Changes In Mating System Research Articles

Differences in fine-scale spatial genetic structure across the distribution range of the distylous forest herb Pulmonaria officinalis (Boraginaceae)

BackgroundGeographical ranges of plants and their pollinators do not always entirely overlap and it has been suggested that the absence of specialized pollinators at range margins may induce changes in mating systems. Because a species’ mating system is known to have a considerable effect on within-population pollen movement, the extent of fine-scale spatial genetic structure (SGS) can be expected to differ between populations located at different parts of their geographical range. To test this prediction, we compared the fine-scale SGS between two core and two disjunct populations of the distylous forest herb Pulmonaria officinalis. Because in disjunct populations of this species the heteromorphic self-incompatibility system showed relaxation in the long-styled morph, but not in the short-styled morph, we also hypothesized that the extent of fine-scale SGS and clustering differed between morphs.ResultsSpatial autocorrelation analyses showed a significant decrease in genetic relatedness with spatial distance for both core and disjunct populations with the weakest SGS found in one of the core populations (Sp = 0.0014). No evidence of stronger SGS in the long-styled morph was found in the center of the range whereas one disjunct population showed a significantly (P = 0.029) higher SGS in the long-styled morph (SpL = 0.0070) than in the short-styled morph (SpS = 0.0044).ConclusionsConsistent with previous analyses on distylous plant species, we found weak, but significant spatial genetic structure. However, the extent of SGS varied substantially between populations within regions, suggesting that population characteristics other than mating system (e.g. local pollinator assemblages, population history) may be as important in determining variation in SGS.

Book review: Plants and Islands revisited

news and update ISSN 1948‐6596 book review Plants and Islands revisited The Biology of Island Floras, edited by David Bramwell & Juli Caujape‐Castells, 2011, Cambridge University Press, 522 pp. $120 (hardback) ISBN: 978‐0‐521‐11808‐8; http://www.cambridge.org In April 1977, David Bramwell, at the time recently appointed as director of the Canarian Botanical Garden Viera y Clavijo in Gran Canaria (Canaries, Spain), hosted a scientific meeting commemorat‐ ing the 25 th anniversary of the garden. Two years later this yielded an excellent book, appropriately entitled Plants and Islands (Bramwell, 1979), which became a classic reference on the state of knowledge on island evolution and conservation. It signalled the end of a golden age of island‐ themed books that began with Sherwin Carlquist's Island Life (1965), and included Robert MacArthur and Edward O. Wilson’s The Theory of Island Bio‐ geography (1967), Carlquist’s Island Biology (1974) and David Lack’s posthumous Island Biol‐ ogy (1976). Thirty‐two years later, close to his retire‐ ment, David Bramwell, together with the current Head of the Department of Molecular Biodiversity at the Botanical Garden, Juli Caujape‐Castells, have edited a new text. In it they revisit the sce‐ nario of island plants, but now in light of the geo‐ logical, molecular and phylogeographic tools de‐ veloped since the publication of the original Plants and Islands which have impressively enhanced our understanding of the biology of island floras. The book encompasses 21 chapters written by 48 relevant authors (of these only two are sur‐ vivors of Plants and Islands: David Bramwell and Vernon Heywood) plus a preface and subject in‐ dex. An introduction prepared by David Bram‐ well, in which the structure of the book is ex‐ plained, is followed by two interesting generalist chapters. These reflect the development, over the last three decades, of the state of knowledge on “The reproductive biology of island plants” and “Spatial methodologies in historical biogeography of islands.” In the former, Daniel Crawford et al. present an updated and easy‐to‐read synthesis, stressing the importance of comparing island plants with their continental relatives for under‐ standing the changes in mating systems induced by the selective pressure existing on islands. In the latter, Paula Posadas and colleagues provide a simple and straightforward example‐based expla‐ nation of the new possibilities that appear when phylogenetic data and spatial distribution tech‐ niques in biogeography are combined. In the central part of the book, the present knowledge on the origin and evolution of the flora of the more important islands and archipelagos worldwide are given, including Hawai’i, Galapa‐ gos, the Caribbean islands, Madagascar, Socotra, Macaronesia, Pitcairn, New Zealand and New Caledonia. From this part I would like to highlight the high level of all the contributions. Those of par‐ ticular interest may include the highly informative “Origins and evolution of the Galapagos vascular plants” by Alan Tye & Javier Francisco‐Ortega, and the chapters written by Juli Caujape‐Castells (“Jesters, red queen, boomerangs and surfers: a molecular outlook on the diversity of the Canarian endemic flora”) and by Alain Vanderpoorten et al. (“Dispersal, diversity and evolution of the Macaronesian cryptogamic floras”) in which an audacious interpretation of the role of Macarone‐ sia, and hence of islands worldwide, as climate refugia for continental back‐colonisation is sup‐ ported. Unfortunately the list of islands included is incomplete. Although a plethora of literature has been recently published in this respect, an up‐to‐ date summary of present knowledge on the origin and evolutionary features of many islands and archipelagos that have contributed significantly to our understanding of insular floras is missing. Among them are the Mediterranean islands, Sey‐ chelles, Mascarenes, Juan Fernandez, South Atlan‐ tic islands and French Polynesia. The conservation‐focused last part of the book, for me the less polished, includes several general chapters, such as “Invasive alien species © 2012 the authors; journal compilation © 2012 The International Biogeography Society — frontiers of biogeography 4.3, 2012

