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M Enquist

Publications and source records attributed to M Enquist.

18 recordsLinked to original sources
PubMed2000

Step-parents and infanticide: new data contradict evolutionary predictions.

Evolutionary psychologists have hypothesized, inspired by evolutionary biology, that parents should care less for children with whom they are not genetically related since these young do not contribute to the genetic fitness of the parents. Based on this, evolutionary psychologists have predicted that there will be an overrepresentation of step-parents as offenders in family-related killings of children. Data on child homicide, particularly from Canada, have supported this prediction in that the frequency of children killed was relatively high in families where one of the two parents was a step-parent. Here we present a survey of all child homicide that occurred in Sweden between 1975 and 1995. In contrast to the Canadian data, children in Sweden living in families with a step-parent were not at an increased risk compared with children living together with two parents to whom they were genetically related. In addition, there were no other indications that step-parents are overrepresented as offenders.

Adolescent↗
PubMed1999

The geometry of stimulus control.

Many studies, both in ethology and comparative psychology, have shown that animals react to modifications of familiar stimuli. This phenomenon is often referred to as generalization. The majority of modifications lead to a decrease in responding, but to certain new stimuli an increase in responding is observed. This holds for both innate and learned behaviour. Here we propose a heuristic approach to stimulus control, or stimulus selection, with the aim of explaining these phenomena. The model has two key elements. First, we choose the receptor level as the fundamental stimulus space. Each stimulus is represented as the pattern of activation it induces in sense organs. Second, in this space we introduce a simple measure of 'similarity' between stimuli by calculating how activation patterns overlap. The main advantage in this approach is that the generalization of acquired responses emerges from a few simple principles that are grounded in the recognition of how animals actually perceive stimuli. Many traditional problems that face theories of stimulus control (e.g. the Spence-Hull theory of gradient interaction or ethological theories of stimulus summation) do not arise in the present framework. These problems include the amount of generalization along different dimensions, peak shift phenomena (with respect to both positive and negative shifts), intensity generalization and generalization after conditioning on two positive stimuli. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1998

Instability of signaling resolution models of parent-offspring conflict.

Recent signaling resolution models of parent-offspring conflict have provided an important framework for theoretical and empirical studies of communication and parental care. According to these models, signaling of need is stabilized by its cost. However, our computer simulations of the evolutionary dynamics of chick begging and parental investment show that in Godfray's model the signaling equilibrium is evolutionarily unstable: populations that start at the signaling equilibrium quickly depart from it. Furthermore, the signaling and nonsignaling equilibria are linked by a continuum of equilibria where chicks above a certain condition do not signal and we show that, contrary to intuition, fitness increases monotonically as the proportion of young that signal decreases. This result forces us to reconsider much of the current literature on signaling of need and highlights the need to investigate the evolutionary stability of signaling equilibria based on the handicap principle.

Animals↗
PubMed1998

The logic of Ménage à Trois.

Recent studies of socially monogamous species have shown that in many cases females do not copulate exclusively with their pair mates, but are also receptive to other males. The explanation usually given for unfaithful female behavior is that most females are unable to bond with a male they would prefer as genetic father to their offspring. To secure male assistance the female therefore pairs with an available male but also copulates with males of supposedly higher genetic quality. Here we offer an alternative evolutionary explanation for female infidelity, which does not rely upon this 'Good Genes hypothesis of female choice. We show with a simple model that in an evolutionary game between three players, a male, a female and a male lover, solutions exist in which the female can secure more assistance from her mate by being receptive to other males. We conclude that female sexuality can have a decisive role in regulating social behaviour, in which the fertile female is the driving force.

Animals↗
PubMed1998

Artificial neural networks as models of stimulus control.

We evaluate the ability of artificial neural network models (multilayer perceptrons) to predict stimulus-response relationships. A variety of empirical results are considered, such as generalization, peak shift (supernormality) and stimulus intensity effects. The networks were trained on the same tasks as the animals in the experiments considered. The subsequent generalization tests on the networks showed that the model replicates correctly the empirical results. We conclude that these models are valuable tools in the study of animal behaviour. (c) 1998 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1995

Conflict, receiver bias and the evolution of signal form.

