Sibling recognition in the prairie vole, Microtus ochrogaster

Since mating is seldom observed between sibling prairie voles, Microtus ochrogaster, this, behavioural discrimination was used to investigate sibling recognition in this species. Cross-fostering of 1–3-day-old pups demonstrated that unrelated pups reared together did not breed, whereas siblings reared apart bred readily when paired at weaning. When unrelated voles were paired at 14 days of age, prior to sexual maturity, significantly fewer pairs bred than when strangers were paired at 21 days of age. Separation of 21-day-old siblings for 8 days before pairing overcame incest avoidance; a 15-day separation was required for breeding by siblings that had remained together until they were 50 days old. These results indicate that sibling recognition and, consequently, incest avoidance depend on association prior to weaning.     

Sexual reproduction in Daphnia pulex (Crustacea: Cladocera): observations on male mating behaviour and avoidance of inbreeding

1.  Mating behaviour in Daphnia appears to rely on random contact between males and sexual females rather than diffusible pheromones. Males may be able to discriminate sexually receptive females from females in other developmental stages and increase their mating efficiency. Males may also use chemical signals to avoid mating with females from the same clone and avoid the severe inbreeding depression that has been documented for intraclonal mating. The present study used experiments to test for the avoidance of intraclonal mating and assess male mating efficiency in D. pulex .
2.  Three clones were examined for the avoidance of intraclonal mating by providing males with an opportunity to mate with females of the same or two different clones. The proportion of intraclonal matings did not differ from the proportion of interclonal matings, suggesting that D. pulex males do not use kin discrimination to avoid mating with females from the same clone.
3.  The proportion of mated females decreased with increasing numbers and density of sexual females when exposed to a single male. This observation suggests that a male spends more time pursuing and copulating with sexually receptive females than non-receptive females and there is insufficient time to mate with all sexual females. The decrease in proportion of females mated could also be the result of sperm depletion in the male. Sperm depletion is unlikely to occur in nature because sexually receptive females are much rarer than in the experimental conditions.     

Same-Sex Avoidance Relations and What They Reveal about Male Competition

Avoidance relations between male kin are a pervasive social phenomenon, yet the subject has received comparatively little treatment in the anthropological literature. When anthropologists have addressed it, they have usually done so indirectly, or put forward theories better suited to explaining other social phenomena. The most common explanations one comes across in the anthropological literature to account for avoidance relationships between male kin, or what I also describe as same-sex avoidance relations in the paper, are the incest taboo and Radcliffe-Brown's theory of respect. In contrast to these explanations, I propose to demonstrate that the reason male kin avoid each other in certain types of settings is not just to maintain a sense of authority and precedence as Radcliffe-Brown's theory implies, or to avoid contravening incest prohibitions, which as Robert Lowie pointed out long ago is incorrect. Rather, because closely related male kin should not compete with each other, as this would contravene the ideology of descent which demands loyalty to one's kin.     

Reproductive roles in the cooperatively breeding acorn woodpecker: incest avoidance versus reproductive competition

Incest is rare in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) despite a polygynandrous mating system in which nearly all group members are close relatives. Here we test the relative importance of avoiding matings between close relatives (incest avoidance) and within-sex competition for breeding opportunities (reproductive competition) in determining the mating system of acorn woodpeckers by examining how reproductive roles change following breeding vacancies. In 83% of cases in which helpers of the same sex were present in the group, reproductive vacancies were resolved when new unrelated immigrants filled the vacancy to the exclusion of resident same-sex helpers, who generally emigrated or did not breed while they remained in the group. Helpers of the opposite sex, especially when male, were significantly more likely to remain in their natal group and in about half the cases inherited and bred following reproductive vacancies. This result was not explainable by reproductive competition, since the number of immigrants was often less than or equal to the number of same-sex helpers in the group. Apparent incest resulted in 5% of cases. The time required to resolve reproductive vacancies was significantly longer for groups with helpers of the same sex as the vacancy. These results confirm that both incest avoidance and reproductive competition are important factors determining reproductive roles within groups of this highly social species.     