Co-evolution between transposable elements and their hosts: a major factor in genome size evolution?

Most models of genome size evolution emphasize changes in relative rates of and/or the efficacy of selection on insertions and deletions. However, transposable elements (TEs) are a major contributor to genome size evolution, and since they experience their own selective pressures for expansion, genome size changes may in part be driven by the dynamics of co-evolution between TEs and their hosts. Under this perspective, predictions about the conditions that allow for genome expansion may be altered. In this review, we outline the evidence for TE-host co-evolution, discuss the conditions under which these dynamics can change, and explore the possible contribution to the evolution of genome size. Aided partly by advances in our understanding of the mechanisms of TE silencing via small RNAs, there is growing evidence that the evolution of transposition rates can be important in driving genome expansion and contraction. Shifts in genome size and transposon abundance associated with interspecific hybridization and changes in mating system are consistent with an important role for transposition rate evolution, although other possible explanations persist. More understanding of the potential for the breakdown of host silencing mechanisms and/or the potential for TEs to evade host immune responses will improve our understanding of the importance of changes in TE activity in driving genome size evolution.

Mating system change reduces the strength of sexual selection in an American frontier population of the 19th century

Sexual selection, or competition among members of one sex for reproductive access to the other, is one of the strongest and fastest evolutionary processes. Comparative studies support the prediction that sexual selection is stronger in polygamous than in monogamous species. We report the first study of the effect on sexual selection of a change in mating system, from polygyny to monogamy, within a historical human population. Here we show that over the reproductive lifetimes of Utahns born between 1830 and 1894, socially induced reductions in the rate and degree of polygamy correspond to a 58% reduction in the strength of sexual selection. Polygyny conferred a strong advantage to male fitness as well as a weak disadvantage to female fitness. In contrast, mating with multiple males provided little benefit to females in this population. Polygamy benefitted males by increasing reproductive rates and by lengthening reproductive tenure. Each advantage contributed to roughly half of the increased total lifetime reproductive success. This study illustrates both the potency of sexual selection in polygynous human populations and the dramatic influence that short-term societal changes can have on evolutionary processes.

Pedigree analysis for conservation of genetic diversity and purging

We present an approach to describe and evaluate changes in genetic diversity and to calculate bounds for improvement. This pedigree-based analysis was applied to the Kromfohrländer dog (FCI Gr9 Sec10). Pedigrees trace back to the foundation of the breed and were available for 5527 individuals. Based on this dataset the population structure and historical bottlenecks were studied. Distributions of allele frequencies were estimated by Monte Carlo simulation. To monitor changes in mating systems throughout the breeding history, the homozygosity of alleles was compared with their expectations in Hardy-Weinberg equilibrium. Different breeding lines were identified by hierarchical cluster analysis and were characterized by ancestor contributions. Our calculations showed that the founder event in 1945 was followed by two bottlenecks. One was caused by strong selection in a very small population, and the other was triggered by rigorous disease management. The necessary amount of purging that arised due to the bottlenecks was also discussed.

Modification of flower architecture during early stages in the evolution of self-fertilization.

