Contest Competition Research Articles

Reproductive competition promotes the evolution of female weaponry

Secondary sexual traits in females are a relatively rare phenomenon. Empirical studies have focused on the role of male mate choice in their evolution; however, recently it has been suggested that secondary sexual traits in females are more likely to be under selection via reproductive competition. We investigated female competition and the influence of female phenotype on fitness in Onthophagus sagittarius, a species of dung beetle that exhibits female-specific horns. We compared reproductive fitness when females were breeding in competition versus breeding alone and found that competition for breeding resources reduced fitness for all females, but that smaller individuals suffered a greater fitness reduction than larger individuals. When females were matched for body size, those with the longest horns gained higher reproductive fitness. The fitness function was positive and linear, favouring increased horn expression. Thus, we present evidence that female body size and horn size in O. sagittarius are under directional selection via competition for reproductive resources. Our study is a rare example of female contest competition selecting for female weaponry.

The socioecology of adult female patas monkeys and vervets in Kenya

CH. 1: The Research Question Study species - Vervets and patas monkeys Questions and hypotheses Socioecological theory - Food resources and primate behavior Scramble and contest competition Social dominance Models of female social relationships Testing the models on Segera: A natural ecological experiment Holding habitat constant Other factors affecting female social behavior Project goals revisited Study groups Study subjects CH. 2. primate behavior and ecology Measuring ecology food availability Commonly used methods Primates' perceptions of the foods available to them Clumped resources and contest competition CH. 3: Foods available to vervet and patas monkeys Hypotheses and predictions Defining a food patch food availability Large-scale food availability Methods Results Small-scale food availability Methods Results Swollen thorn foods Foods in the riverine habitat Herbaceous level food availability Food availability on Segera CH. 4: Feeding behavior of vervets and patas monkeys Studying the feeding behavior of sympatric species Vervets and patas monkeys: Expected differences and similarities Whistling-thorn Acacia: Why focus on one food species? Feeding behavior Continuous sampling Bout sampling Rank-related differences in feeding behavior Patch depletion Monkeys, Acacia, and ants Species differences Within-species differences according to rank CH. 5: Contest competition and dominance in vervets The concept of dominance Testing hypotheses Results: Contest competition and dominance in vervets Dominance patterns and feeding competition in vervets The significance of dominance to vervets Dominance style in Segera vervets Typical cercopithecines? Why a stable, linear dominance hierarchy on Segera? CH. 6: Comparing vervet and patas monkeys in the same habitat. Questions and Predictions Agonistic and dominance behavior Food contestability in vervets and patas monkeys Dominance in adult females Feeding competition and whistling-thorn foods Significance of feeding competition to vervet and patas monkeys CH. 7: Food availability, feeding competition and dominance in vervet and patas monkeys. Female contest competition and dominance in vervets and patas monkeys on Segera How do the models rate? Summary and implications for future research

The mechanistic basis of discrete-time population models: The role of resource partitioning and spatial aggregation

The purpose of this paper is to present a unified view to understand mechanistic basis of various discrete-time population models from the viewpoints of resource partitioning and spatial aggregation of individuals. A first-principles derivation is presented of a new population model which incorporates both scramble and contest competition using a site-based framework in which individuals are distributed over discrete resource sites. The derived model has parameters relating to the way of resource partitioning and the degree of spatial aggregation of individuals, respectively. The model becomes various population models in various limits in these parameters. This model thus provides a unified view to understand how various population models are interrelated. The dependence of the stability of the model on the parameters is also examined.

Sexual dimorphism and the mating ecology of polar bears (Ursus maritimus) at Svalbard

We assessed the role of size, mass, and age in mating and non-mating polar bears (Ursus maritimus) at Svalbard, Norway, during the spring breeding season. The ratio of male to female mass, in male-female pairs, ranged from 1.00 to 3.02 (\( \overline x = 1.99 \)) indicating that mating males were larger than mating females but with substantial variation. Paired males were older than unpaired males and male mass was related to age. However, males paired with females were not significantly different in body mass from those males caught alone. Wounds and scars resulting from fights between males began at about 6 years of age and peaked at about 17 and 20 years of age, respectively. The frequency of broken canines in males, presumably due to increased male-male conflicts, increased with age but showed little increase in females. The wide range of male size in male-female pairs and the age-related signs of injury suggest that male polar bears engage in both scramble competition and contest competition for access to breeding females. The mating system of polar bears is variable but is best described as female defense polygyny or serial monogamy.