Homosexual incest avoidance among females in captive Japanese macaques

This paper reports a systematic pattern of homosexual incest avoidance among females in a captive group of Japanese macaques (Macaca fuscata) culled from the Arashiyama-West troop known for its high rates of female homosexuality. The study group included three matrilines and two generations. Between eight and 11 females were sexually active over four consecutive mating seasons, and all engaged in both heterosexual and homosexual activity. While all females performed homosexual acts with almost all possible non-kin partners, they systematically avoided homosexual interactions with their mother, daughters, and sisters. This pattern could not be explained either in terms of kin not being simultaneously in estrus, kin avoiding affiliative interactions in general, or non-kin utilizing the tension-reducing effect of estrus to affiliate exclusively with each other. In contrast to homosexual females, heterosexual pairs of relatives (brother-sister, mother-son) were sometimes incestuous. Assuming that female homosexuality expresses the reproductive strategy of females unconstrained by male influence, the present results point to the strong tendency of females to avoid incest and suggest that males are primarily responsible for the reported exceptions to incest avoidance.     

Brother-sister and parent-child marriage outside royal families in ancient egypt and iran: A challenge to the sociobiological view of incest avoidance?

Copious and unequivocal evidence of legally condoned and socially favored brother-sister and parent-child marriage among common people from Roman Egypt (first to third centuries, A.D.) and Zoroastrian Iran (fifth century, B.C. to 11th century, A.D.) can be taken to pose a challenge to the sociobiological case for universal evolved incest avoidance within the nuclear family, triggered by early childhood proximity (the Westermarck effect). Official census documents from Roman Egypt show a high incidence of full sibling unions with relatively small age gaps between the spouses and no indication of reduced marital fertility, sexual aversion, or increased infant and child mortality. Zoroastrian religious tracts actively encourage nuclear family incest and extol its meritorious nature and supernatural benefits. A schematic assessment of the likely extent of inbreeding depression in such families under conditions of very high mortality through other causes makes reproduction at replacement level seem difficult to accomplish. But given the lack of information on the frequency of deleterious recessive genes in these populations, this reconstruction is fraught with uncertainty; pertinent ancient evidence is suggestive of some incidence of inbreeding depression but remains inconclusive. Aversion and revulsion between incestuous spouses proves a similarly elusive issue. Although these cases from antiquity do not clearly contradict the view of incest avoidance as an evolved mechanism that engenders sexual indifference and normally translates into corresponding cultural norms, they demonstrate the need for a more comprehensive consideration of the available historical record in the testing of evolutionary rules.     

Early separation and sibling incest: A test of the revised Westermarck theory

This was a follow-up study of earlier reported findings by the present investigators suggesting, albeit equivocally, that separation during early childhood inhibited later sexual acts of a potentially procreative nature between siblings but did not deter other sexual activity. The present study surveyed 170 subjects, mostly in the Toronto area, by telephone and mail. Respondents reporting potentially procreative, postchildhood sexual acts (attempted or completed genital intercourse) with siblings were compared with those reporting sexual relationships excluding procreative acts, and a third sample reporting no postchildhood sibling sexual behavior. Consonant with expectations from the earlier study, prolonged separation during early childhood was associated with procreative postchildhood sexual activity but not with other postchildhood sexual activity. Contrary to predictions, however, both sexual activity groups reported significantly more nudity and physical contact with siblings during childhood than subjects reporting no sexual activity. The findings are discussed in terms of a revised version of the Westermarck hypothesis, which is consistent with a domain-specific approach to evolved incest avoidance mechanisms.     

Early proximity and intimacy between siblings and incestuous behavior: A test of the Westermarck theory

This study was based on the Westermarck hypothesis, which maintains that the proximate mechanism for sibling incest avoidance is continued proximity in early childhood. Using a survey method, we compared university students reporting sibling incest with a matched sample of those reporting no such experience on measures of both early intimacy of the sibling pair (e.g., sleeping arrangements, practices regarding nudity) and early separations between them. Intimacy variables did not relate to incestuous behavior. Separation for more than a year during the first six years was significantly related to sexual activities which culminated in anal, oral, or attempted or actual genital intercourse (labeled consummatory behaviors), but this difference in regard to nonconsummatory sexual activities was in the opposite direction. It was suggested that previous interpretations of the Westermarck effect implying that early cohabitation produces sexual disinterest may require revision. Early association may function instead to provide a barrier to more mature, consummatory sexual activities and, hence, reduce procreative potential.     