The evolution of selfing from outcrossing is characterized by a series of morphological changes to flowers culminating in the selfing syndrome. However, which morphological traits initiate increased self-pollination and which are accumulated after self-fertilization establishes is poorly understood. Because the expression of floral traits may depend on the conditions experienced by an individual during flower development, investigation of changes in mating system should also account for environmental and developmental factors. Here, early stages in the evolution of self-pollination are investigated by comparing floral traits among Brazilian populations of Eichhornia paniculata (Pontederiaceae), an annual aquatic that displays variation in selfing rates associated with the breakdown of tristyly to semi-homostyly. Thirty-one Brazilian populations under uniform glasshouse conditions were compared to investigate genetic and environmental influences on flower size and stigma-anther separation (herkogamy), two traits that commonly vary in association with transitions to selfing. Within-plant variation in herkogamy was also examined and plants grown under contrasting environmental conditions were compared to examine to what extent this trait exhibits phenotypic plasticity. In E. paniculata a reduction in herkogamy is the principal modification initiating the evolution of selfing. Significantly, reduced herkogamy was restricted to the mid-styled morph and occurred independently of flower size. Significant genetic variation for herkogamy was detected among populations and families, including genotypes exhibiting developmental instability of stamen position with bimodal distributions of herkogamy values. Cloned genets exposed to contrasting growth conditions demonstrated environmental control of herkogamy and genotypic differences in plasticity of this trait. The ability to modify herkogamy independently of other floral traits, genetic variation in the environmental sensitivity of herkogamy, and the production of modified and unmodified flowers within some individuals, reveal the potential for dynamic control of the mating system in a species that commonly confronts heterogeneous aquatic environments.

Genetic structure of Cirsium palustre (Asteraceae) and its role in host diversification of Tephritis conura (Diptera: Tephritidae)

Whether or not a host plant is incorporated into a phytophagous insect's diet depends on both the insect's ability to colonize the novel host and the host plant's susceptibility to the insect. The latter, again, will be influenced by the genetic structure of the host plant. Cirsium palustre (marsh thistle) is heavily infested by the tephritid fly Tephritis conura in northern Britain, whereas infestation is not only absent in southern England, but also absent on the European continent where T. conura is common on other Cirsium. To understand why regional infestation patterns evolve, we studied how genetic structure and phylogeographic ancestry of C. palustre are related to the probability of infestation in Britain. The probability of infestation in Britain was related neither to phylogeographic lineage (two lineages were observed in Europe), nor to introgressive hybridization with the original host of T. conura, C. heterophyllum. However, infested British C. palustre were strongly genetically deprived. British C. palustre showed continuous loss of genetic variability towards the northern range edge. The loss was explained best by genetic drift during geographic range expansion, rather than by genetic drift in isolated patches or mating system change from predominately outcrossing to selfing. The phylogeographic pattern of C. palustre strongly suggests Scotland as the geographic origin of a recent host-plant expansion of T. conura. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 221–232.

Agricultural management affects evolutionary processes in a migratory songbird

Hay harvests have detrimental ecological effects on breeding songbirds, as harvesting results in nest failure. Importantly, whether harvesting also affects evolutionary processes is not known. We explored how hay harvest affected social and genetic mating patterns, and thus, the overall opportunity for sexual selection and evolutionary processes for a ground-nesting songbird, the Savannah sparrow (Passerculus sandwichensis). On an unharvested field, 55% of females were in polygynous associations, and social polygyny was associated with greater rates of extra-pair paternity (EPP). In this treatment, synchrony explained variation in EPP rates, as broods by more synchronous females had more EPP than broods by asynchronous females. In contrast, on a harvested field, simultaneous nest failure caused by haying dramatically decreased the overall incidence of EPP by increasing the occurrence of social monogamy and, apparently, the ability of polygynous males to maintain paternity in their own nests. Despite increased social and genetic monogamy, these haying-mediated changes in mating systems resulted in greater than twofold increase in the opportunity for sexual selection. This effect arose, in part, from a 30% increase in the variance associated with within-pair fertilization success, relative to the unharvested field. This effect was caused by a notable increase (+110%) in variance associated with the quality of social mates following simultaneous nest failure. Because up to 40% of regional habitat is harvested by early June, these data may demonstrate a strong population-level effect on mating systems, sexual selection, and consequently, evolutionary processes.

