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Biomedical subjects

M Mangel

Publications and source records attributed to M Mangel.

16 recordsLinked to original sources
PubMed2001

Complex adaptive systems, aging and longevity.

Mortality and reproduction are intimately entwined in the study of aging and longevity. I apply the modern theory of complex adaptive systems (nonlinear, stochastic, dynamic methods) to questions of aging and longevity. I begin by highlighting major questions that must be answered in order to obtain a deeper understanding of aging. These are: (i) What should (in an evolutionary sense) mortality trajectories look like? (ii) Why does caloric restriction slow aging? (iii) Why does reproduction cause delayed mortality? (iv) Why does compensatory growth cause delayed mortality? I show how dynamic state variable models based on stochastic dynamic programming (Clark & Mangel, 2000) can be used to embed genetic theories of senescence (either mutation accumulation or antagonistic pleiotropy) in the somatic environment, as George Williams called for in 1957, and how they make the disposable soma theory of aging operational. Such models will allow unification of genetic and phenotypic theories of aging.

Adaptation, Physiological↗
PubMed2001

Age and longevity in fish, with consideration of the ferox trout.

In the first part of this paper, we review the evolutionary aspects of age and longevity in fish and then summarize the theory of maturity due to Ray Beverton. This theory allows one to predict age at maturity (and thus a putative point for the onset of senescence) from information on growth rate and mortality rate. We illustrate the application of this theory with data on tilapia species and then discuss the limitations of the theory. In the second part of the paper, we develop an individual based model for the ferox trout. This is a morph of brown trout Salmo salar that is an exception to the common notion that caloric restriction extends lifespan, in the sense that ferox trout achieve long life by eating more, not less. The model allows one to identify the role that ecological and biochemical adaptations play in the longevity of the ferox trout.

Age Factors↗
PubMed2000

Egg maturation, egg resorption and the costliness of transient egg limitation in insects.

Although there is widespread agreement that the cost of oviposition underlies selective oviposition in insects, there is no consensus regarding which factors mediate the cost of oviposition. Models have suggested that egg costs are often paramount in those insects that do not continue to mature eggs during the adult stage (pro-ovigenic insects). Here we address the hypothesis that egg costs are generally less significant in synovigenic insects, which can replenish oocyte supplies through continuous egg maturation. A dynamic optimization model based on the biology of a highly synovigenic parasitoid, Aphytis aonidiae, suggests that the maximum rate of egg maturation is insufficient to balance the depletion of eggs when opportunities to oviposit are abundant. Transient egg limitation therefore occurs, which imposes opportunity costs on reproducing females. Thus, whereas the most fundamental constraint acting on the lifetime reproductive success of pro-ovigenic species is the fixed total number of eggs that they carry at eclosion, the most fundamental constraint acting on a synovigenic species is the maximum rate of oocyte maturation. Furthermore, the ability of synovigenic species to reverse the flow of nutrients from the soma to oocytes (i.e. egg resorption) has a dramatic influence on the cost of oviposition. Whereas females in hostrich environments may experience oviposition-mediated egg limitation, females in host-poor environments may experience oosorption-mediated egg limitation. Both forms of egg limitation are costly. Contrary to initial expectations, the flexibility of resource allocation that typifies synovigenic reproduction actually appears to broaden the range of conditions under which costly egg limitation occurs. Egg costs appear to be fundamental in mediating the trade-off between current and future reproduction, and therefore are an important factor favouring selective insect oviposition.

Animals↗
PubMed1999

Messages from mortality: the evolution of death rates in the old.

Ageing is an increase in mortality and/or decline in fertility with advancing age. Evolutionary theories predict that ageing will evolve in response to the pattern of externally imposed hazards to survival and fertility; a prediction confirmed in new empirical studies. Recent studies of large cohorts of experimental animals and of humans have revealed that mortality rates do not continue to accelerate at very advanced ages. It has been suggested that evolutionary theories cannot account for these mortality patterns; however, this challenge is more apparent than real. Heterogeneity between individuals can shape mortality trajectories for populations, and recent evolutionary theory can both account for such heterogeneity and accommodate late-age mortality patterns.

Journal Article↗
PubMed1993

A simple direct method for finding persistence times of populations and application to conservation problems.

The computation of persistence times of populations has become a central focus in conservation biology. We describe a simple, direct method for finding the statistics of persistence times by assuming that there is a maximum population size. Thus, even though the population dynamics may be very complex for population sizes below the maximum, it is possible to write a finite set of equations from which the mean and second moment of the persistence time can be found by using simple, algebraic methods. We apply the method to compute the mean and coefficient of variation of persistence times of populations that suffer large decrements (catastrophes). Our results show that in the presence of catastrophes, the increase in mean persistence time with large populations is not nearly as rapid as other theories suggest and that catastrophes occurring at even modest rates can considerably increase the risk of extinction.

