Search PubMed⌕ Search

Biomedical subjects

R M Sibly

Publications and source records attributed to R M Sibly.

11 recordsLinked to original sources
PubMed2001

Effects of stone chewing by outdoor sows on their teeth and stomachs.

Stone chewing is a common behaviour in outdoor sows, but its effects on their teeth and stomachs have not been investigated. Tooth wear and damage was assessed by examining the heads of 58 sows culled from outdoor units and 23 culled from indoor units. Tooth damage was found in 28 per cent of outdoor and 30 per cent of the indoor sows, and tooth wear affected 88 per cent of the outdoor and 91 per cent of the indoor sows. The outdoor sows were more prone to wear on the lower molars and premolars, a pattern of wear associated with stone chewing. The stomachs of 152 outdoor sows and 47 indoor sows were examined. Stones were found in 59 of the outdoor sows but in none of those kept indoors. There was no evidence that the presence of stones damaged the stomach, or that stone chewing affected the health of the sows. The teeth of the outdoor sows were worn, but probably not sufficiently to affect their ability to eat during their relatively short productive lives.

Animals↗
PubMed2001

A maximum-likelihood approach to fitting equilibrium models of microsatellite evolution.

Here, we develop a new approach to Markov chain modeling of microsatellite evolution through polymerase slippage and introduce new models: a "constant-slippage-rate" model, in which there is no dependence of slippage rate on microsatellite length, as envisaged by Moran; and a "linear-with-constant" model, in which slippage rate increases linearly with microsatellite length, but the line of best fit is not constrained to go through the origin. We show how these and a linear no-constant model can be fitted to data hierarchically using maximum likelihood. This has advantages over previous methods in allowing statistical comparisons between models. When applied to a previously analyzed data set, the method allowed us to statistically establish that slippage rate increases with microsatellite length for dinucleotide microsatellites in humans, mice, and fruit flies, and suggested that no slippage occurs in very short microsatellites of one to four repeats. The suggestion that slippage rates are zero or close to zero for very short microsatellites of one to four repeats has important implications for understanding the mechanism of polymerase slippage.

Animals↗
PubMed2001

Are current species extrapolation models a good basis for ecological risk assessment?

Extrapolating the effects of toxicants with either the fixed application factor approach or one of the species sensitivity distribution models currently in widespread use presumes that toxicant effects on single, individual-level endpoints (e.g., survival, fecundity, and growth) reflect effects at the population level. Here, we consider if extrapolations derived on the basis of individual-level endpoints might be misleading with regard to risk assessment and, hence, risk management decisions for ecosystems. Both analytically and by simulation, we demonstrate that for populations with multiplication rates close to one, effects of toxicants at the population level likely are less than or equal to effects on individual life-cycle traits, suggesting that risk assessments based on the latter likely are protective of population-level impacts. We used Monte Carlo simulations to explore how both the frequency of different life-cycle types in a community as well as their relative sensitivity to toxicants influence the toxicant sensitivity distribution of the community as a whole. We compared the output of our simulations with predicted no-effect concentrations derived by an application factor approach and a log-normal distribution-based model, using no-observed-effect concentrations for juvenile survival as input variables in both cases. Our analyses suggest that current extrapolation approaches appear to be protective, and may often be very overprotective, but we have identified conditions in which this may not be the case. We recommend that additional consideration be given to the relative frequency of different life-cycle types, to the proportions of sensitive and insensitive taxonomic groups in communities, and to the role of density-dependent influences on population dynamics to ensure that we develop environmental quality criteria that are sufficiently--but not overly--protective.

Ecology↗
PubMed1996

Estimation of the weight and body condition of ostriches (Struthio camelus) from body measurements.

