The Evolution of Parental Care

The Evolution of Parental Care

Nick J. Royle & Per T. Smiseth
Published in:
Oxford University Press
Release Year: 2012
ISBN: 978–0–19–969257–6
Edition: First Edition
File Size: 3 MB
File Type: pdf
Language: English

Description of The Evolution of Parental Care

Parental care is a trait that shows tremendous diversity both within and across different animal taxa and is an important topic in evolutionary biology and behavioral ecology. Parental care forms an integral part of an organism’s reproduction, development, and life-history, and because caring for offspring means that parents have less time, resources, or energy available to search for or attract mates, the evolution of parental care is closely linked with sexual selection. In addition, the evolution of parental
care represents an important step in the evolution of sociality as it leads to the formation of family groups, which provides a bridge to more complex forms of social structures. But because parents and offspring share only some of their genes, conflicts emerge in sexually reproducing organisms that shape the evolution of parental care and offspring strategies to demand care. As a consequence, the family also constitutes a model to understand the evolutionary tension between cooperation and conflict.
The importance of parental care in evolutionary biology has only been recognized relatively recently. Darwin did not consider parental care in great detail, except when speculating on the development of a moral sense and the role of selection operating on families in the evolution of more complex sociality: ‘With respect to the origin of the parental and filial affections, which apparently lie at the base of the social instincts, we know not the steps by which they have been gained; but we may infer that it has been to a large extent through natural selection’ (Page 105; The Descent of Man, Darwin 1871). The development of the robust conceptual framework that we have today for understanding the evolution of parental care was dependent on numerous innovations in the wider field of ecology and evolution. In particular: (i) the incorporation of kin selection to evolutionary thinking (Hamilton 1964), (ii) an appreciation of the relationship between parental care and the ecology and life history of organisms (Lack 1968), (iii) the recognition that specific ecological conditions can drive the evolution of parental care and sociality (Wilson 1971), (iv) the insight of Trivers (1972) in making the connection between parental care, parental investment, and sexual selection, (v) his application of kin selection logic to derive genetic conflicts between parents and offspring over the amount and duration of parental care (Trivers 1974), and (vi) the introduction of evolutionary game theory to study the evolution of parental care and family conflicts by Maynard-Smith (1977). In particular, the concept of parental investment (Trivers 1972) was vital in triggering a large amount of research on sexual selection and mating system evolution. As a result,
Tim Clutton-Brock noted in his book The Evolution of Parental Care, published in 1991, that ‘few areas of evolutionary biology [. . . ] progressed as rapidly over the past two decades as our understanding of animal breeding systems’ (p. 3). The underlying /rationale for writing a book on the evolution of parental care by Clutton-Brock, therefore, centered on the importance of parental care in determining the strength of sexual selection. Since then the study of the evolution of parental care has progressed and diversified substantially.
The idea for the present book arose from discussions among the three of us about parental care research, and the realization that more than 20 years had passed since Clutton-Brock’s (1991) classic book on this topic. In the intervening years since this book was published there has been growing recognition of the central importance of parental care research in behavioral and evolutionary biology, with an increasing number of papers being published and new fields of study steadily emerging. For example, there has been notable progress in the study of some aspects of parental care that were newly emerging at the time of the publication of Clutton-Brock’s book, such as the physiology of maternal effects or the effect of parentage on parental care. However, there has also been notable progress in other topics that, although they had emerged at the time, had progressed rather slowly, such as within-family conflicts, cooperative breeding, and brood parasite-host co-evolution.
More recently new areas of research altogether have emerged, such as the evolutionary and molecular genetics of parental care. It, therefore, seemed high time to have a go at synthesizing these exciting developments in the study of the evolution of parental care. The aim of this book is to provide a comprehensive, fresh overview of research on the evolution of parental care in animals. The book integrates the major advances in the field over the last two decades since Clutton-Brock’s (1991) book, focusing on establishing key concepts and on drawing general principles whilst emphasizing a broad taxonomic approach throughout. There are three main sections that represent major themes in the evolution of parental care and 18 chapters. The chapters and sections are arranged in a logical order to encourage the reading of the book from front to back. Nevertheless, each chapter has been written so that
it can easily be read in isolation, too.
