Reproductive Competition Research Articles

Circumstantial evidence of gibel carp, Carassius gibelio, reproductive competition exerted on native fish species in a mesotrophic reservoir

Abstract Gibel carp, Carassius gibelio (Bloch), impacts on native fish species have been reported but little studied despite a long history of introductions in Europe. This species is able to reproduce gynogenetically, which involves the use of sperm from males of other species to activate egg development, so reproductive competition is a likely but virtually unstudied impact of gibel carp on native fishes. This study evaluates the impact of introduced C. gibelio on the population biology of native fishes over a 6‐year period in a mesotrophic drinking water reservoir in north‐western Turkey. A dramatic decrease in the relative density (i.e. catch per unit effort) of native species correlated significantly with an increase in C. gibelio relative density. Growth characteristics (back‐calculated ages, growth index and relative condition) and length at maturity did not differ significantly among years in C. gibelio and native fishes. Relative density, duration of spawning, reproductive effort and gonado‐somatic index of C. gibelio increased with some water quality variables [total phosphorus (TP); chlorophyll‐a (Chl‐a)] and coincided with decreasing trends for natives. However, TP and Chl‐a were not correlated with growth features in C. gibelio or natives fishes. The results suggest that the decline in the reservoir’s native cyprinid populations is likely due to a combination of degrading environmental conditions and a disparity in reproductive effort, with introduced C. gibelio invasion facilitated by gynogenetic reproduction and an observed interference with native fishes during spawning.

Female reproductive competition within families in rural Gambia

Many studies show that the extended human family can be helpful in raising offspring, with maternal grandmothers, in particular, improving offspring survival. However, less attention has been given to competition between female kin and co-residents. It has been argued that reproductive conflict between generations explains the evolution of menopause in cooperatively breeding species where females disperse, and that older females are related to the offspring of younger females through their sons, whereas younger, incoming females are unrelated to older females. This means the pattern of help will be asymmetric, so older females lose in reproductive conflict and become 'sterile helpers'. Here, we seek evidence for female reproductive competition using longitudinal demographic data from a rural Gambian population, and examine when women are helping or harming each other's reproductive success. We find that older women benefit and younger women suffer costs of reproductive competition with women in their compound. But the opposite is found for mothers and daughters; if mother and daughter's reproductive spans overlap, the older woman reduces her reproduction if the younger woman (daughter) reproduces, whereas daughters' fertility is unaffected by their mothers' reproduction. Married daughters are not generally co-resident with their mothers, so we find not only competition effects with co-resident females, but also with daughters who have dispersed. Dispersal varies across human societies, but our results suggest reproductive conflict could be influencing reproductive scheduling whatever the dispersal pattern. A cultural norm of late male marriage reduces paternal grandmother/daughter-in-law reproductive overlap almost to zero in this population. We argue that cultural norms surrounding residence and marriage are themselves cultural adaptations to reduce reproductive conflict between generations in human families.

Better off alone! Reproductive competition and ecological constraints determine sociality in the African striped mouse (Rhabdomys pumilio)

1. While the reasons for group-living have been studied for decades, little is known about why individuals become solitary. 2. Several previous experimental studies could demonstrate that group-living can arises as a consequence of ecological constraints. 3. It has been argued that reproductive competition between group members leads to significant costs of group-living, being a main reason of solitary-living. However, so far, no studies tested experimentally whether reproductive competition can explain solitary-living. 4. Using a socially flexible species, the African striped mouse (Rhabdomys pumilio), we tested experimentally in the field whether dispersal and solitary-living are more likely to occur when reproductive competition is present. 5. We investigated ecological constraints, here expressed as a function of population density, by removing groups of striped mice and creating vacant territories. To control for the effect of reproductive competition, which occurs only during the breeding season, we performed experiments during both the breeding and the non-breeding season. This is the first removal experiment performed in a species with communal breeding during the non-breeding season. 6. During the breeding season, when population density was low, more striped mice from experimental groups moved into the vacant territories and became solitary than striped mice from control groups. This is in support of the ecological constraints hypothesis. 7. During the non-breeding season, striped mice remained group-living despite the availability of free territories. Significantly, more striped mice became solitary-living during the breeding than during the non-breeding season. This is the first experimental support for the reproductive competition hypothesis explaining solitary-living. 8. Analysis of the sexual maturity of males showed that males which became solitary had a higher reproductive potential than males that remained group-living. Analysis of the body mass data of females showed that more solitary females reproduced than group-living females. These results indicate that by becoming solitary individuals of both sexes avoided costs of reproductive competition within groups. 9. Our study provides experimental evidence that reproductive competition within groups can lead to dispersal and solitary-living.

