Evolutionary dynamics of cooperation in multiplayer games

Tese de mestrado em Física (Física Estatística e Não Linear), apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2011Cooperation is an act where individuals can contribute something, at a cost to themselves, to provide a benefit for others. We can find situations where this kind of act can arise at almost every layer of human societies and also in the animal world. Such a widespread behavior can be studied using Game Theory, a mathematical formulation of multi-optional interactions. The theory takes its predictions from social games between rational individuals that want to maximize their profit. When applied to dilemmas of cooperation, the theory predicts the demise of those who contribute, thus contradicting extensive empirical evidence. Evolutionary Game Theory was created to counteract this shortcoming. It drops the rationality assumption of Game Theory and instead places individuals in a dynamical context where natural selection applies. Thus, in view of this theory, the most successful individuals (who profit the most from games) are more likely to be leave descendants or be imitated by others, depending on the kind of dynamics being considered. Nevertheless, this process by itself does not yet explain the emergence of cooperation and only by adding other mechanisms to the dynamics can we make this property appear. In this thesis we present three such mechanisms all in the context of Public Goods Games. In these games, individuals in a group choose if they want to contribute to a common good that is then transformed into a benefit to be equally shared by all group members. The first mechanism applies to infinite well-mixed population dynamics and consists of a detaching in time the impact of the game on the system. By applying a time delay to the profits, we can modify a specific public goods game in order to give cooperation a chance. The other mechanisms are somewhat similar and apply to finite structured populations where individuals are restricted to interacting with specific opponents. They both consider that this interaction structure is shaped by the game dynamics and can evolve over time. The second and third mechanisms are thus a numerical and analytical implementation, respectively, of this co-evolutionary dynamics.A Cooperação é uma accão em que temos indivíduos que podem contribuir com algo para beneficiar outros, pagando os primeiros um custo. Exemplos de situações onde a cooperação pode surgir encontram-se em quase todos as camadas da sociedade: desde pessoas a terem uma conversa ou a realizar um debate político, a jogar no mercado bolsista, a caçar ou a arranjar mantimentos ou até a construir um edifício ou uma comunidade inteira. No mundo animal também pode surgir cooperação, no contexto de caçadas, protecção contra predadores ou em lideranças de grupo. Um comportamento tão vasto pode ser estudado utilizando Teoria de Jogos, uma formulação matemática de interacções multi-estratégicas. A teoria faz previsões através da realização de jogos sociais entre indivíduos racionais que querem maximizar o seu lucro. Quando aplicada a dilemas de cooperação, a teoria prevê a extinção da cooperação, o que contradiz vastos dados empíricos. A Teoria de Jogos Evolutiva for criada com o intuito de corrigir essa falha. Esta teoria deixa cair o postulado da racionalidade da teoria anterior e considera então que os indivíduos estão num contexto dinâmico sendo estes afectados pela selecção natural. Portanto, de acordo com a teoria, os indivíduos mais fortes (que obtêm mais lucros dos jogos) são os que mais hipóteses têm de deixar descendentes ou de ser imitados, consoante o tipo de dinâmicas consideradas. No entanto, este processo por si só não explica ainda a emergência da cooperação. É preciso adicionar outros mecanismos para fazer aparecer esta propriedade. Nesta tese apresentamos três destes mecanismos, todos no contexto dos Jogos de Bem Público. Nestes jogos, indivíduos juntam-se num grupo e escolhem se querem ou não contribuir para um bem público que depois é transformado e dividido igualmente por todos os membros desse grupo. O primeiro mecanismo aplica-se a dinâmicas de populações infinitas well-mixed onde qualquer indivíduo interage com qualquer outro com a mesma probabilidade, e consiste em separar temporalmente o impacto do jogo no sistema considerado. Aplicando um atraso temporal aos lucros podemos modificar um tipo específico de jogo de bem público de modo a que a cooperação tenha uma hipótese de subsistir. Os outros mecanismos são algo similares e aplicam-se a populações finitas e estruturadas onde os indivíduos estão restringidos a interagir apenas com determinados oponentes. Ambos consideram que esta estrutura de interacção é moldada pelo jogo e pode evoluir ao longo do tempo. O segundo e terceiro mecanismos são portanto uma implementação numérica e analítica, respectivamente, desta dinâmica co-evolutiva

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved

Safety of hospital discharge before return of bowel function after elective colorectal surgery