Chapter 1: The introductory chapter by Smiseth, Kölliker, and Royle set the stage for the book by reviewing the diversity of parental care across taxa, providing definitions of key terms and discussing
some of the central concepts in the evolution of parental care. Section I is on the Origin and evolution of
parental care. This section deals broadly with the factors that promoted the early evolution of parental care. In Chapter 2, Klug, Alonzo, and Bonsall review theory and describe under which conditions parental care can evolutionarily originate, and how life-history and ecology interact to determine the favourable conditions for its spread. In Chapter 3, Alonso-Alvarez and Velando review the causes of variation in the evolutionary benefits and costs of parental care, especially with respect to the physiological mechanisms mediating these fitness consequences. Although we have advocated a question-driven approach throughout, and treatment of a biological phenomenon such as parental care also requires appropriate coverage of its natural history and diversity. Chapter 4 by Balshine covers the diversity and distribution of forms of are among vertebrates, and Chapter 5 by Trumbo the very diverse and sometimes striking and peculiar forms of care that have evolved among invertebrates. Finally, Chapter 6 by Kokko and Jennions
explains the multiple and complex evolutionary relationships between sexual selection and the sex roles in parental care.
Section II is concerned with Cooperation and conflict in parental care and covers the tension between conflict and cooperation that emerges in the context of parental care due to sexual reproduction and the resulting asymmetries in fitness consequences of care and/or genetic relatedness. Kilner and Hinde (Chapter 7) discuss how the resolution of parent-offspring conflict affects parent and offspring strategies and review the experimental evidence. Chapter 8 by Roulin and Dreiss explores conflicts and cooperation between siblings and how these are resolved via mechanisms such as aggression, signalling, and negotiation. Lessells (Chapter 9) explores the theoretical underpinning and empirical evidence for conflicts between parents over the provision of care. In Chapter 10, Komdeur presents the ecological and social factors that alter the fitness returns of investment in sons versus daughters and the evidence for adaptive sex allocation, and Chapter 11 by Alonzo and Klug reviews the central importance of parentage in the evolution of parental care. The two extremes of cooperation and conflict in animal families are covered in Chapter 12 by Cant, on the evolution of cooperative breeding, and Chapter 13 by Spottiswoode, Kilner, and Davies on the manipulation of parents and exploitation of parental care by brood parasites.
Section III covers the Evolutionary genetics of parental care. This section takes a change of perspective from the majority of chapters in sections I and II, which focus on how selection acts on phenotypes, towards establishing how phenotypic variation in parent and offspring traits are generated and maintained by genetic and non-genetic  factors, how this variation is exposed to natural selection, and what the molecular and quantitative genetic trait architecture of parental care are.
Uller (Chapter 14) describes how parental care can enhance adaptive evolution by generating environ-mentally induced variation in offspring phenotypes through processes such as phenotypic and genetic accommodation. In Chapter 15, Hadfield discusses the mathematical framework of quantitative genetic parental effect models to study the co-evolutionary dynamics of parental effects and offspring phenotypes. Chapter 16 by Kölliker, Royle, and Smiseth outlines the theory and experimental research exploring how the co-evolution of parents and offspring lead to co-adapted strategies individuals use as
offspring and as parents. While quantitative genetic results are discussed across the previous chapters, Chapter 17 by Champagne and Curley presents a comprehensive overview of the current knowledge of the molecular genetics and epigenetics of parental care. Finally, in the summarizing Chapter 18 by Royle, Smiseth, and Kölliker we offer a summary of the previous 17 chapters, draw conclusions and discuss promising avenues for future research. Our main target audiences are new and established researchers and students in behaviour, ecology, genetics, development, and evolution; although we hope the book may also appeal to academics and students in other, related disciplines, such as psychology and sociology.
This project would not have been possible without the contributions of the authors. The editing of this book has been a highly enjoyable process as it has allowed us to interact with some of the most talented researchers working on parental care. We would like to thank all the authors for their hard work, efforts to meet deadlines, and willingness to buy into the project with such enthusiasm. We would also like to extend our gratitude to the many reviewers, some of whom read and commented on several chapters: Kate Arnold, Matt Bell, Jon Blount, Kate Buchanan, Tim Clutton-Brock, James Curley, Sasha Dall, Jeremy Field, Scott Forbes, Simon Griffith, Uri Grodzinski, Reinmar Hager, David Haig, Ian Hardy, Ben Hatchwell, Megan Head, Fabrice Helfenstein, Camilla Hinde, Andy Horn, Clarissa House, Rufus Johnstone, Charlotta Kvarnemo, Tim Linksvayer, Clause Machado, Joah Madden, Dario Maestripieri, Joel McGlothlin, John McNamara, Allen Moore, Geoff Parker, Tom Pike, Sarah Pryke, Eivin Røskraft, Andy Russell, Spencer Sealy, Ben Sheldon, Emilie Snell-Rood, Bård Stokke, Tamas Székely, Fritz Trillmich, Tobias Uller, Mary Jane West-Eberhard, and Jon Wright. Finally, we wish to thank the publishers at OUP, especially Helen Eaton and Ian Sherman, who provided invaluable help and advice during the whole process.