Investigating Xenopus laevis social structure through an analysis of clasping behavior

Event Abstract Back to Event Investigating Xenopus laevis social structure through an analysis of clasping behavior Rachel Stevenson1* and Heather J. Rhodes1 1 Denison University, Biology, United States Male Xenopus laevis frogs frequently engage in amplexus-like clasping with other males, but the function of this male-male behavior in Xenopus social structure or reproductive competition is not clear. We tested whether analyses of clasping behavior support (1) an indiscriminate clasping model where males clasp all conspecifics regardless of sex, (2) a dominance hypothesis, in which male-directed clasping in the absence of a female grants future success in competition for mates, or (3) a “tag-along” hypothesis, in which inferior males clasp more attractive males and thus tag along to a breeding event secured by the claspee. Pairs of males were video-recorded for one night then a female was introduced for a second night of recording. The time each male spent in male-directed and female-directed clasping was quantified. Frogs preferentially clasped tankmates of a particular sex, showing that clasping was not indiscriminate. The amount of male-directed clasping displayed by a frog in the absence of a female was negatively correlated with the time that particular frog spent clasping the female on the second night, but was positively correlated with its time spent clasping the other male on that second night. This suggests that clasping another male does not secure greater success in future competitions for females, refuting the dominance hypothesis. The data are consistent with, but cannot yet confirm, the “tag-along” hypothesis. Further testing is needed to analyze behavior during oviposition and reproductive success among competing males. Acknowledgements This research was supported by the Denison University Department of Biology and the Anderson Endowment Program. Keywords: clasping, social structure, Xenopus laevis Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster (but consider for student poster award) Topic: Social Behavior Citation: Stevenson R and Rhodes HJ (2012). Investigating Xenopus laevis social structure through an analysis of clasping behavior. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00106 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 24 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Ms. Rachel Stevenson, Denison University, Biology, Granville, Ohio, 43023, United States, stevenson.rachel2@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Rachel Stevenson Heather J Rhodes Google Rachel Stevenson Heather J Rhodes Google Scholar Rachel Stevenson Heather J Rhodes PubMed Rachel Stevenson Heather J Rhodes Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Reproductive competitiveness of the pine wood nematode Bursaphelenchus mucronatus and Polish populations of B. xylophilus develop in a common environment (in vitro)Konkurencyjność reprodukcyjna Bursaphelenchus mucronatus i B. xylophilus we wspólnym środowisku rozwoju (in vitro)

Summary The objective of this study was to examine the reproductive competitiveness of the pine wood nematode Bursaphelenchus xylophilus and a series of Polish populations of B. mucronatus (isolates: Mdz-01, NTo-01 i Wro-01) developing together in a common environment (in vitro). The experiments did not show superiority of any of these nematodes. Both B. xylophilus, and B. mucronatus were able to dominate the second species. However, based on the analysis of the obtained results it could be stated that the females of B. xylophilus had a higher percentage share than the feamles of B. mucronatus. Interestingly, it was also found that the offspring populations obtained from the cultures of B. xylophilus and B. mucronatus reared together were significantly greater as compared to the populations of each species reared separately. However, mechanisms of this phenomenon still remain unclear.

Reproductive conflicts in polyandrous and polygynous ant Formica sanguinea

The occurrence of multiple reproductives within an ant colony changes the balance between indirect fitness benefits and reproductive competition. We test whether the number of matings by an ant queen (polyandry) correlates negatively with the number of reproductive queens in the colony (polygyny), whether the patrilines and matrilines differ in their contribution to the sexual and worker progeny and whether there is an overall reproductive skew. For these aims, we genotyped both worker and sexual offspring from colonies of the ant Formica sanguinea in three populations. Most colonies were monogynous, but eight (11%) were polygynous with closely related queens. Most queens in the monogynous colonies (86%) had mated with multiple males. The effective paternity was lower than the actual number of mates, and the paternity skew was significant. Furthermore, in some monogynous colonies, the patrilines were differently represented in the worker pupae and sexual pupae produced at the same time. Likewise, the matrilines in polygynous colonies were differently present in worker pupae and male offspring. The effective number of matings by a queen was significantly lower in polygynous colonies (mean m(e) = 1.68) than in monogynous colonies (means 2.06-2.61). The results give support to the hypotheses that polyandry and polygyny are alternative breeding strategies and that reproductive competition can lead to different representation of patrilines and matrilines among the sexual and worker broods.

