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Biomedical subjects

G D Ruxton

Publications and source records attributed to G D Ruxton.

7 recordsLinked to original sources
PubMed2000

Fish shoal composition: mechanisms and constraints.

Observations were made on three fish species (banded killifish (Fundulus diaphanus), golden shiner (Notemigonus crysoleucas) and white sucker (Catostomus commersoni)) in a temperate lake (New Brunswick, Canada) in order to investigate the relationship between shoal choice behaviour of individual fishes and shoal composition. Encounters between shoals were observed to take place every 1.1 min per shoal and an encounter lasted 3.7 s on average. The duration of shoal encounters was influenced by shoal size but not by differences between shoals in either body length or species. Conversely, the outcome of shoal encounters (i.e. ences between shoals in either body length or species. Conversely, the outcome of shoal encounters (i.e. whether or not an individual changes shoal) was influenced by body length and species differences but not by shoal size. Together, these results suggest that encounter duration itself is unlikely to have an important influence on encounter outcome. The collection of ten entire fish shoals showed that they were assorted by species and body length. A simulation model demonstrated that individual shoal choice behaviour alone could account for the generation and maintenance of the observed levels of size assortedness of shoals without invoking the existence of other sorting mechanisms such as differential swimming speeds. However, the generation of species assortedness was not predicted by the model. Furthermore, our data suggest that fish density acts as a constraint on shoal choice, influencing both shoal size and composition. This work has implications for studies on information transfer and reciprocal altruism within populations.

Animals↗
PubMed2000

Testing Müllerian mimicry: an experiment with wild birds.

Experiments with wild birds feeding on pastry 'prey' were performed to test competing theories of Müllerian mimicry Conventional theories predict that all resemblances between defended prey will be mutually advantageous and, hence, Müllerian. In contrast, unconventional theories predict that, if there are inequalities in defences between mimetic species, the less well-defended prey may dilute the protection of the better defended species in a quasi-Batesian manner. This unconventional prediction follows from an assumption that birds learn about the edibilities of prey using rules of Pavlovian learning. We report on two experiments, each lasting 40 days, which showed that a moderately defended prey can dilute the protection of a better defended mimic in a quasi-Batesian fashion, but can add protection to a mimic which has the same moderate levels of defence. These results match predictions of unconventional theories of mimicry and go some way to resolving the long-running arguments over the nature of Müllerian mimicry.

Animals↗
PubMed2000

Evidence for a rule governing the avoidance of superfluous escape flights.

When an imminent attack by a predator on a group of birds is signalled to non-detectors only by the departure of the detector, non-detectors may make time-wasting false-alarm flights in response to mistaken or non-predator-driven departures. The frequency of false-alarm flights might be reduced if group members assess the reason for single departures before responding. Immediate flights should only occur after multiple simultaneous departures, because these are only likely to be generated by an attack. The response delay between the detectors' departure and the next birds that respond should then be dependent on the number of detectors. On sparrowhawk attack, response delays in redshanks decreased significantly as detector number increased, controlling for raptor conspicuousness and proximity, and flock size and spacing. If response delay is modified because of risk dilution, it should increase with flock size and, consequently, the rate of alarm flights due to mistakes should decrease. However, response delay did not increase and flight frequency due to misidentification of non-raptors or non-predator-driven departures did not decrease with flock size. Significantly more feeding time was lost by birds in small flocks, suggesting that the dilution effect decreased the cost of each false-alarm flight rather than their frequency.

Animal Communication↗
PubMed2000

The importance of stable schooling: do familiar sticklebacks stick together?

Preferences for rejoining shoals composed of familiar individuals have recently been documented in a variety of small, shallow-water fish species. Such preferences are assumed to be adaptive, since familiar groups have improved anti-predator defences and more stable dominance hierarchies. However, the design of these studies may have created conditions that elevate preferences for familiar individuals. Furthermore, in natural habitats, where significant opportunities for inter-shoal transfer may exist, it is unclear whether shoals stay together long enough for such preferences to develop. Here we present the results of a laboratory study examining whether prior familiarity influences the subsequent shoal composition of sticklebacks (Gasterosteus aculeatus) allowed to re-assort freely in a large arena tank. We show that fish from different familiarity groups associate with familiar conspecifics significantly more than predicted by a model of random assortment, suggesting that even when there is ample opportunity for inter-group transfer, shoal composition can remain stable. We discuss the phenomena that may lead to the formation of familiar groups in natural habitats. In addition, we suggest that familiarity benefits may reduce the relative value of transferring to otherwise more attractive (e.g. larger or more phenotypically matched) groups, and thereby stabilize shoal structure.

Animals↗
PubMed2000

Resource allocation between reproductive phases: the importance of thermal conditions in determining the cost of incubation.

Changes in the resources allocated to particular stages of reproduction are expected to influence allocation to, and performance in, subsequent reproductive stages. Experimental manipulation of individual investment patterns provides important evidence that such physiological trade-offs occur, and can highlight the key environmental variables that influence reproductive costs. By temporarily altering the thermal properties of starling nests, we reduced the energetic demand of first-clutch incubation, and examined the effect of this manipulation on performance during the same and the subsequent reproductive attempts. Compared with controls, starlings investing less in incubation were more successful in fledging young, and were more likely to hatch all their eggs if a subsequent reproductive attempt was made. Our results show that incubation demands can limit reproductive success, and that resources saved during incubation can be reallocated to later stages of the same reproductive attempt and to future reproductive attempts. This study also shows that small changes in thermal environment can affect breeding success by altering the energetic demands imposed on incubating parents, independently of the effect of temperature on other environmental variables such as food supply.

Animals↗
PubMed1999

Dispersal-induced instabilities in host-parasitoid metapopulations.

We present a general host-parasitoid metapopulation model and, using analytical techniques (supported by numerical simulations), investigate the effects of dispersal on the equilibrium stability of local populations. As has been demonstrated previously, if the intrinsic dynamics of local populations are unstable, then passive dispersal cannot stabilise. Extreme asymmetry in the dispersal fractions between the two species can, however, destabilise the metapopulation equilibrium state. Our key conclusion is that the precise effects of dispersal on stability depend critically on the underlying ecology of the interaction within each population. The presence of regulatory mechanisms, be they in the form of density-dependent host reproduction or the presence of host refugia, decreases the likelihood of observing dispersal-induced instabilities. Indeed, if the stabilising effects are sufficiently strong, then dispersal cannot be destabilising, no matter how asymmetric the dispersal fractions are. On the other hand, positive feedbacks arising from threshold effects in host reproduction or inversely density-dependent patterns of parasitism (due to, for example, long handling times or egg limitation) amplify the destabilising effects of dispersal.

Animals↗