Nick Royle: On a professional level I would especially like to thank Ian Hartley and Geoff Parker, my supervisors and mentors during my first postdoc. I never realized research on conflicts could be so much fun, as well as being so educational. Thanks also to Jan Lindström, Neil Metcalfe, Craig Walling, Jason Wolf, Scott Forbes, Maggie Hall, Jon Blount, Josie Orledge, Tom Pike, Sasha Dall, Wiebke Schütt, Allen Moore, Megan Head, Paul Hopwood, Heinz Richner, Philipp Heeb, and Mathias Kölliker, amongst others, for making subsequent collaborations similarly intellectually stimulating and fun. On a personal level, I need to thank my parents, John and Sheila, and my brother, Phil, for stimulating and encouraging my interest in parental care and family dynamics throughout my life. Last, but certainly not least, I thank my wife, Marieke, and my two boys, Lachlan and Lucas, for their love and support and for providing daily reminders of the joys of being a parent.

Content of The Evolution of Parental Care

1 What is parental care? 1
Per T. Smiseth, Mathias Kölliker, and Nick J. Royle
1.1 Introduction 1
1.2 Forms of parental care 2
1.2.1 Provisioning of gametes 2
1.2.2 Oviposition-site selection 3
1.2.3 Nest building and burrowing 3
1.2.4 Egg attendance 3
1.2.5 Egg brooding 4
1.2.6 Viviparity 5
1.2.7 Offspring attendance 5
1.2.8 Offspring brooding 5
1.2.9 Food provisioning 6
1.2.10 Care after nutritional independence 6
1.2.11 Care of mature offspring 7
1.3 Definition of terms 7
1.4 Assigning fitness to parents and offspring 10
1.4.1 Assigning offspring survival to offspring and parental fitness 10
1.4.2 Assigning costs and benefits of care to offspring and parents 11
1.5 Origin and evolution of parental care 12
1.6 Conclusion 14
Section I Origin and Evolution of Parental Care
2 Theoretical foundations of parental care 21
Hope Klug, Suzanne H. Alonzo, and Michael B. Bonsall
2.1 Introduction 21
2.1.1 Defining parental care 21
2.1.2 The role of modelling in parental care theory 22
2.1.3 General theoretical questions 22
2.2 When should care be provided? 22
2.2.1 Costs and benefits of parental care 22
2.2.2 Life-history, ecology, and the origin of care 24
2.3 Which sex should provide care? 26
2.4 How much care should be provided? 29
2.4.1 Offspring reproductive value 29
2.4.2 How much care the other parent provides 30
2.4.3 Parental residual reproductive value 31
2.5 Why do we see a specific form of care behaviour in a given population
or species? 33
2.5.1 What type of care to provide? 33
2.5.2 Explaining complex patterns of care in a given system: three
examples 33
2.6 Future directions and challenges 34
2.6.1 Linking theory and data 34
2.6.2 Origin versus maintenance of care 35
2.6.3 Ecological and evolutionary feedback 36
3 Benefits and costs of parental care 40
Carlos Alonso-Alvarez and Alberto Velando
3.1 Introduction 40
3.2 Trade-offs and the nature of the parental resources 40
3.2.1 Limiting resources 40
3.2.2 Non-linear relationships between resource allocation and fitness 42
3.2.3 Limitations of the resource allocation trade-off perspective 42
3.2.4 Cost-free resources and resources not involved in care 43
3.3 Benefits of parental care: mechanistic basis 44
3.3.1 Short-term benefits of parental care 44
3.3.2 Long-term benefits of parental care 44
3.3.3 Parental care and offspring phenotypic adjustment 45
3.4 The costs of parental care 46
3.4.1 Non-physiological costs 47
3.4.2 Physiological costs 48
3.5 Costs and benefits in the balance 52
3.5.1 Molecular signals promoting parental effort 53
3.5.2 Pathways inhibiting parental effort 53
3.6 Final remarks 54
4 Patterns of parental care in vertebrates 62
Sigal Balshine
4.1 Introduction 62
4.2 Forms of care 63
4.2.1 Preparation of the physical rearing environment 63
4.2.2 Defence of offspring 65
4.2.3 Provisioning 66
4.3 Transitions in care 69
4.3.1 Parental care in fishes 69