What is sexual selection and the short herstory of female trait variation

What is sexual selection and the short herstory of female trait variation

Fire-stimulated flowering among resprouters and geophytes in Australia and South Africa

Data on 386 species with fire-stimulated flowering (fsf) in Australasia and South Africa/Madagascar were collated to show that they occur under a wide range of fire regimes, with 71% confined to the mediterranean-climate regions. About 40% only flower up to 2 years after fire (obligate), while the rest continue at a low rate until the next fire (facultative). Peak flowering occurs 5–18 months after fire in the mediterranean regions but at 1–7 months in savannas. Fsf is recorded in 34 families, headed by terrestrial orchids (45% of species), spread throughout the seed-plant phylogeny from cycads to daisies. Tuberous geophytes (essentially orchids) dominate (51%), but other resprouting growth forms include lignotuberous shrubs and forbs, rhizomatous and bunch grasses, leaf succulents, grasstrees, epicormic trees, and hemiparasites. Most have wind-dispersed diaspores (72%), store their diaspores in the soil (93%), and seeds that do not germinate until the next fire (72%). Fsf in association with resprouting takes advantage of optimal resources and minimal competition for growth and reproduction, conditions that favor wind dispersal and maximize the interval for seed accumulation before the next fire and build-up of fire-tolerant organs. Reduced herbivory has little role in accounting for its benefits. The proximal causes of fsf center around cueing factors (direct effects such as ethylene), resource factors (direct and indirect effects, e.g., extra nutrients), and predisposing factors (circumstantial effects, e.g., fire interval). The evolutionary history of fsf has been explored recently in orchids, proteas, blood roots, droseras, and mistletoes and shown to stretch back over a period of at least 50 million years, indicating that flowering in many groups has a long association with fire as an agent of natural selection.

Social flexibility and social evolution in mammals: a case study of the African striped mouse (Rhabdomys pumilio)

Environmental change poses challenges to many organisms. The resilience of a species to such change depends on its ability to respond adaptively. Social flexibility is such an adaptive response, whereby individuals of both sexes change their reproductive tactics facultatively in response to fluctuating environmental conditions, leading to changes in the social system. Social flexibility focuses on individual flexibility, and provides a unique opportunity to study both the ultimate and proximate causes of sociality by comparing between solitary and group-living individuals of the same population: why do animals form groups and how is group-living regulated by the environment and the neuro-endocrine system? These key questions have been studied for the past ten years in the striped mouse Rhabdomys pumilio. High population density favours philopatry and group-living, while reproductive competition favours dispersal and solitary-living. Studies of genetic parentage reveal that relative fitness of alternative reproductive tactics depends on the prevailing environment. Tactics have different fitness under constrained ecological conditions, when competitive ability is important. Under conditions with relaxed ecological constraints, alternative tactics can yield equal fitness. Both male and female striped mice display alternative reproductive tactics based on a single strategy, i.e. all individuals follow the same decision rules. These changes are regulated by endocrine mechanisms. Social flexibility is regarded as an adaptation to unpredictably changing environments, selecting for high phenotypic flexibility based on a broad reaction norm, not on genetic polymorphism for specific tactics.

Female reproductive competition in Eulemur rufifrons: eviction and reproductive restraint in a plurally breeding Malagasy primate

In mammals with female philopatry, co-resident females inevitably compete with each other for resources or reproductive opportunities, thereby reducing the kin-selected benefits of altruism towards relatives. These counteracting forces of cooperation and competition among kin should be particularly pronounced in plurally breeding species with limited alternative breeding opportunities outside the natal group. However, little is still known about the costs of reproductive competition on females' fitness and the victims' potential counter-strategies. Here we summarize long-term behavioural, demographic and genetic data collected on a plurally breeding primate from Madagascar to illuminate mechanisms and effects of female reproductive competition, focusing on forcible eviction and potential reproductive restraint. The main results of our study indicate that females in groups of redfronted lemurs (Eulemur rufifrons) above a critical size suffer from competition from their close relatives: females in larger groups face an increased probability of not giving birth as well as a higher probability of being evicted, especially during the annual mating and birth seasons. Eviction is not predicted by the number of adult females, the number of close female relatives, female age or inter-annual variation in rainfall but only by total group size. Thus, eviction in this species is clearly linked with reproductive competition, it cannot be forestalled by reproductive restraint or having many relatives in the group, and it occurs in the absence of a clear dominance hierarchy. Our study therefore also underscores the notion that potential inclusive fitness benefits from living with relatives may have been generally over-rated and should not be taken for granted.