Pre- and post-mating sexual selection both favor large males in a rainbowfish

Sexual selection can act through female choice and male–male competition. Although both processes can act simultaneously, they are typically studied independently. Here, we adopt a more integrated approach to studying sexual selection by incorporating measures of both processes using the western rainbowfish Melanotaenia australis, a freshwater fish endemic to northwestern Australia. We assessed male–male competition and female choice separately while measuring the performance of individual males under both processes and used paternity analyses to estimate male reproductive success. We then related the performance of males during each of these stages to their phenotype, which was described using linear measures of size and color pattern traits, and spectrographic measures of the reflectance of color patches. We found that female choice favored relatively large males and that these preferences were consistent within individual females and repeatable between different females. Larger males were also more dominant in the competition trials and sired the majority of offspring produced when females spawned. There was little evidence to suggest that sexual selection acted on male color patterns either via female choice or male contest competition or during subsequent post-mating episodes of sexual selection. We conclude, therefore, that male–male competition and female choice act concordantly to favor relatively large males and that these patterns of mating success are reflected during post-mating episodes of sexual selection.

Contest behaviour of maize weevil larvae when competing within seeds

Food limitation induces severe competition for obligate seed-feeding insect larvae that are unable to leave the seed selected by their mother. The number of eggs laid per seed and the number of larvae hatched from the eggs are important determinants of whether larval behaviour within the seed will be of the scramble or the contest type. In maize weevils, Sitophilus zeamais, few adults emerge per seed regardless of the number of eggs laid, which may arise from scramble competition, if the optimum egg density (i.e. egg density leading to maximum total larval fitness) is low compared to the number of eggs laid per seed, or from contest competition due to direct interference among the larvae. The behavioural process and the ecological outcome of competition were assessed in two strains of the maize weevil. Neither strain showed a reduction in body mass with increased competition, and they had similar optimal egg densities (two eggs per seed). There was a hump in the larval fitness curve suggesting a scramble competition, but this conclusion is compromised because the optimum egg density was small (two eggs per seed). X-ray imaging of seeds was used to observe interactions of larvae within the seed and showed direct interference, with aggression among the larvae. This provides evidence of contest-like competition within seeds even when egg density was low. Hence, one should be cautious in inferring the underlying type of competitive behaviour from variables such as body mass and initial egg density per seed. Direct observation of behaviour is required to make such an inference.

Mate choice in the dung beetle Onthophagus sagittarius: are female horns ornaments?

Sexual selection typically operates via male contest competition and female choice, favoring the evolution of secondary sexual traits in males. However, there are numerous taxa in which females possess secondary sexual traits and the selective pressures operating on female ornamentation are not well understood. We examined sexual selection operating via mate choice in the dung beetle Onthophagus sagittarius, a species exhibiting sex-specific ornamentation. Precopulatory mate preferences and their subsequent effects on breeding success were examined. Female preferences favored large males and when paired with small males, males with high courtship rates. Large females were choosier than small females. No overt male preferences for female size or ornamentation were found; thus, we conclude that female horns in this species are unlikely to have evolved as ornaments via precopulatory male mate choice. Relative horn length in females determined brood ball productivity, whereas female body size and an interaction between male courtship rate and male body size determined brood ball weight. Our results provide evidence for female but not male mate choice. They suggest that attractive males increase reproductive performance, but it is unclear whether the effect of male phenotype is mediated via differences in paternal investment or female differential allocation toward attractive males. Key words: female horns, mutual mate choice, parental provisioning. [Behav Ecol]

Intratree vertical variation of fruit density and the nature of contest competition in frugivores