4.3.2 Parental care in amphibians 69
4.3.3 Parental care in reptiles 71
4.3.4 Parental care in birds 71
4.3.5 Parental care in mammals 72
4.4 Parental care in humans 72
4.4.1 Exceptionally long parental care duration 73
4.4.2 Male care and support 73
4.4.3 Support from constellations of kin 74
4.4.4 Humans as a study system for parental care 74
4.5 Concluding remarks 75
5 Patterns of parental care in invertebrates 81
Stephen T. Trumbo
5.1 Introduction 81
5.2 Forms of care 82
5.2.1 Trophic eggs 82
5.2.2 Attending eggs and offspring 82
5.2.3 Protection and facilitating feeding of mobile young 83
5.2.4 Brooding behaviour and viviparity 83
5.2.5 Nest building and burrowing 84
5.2.6 Food provisioning 84
5.3 Origins and transitions of parental care 85
5.3.1 Factors promoting care 85
5.3.2 Male versus female care 87
5.3.3 Biparental care 89
5.3.4 The loss of parental care 92
5.4 Microbiology of care 93
6 Sex differences in parental care 101
Hanna Kokko and Michael D. Jennions
6.1 Introduction 101
6.2 Why does an individual’s sex predict its behaviour? 102
6.3 The first sex difference in parental care: anisogamy 103
6.4 What happened next? Sex roles in post-zygotic parental care 105
6.5 Uncertain parentage reduces male care 106
6.5.1 Why exactly does paternity matter? 107
6.5.2 Behavioural and evolutionary time scales are not
equivalent 107
6.5.3 Traits that protect paternity can co-evolve with care 108
6.6 Don’t bother caring till the going gets tough: the OSR 108
6.7 Orwell was right, not all animals are equal: sexual selection and
the adult sex ratio 111
6.8 Conclusions 112
Section II Conflict and Cooperation in Parental Care
7 Parent-offspring conflict 119
Rebecca M. Kilner and Camilla A. Hinde
7.1 Introduction 119
7.2 The theory of parent-offspring conflict 119
7.3 Evidence of parent-offspring conflict: battlegrounds for antagonistic
interests 120
7.3.1 Behavioural disputes are not evidence of evolutionary conflict 120
7.3.2 Parent-offspring conflict in natural vertebrate populations 121
7.3.3 IGF-II and other examples of genomic imprinting 121
7.3.4 Sex ratio wars in the social insects 122
7.4 Co-evolution of traits in offspring and their parents 122
7.4.1 The co-evolution of supply and demand 123
7.4.2 Plasticity 124
7.5 What is the outcome of co-evolution between offspring and parents? 125
7.5.1 Unstable outcomes 126
7.5.2 Stable outcomes 127
7.6 Future directions 129
8 Sibling competition and cooperation over parental care 133
Alexandre Roulin and Amélie N. Dreiss
8.1 Introduction 133
8.2 Forms of sibling competition and cooperation 135
8.2.1 Sub-lethal sibling competition 136
8.2.2 Lethal sibling competition 138
8.2.3 Sib–sib cooperation 139
8.2.4 Sibling negotiation 139
8.3 Conditions promoting sibling competition 140
8.3.1 Food amount 141
8.3.2 Weapons 141
8.3.3 Age difference between siblings 141
8.3.4 Parental manipulation of sibling competition 142
8.3.5 Parental strategies to reduce sibling competition 143
8.4 Conditions promoting sibling cooperation 144
8.5 Perspectives 145
9 Sexual conflict 150
C. M. Lessells
9.1 Introduction 150
9.2 How is sexual conflict over parental care resolved? 151
9.2.1 Evolution of manipulation 151
9.2.2 Coevolution of parental care in the two sexes without manipulation 152
9.3 How long to care? Offspring desertion 153
9.3.1 Experimental studies: sex differences in the benefits of care and
desertion 158
9.3.2 Experimental evidence for a trade-off between the benefits of
parental care and desertion 159
9.4 How much to care? 160
9.4.1 Negotiation 161
9.4.2 Experimental changes in partner effort: mate removal and
manipulation 162
9.4.3 Brood division 165
9.4.4 Complementarity and task specialization 166
9.5 Interaction with other evolutionary conflicts within the family 166
9.6 Conclusions and prospects 167
10 Sex allocation 171
Jan Komdeur
10.1 Introduction 171
10.2 Sex ratio adjustment in birds and mammals 172