Competitive reduction by satyrization? Evidence for interspecific mating in nature and asymmetric reproductive competition between invasive mosquito vectors.

Abstract. Upon mating, male mosquitoes transfer accessory gland proteins (Acps) that induce refractoriness to further mating in females. This can also occur because of cross-insemination by males of related species, a process known as mating interference (satyrization). This mechanism could explain the competitive displacement of resident Aedes aegypti by the invasive Aedes albopictus where they co-occur. We tested this hypothesis in mosquito populations in Florida. A new polymerase chain reaction species diagnostic applied to sperm dissected from 304 field-collected females revealed bidirectional cross-mating in five (1.6%) individuals. Cross-injections of females with Acps showed that Ae. albopictus males induced monogamy in heterospecific females but not Ae. aegypti males. Despite its low frequency in the areas under study, the first evidence of cross-mating in nature and the asymmetric effect of Acps on mating suggest that satyrization may have initially contributed to the observed competitive reduction of Ae. aegypti by invasive Ae. albopictus in many areas.

The monogamous reproductive strategy in Lepidoptera

On the basis of literary sources and experimental data, the advantages of a monogamous reproductive strategy are analyzed. Monogamous species of Lepidoptera have no postcopulative mechanisms of control over the success of reproductive sperm competition, cryptic female choice, and, probably, the male's nuptial gift. Selection of the sex partner and of the quality of the future generation is determined by the precopulative and copulative mechanisms of the reproductive behavior, which include chemocommunication and a "lock-and-key" mechanism. In an experiment with one male and one female, the quota of successful mating can depend on the morphological complication of the genital apparatus of males. Cases are considered of repeated mating for some, usually monogamous, species. In comparison with the variant of the experiment with one male x one female, the frequency of repeated mating increases with the addition of two males to one female but decreases in the presence of two females with one male. A possible reason for the appearance of repeated mating may be the weight of the insect's body exceeding the average.

Cross-species correlation between queen mating numbers and worker ovary sizes suggests kin conflict may influence ovary size evolution in honeybees

During social evolution, the ovary size of reproductively specialized honey bee queens has dramatically increased while their workers have evolved much smaller ovaries. However, worker division of labor and reproductive competition under queenless conditions are influenced by worker ovary size. Little comparative information on ovary size exists in the different honey bee species. Here, we report ovariole numbers of freshly dissected workers from six Apis species from two locations in Southeast Asia. The average number of worker ovarioles differs significantly among species. It is strongly correlated with the average mating number of queens, irrespective of body size. Apis dorsata, in particular, is characterized by numerous matings and very large worker ovaries. The relation between queen mating number and ovary size across the six species suggests that individual selection via reproductive competition plays a role in worker ovary size evolution. This indicates that genetic diversity, generated by multiple mating, may bear a fitness cost at the colony level.

Male cleaner wrasses adjust punishment of female partners according to the stakes

Punishment is an important deterrent against cheating in cooperative interactions. In humans, the severity of cheating affects the strength of punishment which, in turn, affects the punished individual's future behaviour. Here, we show such flexible adjustments for the first time in a non-human species, the cleaner wrasse (Labroides dimidiatus), where males are known to punish female partners. We exposed pairs of cleaners to a model client offering two types of food, preferred 'prawn' items and less-preferred 'flake' items. Analogous to interactions with real clients, eating a preferred prawn item ('cheating') led to model client removal. We varied the extent to which female cheating caused pay-off reduction to the male and measured the corresponding severity of male punishment. Males punished females more severely when females cheated during interactions with high value, rather than low value, model clients; and when females were similar in size to the male. This pattern may arise because, in this protogynous hermaphrodite, cheating by similar-sized females may reduce size differences to the extent that females change sex and become reproductive competitors. In response to more severe punishment from males, females behaved more cooperatively. Our results show that punishment can be adjusted to circumstances and that such subtleties can have an important bearing on the outcome of cooperative interactions.

