Biomedical subjects

Iain J Gordon

Publications and source records attributed to Iain J Gordon.

10 recordsLinked to original sources

PubMed2007

Associations between basal metabolic rate and reproductive performance in C57BL/6J mice.

Basal metabolic rate (BMR) is highly variable, both between and within species. One hypothesis is that this variation may be linked to the capacity for sustained rate of energy expenditure, leading to associations between high BMR and performance during energy-demanding periods of life history, such as reproduction. However, despite the attractive nature of this hypothesis, previous studies have failed to show an association between BMR and fecundity. Our approach was to mate 304 C57BL/6J mice and allow them to wean pups before measuring BMR by indirect calorimetry. We did not find an association between BMR and litter mass, size or pup mass at birth or weaning that could not be accounted for by the body mass of the dam. There was also no relationship between BMR (or BMR corrected for body mass) and birth or weaning success, losses during weaning, or sex ratio. However, a significant relationship was found between BMR and gestational weight loss indicative of foetal resorption. This suggests that during pregnancy the available energy may be limited and partitioned away from the growing foetus and towards maintenance of the mother. In this context, a high BMR may actually be disadvantageous, conflicting with the idea that high BMR may bring reproductive benefits.

Animals↗

PubMed2006

A theory of associating food types with their postingestive consequences.

Animals often face complex and changing food environments. While such environments are challenging, an animal should make an association between a food type and its properties (such as the presence of a nutrient or toxin). We use information theory concepts, such as mutual information, to establish a theory for the development of these associations. In this theory, associations are assumed to maximize the mutual information between foods and their consequences. We show that associations are invariably imperfect. An association's accuracy increases with the length of a feeding session and the relative frequency of a food type but decreases as time delay between consumption and postingestive consequence increases. Surprisingly, the accuracy of an association is independent of the number of additional food types in the environment. The rate of information transfer between novel foods and a forager depends on the forager's diet. In light of this theory, an animal's diet may have two competing goals: first, the provision of an appropriate balance of nutrients, and second, the ability to quickly and accurately learn the properties of novel foods. We discuss the ecological and behavioral implications of making associational errors and contrast the timescale and mechanisms of our theory with those of existing theory.

Animal Nutritional Physiological Phenomena↗

PubMed2006

Spatial and temporal variability modify density dependence in populations of large herbivores.

A central challenge in ecology is to understand the interplay of internal and external controls on the growth of populations. We examined the effects of temporal variation in weather and spatial variation in vegetation on the strength of density dependence in populations of large herbivores. We fit three subsets of the model ln(Nt) = a + (1 + b) x ln(N(t-1)) + c x ln(N(t-2)) to five time series of estimates (Nt) of abundance of ungulates in the Rocky Mountains, USA. The strength of density dependence was estimated by the magnitude of the coefficient b. We regressed the estimates of b on indices of temporal heterogeneity in weather and spatial heterogeneity in resources. The 95% posterior intervals of the slopes of these regressions showed that temporal heterogeneity strengthened density-dependent feedbacks to population growth, whereas spatial heterogeneity weakened them. This finding offers the first empirical evidence that density dependence responds in different ways to spatial heterogeneity and temporal heterogeneity.

Animals↗

PubMed2006

Pastoralists' responses to variation of rangeland resources in time and space.

We explore the response of pastoralists to rangeland resource variation in time and space, focusing on regions where high variation makes it unlikely that an economically viable herd can be maintained on a single management unit. In such regions, the need to move stock to find forage in at least some years has led to the evolution of nomadism and transhumance, and reciprocal grazing agreements among the holders of common-property rangeland. The role of such informal institutions in buffering resource variation is well documented in some Asian and African rangelands, but in societies with formally established private-property regimes, where we focus, such institutions have received little attention. We examine agistment networks, which play an important role in buffering resource variation in modern-day Australia. Agistment is a commercial arrangement between pastoralists who have less forage than they believe they require and pastoralists who believe they have more. Agistment facilitates the movement of livestock via a network based largely on trust. We are concerned exclusively with the link between the characteristics of biophysical variation and human aspects of agistment networks, and we developed a model to test the hypothesis that such a link could exist. Our model builds on game theory literature, which explains cooperation between strangers based on the ability of players to learn whom they can trust. Our game is played on a highly stylized landscape that allows us to control and isolate the degree of spatial variation and spatial covariation. We found that agistment networks are more effective where spatial variation in resource availability is high, and generally more effective when spatial covariation is low. Policy design that seeks to work with existing social networks in rangelands has potential, but this potential varies depending on localized characteristics of the biophysical variability.

