Sexual Segregation in Vertebrates

by Kathreen Ruckstuhl, Peter Neuhaus

Overview

An understanding of sexual segregation is important in the explanation of life history and social preference, population dynamics and the conservation of rare species. This book explores the reasons why this behaviour has evolved and what factors contribute to it. Males and females of many species can, and do, live separately for long periods of time. This sexual segregation is widespread and can be on social, spatial or habitat scales.

Product Details

ISBN-13:	9780521184212
Publisher:	Cambridge University Press
Publication date:	02/17/2011
Edition description:	New Edition
Pages:	502
Product dimensions:	6.00(w) x 8.90(h) x 1.40(d)

About the Author

Dr Ruckstuhl is currently Assistant Professor at the University of Calgary, Ecology division, in the Department of Biological Sciences.

Dr Neuhaus is part time Research Assistant at the University of Neuhâtel, and Professor at the University of Calgary.

Read an Excerpt

Cambridge University Press
0521835224 - Sexual Segregation in Vertebrates - Ecology of the Two Sexes - Edited by K. E. Ruckstuhl and P. Neuhaus
Excerpt

Part I Overview

1
Sexual segregation and the ecology of the two sexes

According to Greek mythology, Amazons were female warriors who lived on an island. They occasionally met with men of another people to mate, keeping female offspring and sending male offspring back to their fathers. In many animals too, males and females live apart for most of the year, only gathering for mating. These include many fish species (Sims et al., 2001a), birds (Myers, 1981), lizards and snakes (Parmelee & Guyer, 1995; Shine et al., 2003a), and most mammals with a pronounced sexual dimorphism in body size (Ruckstuhl & Neuhaus, 2000). This book aims at synthesizing our current understanding of the evolution of sexual segregation in different vertebrates, focusing on taxa in which there is sufficient evidence to investigate causes of sexual segregation.

Sexual segregation has caused confusion in the literature, as it can occur at different levels (see Chapter 2 for details). Why, for example, do house mice, Mus musculus, embryos segregate by sex in their mother's womb (Terranova & Laviola, 1995)? Why do African ground squirrels, Xerus inauris, live in separate male and female groups within the same area (Waterman, 1997)? Why do female dogfish, Scyliorhinus canicula, use different feeding and resting areas than males (Chapter 8)? Or why do humans prefer to socialize with same-sex peers (Chapter 12; Maccoby, 1998)? Some species show social segregation of the sexes: males and females are found in different groups outside the mating season, but use the same areas and habitat types (see Chapters 9 and 10). In other cases, males and females may use different habitats, either within the same or in different areas. This type of segregation is referred to as habitat segregation.

What causes segregation of the sexes, and what are its adaptive advantages? Sexual segregation has been an important research focus in ungulate ecology, but until recently only a few studies had been done on other vertebrate species. There may be more than one reason why the sexes segregate, and the factors responsible for segregation may vary within and across the different vertebrate taxa. It has been suggested that social segregation is caused by habitat segregation but this need not necessarily be so (see Chapters 2, 8, 10 and 11). Let us start by looking at what could cause habitat segregation and social segregation separately, and then we will discuss how they could affect or be independent of each other.

The causes of habitat segregation may vary. In some cases, for example, sex differences in body size may be responsible for habitat segregation. Dimorphism in body size could result in divergent nutritional and energetic requirements, reproductive strategies, activity budgets and social affinities (Main & Coblentz, 1990; Bon, 1991; Ruckstuhl & Neuhaus, 2002). Comparative analyses on ungulates confirm that size-monomorphic social species generally live in mixed-sex groups or pairs displaying similar social and habitat preferences and behaviours, while sexually dimorphic species segregate outside the breeding season (Mysterud, 2000; Ruckstuhl & Neuhaus, 2002). Sexual dimorphism in body length or body size could also allow males and females to access different food items. Male giraffe, Giraffa camelopardalis, or elephants, Loxodonta africana, are able to feed higher up in the canopy than the smaller females (Ginnett & Demment, 1999; see also Chapter 2). Other examples including diving birds (i.e. some penguins, see Chapter 18), Antarctic fur seals, Arctocephalus gazella (Chapter 4), or larger whales (Chapter 16). Males in these species might be able to dive deeper and exploit other food sources than their smaller female counterparts, who are limited to foraging closer to the surface. In such cases sexual segregation in diet would not be adaptive, but a by-product of sexual differences in body size.