Mating Consequences of Polyploid Evolution in Flowering Plants: Current Trends and Insights from Synthetic Polyploids

The evolutionary transition from diploidy to polyploidy is prevalent in flowering plants and may result in correlated changes in mating system (outcrossing rate). Most theory predicts a shift toward self‐fertilization (decrease in outcrossing) in polyploids, but empirical evidence for this pattern and its underlying mechanisms is inconclusive or restricted to a few cases. In an analysis of variation in outcrossing rates among diploid‐polyploid species pairs from the literature, polyploids had lower outcrossing rates (higher selfing; $$t=0.23$$, $$\mathrm{SE}\,=0.09$$) than diploids ($$t=0.52$$, $$\mathrm{SE}\,=0.12$$). Among polyploids, however, allopolyploids were predominantly selfing ($$t=0.20$$, $$\mathrm{SE}\,=0.099$$), whereas autopolyploids had significantly higher outcrossing rates ($$t=0.64$$, $$\mathrm{SE}\,=0.087$$), raising the question of what limits the evolution of selfing in autopolyploids. To address this, we examined the magnitude of inbreeding depression in synthetic polyploids of the plant Chamerion angustifolium. The intrinsic cost of selfing in newly formed polyploids was negligible compared with extant polyploids, thus promoting the spread of selfing. However, there was weak evidence that inbreeding depression increases with history of inbreeding, suggesting that the rise in selfing may be ephemeral and that selection ultimately favors mixed or outcrossed mating systems in autopolyploids. Such constraints on selfing have some theoretical support, but additional research on patterns of variation and genetic mechanisms governing polyploid mating systems are needed.

Role of sexual and natural selection in evolution of body size and shape: a phylogenetic study of morphological radiation in grouse

We use standardized independent contrasts (SICs) to elucidate the effect of ecology and mating systems on morphological radiation in grouse. The analysis of SICs for 38 skeletal measurements from 20 taxa, showed that changes in mating system had a significant effect on body size of both sexes. Sexual size dimorphism in grouse is consistent with Rensch's rule; the slope of the regression of male vs. female size SICs was 1.4, significantly >1. Changes in habitat were associated with accelerated rates of evolution of body proportions. SICs for male and female scores of size independent factors were directly proportional to each other (slope = 1), indicating extreme similarities between male and female ecology. Females, however, were better adapted to longer, more energy efficient flight than males. Size independent morphological differences among grouse are adaptive and are related to the differences in habitat and foraging behaviour among the species.

Variations in bumble bee preference and pollen limitation among neighboring populations: comparisons between Phyllodoce caerulea and Phyllodoce aleutica (Ericaceae) along snowmelt gradients

Two alpine-snowbed shrubs, Phyllodoce caerulea and P. aleutica (Ericaceae), co-occur in locales in northern Japan with early to late snowmelt, but they have different mating systems. Phyllodoce caerulea is an obligate outcrosser in any population, whereas the selfing ability of P. aleutica is highly variable among neighboring populations along snowmelt gradients: it shows high self-compatibility in early to middle snowmelt populations but low self-compatibility in late snowmelt populations. We investigated the relationships between pollinator availability and mating systems of these species along three snowmelt gradients. Relative abundance of flowers and nectar standing crop of P. caerulea decreased from early to late snowmelt plots. Bumble bees preferred P. caerulea to P. aleutica in early and middle snowmelt plots, while their preference shifted to P. aleutica in late snowmelt plots. Pollen limitation was severe in P. aleutica in early to middle snowmelt plots but it was severe in P. caerulea in late snowmelt plots. Seed-set success under natural conditions of P. aleutica was higher than that of P. caerulea in all plots. Thus, we infer that the selfing ability of P. aleutica under pollinator limitation acts as a reproductive assurance. We conclude that the interaction through pollination between the sympatric species is strong enough to cause a phenotypic change in mating system even within a local area.

Ecology and evolution of extravagant feather ornaments

Abstract The ancestral conditions that permit the evolution of extravagant secondary sexual characters are of considerable theoretical and empirical interest because they allow identification of necessary ecological conditions, but also allow empirical tests of models of female mate preferences. We investigated the ancestral and derived state of a range of ecological and evolutionary variables that might have been implicated in the evolution of secondary sexual characters. Extravagant feather ornaments have evolved independently at least 70 times in birds, and the context of these evolutionary events was investigated statistically. The acquisition of feather ornaments was significantly associated with a change in social mating system from monogamy to polygyny or lekking. This association is consistent with the Fisherian mechanism of sexual selection. However, very often also the acquisition of feather ornaments occurred without change in mating system. Therefore, ornamentation can develop for reasons other than polygyny. We did not find any indication of male parental care, kind of food, foraging mode, coloniality, nest site, migration or body mass being significantly associated with a change in the state of ornamentation.