Recently, we demonstrated that the highest densities of fruit pulp are located in the uppermost zones of tree crowns. Since heterogeneous distributions of depletable food is theorized to foster contest competition, we tested three hypotheses involving rank differences among species of arboreal frugivores: (1) In the absence of competitors, species tend to feed in higher strata of tree crowns; (2) interspecific contest competition occurs through monopolization and usurpation of feeding sites in these higher strata; and (3) subordinate species decrease their feeding height and ingestion rate when dominants enter the food patch. To test these hypotheses, we observed chimpanzees (Pan troglodytes), red-tailed monkeys (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), and gray-cheeked mangabeys (Lophocebus albigena) in Kibale National Park, Uganda. We found that: (1) all four primates fed preferentially in upper tree crowns when alone, (2) dominant species monopolized and aggressively usurped the upper crown when co-feeding with subordinates and the latter retreated below the middle of tree crowns, (3) in the presence of dominant species, subordinate species showed lower standardized feeding height and modified their food intake rates, while dominants were not affected by the subordinate species, (4) subordinates moved down at the arrival of and up at the departure of dominants, and (5) the presence of folivores in the tree did not affect the feeding height of a frugivore, even through folivores were socially dominant. Contrary to expectations, we found that red-tailed monkeys decreased their movements between successive fruits that they ate in the presence of blue monkeys compared to when they were feeding alone, perhaps to avoid disturbing dominants and attracting aggression or because they ingested more semi-ripe and green unripe fruits, i.e., more food of lower quality.

Resurrecting the differential mortality model of sexual size dimorphism

In some animal groups males may be several times smaller than females. One of the hypotheses proposed to explain the evolution of this extreme sexual size dimorphism (SSD) is the differential mortality model (DMM), which is based on the assumption that when males are the searching sex, higher male mortality relaxes male-male contest competition, leading to the adaptive evolution of early-maturing, small males that are favoured by viability selection. Evidence for the main prediction of this model, i.e. that there is a negative relationship between differential mortality and SSD, has remained elusive. Using sex differences in pitfall trap catches - a proxy of sex differences in mobility and mortality - across 40 spider species, and using the evolutionary comparative method, we found significant negative relationships between differential mortality and SSD for three size traits. Thus, the DMM can still explain part of the observed variance in SSD.

Scramble or contest competition over food in solitarily foraging mouse lemurs (Microcebus spp.): New insights from stable isotopes

The relationships between resource distribution, type of competition, and consequences for social organization have been formalized in the socioecological model (SEM) which predicts that ecological factors are the main determinants of female distribution. We tested this basic prediction in two solitary primates (Microcebus berthae and M. murinus) which differ in female association patterns. Using stable nitrogen and carbon isotope data of hair samples and food sources we quantified inter-specific differences in diet. delta(13)C in M. berthae reflected a diet composed mainly of insect secretions. Higher within-species as well as seasonal variation in delta(13)C of M. murinus indicated a wider trophic niche including plant and animal source food. Constantly elevated delta(15)N in M. murinus most likely reflected extended torpor during the lean season. This energy-saving strategy together with a wider, more opportunistic feeding niche might reduce female competition in this species, facilitating smaller female ranges, and a higher association potential. In contrast, delta(15)N fluctuated seasonally in M. berthae, most likely indicating varying amounts of arthropod food in the diet. Intense scramble competition over small and seasonally limited resources might lead to female spatial avoidance and a reduced association potential in M. berthae. Thus, differences in female association patterns between these two solitary foragers are due to different types of competition and overall intensities of intra-specific competition.

WolbachiaInfection and Resource Competition Effects on ImmatureAedes albopictus(Diptera: Culicidae)

Wolbachia pipientis Hertig and Wolbach (Rickettsiales: Rickettsiaceae) are intracellular alpha-proteobacteria that occur naturally in Aedes albopictus (Skuse) (Diptera: Culicidae) and numerous other invertebrates. These endosymbionts can invade host populations by manipulating host reproduction. Wolbachia infections have been shown to impart both costs and benefits to hosts in terms of development, survival, and fecundity. Here, we monitor intraspecific competition among independent cohorts of infected or uninfected larvae. Levels of competition are manipulated by varying initial larval densities and food levels. Although larval density is observed to have major impacts on immature survivorship, sex ratio of eclosing adults, and developmental rates, the Wolbachia infection status had minimal impact on male immatures and no effect on immature females under these experimental conditions. Female and male immatures were observed to respond differently to competitive pressure, with the functional relationships of females and males consistent with scramble and contest competition, respectively. The results are discussed in relation to the evolution of naturally occurring Wolbachia infections in Ae. albopictus (i.e., natural population replacement events) and public health strategies that propose the manipulation of Wolbachia infections in Ae. albopictus populations.