Body size dimorphism seems a likely candidate to explain sex differences in ecology. There are, however, notable exceptions, which include some species of bats (see Chapter 15), in which sexes segregate despite a lack of size dimorphism, chimpanzees, Pan troglodytes (Chapter 17), many social carnivores, and killer whales, Orcinus orca (Chapter 16), where size-dimorphic males and females are found in mixed-sex groups. Resource limitations (Oli, 1996), the distribution and receptivity of females (chimpanzees and whales), physiological limitations (bats), predator avoidance and other factors may well explain these departures from the rule.

What causes social segregation? Although habitat segregation will invariably lead to social segregation, social segregation can occur in the absence of habitat segregation (see Chapters 2, 10, 11). Differences in social affinity (Chapter 11), sexual avoidance (Geist & Bromley, 1978), sexual differences and asynchrony in activity budgets (Chapter 10) have all been suggested to lead to social segregation in the absence of habitat segregation. Ruckstuhl (1998) and Conradt (1998a) have independently suggested that sexual differences in activity budgets could lead to social segregation. If male and female activities differ considerably the cost of synchronizing activities could be too high, leading to the fission of mixed-sex into uni-sex groups. According to this hypothesis social segregation could even occur on different levels and lead to groups assorted by sex, age and reproductive status. Bon (1991), on the other hand, suggested that innate social preferences for the same sex lead to social segregation in adulthood. Geist and Bromley (1978) further suggested that male deer, for example, would segregate from females when they had antlers and only return into female groups once they had shed their antlers. They argued that males with antlers are conspicuous to predators, while males without antlers are using female mimicry to avoid predation. Lastly, males and females might avoid each other due to increased aggression in mixed-sex groups. Increased intra-sexual aggression was shown in the Roosevelt elk, Cervus elaphus roosevelti, where females were more aggressive towards each other in the presence than in the absence of males in their group (Weckerly et al., 2001).

What then, if at all, is the relationship between social and habitat segregation? There are three equally likely scenarios one could propose. Firstly, habitat segregation leads to social segregation (Main et al., 1996). Secondly, it is possible that habitat and social segregation are two independent phenomena (Conradt, 1999). Thirdly, social segregation leads to habitat segregation, as suggested in the model by Ruckstuhl and Kokko (2002). We will not investigate these different scenarios here, as they will be dealt with in more detail in Parts II to VI of the book.

The factors that cause or constrain sexual segregation appear to vary and are difficult to differentiate. Sexual segregation could be caused by a multitude of factors. Or is it not? Is there a single or a defined set of factors that could explain sexual segregation in all vertebrates? Although it may seem unlikely, there are at least some factors that can explain social and habitat segregation across taxa. These potential candidates are sexual differences in energy requirements and sensitivity to temperatures (Chapters 2, 3, 4, 15), sexual differences in reproductive strategies and predator vulnerability (Chapters 7, 8, 9, 13 and 18) and sexual differences in social affinities and activity budgets (Chapters 10 and 11), all of which are discussed in depth in this book.

What are the ecological consequences of sexual segregation? The study of sexual segregation has important implications for studies on population viability, gene flow, management of economically relevant species and the conservation of rare species (Clemmons & Buchholz, 1997; Komdeur & Deerenberg, 1997). The abundance and spatial distribution of food may affect the spacing or habitat use of males and females differently. Hence, the further male and female ranges are apart the less likely an encounter for breeding will occur. It is therefore conceivable that gene flow in small or reintroduced populations is limited if only subgroups of males and females of adjacent home ranges meet during breeding cycles (Komdeur & Deerenberg, 1997). While differences in body size may allow males and females to exploit resources differently (as mentioned earlier), this might also expose one sex to a higher predation risk than the other. Asian elephant bulls, Elephas maximus, for example, raid crops in India and are sometimes killed because of their transgressions, while females avoid human settlements (Sukumar & Gadgil, 1988). Female wandering albatross, Diomedea exulans (see Chapter 5), or females of some fish species (Wirtz & Morato, 2001) are at higher risk than males, because their foraging range overlaps with fishery activities, where they often get trapped and killed. Such cases of sex-biased mortality could lead to local extinctions of a species, particularly if female numbers are low or populations are small (see Chapters 5 or 18). It is therefore imperative to better understand the requirements of each sex of a species in order to protect the species effectively. Chapters 18 and 19 will focus on these conservation aspects. To summarize, it is possible that sexual segregation has many different ecological consequences that need to be taken into account by conservation groups and management, such as affecting sex specific carrying capacity, sex ratio in space and time, population distribution, variability in emigration and immigration and gene flow.