Intrasexual selection and testis size in strepsirhine primates

The main objective of this study was to investigate two predictions of sexual selection theory concerning interspecific variation in testis size among strepsirhine primates (Lemuriformes and Lorisiformes). First, the unique evolutionary history of lemurs provides an opportunity for an independent test of the predictions of sperm competition theory regarding die relationship between mating system and relative testis size. Second, I examined the evolutionary relationship between the morphological correlates of preand postcopulatory competition (Le., between sexual dimorphism and testis size) because polygamous lemurs, in contrast to other polygamous primates, lack sexual dimorphism. Based on measurements from 174 captive strepsirhines from 24 species, I found that multi-male species had significantly larger testes than pair-Irving ones, but that they did not differ significantly from solitary species. This result deviates from theoretical expectations, but may be the result of yet-unknown heterogeneity in mechanisms of male-male competition in both multi-male and solitary species. There was no difference in relative testis size between nonmonogamous lemurs and lorises, indicating that presumably lower levels of precopulatory competition are not necessarily compensated by more intense sperm competition. Body size and phylogenetic effects were also found to considerably affect interspecific variability in testis size. Analyses of independent contrasts revealed that evolutionary changes in mating system, testis size, sexual size, and canine dimorphism were not, or only weakly, associated in this monophyletic group of primates. Additional comprehensive comparative studies of sexual dimorphism, testis size, mating system, and copulatory behavior in these and other taxa are indicated to illuminate general patterns and causes of covariation among these traits. Kty words: mating system, primates, sexual dimorphism, sexual selection, sperm competition, testis. [Behav Ecot 8:10-19 (1997)]

EFFECTIVE POPULATION SIZE AND GENETIC DRIFT IN TRISTYLOUS EICHHORNIA PANICULATA (PONTEDERIACEAE).

Populations of the tristylous, annual Eichhornia paniculata are markedly differentiated with respect to frequency of mating types. This variation is associated with evolutionary changes in mating system, from predominant outcrossing to high self-fertilization. To assess the potential influence of genetic drift acting on this variation, we estimated effective population size in 10 populations from northeastern Brazil using genetic and demographic methods. Effective size (Ne ) was inferred from temporal changes in allele frequency at two to eight isozyme loci and also calculated using five demographic variables: 1) the number of flowering individuals (N); 2) temporal fluctuations in N; 3) variance in flower number; 4) frequency of mating types; and 5) selfing rate. Average Ne based on isozyme data was 15.8, range 3.4-70.6, and represented a fraction (mean Ne /N = 0.106) of the census number of individuals (mean N = 762.8; range: 30.5-5,040). Temporal variation in N and variance in flower number each reduced Ne to about a half of N whereas mating type frequencies and selfing rate caused only small reductions in Ne relative to N. All estimates of Ne based on demographic variables were considerably larger than those obtained from genetic data. The two kinds of estimates were in general agreement, however, when all demographic variables were combined into a single measure. Monte Carlo simulations indicated that effective size must be fewer than about 40 for drift to overcome the frequency-dependent selection that maintains the polymorphism for mating type. Applying the average Ne /N value to 167 populations censused in northeastern Brazil indicated that 72% had effective sizes below this number. This suggests that genetic drift is likely to play a dominant role in natural populations of E. paniculata.

SEXUAL SELECTION AND BUTTERFLY DESIGN-A COMPARATIVE STUDY.

Temperate butterflies of 44 species were examined to determine if their mating system (perching and patrolling) affected flight design. To control for spurious effects due to ancestry, 25 of these species were assigned to eight contrasts within which a change in mating system had occurred. In perching species sexual selection was predicted to favor traits associated with high acceleration ability and speed, while in patrolling species traits associated with flight endurance were predicted. In conformance with these expectations males of perching species had larger thorax/body mass ratios, higher wing loadings, and higher aspect ratios than patrolling species. The male mating system affected females in the same direction in the same variables as males. This could be explained by a genetic correlation with males. When removing the covariance between the sexes, only male design was explained by the mating system. The mating system was also associated with different degrees of sexual dimorphism in wing size. This supported the hypothesis that male design was affected by the mating system.

A phylogenetic interpretation of sexual dimorphism in body size and ornament in relation to mating system in birds

AbstractSexual dimorphism in body size and ornament is commonly assumed to be a result of sexual selection. Therefore, a pattern among species is expected where the type of mating system correlates with the degree of sexual dimorphism. This was analysed using comparative data on birds using an autocorrelation model. Mating system did correlate with sexual dimorphism in ornament (tail), but was only weakly correlated with sexual size (tarsus) dimorphism. Polygynous species were significantly larger than monogamous species. After correcting for size, there was no difference in size dimorphism between polygynous and monogamous species. The reverse was true for tail dimorphism; polygynous species exhibited a larger degree of tail dimorphism, but there was no difference in absolute size of the tail. This pattern suggests that an evolutionary change in mating system is associated with a change in overall size, and in degree of dimorphism in secondary sexual traits.