Beyond the Taboo: Imagining Incest

This article provides an overview of anthropology's 150-year discussion of the incest taboo in light of the last 30 years of feminist and psychoanalytic discoveries about the incestuous abuse of children, it invites anthropologists to explore incest ethnographically and offers three suggested ways: one biosocial, a second social relational, and a third psychoanalytic, focusing on a connection between what psychologists call dissociation and what anthropologists call trance or possession. [Key words: incest taboo, childhood sexual abuse, dissociation, trance/possession, innate avoidance mechanism]     

Avoidance of close consanguineous inbreeding in captive groups of rhesus macaques

In three captive groups of rhesus macaques (Macaca mulatto) the intensity of inbreeding avoidance was directly correlated with the closeness of kinship. The incidence of inbreeding between matrilineal relatives was lower than expected were mating to occur randomly with regard to matrilineal affiliation. The avoidance of mating between sons and their mothers and between matrilineal sibs contributed about squally to this outcome. The incidence of mating between more remote matrilineal relatives was lower than, but could not be shown to differ significantly from, that expected at the 0.05 level of probability. Familiarity fostered by interactions among matriline members probably provides a focus for avoidance of matrilineal inbreeding, and might influence male dispersal. Results of this study are consistent with reports of female rhesus macaques' avoidance of sexual activity with matrilineal male relatives as predicted by parental investment theory The observed incidence of inbreeding between patrilineal relatives was not different from that expected, but significantly fewer of the patrilineally inbred matings involved father/daughter matings than expected were mating random. Recognition of phenotypic similarities might provide a focus for avoidance of inbreeding between fathers and their daughters, but is loss reliable than strategies for avoiding matrilineal inbreeding. Adaptations for avoiding matrilineal inbreeding that are more effective than those for avoiding patrilineal inbreeding might have evolved because the risk of patrilineal inbreeding, and hence the resulting loss in fitness, is marginal compared to that for matrilineal inbreeding in free-ranging groups. © 1995 Wiley-Liss, Inc.     

SENSUAL ATTACHMENT AND INCEST AVOIDANCE IN HUMAN EVOLUTION AND CHILD DEVELOPMENT

The authors take issue with the critique of psychoanalysis and the depiction of human sexuality and incest avoidance in evolutionary psychology. Drawing on human neurobiology and evolutionary anthropology, they show that human beings have an evolved disposition toward pair-bonding and evolved capacities for self-regulation of sexual and aggressive impulses. The realization that these characteristics are not only important but also interrelated leads to a reassessment of the Oedipus complex, a new model of incest avoidance in humans, and a fresh perspective on the relation between reproductive behaviour and environmental conditions.     

Social cues elicit sexual behavior in subordinate Damaraland mole-rats independent of gonadal status

Damaraland mole-rats (Fukomys damarensis) are among a small number of eusocial mammals. Eusociality is a social system where only a few individuals within a colony engage in direct reproduction, while remaining subordinate members are non-breeders and support reproductive efforts of breeding individuals. Inbreeding avoidance precludes mating between subordinate siblings and between offspring and parents. Interestingly, non-breeders readily attempt to mate with unrelated opposite-sex individuals. This is unusual since the non-breeding females do not attain puberty while in their natal colony. Based on this finding, the present study investigated the role of the gonads in the regulation of mating behaviors in this species and identified the mechanism of inbreeding avoidance. Gonadal-intact and gonadectomized non-breeders from different colonies were removed from their colonies and tested for the expression of sexual behavior. Results indicated that gonadal status had only minor effects on the expression of sexual behavior in either males or females. In a second experiment, sexual behaviors were absent between opposite-sex siblings so long as they had frequent contact with each other; however, following 5 weeks of separation, sexual behavior between these siblings was robustly expressed. Thus, Damaraland mole-rats avoid establishing mating relationships with familiar individuals but will readily mate with unfamiliar individuals of the opposite sex, with genetic relatedness apparently playing little role. The initiation of sexual behavior in Damaraland mole-rats does not require the presence of the gonads, but does require that the members of the pair have not been in contact with one another for at least several weeks.     

