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GD Ruxton

Publications and source records attributed to GD Ruxton.

11 recordsLinked to original sources
PubMed1999

Modelling territorial behaviour of animals in variable environments.

We present an individual-based model describing the distribution and resource gain of territorial individuals in situations where the rank order of territory quality changes over time. The model integrates both competitive (territory-holding ability) asymmetries and a memory function. A balance of effects resulted in a peak in movement rates, but not resource gain, for individuals of intermediate ability. Furthermore, when the system was reduced to a linear array of territories (as commonly used in empirical studies) the model generated quite different predictions because of the severe limitation in movement that the linear array imposes. We suggest that the model can be used to generate testable predictions for territorial species such as salmonids, and that future empirical work should take into account the consequences of reductions in movement imposed by a linear array of territories. Copyright 1999 The Association for the Study of Animal Behaviour.

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PubMed1999

The effect of clutch cooling rate on starling, Sturnus vulgaris, incubation strategy.

In avian species where only one parent incubates, that parent must divide its time between the mutually exclusive activities of incubation and foraging in such a way as to maintain both body condition and clutch temperature within certain limits. In a uniparental incubator, the starling, we experimentally reduced the rate at which unattended clutches of eggs cooled down and monitored the resulting changes in the parent's incubation strategy. Opposite to the predictions of standard models of time allocation during incubation, parents spent a much greater percentage of each 24-h period incubating when the rate of clutch cooling was reduced. Incubation bouts lasted significantly longer on experimental nests than on control nests, both during the daytime and overnight. Mean foraging bout duration did not differ between the two groups of nests. These results are consistent with the hypotheses that parental foraging success cues the end of a foraging bout, and that parental energy level cues the end of an incubation bout. However, most previous studies suggest that parents spend less time incubating when the rate of clutch cooling is slow. If parental energy level cues departure, these results can be explained only if the amount of time available for incubation is constrained in these cases by the time a parent must spend foraging in order to maintain body condition. Such parents should take more time away from incubation when the unattended clutch cools slowly, as this is when the cost of being absent is minimized. Copyright 1999 The Association for the Study of Animal Behaviour.

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PubMed1999

Patch choice and risk: relative competitive ability is context dependent.

The relative abilities of individual cichlids, Tilapia zillii to obtain food under scramble competition was highly repeatable between trials using a single input source, regardless of whether the input was constant or variable. However, when given a choice between two patches differing only in their temporal variability in input about an identical mean, an individual's rank based on intake in one patch was uncorrelated with either its intake in the other patch or its intake in the single-patch trials. In the two-patch trials, certain individuals both spent more time in food patches and visited patches more often than others, and overall the fish spent more time in the constant rate patch than the variable patch, leading to more items being consumed from the constant rate patch. We discuss possible causes and consequences of this dependence of relative competitive ability on the context of the foraging situation. Copyright 1999 The Association for the Study of Animal Behaviour.

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PubMed1998

Avian diving, respiratory physiology and the marginal value theorem.

Behavioural studies of diving birds have reported that the ratio of dive duration to the duration of the subsequent period on the surface displays a characteristic relation to dive duration. For short dives, the dive to surface ratio increases with dive duration, whereafter the relation peaks, and for longer dives decreases with increasing dive duration. Such a relationship is not a general prediction of existing marginal value models which have been used to predict optimal diving behaviour. This may be because the smooth curve used to describe the oxygen gain rate of individuals after surfacing is not a good reflection of the respiratory physiology of birds. Here we argue that on physiological grounds, the oxygen gain curve for avian divers will not be smooth, but will have two distinct regions (representing oxygen recovery in the respiratory tract, and in haemoglobin and myoglobin, respectively). Modifying two of the classical diving models by incorporating such a kinked curve causes them to predict the humped relationship between dive to surface ratio and dive duration under many circumstances. We also present data on the duration of dives and surface periods from three species of diving seabirds: the shag, Phalacrocorax aristotelis, the black guillemot, Cepphus grylle and the common guillemot, Uria aalge. All three species showed a humped relationship for dive to surface ratio as a function of dive duration. In line with the predictions of our model, when oxygen stores on surfacing were greatly depleted, the dive to surface ratio peaked at short dive durations. Copyright 1998 The Association for the Study of Animal Behaviour.

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PubMed1998

Temporal prey distribution affects the competitive ability of parasitized sticklebacks.

Parasitized animals are often reported to have a reduced competitive ability in experimental studies designed to examine foraging success under a specific type of competitive interaction; however, since animals compete under a range of competition regimes in natural situations, and because success is likely to require different foraging skills under each, it is unclear whether infected animals should be equally poor competitors under all competitive scenarios. We studied the foraging success of three-spined sticklebacks, Gasterosteus aculeatus, infected with plerocercoids of a cestode, Schistocephalus solidus, in competition with uninfected conspecifics. When pairs of differentially infected sticklebacks were presented with sequentially introduced items, the numbers of available prey taken by infected and uninfected competitors did not differ significantly, although nonparasitized fish were more successful at taking items over which there was direct competition. In contrast, when prey items were presented simultaneously in a locally dense patch, nonparasitized fish ingested significantly more of the available food than their infected counterparts: an apparent consequence of their greater ability to take items in rapid succession. Our results show that the type of competition conditions generated as a result of specific prey distribution patterns plays a role in determining the relative foraging success of parasitized sticklebacks, and suggest that this may have consequences for the distribution of different infection classes in natural, heterogeneous environments. (c) 1998 The Association for the Study of Animal Behaviour.

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PubMed1998

Population Floors and the Persistence of Chaos in Ecological Models.

Chaotic dynamics have been observed in a wide range of population models. Here we describe the effects of perturbing several of these models so as to introduce a non-zero minimum population size. This perturbation generally reduces the likelihood of observing chaos, in both discrete and continuous time models. The extent of this effect depends on whether chaos is generated through period-doubling, quasiperiodicity, or intermittence. Chaos reached via the quasiperiodic route is more robust against the perturbation than period-doubling chaos, whilst the inclusion of a population floor in a model exhibiting intermittent chaos may increase the frequency of population bursts although these become non-chaotic. Copyright 1998 Academic Press.

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PubMed1997

Mortality During Dispersal an the Stability of a Metapopulation

We use a coupled map lattice to investigate the dynamics of a system of populations linked by dispersal, when dispersal incurs an additional mortality cost. Considering a single isolated population first, we show analytically that imposing an additional mortality term can stabilise the non-trivial steady state only under certain conditions. We demonstrate algebraically that in the absence of mortality during dispersal, linking a number of similar populations does not affect whether or not the equilibrium will be stable, although it can affect the nature of any unstable dynamics. Adding a fixed mortality rate during dispersal has a strong stabilising effect on system dynamics. We show analytically that for any combination of intrinsic reproduction parameters, a range of dispersal rates and dispersal mortalities can be found which together act to stabilise the equilibrium. Our results are shown numerically to be robust against a number of perturbations. Hence dispersal mortality has a strong stabilising effect on dynamics. In natural systems, some losses during long-distance dispersal are likely, and so we suggest that this factor could be an important determinant of the strength of observed population fluctuations. Copyright 1997 Academic Press Limited

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