Sexual size dimorphism in Asian colobines revisited

Asian colobines exhibit a wide range of sexual dimorphism in body mass. Some species are monomorphic, whereas others are strongly dimorphic. Strong sexual dimorphism is generally viewed as the consequence of intense male contest competition over access to mates, but this idea appears not to explain variation in sexual dimorphism in Asian colobines. Our results show that modular colobines, i.e. species in which social units aggregate into higher-level bands or often associate, have significantly higher levels of sexual dimorphism in body mass than the nonmodular ones. This finding was corroborated by means of phylogenetically controlled methods and multiple regression analyses. The results suggest that living in a modular society intensifies the contest competition among males, which is further exacerbated by the continuous presence of all-male units.

Factors underlying party size differences between chimpanzees and bonobos: a review and hypotheses for future study

Differences in party size and cohesiveness among females have been primary topics in socio-ecological comparisons of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). This paper aims to review previous studies that attempted to explain these differences and propose some hypotheses to be tested in future studies. Comparisons of recent data show that relative party size (expressed as a percentage of total group size) is significantly larger for bonobos than chimpanzees. Although the prolonged estrus of females, close association between mother and adult sons, female social relationships including unique homosexual behavior, and high female social status might be related to the increased party size and female cohesiveness of bonobos, these social and behavioral factors alone do not appear to explain the differences between the two species. Differences in ecological factors, including fruit-patch size, density of terrestrial herbs, and the availability of scattered foods that animals forage as they travel between large fruit patches could also contribute to the differences between chimpanzees and bonobos. However, these factors cannot fully account for the increased party size and female cohesiveness of bonobos. The higher female cohesiveness in bonobos may be explained by socio-ecological systems that reduce the cost in feeding efficiency incurred by attending mixed-sex parties. These systems may include female initiatives for party ranging movements as well as the factors mentioned above. Because of their geographical isolation, the two species probably evolved different social systems. Chimpanzees, whose habitats became very dry during some periods in the Pleistocene, likely evolved more flexible fission-fusion social systems to cope with seasonal and annual variation in food availability. On the other hand, bonobos had a large refugia forest in the middle of their range even during the driest periods in the Pleistocene. Therefore bonobos, whose habitats had more abundant food and smaller variation in food availability, probably evolved systems that help females stay in mixed parties without incurring large costs from contest and scramble competition.

Females go where the food is: does the socio-ecological model explain variation in social organisation of solitary foragers?

The socio-ecological model (SEM) links ecological factors with characteristics of social systems and allows predictions about the relationships between resource distribution, type of competition and social organisation. It has been mainly applied to group-living species but ought to explain variation in social organisation of solitary species as well. The aim of this study was to test basic predictions of the SEM in two solitary primates, which differ in two characteristics of female association patterns: (1) spatial ranging and (2) sleeping associations. Beginning in August 2002, we regularly (re-)captured and marked individuals of sympatric populations of Madame Berthe's and grey mouse lemurs (Microcebus berthae, Microcebus murinus) in Kirindy Forest (Madagascar). We recorded data on spatial patterns, feeding and social behaviour by means of direct observation of radio-collared females. The major food sources of M. berthae occurred in small dispersed patches leading to strong within-group scramble competition and over-dispersed females with a low potential for female associations. In contrast, M. murinus additionally used patchily distributed, high-quality (large) resources facilitating within-group contest competition. The combined influence of less strong within-group scramble and contest as well as between-group contest over non-food resources allowed females of this species to cluster in space. Additionally, we experimentally manipulated the spatial distribution of food sources and found that females adjusted their spatial patterns to food resource distribution. Thus, our results support basic predictions of the SEM and demonstrated that it can also explain variation in social organisation of solitary foragers.

Evolution of the scale and manner of brother competition in pollinating fig wasps

Contest competition and male dispersal have evolved a number of times in pollinating fig wasps. Given their highly female-biased sex ratios and high relatedness between competing males, this phenomenon is surprising. Using a comparative approach, we found a significant association between high adult sex ratios (male:female), fighting and dispersal. We conclude that a relatively high proportion of males could have led to the evolution of fighting and to excess males that avoid kin conflict through dispersal. These wasps respond to increased competition between relatives by altering the scale at which they compete, but also by competing more intensely. In support of the new generation of kin selection models, we found that relatedness of competing males has no effect on the likelihood of fighting. Even so, selection at a higher hierarchical level has selected for dispersal and hence a larger scale at which competition occurs. We found that although the sex ratio gives a rough indication of the mating system it fails to be accurate at a finer scale.