The body dimensions and weights of over 100 ostriches were analysed to investigate which body measurements provided the best estimators of bodyweight. The data were divided between growing and adult birds (below and above two years old) for regression analyses. The residual standard deviation of weight was 0.117 for growing birds, using tibiotarsal length and abdominal girth as predictors. The analogous figure for adults was 0.078 using back length and abdominal girth. An assessment of body condition was made by calculating the average weight-for-size of a bird from skeletal measurements which are unaffected by gross changes in weight, and then comparing it with its estimated or actual weight. These estimates of the weight of a typical ostrich should be useful in veterinary practice, particularly when the weight of a bird is required for the administration of medication.

Animals↗
PubMed1993

An allelocentric view of life-history evolution.

For the case of weak selection, random assortment of gametes, and density-independent population regulation, we here establish the conditions under which an allele will spread in a population, with particular reference to the life-history effects of the allele, its level of dominance, and sex differences in its effects. Our treatment is simpler than that of Charlesworth (1980), but the results are essentially the same. We show that two quantities govern the selective dynamics of a two-allele single-locus system; these are level of dominance, and the difference between the per copy rates of increase of the alleles in homozygous populations. Our main conclusion is that the eventual outcome of evolution is unaffected by an allele's level of dominance, or sex differences in its effects, provided there is no overdominance. However, speed of progress to fixation is, of course, affected by these factors, and equations are derived to show how level of dominance affects speed of progress to fixation. When a dominant allele only affects the life history of one sex, its rate of spread is half that if both sexes are affected. The relationship between actual and intrinsic rates of increase is discussed and formulae are given showing the relationship for the case of weak selection.

Aging↗
PubMed1991

Testing life-cycle theory by computer simulation--I. Introduction of genetical structure.

Computer simulation was used to relax assumptions of analytical life-cycle theory about the eventual outcome of evolution in a constant environment. The computer simulation models, of diploid one-locus genetic systems, are described in detail. Good agreement was obtained between the analytical and simulation outcomes, except in some cases of discrepancy between the male and female life cycles.

Animals↗
PubMed1991

Testing life-cycle theory by computer simulation--II. Bet-hedging revisited.

Analytical and computer models were used to reexamine bet-hedging, the reduction in fecundity that is evolutionarily advantageous in conditions of greater variation in juvenile survivorship or less variation in adult survivorship. The computer simulation models represent diploid one-locus genetic systems with semidominance. Schaffer's (1974) predictions proved remarkably robust when variations were symmetrical, and a simple modification allowed successful prediction for the asymmetries that occurred in the computer simulations when variations were large.

Animals↗
PubMed1987

Gas diffusion through non-tubular pores.

An equation is derived for quantifying the diffusion of gases through the funnel-shaped pores of eggshells. Calculations based on this approach indicate that diffusion in such pores is largely determined by the inner pore radius and that the thickness of the eggshell and its cuticle are relatively unimportant. The approach is extended to include other noncylindrical pores.

Animals↗
PubMed1975

The behavioural final common path.

In this paper it is argued that any model of the motivational (i.e. reversible) processes governing the behaviour of an animal can be represented by means of isoclines in a multidimensional 'causal-factor space'. The argument is axiomatic, based upon the two prime assumptions: that (1) it is always possible to classify the behavioural repertoire of a species in such a way that the classes are mutually exclusive in the sense that the members of different classes cannot occur simultaneously, and (2) these incompatible actions are uniquely determined by a particular set of causal factors. The isoclines join all points in the space which present a given 'degree of competitiveness' of a particular 'candidate' for overt behavioural expression. The competition between candidates is an inevitable consequence of the fact that animals cannot 'do more than one thing at a time', and is envisaged as taking place in the behavioural final common path. An empirical method of determining the motivational state (i.e. point in causal-factor space) is outlined. This is a 'relative' method, independent of the arbitrary calibration of the axes of the causal-factor space. It is shown that an arbitrary scale of measurement along any two axes of the causal-factor space is all that is necessary for empirical determination of the shape of a motivational isocline. Experiments in which this method has been applied to the measurement of hunger and thirst in doves are outlined, and the results are discussed in terms of their implications for motivation theory in general.

Animals↗