© 2020 BJS Society Ltd Published by John Wiley & Sons LtdBackground: Ileus is common after colorectal surgery and is associated with an increased risk of postoperative complications. Identifying features of normal bowel recovery and the appropriateness for hospital discharge is challenging. This study explored the safety of hospital discharge before the return of bowel function. Methods: A prospective, multicentre cohort study was undertaken across an international collaborative network. Adult patients undergoing elective colorectal resection between January and April 2018 were included. The main outcome of interest was readmission to hospital within 30 days of surgery. The impact of discharge timing according to the return of bowel function was explored using multivariable regression analysis. Other outcomes were postoperative complications within 30 days of surgery, measured using the Clavien–Dindo classification system. Results: A total of 3288 patients were included in the analysis, of whom 301 (9·2 per cent) were discharged before the return of bowel function. The median duration of hospital stay for patients discharged before and after return of bowel function was 5 (i.q.r. 4–7) and 7 (6–8) days respectively (P < 0·001). There were no significant differences in rates of readmission between these groups (6·6 versus 8·0 per cent; P = 0·499), and this remained the case after multivariable adjustment for baseline differences (odds ratio 0·90, 95 per cent c.i. 0·55 to 1·46; P = 0·659). Rates of postoperative complications were also similar in those discharged before versus after return of bowel function (minor: 34·7 versus 39·5 per cent; major 3·3 versus 3·4 per cent; P = 0·110). Conclusion: Discharge before return of bowel function after elective colorectal surgery appears to be safe in appropriately selected patients

Safety of hospital discharge before return of bowel function after elective colorectal surgery

Background: Ileus is common after colorectal surgery and is associated with an increased risk of postoperative complications. Identifying features of normal bowel recovery and the appropriateness for hospital discharge is challenging. This study explored the safety of hospital discharge before the return of bowel function.Methods: A prospective, multicentre cohort study was undertaken across an international collaborative network. Adult patients undergoing elective colorectal resection between January and April 2018 were included. The main outcome of interest was readmission to hospital within 30 days of surgery. The impact of discharge timing according to the return of bowel function was explored using multivariable regression analysis. Other outcomes were postoperative complications within 30 days of surgery, measured using the Clavien-Dindo classification system.Results: A total of 3288 patients were included in the analysis, of whom 301 (9.2 per cent) were discharged before the return of bowel function. The median duration of hospital stay for patients discharged before and after return of bowel function was 5 (i.q.r. 4-7) and 7 (6-8) days respectively (P &lt; 0.001). There were no significant differences in rates of readmission between these groups (6.6 versus 8.0 per cent; P = 0.499), and this remained the case after multivariable adjustment for baseline differences (odds ratio 0.90, 95 per cent c.i. 0.55 to 1.46; P = 0.659). Rates of postoperative complications were also similar in those discharged before versus after return of bowel function (minor: 34.7 versus 39.5 per cent; major 3.3 versus 3.4 per cent; P = 0.110).Conclusion: Discharge before return of bowel function after elective colorectal surgery appears to be safe in appropriately selected patients

Timing of nasogastric tube insertion and the risk of postoperative pneumonia: an international, prospective cohort study

Aim: Aspiration is a common cause of pneumonia in patients with postoperative ileus. Insertion of a nasogastric tube (NGT) is often performed, but this can be distressing. The aim of this study was to determine whether the timing of NGT insertion after surgery (before versus after vomiting) was associated with reduced rates of pneumonia in patients undergoing elective colorectal surgery. Method: This was a preplanned secondary analysis of a multicentre, prospective cohort study. Patients undergoing elective colorectal surgery between January 2018 and April 2018 were eligible. Those receiving a NGT were divided into three groups, based on the timing of the insertion: routine NGT (inserted at the time of surgery), prophylactic NGT (inserted after surgery but before vomiting) and reactive NGT (inserted after surgery and after vomiting). The primary outcome was the development of pneumonia within 30 days of surgery, which was compared between the prophylactic and reactive NGT groups using multivariable regression analysis. Results: A total of 4715 patients were included in the analysis and 1536 (32.6%) received a NGT. These were classified as routine in 926 (60.3%), reactive in 461 (30.0%) and prophylactic in 149 (9.7%). Two hundred patients (4.2%) developed pneumonia (no NGT 2.7%; routine NGT 5.2%; reactive NGT 10.6%; prophylactic NGT 11.4%). After adjustment for confounding factors, no significant difference in pneumonia rates was detected between the prophylactic and reactive NGT groups (odds ratio 1.03, 95% CI 0.56–1.87, P = 0.932). Conclusion: In patients who required the insertion of a NGT after surgery, prophylactic insertion was not associated with fewer cases of pneumonia within 30 days of surgery compared with reactive insertion