is particularly pronounced under extreme intraspecific competition. Our results supported the first two of these assumptions, but provided mixed evidence on the third. On balance, we suggest that the Dominance Hypothesis is a plausible mechanism for the evolution of large size of island passerines, but urge further empirical tests on the role of intraspecific competition on oceanic islands versus that on mainlands.

Variation in body size and sexual dimorphism across geographical and environmental space in the frogs Limnodynastes tasmaniensis and L. peronii

This study aimed to identify potential factors responsible for geographically structured morphological variation within the widespread Australian frogs Limnodynastes tasmaniensis Gunther and L. peronii Dumeril & Bibron. There was support for James's rule, and both latitude and present climate explained large amounts of the variation in body size and shape (particularly in L. peronii). There was also some support for the influence of several biogeographical barriers. Finally, both species were sexually dimorphic for body size and the degree of sexual size dimorphism (SSD) varied geographically. Climate was an important explanation for SSD variation in L. peronii, while latitude was most important for L. tasmaniensis. Geographical variations in sexual selection via male-male physical competition and climate-related resources are suggested as potential explanations for SSD variation in L. peronii. (C) 2004 The Linnean Society of London.

"mind the gap": The size-distance dissociation in visual neglect is a cueing effect

There is a growing body of evidence that the processes mediating the allocation of spatial attention within objects may be separable from those governing attentional distribution between objects. In the neglect literature, a related proposal has been made regarding the perception of (within-object) sizes and (between-object) distances. This proposal follows observations that, in size-matching and bisection tasks, neglect is more strongly expressed when patients are required to attend to the sizes of discrete objects than to the (unfilled) distances between objects. These findings are consistent with a partial dissociation between size and distance processing, but a simpler alternative must also be considered. Whilst a neglect patient may fail to explore the full extent of a solid stimulus, the estimation of an unfilled distance requires that both endpoints be inspected before the task can be attempted at all. The attentional cueing implicit in distance estimation tasks might thus account for their superior performance by neglect patients. We report two bisection studies that address this issue. The first confirmed, amongst patients with left visual neglect, a reliable reduction of rightward error for unfilled gap stimuli as compared with solid lines. The second study assessed the cause of this reduction, deconfounding the effects of stimulus type (lines vs. gaps) and attentional cueing, by applying an explicit cueing manipulation to line and gap bisection tasks. Under these matched cueing conditions, all patients performed similarly on line and gap bisection tasks, suggesting that the reduction of neglect typically observed for gap stimuli may be attributable entirely to cueing effects. We found no evidence that a spatial extent, once fully attended, is judged any differently according to whether it is filled or unfilled.

Advantage of single-trial models for response to selection in wheat breeding multi-environment trials

An investigation was conducted to evaluate the impact of experimental designs and spatial analyses (single-trial models) of the response to selection for grain yield in the northern grains region of Australia (Queensland and northern New South Wales). Two sets of multi-environment experiments were considered. One set, based on 33 trials conducted from 1994 to 1996, was used to represent the testing system of the wheat breeding program and is referred to as the multi-environment trial (MET). The second set, based on 47 trials conducted from 1986 to 1993, sampled a more diverse set of years and management regimes and was used to represent the target population of environments (TPE). There were 18 genotypes in common between the MET and TPE sets of trials. From indirect selection theory, the phenotypic correlation coefficient between the MET and TPE single-trial adjusted genotype means [r(p(MT))] was used to determine the effect of the single-trial model on the expected indirect response to selection for grain yield in the TPE based on selection in the MET. Five single-trial models were considered: randomised complete block (RCB), incomplete block (IB), spatial analysis (SS), spatial analysis with a measurement error (SSM) and a combination of spatial analysis and experimental design information to identify the preferred (PF) model. Bootstrap-resampling methodology was used to construct multiple MET data sets, ranging in size from 2 to 20 environments per MET sample. The size and environmental composition of the MET and the single-trial model influenced the r(p(MT)). On average, the PF model resulted in a higher r(p(MT)) than the IB, SS and SSM models, which were in turn superior to the RCB model for MET sizes based on fewer than ten environments. For METs based on ten or more environments, the r(p(MT)) was similar for all single-trial models.

Contrasting mutual sexual selection on homologous signal traits in Drosophila serrata

The nature of male mating preferences, and how they differ from female mating preferences in species with conventional sex roles, has received little attention in sexual selection studies. We estimated the form and strength of sexual selection as a consequence of male and female mating preferences in a laboratory-based population of Drosophila serrata. The differences between sexual selection on male and female signal traits (cuticular hydrocarbons [CHCs]) were evaluated within a formal framework of linear and nonlinear selection gradients. Females tended to exert linear sexual selection on male CHCs, whereas males preferred intermediate female CHC phenotypes leading to convex (stabilizing) selection gradients. Possible mechanisms determining the nonlinear nature of sexual selection on female CHCs are proposed.