Female competition and its evolutionary consequences in mammals

Following Darwin's original insights regarding sexual selection, studies of intrasexual competition have mainly focused on male competition for mates; by contrast, female reproductive competition has received less attention. Here, we review evidence that female mammals compete for both resources and mates in order to secure reproductive benefits. We describe how females compete for resources such as food, nest sites, and protection by means of dominance relationships, territoriality and inter-group aggression, and by inhibiting the reproduction of other females. We also describe evidence that female mammals compete for mates and consider the ultimate causes of such behaviour, including competition for access to resources provided by mates, sperm limitation and prevention of future resource competition. Our review reveals female competition to be a potentially widespread and significant evolutionary selection pressure among mammals, particularly competition for resources among social species for which most evidence is currently available. We report that female competition is associated with many diverse adaptations, from overtly aggressive behaviour, weaponry, and conspicuous sexual signals to subtle and often complex social behaviour involving olfactory signalling, alliance formation, altruism and spite, and even cases where individuals appear to inhibit their own reproduction. Overall, despite some obvious parallels with male phenotypic traits favoured under sexual selection, it appears that fundamental differences in the reproductive strategies of the sexes (ultimately related to parental investment) commonly lead to contrasting competitive goals and adaptations. Because female adaptations for intrasexual competition are often less conspicuous than those of males, they are generally more challenging to study. In particular, since females often employ competitive strategies that directly influence not only the number but also the quality (survival and reproductive success) of their own offspring, as well as the relative reproductive success of others, a multigenerational view ideally is required to quantify the full extent of variation in female fitness resulting from intrasexual competition. Nonetheless, current evidence indicates that the reproductive success of female mammals can also be highly variable over shorter time scales, with significant reproductive skew related to competitive ability. Whether we choose to describe the outcome of female reproductive competition (competition for mates, for mates controlling resources, or for resources per se) as sexual selection depends on how sexual selection is defined. Considering sexual selection strictly as resulting from differential mating or fertilisation success, the role of female competition for the sperm of preferred (or competitively successful) males appears particularly worthy of more detailed investigation. Broader definitions of sexual selection have recently been proposed to encompass the impact on reproduction of competition for resources other than mates. Although the merits of such definitions are a matter of ongoing debate, our review highlights that understanding the evolutionary causes and consequences of female reproductive competition indeed requires a broader perspective than has traditionally been assumed. We conclude that future research in this field offers much exciting potential to address new and fundamentally important questions relating to social and mating-system evolution.

Cephalic modifications in dimorphic dwarf spiders of the genus oedothorax (Erigoninae, Linyphiidae, Araneae) and their evolutionary implications

Reproductive competition among males selects for a broad variety of strategies and traits from mate guarding to nuptial food gifts. Males of many dwarf spider species possess conspicuous secondary cephalic modifications, and the few studies available suggest that these cephalic structures are connected to extensive glandular tissue. Because females were observed to contact the male head structures during mating, these traits may have evolved in the context of sexual selection. We investigated the structure, glandular equipment, and sensory equipment of the cephalic regions of several species of the dwarf spider genus Oedothorax with varying degrees of sexual dimorphism using light and electron microscopy. In one Oedothorax species, there are two male morphs that exhibit a cephalic modification (O. gibbosus gibbosus) or not (O. gibbosus tuberosus). Our study demonstrates that all males investigated produce cephalic secretions, irrespective of the morphology of their cephalic region, however, they may differ in amount of secretion and in cellular organization. In males of O. apicatus, O. gibbosus gibbosus and O. retusus the gland cells are very abundant in the area of a cephalic hump, whereas in the less conspicuous O. agrestis, and O. gibbosus tuberosus the gland cells are restricted to a small area behind the ocular region or include the ocular region as in O. fuscus. The glandular tissue consists of two gland types in O. agrestis, O. fuscus, O. gibbosus tuberosus and O. retusus and of only one type in O. apicatus and O. gibbosus gibbosus. The setae present on the head structure of all species seem to function as mechano- and/or chemoreceptors. The implications of our findings for the evolution of secretory head structures are discussed along with their potential for driving speciation.

Directionality of male northern elephant seal (Mirounga angustirostris) threat calls and how it influences receiver behavior.

