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DIVERSITY AND EVOLUTION OF MATING SYSTEMS
The term mating system is commonly defined as the general behavioural strategies adopted by individuals of both sexes in obtaining mates (Emlen & Oring 1977; Clutton-Brock 1989). It encompasses such features as the number of mates acquired, the manner of mate acquisition, the presence and characteristics of any pair bonds, and the patterns of parental care provided by each sex (Emlen & Oring 1977).
Parental care and party size
Although different mating systems have long been recognized in evolutionary biology, there is no strict agreement on their classification (Andersson 1994). The number of mating partners per male and female (also called party size) is one of the main criteria (Krebs & Davies 1993). It is linked to the ability of a portion of the population to control the access of others to potential mates, which depends in large part on the degree of parental care required for successful rearing of young (Emlen & Oring 1977). Four different mating systems have thus been distinguished in relation to the different patterns of parental care: (i) Monogamy (i.e. a male and a female form a pair bond) should be favored mainly when males and females share parental care for their young after birth and, in some cases, for the eggs. This is the case in the great majority of bird species (90% of birds: Lack 1968; e.g., in arctic skua Stercorarius parasiticus: O’Donald 1983), but in very few mammals (less than 3% of mammals, e.g., in several species of bats and foxes: Kleiman 1977). (ii) Polygyny (i.e. a male mates with several females, while each female mates with only one male) should be favored particularly when females are strongly predisposed to care for their young (for example, because of the constraints of a prolonged period of gestation and lactation). This is the case in most mammals for which males usually provide little parental care after birth and so females take sole responsibility for the care of the young (e.g., in northern elephant seal Mirounga angustrirostris: LeBoeuf 1974). (iii) Polyandry (i.e. a female mates with several males, while each male mates with only one female) is more infrequent and mainly appears when the male provides most parental care (e.g., in pipefishes and seahorses: Berglund et al. 1986; in spotted sandpipers Actitis macularia: Oring & Lank 1982). (iv) Promiscuity (i.e. both male and female mate several times with different individuals so that there is a mixture of polygyny and polyandry) should develop when either sex may care for the eggs or young (e.g., in Soay sheep Ovis aries: Coltman et al. 1999a).
Strategy of monopolization of mates and evolution of mating systems
In mammals, which are commonly polygynous since females commonly provide most of the parental care, the reproductive success of females is mainly limited by access to the resources necessary to breed and to meet the energy requirements of gestation and lactation (e.g., food, breeding sites), whereas the reproductive success of males is limited more by access to females (Trivers 1972). As a consequence, while the distribution of females should depend primarily on resource dispersion (modified by predation pressure and the costs and benefits of associating with other individuals), males should distribute themselves in relation to female dispersion (modified by male density) in order to gain access to as many mates as possible (Emlen & Oring 1977; see Figure 3).
THE THEORY OF SEXUAL SELECTION
Mating systems, as defined by Emlen & Oring (1977), actually encompass both sexual selection (specifically the number of mates obtained and the manner in which they are acquired) and parental care (Reynolds 1996). Hence, if we want to better understand the evolution of mating systems, the study of sexual selection is pivotal (Emlen & Oring 1977).
Origin of the competition over mates
Sexual reproduction is commonly anisogamous: females produce a few large macrogametes rich in energy (eggs), whereas males produce a lot of small, highly mobile microgametes (sperm). As a consequence, while females need only one male to be fertilized and father a particular litter of young males, males have the capacity to fertilize a number of females (Bateman 1948). Trivers (1972) has expressed this difference in the size of gametes in terms of parental investment between the sexes: females usually allocate more energy to parental investment (including investment in the production of gametes, in the nourishing of the embryo and in the care of offspring) than males and so males can father more young than females. Hence, males usually have a much greater potential rate of reproduction than females. This has consequences in terms of optimal mating strategies for the two sexes. While males should attempt to mate with as many fecund females as possible, females should try to choose the highest quality males as mates. As a result, many sexually active males are searching at a given time for relatively few receptive females. As Bateman (1948) stated, “the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete”. This sets up a competition among males for the scarce resource which is females and allows females to choose their partner, that is manifest as sexual selection. Notice however that in some cases (for example, when males provide nutrition for females, thus enhancing female fecundity), males can be the scarce resource too and then females compete between themselves for access to males (Gwynne & Simmons 1990; Gwynne 1991).
Darwin’s theory of sexual selection
The observation of conspicuous traits (in appearance or behaviour) that defied explanation by ordinary natural selection for improved survival and which occur only in one sex, especially during the breeding season, lead Darwin to the conclusion that these traits may be favored by competition over mates rather than by the struggle for existence (Darwin 1859). Classic examples of such elaborate traits are the train of peacocks (Pavo spp.), the ornaments of birds of paradise and the antlers of deer. These observations were at the origin of Darwin’s theory of sexual selection, later developed in The Descent of Man, and Selection in Relation to Sex published in 1871. The objective of sexual selection theory is thus to explain the evolution of such differences.