Does morality have a biological basis? An empirical test of the factors governing moral sentiments relating to incest.

Kin-recognition systems have been hypothesized to exist in humans, and adaptively to regulate altruism and incest avoidance among close genetic kin. This latter function allows the architecture of the kin recognition system to be mapped by quantitatively matching individual variation in opposition to incest to individual variation in developmental parameters, such as family structure and co-residence patterns. Methodological difficulties that appear when subjects are asked to disclose incestuous inclinations can be circumvented by measuring their opposition to incest in third parties, i.e. morality. This method allows a direct test of Westermarck's original hypothesis that childhood co-residence with an opposite-sex individual predicts the strength of moral sentiments regarding third-party sibling incest. Results support Westermarck's hypothesis and the model of kin recognition that it implies. Co-residence duration objectively predicts genetic relatedness, making it a reliable cue to kinship. Co-residence duration predicts the strength of opposition to incest, even after controlling for relatedness and even when co-residing individuals are genetically unrelated. This undercuts kin-recognition models requiring matching to self (through, for example, major histocompatibility complex or phenotypic markers). Subjects' beliefs about relatedness had no effect after controlling for co-residence, indicating that systems regulating kin-relevant behaviours are non-conscious, and calibrated by co-residence, not belief.     

The opportunity to be misled in studies of sexual selection

It is a challenge to measure sexual selection because both stochastic events (chance) and deterministic factors (selection) generate variation in individuals' reproductive success. Most researchers realize that random events ('noise') make it difficult to detect a relationship between a trait and mating success (i.e. the presence of sexual selection). There is, however, less appreciation of the dangers that arise if stochastic events vary systematically. Systematic variation makes variance-based approaches to measuring the role of selection problematic. This is why measuring the opportunity for sexual selection (I(s) and I(mates)) is so vulnerable to misinterpretation. Although I(s) does not measure actual sexual selection (because it includes stochastic variation in mating/fertilization success) it is often implicitly assumed that it will be correlated with the actual strength of sexual selection. The hidden assumption is that random noise is randomly distributed across populations, species or the sexes. Here we present a simple numerical example showing why this practice is worrisome. Specifically, we show that chance variation in mating success is higher when there are fewer potential mates per individual of the focal sex [i.e. when the operational sex ratio (OSR), is more biased]. This will lead to the OSR covarying with I(s) even when the strength of sexual selection is unaffected by the OSR. This can generate false confidence in identifying factors that determine variation in the strength of sexual selection. We emphasize that in nature, even when sexual selection is strong, chance variation in mating success is still inevitable because the number of mates per individual is a discrete number. We hope that our worked example will clarify a recent debate about how best to measure sexual selection.     

Inbreeding avoidance through non-random mating in sticklebacks

Negative effects of inbreeding are well documented in a wide range of animal taxa. Hatching success and survival of inbred offspring is reduced in many species and inbred progeny are often less attractive to potential mates. Thus, individuals should avoid mating with close kin. However, experimental evidence for inbreeding avoidance through non-random mating in vertebrates is scarce. Here, we show that gravid female three-spined sticklebacks (Gasterosteus aculeatus) when given the choice between a courting familiar brother and a courting unfamiliar non-sib prefer to mate with the non-sib and thus avoid the disadvantages of incest. We controlled for differences in males' body size and red intensity of nuptial coloration. Thus, females adjust their courting behaviour to the risk of inbreeding.     

Acephaline gregarine parasites (Monocystis sp.) are not transmitted sexually among their lumbricid earthworm hosts

The precise transmission mode(s) of acephaline gregarines in their earthworm hosts has long been questioned, yet a rigorous experimental evaluation of sexual transmission is currently lacking. That Monocystis sp., a common gregarine parasite of the earthworm Lumbricus terrestris, infects the sexual organs of its host is suggestive of sexual transmission. Considering the divergent evolutionary consequences of various modes of transmission, excluding or proving sexual transmission in this host-parasite system is critical to fully understanding it. We cultured uninfected earthworms from cocoons and subsequently mated them to either an infected or uninfected partner (from the wild). We then compared these individuals with an orally infected group, which were infected using a newly developed gavage (oral injection) method. Our data have unambiguously established that (1) horizontal sexual transmission does not play a significant role in the transmission of Monocystis sp., and (2) oral transmission through the soil is likely the principal mode of transmission between earthworms. This finding is important to models of mate-choice because infection avoidance does not appear to drive mating decisions. Finally, we further report a successful and relatively simple method to obtain infection-free individuals, which can subsequently be infected via oral gavage and used in empirical studies.     