Animal Husbandry↗

PubMed2005

Gregariousness increases brain size in ungulates.

The brain's main function is to organise the physiological and behavioural responses to environmental and social challenges in order to keep the organism alive. Here, we studied the effects that gregariousness (as a measurement of sociality), dietary habits, gestation length and sex have on brain size of extant ungulates. The analysis controlled for the effects of phylogeny and for random variability implicit in the data set. We tested the following groups of hypotheses: (1) Social brain hypothesis-gregarious species are more likely to have larger brains than non-gregarious species because the former are subjected to demanding and complex social interactions; (2) Ecological hypothesis-dietary habits impose challenging cognitive tasks associated with finding and manipulating food (foraging strategy); (3) Developmental hypotheses (a) energy strategy: selection for larger brains operates, primarily, on maternal metabolic turnover (i.e. gestation length) in relation to food quality because the majority of the brain's growth takes place in utero, and finally (b) sex hypothesis: females are expected to have larger brains than males, relative to body size, because of the differential growth rates of the soma and brain between the sexes. We found that, after adjusting for body mass, gregariousness and gestation length explained most of the variation in brain mass across the ungulate species studied. Larger species had larger brains; gregarious species and those with longer gestation lengths, relative to body mass, had larger brains than non-gregarious species and those with shorter gestation lengths. The effect of diet was negligible and subrogated by gestation length, and sex had no significant effect on brain size. The ultimate cause that could have triggered the co-evolution between gestation length and brain size remains unclear.

Animals↗

PubMed2005

A life history model of somatic damage associated with resource acquisition: damage protection or prevention?

A resource acquisition-allocation model is developed to examine the trade-off between reproduction and somatic protection. Unlike previous studies, resource intake is not assumed to be constrained: instead, resource intake is free to vary, with increased intake being associated with an increased risk of somatic damage. This gives rise to an optimal resource intake as well as an optimal allocation strategy. This paper studies the relative importance of acquisition and allocation strategies in regulating acquisition-related mortality. Under the optimal allocation strategy mortality rate increases with age, in accordance with the disposable soma theory of aging. Contrary to the usual interpretation of the disposable soma theory, this increase in mortality can arise from an increase in the resource acquisition effort rather than a decrease in the resources allocated to protection. At early ages resource acquisition is found to be the primary path for regulating life history costs, whilst allocating resources to protection becomes more important later in life. Models for targeted and non-targeted damage repair are considered and the robustness of our results to the structure and parameterization of the model is discussed. The results from our models are discussed in light of published data. Resource acquisition is shown to be a potentially important mechanism for controlling somatic damage which deserves further study.

Aging↗

PubMed2004

Bush selection along foraging pathways by sympatric impala and greater kudu.

In order to identify the selection mechanism of two sympatric African browsers, we analysed encounter rates and selection of bushes along foraging pathways. We monitored the tracks, left overnight, by kudu and impala on an experimental plot of natural Acacia nilotica and Dichrostachys cinerea in the highveld of Zimbabwe, and recorded the number of bushes attacked in each category. Both ungulates were selective for the bush categories, but kudu were consistently more selective than impala, and showed a higher preference for the larger A. nilotica and D. cinerea bushes, which had a significantly greater number of bites which were not reachable by impala. For both kudu and impala, the probability of attacking larger bushes increased significantly with the proportion of large bushes encountered along the foraging pathways, whereas the consumption of smaller bushes was apparently unpredictable. For the most abundant food item (medium D. cinerea), the probability of attack by impala along a pathway decreased with increasing proportions of larger bushes in the experimental area, but was also dependent on impala group size and season. In addition, we found that encounter rates with larger bushes were significantly higher for kudu than for impala. Experimentally reducing the availability of the larger bushes had little effect on both impala and kudu during the following rainy season. However, during the following cool dry season, kudu showed an increased selectivity with a strong preference for the remaining large bushes (large A. nilotica), followed by a sharp decrease in selectivity in the hot dry season when they also fed from significant numbers of medium trees. Impala had little reaction to the experimental changes in the availability of bush categories in either season. We suggest that both kudu and impala selected bushes on the basis of the potential number of bites they can provide, and this resulted in different search strategies. Kudu focussed on the larger bushes which have a larger number of twigs which are out of reach of impala and kudu also probably directed their path preferentially towards the few larger bushes to maximize encounter rates with this favoured bush category. These differences in bush selection process lead to a low overlap in resource use between the two browsers in this type of savanna.