In a model, conflicts of interest between communicating individuals are shown to have an important influence on the cost and form of signals that evolve. Two types of conflict are considered: competition between senders to obtain a response from the receiver, and conflict between the sender and the receiver. The receiver system is modelled as an artificial neural network whose 'resistance' to signals is represented as a motivational factor that varies independently of the signal. Biases in the receiver system act as the selective force on signals, causing them to become more costly and conspicuous as the intensity of conflict increases. There is some evidence that competition between senders and sender-receiver conflict may have qualitatively different outcomes. We give examples of some situations to which the model might be applied and point out some predictions that could be tested empirically.

Animal Communication↗
PubMed1994

Symmetry, beauty and evolution.

Humans and certain other species find symmetrical patterns more attractive than asymmetrical ones. These preferences may appear in response to biological signals, or in situations where there is no obvious signalling context, such as exploratory behaviour and human aesthetic response to pattern. It has been proposed that preferences for symmetry have evolved in animals because the degree of symmetry in signals indicates the signaller's quality. By contrast, we show here that symmetry preferences may arise as a by-product of the need to recognize objects irrespective of their position and orientation in the visual field. The existence of sensory biases for symmetry may have been exploited independently by natural selection acting on biological signals and by human artistic innovation. This may account for the observed convergence on symmetrical forms in nature and decorative art.

Animals↗
PubMed1993

Selection of exaggerated male traits by female aesthetic senses.

Darwin suggested that many apparently deleterious secondary sexual characters in males, such as bright colours, elaborate ornaments and conspicuous displays, evolved as a result of female choice. Darwin never tried to explain the crucial agent of selection, that females have preferences for exaggerated male traits. Rather, he took it for granted that females of many species possess a 'sense of the beautiful', akin to the aesthetic sense in humans. The question of why such preferences evolve remains a controversial issue. Here we report that mechanisms concerned with signal recognition possess inevitable biases in response that act as important agents of selection on signal form. The existence of such biases may be sufficient to explain the evolution of exaggerated male secondary sexual traits, and elaborate signals in general.

Animals↗
PubMed1991

Enantioselective steady-state kinetics of unbound disopyramide and its dealkylated metabolite in man.

Disopyramide is provided as a racemic mixture of R and S enantiomers, which have different pharmacodynamic and pharmacokinetic characteristics. Five volunteers were given racemic disopyramide 100 mg and 200 mg t.d.s. in a cross-over design. Plasma and urine concentrations of disopyramide and its active metabolite monodesisopropyl-disopyramide (MND) were determined at steady state by an enantioselective HPLC method. Unbound drug in plasma was measured after ultrafiltration. There was enantioselective clearance of unbound disopyramide (0.39 l.h-1.kg-1 for R-disopyramide and 0.58 l.h-1.kg-1 for S-disopyramide after 100 mg t.d.s.). The enantioselectivity was due to differences in the metabolism of disopyramide to MND and in further non-renal clearance, and the renal clearance of disopyramide was not enantioselective. The in vivo protein binding of disopyramide, which was saturable for both enantiomers, was also enantioselective. The difference in binding of the two enantiomers was explained by a difference in apparent binding capacity rather than in apparent binding affinity. The renal clearance of S-MND was significantly higher than R-MND (0.29 and 0.19 l.h-1.kg-1, respectively, after 100 mg t.d.s.). The renal clearance of MND also showed a tendency to saturation at higher concentrations.

Adult↗
PubMed1990

Separation of the enantiomers of beta-receptor blocking agents and other cationic drugs using a CHIRAL-AGP column. Binding properties and characterization of immobilized alpha 1-acid glycoprotein.