Female American goldfinches use carotenoid-based bill coloration to signal status

Interest in female ornamentation has burgeoned recently, and evidence suggests that carotenoid-based female coloration may function as a mate-choice signal. However, the possibility that females may signal status with coloration has been all but ignored. Bill coloration of female American goldfinches (Spinus tristis) changes seasonally, from dull gray in winter to bright orange in the breeding season. We conducted a series of aviary experiments in the breeding season to examine the signaling role of female bill color during both intra- and intersexual contests as well as during male mate choice. We tested for status signaling by examining whether caged females and males avoided feeding adjacent to female taxidermic models as a function of the model’s bill color, which was experimentally augmented or dulled. We tested for a mate signaling function by giving captive males a choice between 2 live females with experimentally altered bill colors. Females avoided feeding near model females with colorful bills, but males showed neither avoidance of nor preference for females with more colorful bills. These results indicate that the female’s carotenoid-based bill coloration signals status during competitive interactions and suggest that female bill color does not function as a mate-choice signal. This represents the first experimental evidence that a carotenoid-based coloration of females functions to mediate contest competition over food. Key words: American goldfinch, bill color, carotenoid signal, intrasexual selection, ornamental female, social selection, Spinus (Carduelis) tristis, status signaling. [Behav Ecol]

Competition strategies and correlated selection on responses to polyandry in the seed beetle Callosobruchus maculatus

Abstract Polyandry reflected in multiple mating with different mates is regarded as favoured by natural selection in males but not necessarily in females, where conflicting effects on fitness components can occur. The present study aims to provide empirical evidence to predict which fitness components may be affected in this sexual conflict using a species that demonstrates potential between‐population variation in their resolution: the cowpea weevil Callosobruchus maculatus. Two strains showing contrasting competition outcomes (scramble × contest) and contrasting life‐history strategies based on trade‐offs between longevity and fecundity are crossed for subsequent selection based on larval‐competition strategy, expecting the production of a correlated response to multiple (polyandrous) mating. Such a response is expected because the scramble strain shows high fecundity (and lower longevity) and would benefit from multiple mating, in contrast with the contest strain, which shows high juvenile mortality. The scramble‐selected lines would evolve a response of increased fecundity and reduced longevity under multiple and potentially polyandrous mating but the contest‐selected lines would not respond to multiple (polyandrous) mating. Instead, both scramble‐ and contest‐selected lines show increased fecundity and reduced longevity with multiple (polyandrous) matings, which did not affect egg weight. Indirect benefits of multiple (polyandrous) mating appear to be relevant for lines showing contest competition among juveniles.

Craig Stanford: Apes of the impenetrable forest. The behavioral ecology of sympatric chimpanzees and gorillas