Journal Article↗
PubMed1990

Dynamic information in uncertain and changing worlds.

A general theory for information processing by organisms living in uncertain and changing worlds is developed. The three fundamental properties of the theory are: (i) the use of a memory parameter that allows the organism to forget the more distant past, (ii) a succinct representation of encounters and information and (iii) flexibility in the estimates of parameters by including the uncertainty in these estimates in a consistent manner. The theory is developed using Bayesian methods (but can also be applied to maximum likelihood estimation) and is applied to the encounter models standardly used in ecology (Poisson, binomial, and negative binomial). Two applications are discussed: (i) patch selection and the matching rule and (ii) superparasitism by a parasitoid.

Animals↗
PubMed1990

Evolutionary optimization and neural network models of behavior.

One of the main challenges to the adaptionist program in general and the use of optimization models in behavioral and evolutionary ecology, in particular, is that organisms are so constrained by ontogeny and phylogeny that they may not be able to attain optimal solutions, however those are defined. This paper responds to the challenge through the comparison of optimality and neural network models for the behavior of an individual polychaete worm. The evolutionary optimization model is used to compute behaviors (movement in and out of a tube) that maximize a measure of Darwinian fitness based on individual survival and reproduction. The neural network involves motor, sensory, energetic reserve and clock neuronal groups. Ontogeny of the neural network is the change of connections of a single individual in response to its experiences in the environment. Evolution of the neural network is the natural selection of initial values of connections between groups and learning rules for changing connections. Taken together, these can be viewed as "design parameters". The best neural networks have fitnesses between 85% and 99% of the fitness of the evolutionary optimization model. More complicated models for polychaete worms are discussed. Formulation of a neural network model for host acceptance decisions by tephritid fruit flies leads to predictions about the neurobiology of the flies. The general conclusion is that neural networks appear to be sufficiently rich and plastic that even weak evolution of design parameters may be sufficient for organisms to achieve behaviors that give fitnesses close to the evolutionary optimal fitness, particularly if the behaviors are relatively simple.

Animals↗
PubMed1990

A dynamic habitat selection game.

A patch selection game is formulated and analyzed. Organisms can forage in one of H patches. Each patch is characterized by the cost of foraging, the density and value of food, the predation risk, and the density of conspecifics. The presence of conspecifics affects the finding and sharing of food, and the predation risk. Optimal foraging theory can be viewed as a "1-person" game against nature in which the optimal patch choice of a specific organism is analyzed assuming that the number of conspecifics in other patches is fixed. In the general game theoretic approach, the behavior of conspecifics is included in the determination of the distinguished organism's strategy. An iterative algorithm is used to compute the solution of the "n-person" game or dynamic ESS, which differs from the optimal foraging theory solution. Experiments to test the proposed theory using rodents and seed trays are briefly discussed.

Animals↗
PubMed1986

Solution of functional difference equations from behavioral theory.

Behavioral models based on Markovian decision processes lead to functional difference equations for quantities such as the mean lifetime of the forager and the probability of reproductive success of the forager. In this paper, asymptotic and iterative methods are developed for the solution of such equations. The asymptotic methods are compared with numerical simulations. The iterative methods can be proved by a simple application of contraction mapping theorems.

Animals↗
PubMed1976

Relationship of the photosensitivity of bilayer lipid membranes and the aqueous acceptor. Studies using complex ions of amino acids.

The photocurrent in photosensitive bilayer lipid membranes has been studied as a function of the aqueous acceptor. Correlations are observed between the relative photocurrent and the position of the complex ion visible absorption band and the dipole moment of the ligand. The effect of the ligands is nondirectional: they may be added to either side of the membrane with a corresponding effect on the photocurrent. The effects of the ligands are interpreted using an energy barrier model.

Amino Acids↗
PubMed1975

Dependence of photosensitivity of bileaflet lipid membranes upon the chlorophyll and carotenoid content.

Bileaflet lipid membranes were formed from solutions containing lecithin, chlorophyll and carotene in various concentrations. If all the above components were present at sufficient concentrations the membranes were photosensitive; i.e., a photocurrent was produced if a redox potential gradient was present across the membranes. The presence of chlorophyll and carotene were essential for the photosensitivity of the membranes. Photoresponse could be elicited by illuminating the membrane with light which did not excite carotene. On the other hand, elimination of the part of the light spectrum which excites chlorophyll led to the abolition of the photoresponse. The findings of this study are consistent with the assumption that the excited chlorophyll chromophores allow electron exchange at the membrane-water interface while the presence of carotene allows electron movement across the "bulk" lipid membrane.

Binding Sites↗