The effect of the immobilization procedure on the conformation of alpha 1-acid glycoprotein (AGP) was investigated by recording the fluorescence spectra of native and immobilized AGP. A 20-nm red shift was obtained for the immobilized form of AGP compared with the emission maximum of 338 nm obtained for native AGP. This demonstrates that the tryptophan residues are exposed on the protein surface after immobilization, indicating that the immobilized form of AGP has a more unfolded structure than the native AGP. The effect of N,N-dimethyloctylamine on the enantioselectivity for some fentiazine derivatives, observed with immobilized AGP, was equal to that obtained with AGP as a chiral complexing agent in the mobile phase. This demonstrates that even though the immobilization procedure affects the conformation of the protein there still exist large similarities between native and immobilized AGP concerning chiral recognition. The adsorption isotherm of (-)-terodiline was studied by use of the breakthrough technique. The adsorption isotherm indicates that (-)-terodiline is adsorbed to one site with high affinity and at least one more site with lower affinity. It was also observed that the enantiomers of amines, acids and non-protolytic compounds compete with the cationic compound, (-)-terodiline, for binding to the same sites. The beta-receptor blocking agents atenolol, metoprolol, pindolol, alprenolol, oxprenolol and propranolol were resolved on a CHIRAL-AGP column. The retention and enantioselectivity are highly influenced by the structure of the solute and the nature of the uncharged mobile phase additives. Separation factors of 1.2-1.8 were obtained for the beta-blockers under the studied conditions.

Adrenergic beta-Antagonists↗
PubMed1990

Is the behavioural effect of diazepam in rats unique to negative secondary stimuli?

Studies of the behavioural effects of benzodiazepines have focused in particular on situations with negative secondary stimuli, i.e., stimuli signalling negative primary events such as punishment or non-reward. The general result is that benzodiazepines attenuate behavioural reactions to this type of stimuli. The aim of the present study was to investigate if there are any differences between positive and negative secondary stimuli in this respect. Rats were treated with diazepam in a modified Skinner box with two levers. One of the levers always gave a small reward. A lamp being ON or OFF was used as a secondary stimulus indicating if the other lever would give a large or no reward. Pretreatment with diazepam (1.0 mg/kg) did not act differently on the response to the positive or negative secondary stimulus. The main effect was a general attenuation of optimal responding with a concomitant decrease in water intake. An alternative hypothesis stating that benzodiazepines alter the reactivity to secondary stimuli more in general is supported.

Animals↗
PubMed1990

Effects of dopamine D-1 and D-2 antagonists on decision making by rats: no reversal of neuroleptic-induced attenuation by scopolamine.

The effects of the dopamine (DA) D-1 antagonist SCH 23390 and the D-2 antagonists haloperidol and sulpiride on decision making in thirsty rats were investigated. The problem given to the rats was to choose between two behaviours, locomoting or lever-pressing, in order to obtain rewards (water). SCH23390 and haloperidol dose-dependently reduced the number of rewards obtained. However, only minor effects were seen on the decision making parameters. DA is thus important for initiation and/or performance of learned behavioural acts, not for information sampling and processing leading to the decision of which behaviour to perform. There were no differences between D-1 and D-2 antagonism in this respect. After sulpiride, no significant effects were observed. A specific behavioural prolife obtained by extinction was not mimicked by any of the doses of neuroleptics tested. It has previously been shown that performance per se of the two behavioural options used in the task can be reinstated by scopolamine after haloperidol attenuation. However, when these options had to be organized into functional sequences to reach a goal, haloperidol-induced attenuation could not be counteracted by scopolamine.

Animals↗
PubMed1989

Comparison between two methods for the determination of the total and free (R)- and (S)-disopyramide in plasma using an alpha 1-acid glycoprotein column.