Darwin (1859) drew a clear distinction between sexual selection and natural selection, because he saw them as forces which are often in opposition. As Darwin (1859) put it, sexual selection depends: « not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. » Sexual selection is nowadays often considered as a special case of natural selection (e.g., Mayr 1972). Natural and sexual selection would be part of an unitary process which can operate on variation in fitness generated for various reasons at several life-history stages (Clutton-Brock 2003).
Darwin (1859, 1871) defined sexual selection as selection acting on individual traits of one sex (usually males) that affects mating success or fertilization and which arises from competition over mates. If the differences in the traits are hereditary, sexual selection will lead to spread of those attributes that enhance reproductive success (named sexually selected traits). Sexual selection of a trait can then be viewed as differences in reproductive success, caused by competition over mates, and related to the expression of the trait (Andersson 1994).
Darwin (1871) recognized two main ways by which sexual selection can act. First, it could arise through competition between males to gain access to females. This form of competition for mates has been later called intra-sexual selection by Huxley (1938). Second, it could arises through female mate choice, which consists of competition between males to attract females and should lead to the elaboration of conspicuous structures or behavioural signals that attract females and that are reliable indicators of male phenotypic quality or sexual vigor (e.g., body condition, ornament, vocalizations, patch color). This form of competition for mates has been later called epigamic selection by Huxley (1938) and inter-sexual selection by others. In practice, these two mechanisms may occur together and may favor the same traits (e.g., the antlers of deer may be considered as weapons but also as epigamic ornamental traits).
The various forms of intra-sexual selection
Intra-sexual selection is not restricted to male-male combat. It can take several forms and favor a wide range of attributes (Andersson 1994):
• contest competition, which consists in direct interactions between males, such as display and/or fight, for access to females, or resources needed to attract mates, and is expected to favor the development of male traits that enhance their strength and their ability to fight and dominate (e.g., body mass or size, weapon size).
• scramble competition, which is linked to the rapidity of location of a mate, and is expected to favor sensory and locomotory organs, or protandry.
• endurance rivalry competition, which is linked to the ability of males to remain reproductively active and maintain a top rank during breeding season, the amount of time that a male can display without leaving for food or water.
• sperm competition (e.g., mate guarding, sequestering, ability of displacing rival sperm; see Eberhard 1985), which should favor production of abundant sperm and structures to store sperms. Male-male competition indeed does not stop when copulation is over. The true determinant of a male’s mating success is not whether he copulates, but whether his sperm actually fertilize eggs. If a female mates with multiple males within a short period of time, there will be a race among the males to successfully fertilize her eggs.
Table of contents :
INTRODUCTION
I. GENERAL INTRODUCTION: CONTEXT OF THE STUDY
II. MATING SYSTEMS AND SEXUAL SELECTION: WHAT THE THEORY TELLS US
II.1. DIVERSITY AND EVOLUTION OF MATING SYSTEMS
II.1.1. Parental care and party size
II.1.2. Strategy of monopolization of mates and evolution of male mating systems
II.2. THE THEORY OF SEXUAL SELECTION
II.2.1. Origin of the competition over mates
II.2.2. Darwin’s theory of sexual selection
II.2.3. The various forms of intra-sexual selection
II.2.4. Inter-sexual selection and evolution of female mate choice
II.3. VARIATION IN SEXUAL SELECTION AMONG SPECIES: THE THEORETICAL POINT OF VIEW
II.3.1. How to measure sexual selection?
II.3.2. Variation in the opportunity for sexual selection
II.3.3. Variation in sexually selected traits and mechanisms of competition over mates
III. MATING SYSTEMS AND SEXUAL SELECTION: NEW INSIGHTS FROM EMPIRICAL STUDIES
III.1. THE IMPORTANCE OF MAMMALS, AND ESPECIALLY UNGULATES, AS A STUDY MODEL