What does this book cover? This book reviews and discusses the different ecological factors affecting sexual segregation in fish (Chapters 7 and 8), reptiles (Chapter 13), birds (Chapters 5, 6 and 18), bats (Chapter 15), ungulates (Chapters 2, 3, 9, 10, 11 and 19), seals (Chapter 4), whales (Chapter 16), marsupials (Chapter 14), monkeys, apes (Chapter 17) and humans (Chapter 12), and discusses important conservation aspects. The book is divided into eight parts. Part I is meant as an introduction to the topic, while Part II deals with definitions and measures of segregation. Part III describes in detail sexual differences in foraging ecology, while Part IV describes cases where predator avoidance and reproductive strategies may cause ecological segregation. Part V discusses sex related activities and social preferences and how these could cause social segregation. Part VI gives an overview of sexual differences in ecology within different taxa of vertebrates, while Part VII deals with implications for conservation and management. Part VIII includes a short conclusion and directions for future research. With this book, we portray and discuss sexual segregation in its many different forms and possible causes. By doing so, we will not only clarify ideas and concepts, but also initiate and focus on promising future research on this topic.

Part II Concepts and methodology

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© Cambridge University Press

Table of Contents

Preface; Part I. Overview: 1. Sexual segregation and the ecology of the two sexes Kathreen E. Ruckstuhl and Tim H. Clutton-Brock; Part II. Concepts and Methodology: 2. Definitions, hypotheses, models and measures in the study of animal segregation Larissa Conradt; Part III. Foraging Ecology: 3. Sex differences in the foraging ecology of large mammalian herbivores Johan T. du Toit; 4. Sexual segregation in seals Ian J. Staniland; 5. Sexual differences in foraging behaviour and diets: a case-study of wandering albatrosses Jose C. Xavier and John P. Croxall; 6. Differences in foraging behaviour and feeding ecology in giant petrels Jacob Gonzáles-Solís and John P. Croxall; Part IV. Predator Avoidance and Reproductive Strategies: 7. Predation risk as a driving factor for size assortative shoaling and its implications for sexual segregation in fish Darren P. Croft, Jens Krause and Richard James; 8. Differences in habitat selection and reproductive strategies of male and female sharks David W. Sims; 9. Sex differences in reproductive strategies affect habitat choice in ungulates Martin B. Main and Johan T. du Toit; Part V. Sex-related Activities and Social Factors: 10. Activity asynchrony and social segregation Kathreen E. Ruckstuhl and Peter Neuhaus; 11. Sexual segregation in ungulates: from individual mechanisms to collective patterns Richard Bon, Jean-Louis Deneubourg, Jean-François Gerard and Pablo Michelena; 12. Sexual segregation in humans Anthony D. Pellegrini, Jeffrey D. Long and Elizabeth A. Mizerek; Part VI. Sexual Differences in Ecology: Comparisons within Different Taxa: 13. Ecological divergence between the sexes in reptiles Richard Shine and Mike Wall; 14. Sexual segregation in Australian marsupials Abigail M. MacFarlane and Graeme Coulson; 15. Social systems and ecology of bats John D. Altringham and Paula Senior; 16. Sociality and ecology of the odontocetes Robert Michaud; 17. Sexual segregation in non-human primates David P. Watts; Part VII. Implications for Conservation: 18. Sexual segregation in birds: patterns, processes and implications for conservation Paulo Catry, Richard A. Phillips and John P. Croxal; 19. Sexual segregation: a necessary consideration in wildlife conservation Esther S. Rubin and Vernon C. Bleich; Part VIII. Outlook: 20. Conclusions and future directions Peter Neuhaus, Kathreen E. Ruckstuhl and Larissa Conradt; References; Index.

Editorial Reviews

“In addition to being a solid review, this first synthesis should serve to encourage discussion and promote further research in this area of behavioral ecology.” —Choice

From the Publisher