Sexual selection in fungi

The significance of sexual selection, the component of natural selection associated with variation in mating success, is well established for the evolution of animals and plants, but not for the evolution of fungi. Even though fungi do not have separate sexes, most filamentous fungi mate in a hermaphroditic fashion, with distinct sex roles, that is, investment in large gametes (female role) and fertilization by other small gametes (male role). Fungi compete to fertilize, analogous to ‘male‐male’ competition, whereas they can be selective when being fertilized, analogous to female choice. Mating types, which determine genetic compatibility among fungal gametes, are important for sexual selection in two respects. First, genes at the mating‐type loci regulate different aspects of mating and thus can be subject to sexual selection. Second, for sexual selection, not only the two sexes (or sex roles) but also the mating types can form the classes, the members of which compete for access to members of the other class. This is significant if mating‐type gene products are costly, thus signalling genetic quality according to Zahavi's handicap principle. We propose that sexual selection explains various fungal characteristics such as the observed high redundancy of pheromones at the B mating‐type locus of Agaricomycotina, the occurrence of multiple types of spores in Ascomycotina or the strong pheromone signalling in yeasts. Furthermore, we argue that fungi are good model systems to experimentally study fundamental aspects of sexual selection, due to their fast generation times and high diversity of life cycles and mating systems.     

Intergenerational incest aversion: self-reported sexual arousal and disgust to hypothetical sexual contact with family members

The biological costs of inbreeding are expected to have shaped human incest aversion. These costs depend on biological sex, relatedness, and age. Whereas previous studies have focused on investigating how these factors modulate incest aversion in siblings and cousins—family members of the same generation— we examined relatives of different generations. In a population-based sample, 2499 respondents reported reactions to imagined sexual behaviors with either a biological child or parent, a niece/nephew or aunt/uncle, or a stepchild or stepparent; these responses were compared to reactions to imagined sexual behaviors involving a friend's child or parent. Replicating prior results, women report stronger incest aversions than do men. We extend previous findings by showing that incest aversions tended to be stronger between close (vs. more distant) intergenerational relatives. Indeed, for biological relatives, decreased degree of relatedness was associated with decreased incest aversion, and for biological relatives, the certainty in relatedness was also positively associated with incest aversion. As expected, age modulated sexual aversion for unrelated, but not related, target individuals. Sexual aversions towards step-relatives did not differ from sexual aversions to biological relatives.     

Rethinking the Taiwanese minor marriage data: evidence the mind uses multiple kinship cues to regulate inbreeding avoidance

Natural experiments such as the Israeli Kibbutzim and Taiwanese minor marriages provide unique opportunities for testing the effects of childhood association on adult sexual attraction. Within these populations, early childhood association leads to the development of a sexual aversion, an effect first proposed by Edward Westermarck. However, recent analysis of Taiwanese minor marriages indicates that only the age at first association (an inverse index of childhood association) of the younger partner predicts marital fertility rates; the age at first association of the older partner does not. Although considered a puzzle, a recent model of human inbreeding avoidance can explain this pattern. This model suggests that the mind uses at least two kinship cues to regulate the development of sibling sexual aversions: (i) childhood coresidence duration, a default cue used mainly by younger siblings in detecting probable older siblings, and (ii) exposure to one's mother caring for a newborn, a cue only available to older siblings and reliable regardless of coresidence duration and, hence, age at first association. Thus, one reason that the age at first association of only the younger partner in minor marriages predicts fertility is that coresidence duration serves as a cue to siblingship mainly for younger partners; older partners use a different kinship cue not influenced by durations of association. When compared to data from psychological investigations of the effects of coresidence duration on opposition to sibling incest, the minor marriage data reveal an identical pattern providing converging lines of evidence that multiple kinship cues mediate sibling detection and inbreeding avoidance in humans.