10.3 Difficulties applying the theory 172
10.4 Sex ratio bias at the population level 174
10.5 Tests of population sex ratio models 175
10.6 Facultative sex ratio variation 176
10.6.1 Food availability 176
10.6.2 Maternal condition or quality 177
10.6.3 Attractiveness or quality of males 178
10.6.4 Social environment 179
10.6.5 Sibling competition 181
10.6.6 Sexual conflict 182
10.7 Concluding remarks and future directions 183
11 Paternity, maternity, and parental care 189
Suzanne H. Alonzo and Hope Klug
11.1 An overview of parentage and parental effort 189
11.1.1 The effect of relatedness on parental effort 189
11.1.2 Variation in maternity and paternity 189
11.1.3 Parentage is important, but it is not fitness 190
11.1.4 Interactions within and between the sexes drive parentage and
parental effort 190
11.2 Theoretical predictions: does higher parentage always favour greater
parental effort? 191
11.2.1 Paternity is expected to affect parental effort 191
11.2.2 The biological importance of model self-consistency 192
11.2.3 A specific self-consistent model relating parentage and care 192
11.2.4 Interactions between the sexes affect parentage and parental
care 193
11.2.5 Information about expected parentage affects predictions 194
11.3 Empirical patterns: what determines the relationship between
parentage and parental effort? 195
11.3.1 Paternity affects parental care in the bluegill sunfish 195
11.3.2 Paternal care is more likely with high sperm competition in the
ocellated wrasse 197
11.3.3 Paternity does not affect paternal effort in western bluebirds 197
11.3.4 Interactions between the sexes affect parentage and parental
care in the dunnock 198
11.3.5 Alloparental care in Canada geese can increase offspring
survival 198
11.3.6 Alloparental care in eiders might increase survival of a caring
mother’s chicks 199
11.3.7 Lessons from comparative studies 199
11.4 Parentage and parental care: what are we missing? 200
11.4.1 A priori predictions should replace post hoc explanations 200
11.4.2 The importance of kin recognition and cues of extra-pair
paternity 200
11.4.3 Co-evolutionary and social feedbacks between the sexes 201
11.4.4 Rigorous experiments will be needed to test how multiple traits
interact 201
11.4.5 Connecting relatedness, sexual selection, social interactions,
and parental care: cooperative breeding as a case study 201
11.4.6 Do we need to change the question? 202
11.5 Conclusions 203
12 Cooperative breeding systems 206
Michael A. Cant
12.1 Introduction 206
12.2 Routes to cooperative breeding 208
12.3 Selection for helping behaviour 209
12.3.1 Demography and indiscriminate altruism 210
12.3.2 Discriminate altruism: kin directed care 213
12.3.3 Direct fitness benefits 214
12.4 Negotiation over help 215
12.5 Reproductive conflict 217
12.6 Conclusion and future research 220
13 Brood parasitism 226
Claire N. Spottiswoode, Rebecca M. Kilner, and Nicholas B. Davies
13.1 Introduction 226
13.2 Who are the brood parasites, how virulent are they? 227
13.3 The egg-laying stage 227
13.4 The incubation stage 229
13.5 The chick-rearing stage 231
13.5.1 How parasitic parents can improve the nestling environment 231
13.5.2 Costs of chick-killing to parasites 232
13.5.3 Virulent chicks: how to solicit a foster-parent 232
13.5.4 Benign chicks: how to compete with foster-siblings 233
13.5.5 How can hosts defend themselves at the chick stage? 235
13.5.6 How are chick adaptations evolutionarily maintained? 236
13.6 Why are host parents often so gullible? 236
13.7 Conclusions and speculations 239
Section III Evolutionary Genetics of Parental Care
14 Parental effects in development and evolution of 247
Tobias Uller
14.1 Introduction 247
14.2 Parental effects and the origins of variation 249
14.2.1 Patterns of phenotypic variation 252
14.3 Parental effects and adaptive evolution 252
14.3.1 Parental effects can increase the functionality of novel variation 253