Sexual selection for male dominance reduces opportunities for female mate choice in the European bitterling (Rhodeus sericeus)

Sexual selection involves two main mechanisms: intrasexual competition for mates and intersexual mate choice. We experimentally separated intrasexual (male-male interference competition) and intersexual (female choice) components of sexual selection in a freshwater fish, the European bitterling (Rhodeus sericeus). We compared the roles of multiple morphological and behavioural traits in male success in both components of sexual competition, and their relation to male reproductive success, measured as paternity of offspring. Body size was important for both female choice and male-male competition, though females also preferred males that courted more vigorously. However, dominant males often monopolized females regardless of female preference. Subordinate males were not excluded from reproduction and sired some offspring, possibly through sneaked ejaculations. Male dominance and a greater intensity of carotenoid-based red colouration in their iris were the best predictors of male reproductive success. The extent of red iris colouration and parasite load did not have significant effects on female choice, male dominance or male reproductive success. No effect of parasite load on the expression of red eye colouration was detected, though this may have been due to low parasite prevalence in males overall. In conclusion, we showed that even though larger body size was favoured in both intersexual and intrasexual selection, male-male interference competition reduced opportunities for female choice. Females, despite being choosy, had limited control over the paternity of their offspring. Our study highlights the need for reliable measures of male reproductive success in studies of sexual selection.

Infracommunity structure of parasites of Hemigymnus melapterus (Pisces : Labridae) from Lizard Island, Australia: The importance of habitat and parasite body size

This Study describes the community of all metazoan parasites from 14 individuals of thicklip wrasse, Hemigymnus melapterus, from Lizard Island, Australia. All fish were parasitized, and 4,649 parasite individuals were found. Twenty-six parasite species were identified although only 6 species were abundant and prevalent: gnathiid isopods, the copepod Hatschekia hemigymni, the digenean Callohelmis pichelinae, and 3 morphotypes of tetraphyllidean cestode larvae. We analyzed whether the body size and microhabitat of the parasites and size of the host affected understanding of the structure of the parasite community. We related the abundance, biovolume, and density of parasites with the host body size and analyzed the abundances and volumetric densities of some parasite species within microhabitats. Although the 2 most abundant species comprised 75% of all parasite individuals, 4 species, each in similar proportion, comprised 85% of the total biovolume. Although larger host individuals had higher richness, abundance, and biovolume of parasites than smaller individuals, overall parasite volumetric density actually decreased with the host body size. Moreover. parasites exhibited abundances and densities significantly different among microhabitats; some parasite species depended on the area available, whereas others selected a specific microhabitat. Parasite and habitat size exhibited interesting relationships that should be considered more frequently. Considerations of these parameters improve understanding of parasite community structure and how the parasites use their habitats.

Habitat selection and breeding success in a forest-nesting Alcid, the marbled murrelet, in two landscapes with different degrees of forest fragmentation

We studied habitat selection and breeding success in marked populations of a protected seabird (family Alcidae), the marbled murrelet (Brachyramphus marmoratus), in a relatively intact and a heavily logged old-growth forest landscape in south-western Canada. Murrelets used old-growth fragments either proportionately to their size frequency distribution (intact) or they tended to nest in disproportionately smaller fragments (logged). Multiple regression modelling showed that murrelet distribution could be explained by proximity of nests to landscape features producing biotic and abiotic edge effects. Streams, steeper slopes and lower elevations were selected in both landscapes, probably due to good nesting habitat conditions and easier access to nest sites. In the logged landscape, the murrelets nested closer to recent clearcuts than would be expected. Proximity to the ocean was favoured in the intact area. The models of habitat selection had satisfactory discriminatory ability in both landscapes. Breeding success (probability of nest survival to the middle of the chick rearing period), inferred from nest attendance patterns by radio-tagged parents, was modelled in the logged landscape. Survivorship was greater in areas with recent clearcuts and lower in areas with much regrowth, i.e. it was positively correlated with recent habitat fragmentation. We conclude that marbled murrelets can successfully breed in old-growth forests fragmented by logging.

Systematic selection of solvents for the fabrication of 3D combined macro- and microporous polymeric scaffolds for soft tissue engineering

In this study, we investigate the fabrication of 3D porous poly(lactic-co-glycolic acid) (PLGA) scaffolds using the thermally-induced phase separation technique. The current study focuses on the selection of alternative solvents for this process using a number of criteria, including predicted solubility. toxicity, removability and processability. Solvents were removed via either vacuum freeze-drying or leaching, depending on their physical properties. The residual solvent was tested using gas chromatography-mass spectrometry. A large range of porous, highly interconnected scaffold architectures with tunable pore size and alignment was obtained, including combined macro- and microporous structures and an entirely novel 'porous-fibre' structure. The morphological features of the most promising poly(lactic-co-glycolic acid) scaffolds were analysed via scanning electron microscopy and X-ray micro-computed tomography in both two and three dimensions. The Young's moduli of the scaffolds under conditions of temperature, pH and ionic strength similar to those found in the body were tested and were found to be highly dependent on the architectures.