Two different high-performance liquid chromatographic systems for the determination of the total and free (R)- and (S)-disopyramide (DP) in plasma and urine were compared. In method I a Nucleosil C8 column was coupled in series with an alpha 1-acid glycoprotein column. Method II consisted of two systems; a LiChrosorb Si 60 column was used for the determination of the racemic drug concentration and the R/S ratio was determined on an alpha 1-acid glycoprotein column. The recovery of (R)- and (S)-DP from plasma was greater than 97% in both methods. The precisions of the (R)- and (S)-DP determinations in plasma are high with both methods. The relative standard deviations for the determination of the free concentration do not exceed 6.5% at 1.59 micrograms/ml racemic DP. Method II is preferred as it can also be used to determine the concentration of (R)- and (S)-monodesisopropyramide. It is also easier to avoid disturbances from endogenous compounds in plasma samples with method II than with method I. It was observed that DP was incorporated into urine sediment during storage. A simple ultrasonic treatment of the urine samples was demonstrated to release DP from the sediment.

Chromatography, High Pressure Liquid↗
PubMed1989

Separation and quantitation of (R)- and (S)-atenolol in human plasma and urine using an alpha 1-AGP column.

A method for the determination of (R)- and (S)-atenolol in human plasma and urine is described. The enantiomers of atenolol are extracted into dichloromethane containing 3% heptafluorobutanol followed by acetylation with acetic anhydride at 60 degrees C for 2 h. The acetylated enantiomers were separated on a chiral alpha 1-AGP column. Quantitation was performed using fluorescence detection. A phosphate buffer pH 7.1 (0.01 M phosphate) containing 0.25% (v/v) acetonitrile was used as mobile phase. The described procedure allows the detection of less than 6 ng of each enantiomer in 1 ml plasma. The relative standard deviation is 4.4% at 30 ng/ml of each enantiomer in plasma. The plasma concentration of (R)- and (S)-atenolol did not differ significantly in two subjects who received a single tablet of racemic atenolol. The R/S ratio of atenolol in urine was approximately 1.

Atenolol↗
PubMed1987

Impairment of decision making in rats by diazepam: implications for the "anticonflict" effects of benzodiazepines.

Benzodiazepines, used in the clinic as anxiolytics, have in animal models been found specifically to attenuate behavioural suppression caused by response contigent aversive stimuli, non-reward or novelty. The effects have been interpreted in more general terms as "behavioural disinhibition" or "response perseveration" or in more specific terms as reduced "reward delay" or as an attenuation of a "behavioural inhibition system". In a recent publication we have described an experimental test in which decision making in the rat can be studied. The model is derived from ethology, in particular from optimal foraging theory. In order to solve the task, the animal must choose correctly between two options. For each option the probability of its resulting in a reward (water) has to be estimated on the basis of available information and to be related to the cost of performing it. We found that diazepam, in a dose that did not significantly affect the ability to perform the options per se, caused a strong impairment when these options, on the basis of available information, had to be combined into functional sequences in a decision making procedure. The results obtained cannot be explained on the basis of disinhibition or response perseveration. The hypothesis is advanced that benzodiazepines alter decision making in a more nonspecific may, by, for example, affecting the evaluation of the learned significance of stimuli in the environment.

Animals↗
PubMed1987

Disruptive effects of low doses of d-amphetamine on the ability of rats to organize behaviour into functional sequences.

A complex laboratory task was used to test the effects of low doses of d-amphetamine on decision making in the white rat. In particular, the animals' ability to organize their behaviour into functional sequences was studied. The rats were required to choose between two options in order to obtain rewards (water). To solve the problem efficiently, the animal must correctly use information currently available about the reward probabilities and the response costs of the two activities. The results showed that already at a dose of 0.2 mg/kg, by comparison with control, the decision rule was significantly affected and the efficiency of the behaviour decreased. At 1 mg/kg, the rats were generally unable to organize their behaviour into functional sequences resulting in rewards even though they were able to perform the separate behavioural responses required to solve the task, as shown in separate control experiments. Low doses of d-amphetamine have previously been described to be "psychomotor stimulant" and, for example, to increase locomotion and exploration. Our conclusion is that these low doses do not increase behavioural output in an adaptive way. In simple tasks where motor output is directly related to a measure of performance, these doses might be interpreted as causing increased efficiency. However, when tested in our complex decision making task, these doses result in suboptimal behaviour.

Adaptation, Psychological↗