14.3.2 Parental effects can increase recurrence of novel variation 253
14.3.3 Parental effects and genetic accommodation 254
14.4 Evolution of trans-generational plasticity 256
14.4.1 Adaptive evolution of trans-generational plasticity 256
14.4.2 Trans-generational plasticity under parent-offspring conflict 259
14.4.3 Mechanisms of trans-generational plasticity 260
14.5 Exploring the evolutionary dynamics of parental effects 261
15 The quantitative genetic theory of parental effects 267
Jarrod Hadfield
15.1 Introduction 267
15.2 The K–L model 268
15.3 An example and its relation to behavioural ecology 273
15.4 The William model 274
15.5 Hamilton’s rule 276
15.6 The evolution of G 277
15.7 General indirect genetic effect models and parental effect models 279
15.8 Discussion 280
16 Parent-offspring co-adaptation 285
Mathias Kölliker, Nick J. Royle, and Per T. Smiseth
16.1 Introduction 285
16.2 Offspring performance traits that convert parental care into offspring
fitness 286
16.3 Trait-based Hamilton’s rule and parent-offspring covariances 286
16.4 Evolution of the parent-offspring covariance 286
16.5 Co-adaptation models 287
16.5.1 Levels of analysis 287
16.5.2 Sources of correlational selection in families 288
16.5.3 Co-adaptation models 289
16.5.4 Co-adaptation and epigenetic inheritance 292
16.5.5 Other implications of parent-offspring co-adaptation 292
16.6 Experimental evidence for parent-offspring co-adaptation 293
16.6.1 Genotype x family environment interactions, social epistasis,
and co-adaptation 294
16.7 Co-adaptation and conflict resolution 297
16.7.1 Genetic covariance and parent-offspring conflict 299
16.8 Conclusions and future directions 299
17 Genetics and epigenetics of parental care 304
Frances A. Champagne and James P. Curley
17.1 Introduction 304
17.2 Genetics of care-giving in honeybees (Apis mellifera) 306
17.2.1 Genetic factors regulating hygienic behaviour 306
17.2.2 Transition from nursing to foraging 307
17.3 Laboratory studies of the genetics of parental care in rodents 308
17.3.1 Strain and species differences in parental care 308
17.3.2 Selective breeding 310
17.3.3 Quantitative trait loci (QTL) analysis 311
17.3.4 Gene knockout (KO) studies 312
17.3.5 Sex chromosomes 313
17.4 Gene polymorphisms in primates and humans 313
17.4.1 Serotonin transporter (5-HTT) 313
17.4.2 Mu-opioid receptor (OPRM1) 314
17.4.3 Dopamine D4 receptor (DRD4) 314
17.4.4 Oxytocin receptor (OXTR) 314
17.5 Epigenetic influences on parental care and offspring development:
genomic imprinting 315
17.6 Epigenetic influences on parental care and offspring development:
nature via nurture 316
17.6.1 Maternal nutrition effects on DNA methylation 317
17.6.2 Post-natal influence of maternal care 317
17.6.3 Transgenerational impact of maternal care 318
17.7 Conclusions 319
Section IV Conclusions
18 The evolution of parental care: summary, conclusions, and implications 327
Nick J. Royle, Per T. Smiseth, and Mathias Kölliker
18.1 Introduction 327
18.2 What is parental care? 328
18.3 Origin and evolution of parental care 328
18.3.1 Costs and benefits 328
18.3.2 Evolutionary origins 329
18.3.3 The role of the social environment 329
18.4 Conflicts and cooperation in parental care 330
18.4.1 Why are conflicts expected? 330
18.4.2 Who should provide care? 331
18.4.3 Sexual conflict over care 331
18.4.4 Sibling competition, cooperation, and parental favouritism 332
18.4.5 Parent–offspring conflict 333
18.5 Co-evolution and correlated responses 333
18.6 Mechanisms and constraints in parental care 337
18.6.1 Physiological mechanisms 337
18.6.2 Genetic mechanisms 338
18.7 Evolutionary implications of parental care 339
18.7.1 Life-history evolution 339
18.7.2 Social evolution 339
18.7.3 Sexual selection 340
18.7.4 Evolution of personality 340
18.8 Future directions 340
18.9 Conclusions 343
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