The evolution of sex-change mechanisms in fishes

Five distinct sex-change mechanisms are identified among sequentially hermaphroditic fishes based on socio-ecological characteristics. The primary determinants of the sex-change mechanisms appear to be social organization and mating system, which in turn depend on resource distribution in space and time. The ability of a single individual to control all mating in the social unit, which is related to the size of the social unit, differentiates three suppression mechanisms from two induction mechanisms. Sex-change suppression, which is characteristic of species with small group size and rigid dominance hierarchies, refers to inevitable sex change in the absence of group dominance. Ability to migrate between resource patches differentiates protogynous suppression (e.g. inLabroides dimidiatus) from protandrous suppression (e.g. inAmphiprion spp.). Early sex change appears to have evolved from protogynous suppression under special conditions involving the loss of mating control by a single dominant individual in certain species (e.g.Centropyge spp. ). Sex-change induction, which is characteristic of species with large social groups lacking rigid dominance hierarchies, refers to the requirement that sex change must be induced by specific characteristics of (or changes in) the social group, regardless of dominance status. Ability to distinguish sex, or its importance, differentiates sex-ratio induction (e.g.Anthias squamipinnis) from size-ratio induction (e.g.Thalassoma spp.). Alternative models account for the possibility that all cases of sex change require stimulation from smaller conspecifics (universal induction-inhibition model) or that all fish have the genetic capacity to switch mechanisms, depending on changing ecological conditions and resulting changes in mating system (behavioral-scaling model). Neurophysiological models suggest that induction mechanisms, which require at least two categories of environmental stimuli, may have evolved from the simpler suppression mechanisms, which require only one kind of input from the environment.

Mating system shift in a pronghorn population

At the National Bison Range (western Montana, USA), pronghorn (Antilocapra american) maintained a territorial mating system from as early as 1965 through 1978. Preliminary observations in 1981 suggested that the mating system had changed. Data from the ruts, 1982–1984 revealed a progressive decay in territoriality. In this paper, data from the “territorial years” 1969–1978 are contrasted with data from the “decay years” 1982–1984, with results as follow: 1. In 1982–84, fewer territories were defended than in 1969–78. This was attributable to a smaller proportion of males defending territories in 1982–84, not to a smaller number of males. 2. In 1982–84, most territory owners either abandoned their territories early in ruts, or lost control of females on them following frequent, persistent intrusions by non-territorial males. Abandonment and loss of control did not occur in 1969–78. 3. In 1982–84, territorial males that maintained control of females on their territories did so by shrinking their zones of defense to small areas around female groups. 4. In 1982–84, following the disruption or severe disturbance of all territories, many females left territories, and mated elsewhere, with non-territorial males. In 1969–78, most females remained on territories throughout rut, and mated with territory owners. —The mating system change followed catastrophic winter mortality, 1978–79, that removed 75% of the males, including all males older than 5 years, and all male fawns, from the population. In 1982–84, the number of males present was not different from the number of males in 1969–1978, but the frequency distribution of male ages was strongly shifted toward younger ages. The small number of older males, 1982–84, likely resulted in smaller proportions of males initially defending territories, and in less effective territory defense. When females then clustered on the few territories where defense was at first successful, they attracted large numbers of non-territorial males. The resulting high rates of raids on these territories, coupled with reduced defense radii by territorial males, allowed females only 12% reclining time (summer percentage was 39%). This increased energy cost, plus an apparently greater risk of injury on weakly defended territories, appeared to prompt many females to seek calmer matings elsewhere. Also, if female pronghorn practiced mate selection based upon horn or body size, they may have reduced their efforts to remain on territories in 1982–84. Males from 1969–78 were larger than males from 1982–84, and showed greater variance in horn size. At least two conditions appear to influence the tendency of males to be territorial. First, males must be at least three years old before they attempt to defend a territory. Second, the declining proportion of males defending territories, 1982–84, that coincided with an increasing number of males three years and older, suggests that males also decide whether or not to attempt territory defense based upon the frequency of territorial defense in the population.

Sexual Selection and Variance in Reproductive Success

Sexual Selection and Variance in Reproductive Success