Animals↗

PubMed2003

African wild dogs test the 'survival of the fittest' paradigm.

Charles Darwin first used the term 'survival of the fittest' in the 5th edition of The origin of species. A literal interpretation implies that predators will selectively prey upon the weakest members of a population. We demonstrate that this is true for African wild dogs hunting impala.

Africa↗

PubMed2003

Can animals use foraging behaviour to combat parasites?

Host-parasite interactions are often seen as an arms race, with parasites attempting to overcome host resistance to infection. Herbivory is a common route of transmission of parasites that represents the most pervasive challenge to mammalian growth and reproduction. The present paper reviews the foraging skills of mammalian herbivores in relation to their ability to exploit plant properties to combat parasites. The starting point is that foraging behaviour may ameliorate the impact of parasitism in three ways; hosts could: (1) avoid foraging in areas contaminated with parasites; (2) select diets which increase their resistance to parasites; (3) select for foods containing anti-parasitic properties (self-medication). Details are given of the pre-requisite skills needed by herbivores if they are to combat parasitism via behaviour, i.e. herbivores are able to: (a) determine their parasitic state and alter their behaviour in relation to that state (behaviours 1, 2 and 3); (b) determine the environmental distribution of parasites (behaviour 1); (c) distinguish plant species or plant parts that increase their resistance to parasites (behaviour 2) or have anti-parasitic properties (behaviour 3). Mammalian herbivores cannot detect the presence of the parasites themselves and must rely on cues such as faeces. Despite the use of these cues contacting parasites may be inevitable and so mechanisms to combat parasitism are necessary. Mammalian herbivores have the foraging skills needed to exploit the heterogeneous distributions of nutrients and parasites in complex foraging environments in order to avoid, and increase their resistance to, parasites. Current evidence for the use of plant secondary metabolites (PSM) by herbivores for self-medication purposes remains equivocal. PSM have both positive (anti-parasitic) and negative (toxic) effects on herbivores. Here details are given of an experimental approach using tri-trophic (plant-herbivore-parasite) interactions that could be used to demonstrate self-medication in animals. There is strong evidence suggesting that herbivore hosts have developed the foraging skills needed to take advantage of plant properties to combat parasites and thus use behaviour as a weapon in the host-parasite arms race.

Animal Feed↗

PubMed2002

A lifetime perspective on foraging and mortality.

Food intake carries many potential risks which may impair an animal's reproductive success not only in the current breeding cycle, but also for the rest of its lifetime. We examine the lifetime trade-off between the costs and benefits of food intake by presenting a simple animal foraging model, where each unit of food eaten carries with it a risk of mortality. We show that the optimal food intake rate over an animal's lifetime, for both semelparous and iteroparous animals, is not maximal. Instead, animals are required to strike a balance between the immediate reproductive benefits of gathering food and the future reproductive costs incurred by the food's mortality risk. This balance depends upon the lifespan of the animal as well as the nature of the risk. Different mortality risks are compared and it is shown that a mortality risk per unit time spent foraging is not, in general, equivalent to a mortality risk per unit of food consumed. The results suggest that a mortality risk per unit of food consumed, such as that presented by the presence of a toxin or of a parasite in the diet, has important consequences for feeding behaviour and is a possible factor involved in food intake regulation.

Animals↗