Many animal sounds are directional in which the sound energy is focused in a direction that depends on the signaler’s orientation. In the 1970s, Ron Schusterman quantitatively showed this in barking California sea lions and dogs. Several investigators have suggested ways that such features might be particularly useful among individuals in acoustic communication networks. However, only a few have tested such hypotheses experimentally and even fewer have investigated how directional signals affect receiver behavior. In this study, we measured directivity patterns of male northern elephant seal threat calls and used an acoustic playback approach to determine how call directionality influenced the responses of male seals in reproductive competition. We collected data on adult and older subadult seals on a breeding rookery (Ano Nuevo State Park) over three field seasons. Threat calls had substantial directionality, particularly at frequencies above 1 kHz and responses to playbacks depended on call directivity patterns. Males moved farther away from the playback source when it simulated a caller oriented toward them compared to when playbacks simulated a caller oriented away from them. These results suggest that threat call directionality provides meaningful information about the auditory scene and spatial orientation of male elephant seals in reproductive competition.

The effect of group size on the interplay between dominance and reproduction in Bombus terrestris.

Social insects provide good model systems for testing trade-offs in decision-making because of their marked reproductive skew and the dilemma workers face when to reproduce. Attaining reproductive skew requires energy investment in aggression or fertility signaling, creating a trade-off between reproduction and dominance. This may be density-dependent because the cost of achieving dominance may be higher in larger groups. We investigated the effect of group-size in B. terrestris queenless workers on two major reproduction-dominance correlates: between-worker aggression, and pheromone production, aiming at mimicking decision-making during the transition of worker behavior from cooperation and sterility to aggressive reproductive competition in whole colonies. Despite the competition, reproductive division of labor in colonies can be maintained even during this phase through the production of a sterility signal by sterile workers that has an appeasement effect on dominant nestmates. Worker-worker aggression, ovary activation, and production of sterility-appeasement signals may therefore constitute components of a trade-off affecting worker reproduction decisions. By constructing queenless groups of different size and measuring how this affected the parameters above, we found that in all groups aggression was not evenly distributed with the α-worker performing most of the aggressive acts. Moreover, aggression by the α-worker increased proportionally with group-size. However, while in small groups the α-worker monopolized reproduction, in larger groups several workers shared reproduction, creating two worker groups: reproductives and helpers. It appears that despite the increase of aggression, this was evidently not sufficient for the α-worker to monopolize reproduction. If we compare the α-worker to the queen in full-sized colonies it can be hypothesized that worker reproduction in B. terrestris colonies starts due to a gradual increase in the worker population and the queen's inability to physically inhibit worker oviposition. This may shift the trade-off between cost and benefit of worker reproduction and trigger the competition phase.

Land inheritance establishes sibling competition for marriage and reproduction in rural Ethiopia

Intergenerational transfer of wealth has been proposed as playing a pivotal role in the evolution of human sibling relationships. Sibling rivalry is assumed to be more marked when offspring compete for limited heritable resources, which are crucial for reproductive success (e.g., land and livestock); whereas in the absence of heritable wealth, related siblings may cooperate. To date, comparative studies undertaken to support this evolutionary assumption have been confounded by other socioecological factors, which vary across populations, e.g., food sharing and intergroup conflict. In this article we explore effects of sibling competition and cooperation for agricultural resources, marriage, and reproduction in one contemporary Ethiopian agropastoralist society. Here recent changes in land tenure policy, altering transfers of land from parents to offspring, present a unique framework to test the importance of intergenerational transfers of wealth in driving sibling competition, while controlling for socioeconomic biases. In households where land is inherited, the number of elder brothers reduces a man's agricultural productivity, marriage, and reproductive success, as resources diminish and competition increases with each additional sibling. Where land is not inherited (for males receiving land directly from the government and all females) older siblings do not have a competitive effect and in some instances may be beneficial. This study has wider implications for the evolution of human family sizes. Recent changes in wealth transfers, which have driven sibling competition, may be contributing to an increased desire for smaller family sizes.

Intrasexual peer aggression and dating behavior during adolescence: an evolutionary perspective

It has been suggested that the use of intrasexual aggression is a form of competition associated with reproductive opportunities. Here the authors investigated the relationship between retrospective dating and flirting behavior and peer aggression and victimization during middle and high school. Results indicate that the use of peer aggression was associated with adaptive dating outcomes in both sexes, whereas experiencing peer victimization was correlated with maladaptive dating behaviors among females only. Females who perpetrated high levels of indirect (i.e. nonphysical) aggression reported that they began dating at earlier ages in comparison to their peers, whereas aggressive males reported having more total dating partners. Experiencing female-female peer victimization was correlated with a later onset of dating behavior, more total dating partners, and less male flirtation while growing up. This report strengthens the connection between adolescent peer aggression and reproductive competition, suggesting a potential functionality to adolescent peer aggression in enhancing one's own mating opportunities at the expense of rivals.