It is a curious but basic fact that all of the better-known behavioural field studies of Pan troglodytes and Gorilla gorilla have been carried out under dubious ecological conditions. That is, whether Bossou, Gombe, Mahale, Tai or Karisoke, among others, the studies at each site present only one of these African ape species, while the other is absent. This is notable because the two closely related taxa almost certainly evolved in sympatry in the Congo Basin; consequently, a full understanding of them requires that the chief competitors be present. Comparative ecological studies of the two hominoid taxa have been done, most notably the pioneering efforts of Caroline Tutin and Michel Fernandez at Lope, Gabon, but behavioural detail was lacking. For example, we do not know at this point even if their competition is contest or scramble. Now comes a new book whose subtitle promises to tackle these issues. Craig Stanford, a field primatologist with experience of Asia and Africa, including the chimpanzees of Gombe, has produced a monograph that tackles the gap. It is the fifth in the series of slim paperback volumes on Primate Field Studies, published by Pearson/ Prentice-Hall, the stated aim of which is to revive the scientific monograph in a form accessible to a wider audience than that of fragmented reports in academic journals. To cut to the chase, this volume succeeds admirably on that front, and is user-friendly to students, even undergraduates. The book opens with personal background on the author, positioning him and the proposed field site for the study to come. The qualifying credentials of both Stanford and Bwindi are convincingly proffered, and the conventional wisdom on chimpanzee–gorilla ecological differences, based on allopatric studies, is robustly challenged. For example, both do show arboreality and frugivory. This sets the stage for the next chapter, which explicitly is a primer on the two species of great apes. Not surprisingly, given the experience of the author, the information presented on the chimpanzees is better grounded. For further contrast, brief treatments on bonobo and orang-utan are included. The chapter closes on the core problem: What do we actually know about chimpanzees and gorillas in sympatry? Here, a worrying trend seems to emerge, that of little attention being paid to previous studies, especially the work by Yamagiwa and colleagues at nearby Kahuzi-Biega, the other high-altitude site where eastern chimpanzees co-exist with eastern gorillas. By far the best chapter in the book, from the point of naive students, is Stanford’s frank account of what it takes to get a long-term field project off the ground. His description dispels the misleading impressions given by television documentaries and even by journal articles, that somehow field-sites just spring magically into existence. Stanford’s efforts are a step-by-step model of how such pioneering needs to be done. This narrative thread continues throughout the book, underpinning the science, which covers the basic aspects of diet, habitat, ranging, shelter construction, etc. Perhaps the most controversial of these topics is his treatment of bipedality which, depending on one’s point of view, provides important new insights (e.g. key role of arboreality) or not (e.g. it is all about posture, not locomotion). But what about that key point about niche overlap or competitive exclusion between the two species? As it turns out, the data for direct (contest) competition are too few to draw conclusions, and W. C. McGrew (&) Leverhulme Centre for Human Evolutionary Studies, Department of Biological Anthropology, University of Cambridge, Cambridge CB2 1QH, UK e-mail: wcm21@cam.ac.uk

Female grooming markets in a population of gray-cheeked mangabeys (Lophocebus albigena)

Primate female allogrooming models based on biological markets theory predict that grooming is “time matched” within bouts, that is, the amount of time the first female grooms predicts the amount of time the second one grooms. The models also predict that when female-female contest competition is weak, grooming is traded for grooming, but when female-female contest competition is strong, grooming may be traded for other commodities such as feeding tolerance, and grooming discrepancy between members of dyads is rank related. We tested these predictions using data collected from adult and subadult female gray-cheeked mangabeys (Lophocebus albigena) (N = 26) in 5 groups in Kibale National Park, Uganda. We found that, overall, females reciprocated in 33% of grooming bouts. Among reciprocated bouts, females in all 5 groups showed time matching. In 2 groups, we also found rank-related grooming discrepancies but showing opposite patterns to each other. Consistent with predictions based on biological markets theory, these groups may have been under greater feeding competition, revealed more by adjustments in ranging behavior than increased agonistic rates. Although these results support current allogrooming models, they also suggest that the models may become more robust if the influence of scramble competition is incorporated. In addition, they emphasize the flexibility and dynamic nature of female competitive relationships within the same population of primates.

No fecundity cost of female secondary sexual trait expression in the horned beetle Onthophagus sagittarius

Typically males bear the products of sexual selection in the form of ornaments and/or weapons used to compete for and attract females. Secondary sexual traits in females have been thought of as the product of correlated responses to sexual selection on males. However, there is increasing phylogenetic evidence that female secondary sexual traits can arise independently of selection on males, and may be subject to sexual selection. Theoretical models of the evolution of female ornamentation via male mate choice have assumed that females suffer a cost of ornament expression via reduced fecundity, and hence female ornaments are less likely to evolve than male ornaments. In the dung beetle Onthophagus sagittarius, there has been an independent evolutionary origin of horns in females that are qualitatively different from the horns produced by males. We use this system as a model to examine the costs of horn expression for females within a life-history context. We identified a longevity cost of reproduction for females that was independent of horn expression. Large females lived longer, and after controlling for lifespan, had a higher lifetime fecundity, and invested more heavily in maternal provisioning than did small females. We found no evidence of a cost to females of investment in horns. Rather, the rate of increase in fecundity and horn expression with body size were equal, so that absolute horn size provides an accurate indicator of body size and maternal quality. The effects we observe were independent of female contest competition and/or male mate choice, which were excluded in our experimental protocol. However, we speculate on the potential functional contributions female horns might make to female fitness.