III.2. A NEW POWERFUL APPROACH BASED ON MOLECULAR TECHNIQUES
III.3. PARTY SIZE, SEXUAL DIMORPHISM AND THE OPPORTUNITY FOR SEXUAL SELECTION: A COMPLEX LINK
III.4. MATING SYSTEMS, SEXUALLY SELECTED TRAITS AND MECHANISMS OF COMPETITION OVER MATES: THE CASE OF THE UNGULATES
III.4.1. Body size and mass
III.4.2. Antler and horn size
III.4.3. Antler and horn asymmetry
III.4.4. Territory size, location, and habitat quality
III.4.5. Chemical signals
III.4.6. Acoustic signals
III.4.7. Visual signals
III.5. CONCLUSION: LACK OF DATA IN SPECIES WITH LOW SEXUAL SIZE DIMORPHISM AND/OR TERRITORIAL MATING SYSTEM IN UNGULATES
IV. THE STUDY: MALE REPRODUCTIVE SUCCESS, THE EVOLUTION OF TERRITORIALITY AND SEXUAL SELECTION IN ROE DEER
IV.1. THE STUDY SPECIES: THE EUROPEAN ROE DEER, A TERRITORIAL SPECIES WITH LOW SEXUAL SIZE DIMORPHISM
IV.2. SUBJECT OF THE STUDY
IV.3. STRUCTURE OF THE STUDY
MATERIALS AND METHODS
I. STUDY POPULATIONS
II. THE METHODS: A COMBINATION OF MOLECULAR TOOLS AND FIELD ECOLOGY STUDIES
II.1. LONGITUDINAL DATA AND POPULATION SAMPLING (all 3 populations)
II.2. MOLECULAR ANALYSES (Bogesund and Trois Fontaines)
II.2.1. Considerations on the lifetime breeding success as proxy of fitness
II.2.2. Tissue sampling
II.2.3. Microsatellite genotyping
II.2.4. Test of the suitability of microsatellite markers
II.2.5. Limitation, identification and quantification of genotyping errors
II.2.6. Paternity analyses
II.2.7. Estimation of male breeding success and the opportunity for sexual selection
II.3. FIELD ECOLOGY STUDIES (Bogesund)
II.3.1. Fawn capture
II.3.2. Pellet-group counts and estimation of a local population abundance
II.3.3. Capture-Mark-Resighting (Petersen method)
II.3.4. Radio-tracking and estimation of the size of males’ territory
II.3.5. Vegetation sampling and map of habitat quality
PART 1: VARIATION IN MALE BREEDING SUCCESS AND OPPORTUNITY FOR SEXUAL SELECTION
PAPER 1: MATING SYSTEM, SEXUAL DIMORPHISM AND THE OPPORTUNITY FOR SEXUAL SELECTION IN A TERRITORIAL UNGULATE
PART 2: AGE-RELATED VARIATION IN MALE BREEDING SUCCESS
PAPER 2: AGE-SPECIFIC VARIATION IN MALE BREEDING SUCCESS AND MATING TACTICS OF ROE DEER
PART 3: PHENOTYPIC TRAITS AS SEXUALLY SELECTED TRAITS
PAPER 3: ANTLER SIZE PROVIDES AN HONEST SIGNAL OF MALE
PHENOTYPIC QUALITY IN ROE DEER
PAPER 4: IS BIGGER BETTER IN A LOW SEXUAL SIZE DIMORPHIC UNGULATE? CONTINUED SEXUAL SELECTION ON MALE BODY MASS AND ANTLER SIZE IN ROE DEER
PART 4: ROLE OF THE TERRITORY IN MALE BREEDING SUCCESS AND EVOLUTION OF TERRITORIALITY
PAPER 5: ACCESS TO MATES IN ROE DEER IS DETERMINED BY THE SIZE OF
A MALE’S TERRITORY, BUT NOT ITS HABITAT QUALITY
PART 5: A FIRST STEP TOWARDS THE FEMALE POINT OF VIEW: FEMALE MULTIPLE MATING
PAPER 6: MULTIPLE PATERNITY IS LIKELY BUT INFREQUENT IN THE
POLYTOCOUS EUROPEAN ROE DEER
DISCUSSION
I. SYNTHESIS OF MAIN RESULTS
II. DISCUSSION AND PERSPECTIVES
II.1. VARIANCE IN MALE BREEDING SUCCESS AND THE OPPORTUNITY FOR SEXUAL
SELECTION IN ROE DEER
II.1.1. First data on male lifetime breeding success in roe deer based on molecular analyses
II.1.2. An attempt to compare estimates of the variance in male lifetime reproductive success between different ungulate species
II.1.3. New insights on the pattern of variation in the opportunity for sexual selection among ungulate species
II.2. DETERMINANTS OF THE VARIANCE IN MALE BREEDING SUCCESS AND SEXUALLY SELECTED TRAITS
II.2.1. Age
II.2.2. Body mass and antler size
II.2.3. The abundance of females in territories
II.2.4. Territory size and habitat quality within the male’s territory
II.2.5. Other potential sexually selected traits
II.2.6. New insights into the adaptive function and evolution of male territoriality
II.3. COMPARISON OF THE EXTENT AND CAUSES OF VARIATION IN BREEDING SUCCESS IN THE TWO SEXES
II.3.1. Comparison of the distribution of lifetime breeding success
II.3.2. Comparison of the effects of age on breeding success and survival
II.3.3. Comparison of the ultimate factors that affect breeding success
II.3.4. New insights into the evolution of the low sexual size dimorphism in roe deer
II.4. ALTERNATIVE MATING TACTICS IN YOUNG AND OLD MALES
II.5. THE ROLE OF FEMALES IN ROE DEER SEXUAL SELECTION
II.6. LET’S GO FURTHER: FROM THEORETICAL KNOWLEDGE ON MATING SYSTEMS AND SEXUAL SELECTION TO APPLIED ISSUES IN CONSERVATION AND POPULATION MANAGEMENT
II.7. CONCLUSION
II.8. PERSPECTIVES
REFERENCES
