Availability Of Mates Research Articles

Mate limitation causes sexes to coevolve towards more similar dispersal kernels

Sex‐specific dispersal behavior has been documented in a wide range of different species. Avoidance of inbreeding and kin competition as well as different benefits of philopatry have been invoked as explanations for these patterns. All of these factors have, however, focused on explaining why dispersal behavior differs between the sexes. In this paper, we make the case that dispersal causes an increase in spatial variability in the sex ratio which can reduce the local availability of mates, and thus feed back to influence the evolution of sex‐specific dispersal and lead to more, rather than less, similar dispersal behavior in the sexes. We investigate this mechanism in two different models, first in a conceptually simple case showing why the coevolutionary effect arises, second in an individual‐based model where we model a population in explicit space with dispersal implemented as dispersal kernels. While our mechanism is not expected to completely remove sex‐bias in dispersal, it can act alongside other selection pressures to reduce such biases. Our model thus shows that dispersal of one sex can have an effect on the selective pressures on the opposite sex, without implementing inbreeding avoidance or differential benefits or costs of dispersal.

DEMOGRAPHY, POLLINATION, AND BAKER'S LAW

Baker's Law states that selfing should commonly be selected during dispersal because bottlenecks during colonization limit the availability of mates. Although this truism has broad intuitive appeal, a recent body of theory (Cheptou and Massol 2009; Massol and Cheptou 2011) casts doubt on whether adaptation favors both selfing and dispersal when both parameters are free to evolve. In these models, the joint evolution of dispersal and the selfing rate are considered in a metapopulation, with a spatially and temporally variable pollination environment. Under these conditions, adaptation favors one of two strategies: the "dispersal/outcrosser" syndrome and the "no dispersal/selfing" syndrome. These results appear to contradict the prediction of Baker's Law. These models clarify how variation in the pollination environment per se cannot generate an association between selfing and dispersal. That being said, demographic factors during dispersal episodes are likely to be important in generating patterns consistent with Baker's law. Determining whether Baker's law maintains its predictive utility requires determining whether seed banks, the perennial habit, multiple introductions, or the simultaneous arrival of many founders weaken selection for selfing during the bottleneck associated with a dispersal event. These issues highlight the many assumptions that are necessary for Baker's law to hold.

Sexual conflicts along gradients of density and predation risk: insights from an egg-trading fish

In principle, the intensity of sexual conflict is best measured as a loss of fitness associated with the expression of conflict-related traits. But because the relevant traits may be difficult to manipulate and fitness difficult to assess, proxy variables linked to conflict intensity may provide important tools for empirical measurement. Here we identify two common types of sexual conflict—one within mating pairs over the less expensive male role, and one between mating pairs and intruders seeking to obtain fertilizations—and consider how they vary in intensity along gradients of population density and predation risk. To do this, we develop and analyze a model of mating dynamics in the chalk bass, an egg-trading simultaneous hermaphrodite that lives on Caribbean coral reefs. In this species, within-pair sexual conflict leads each female-role partner to provide in each mating episode only a subset (parcel) of its egg clutch to its mate for fertilization. Pair-intruder sexual conflict (i.e., sperm competition) increases the proportion of the gonad allocated to male function. In the model, more parceling and greater male allocation both resulted in lower fitness at the ESS, our measure of conflict intensity. Male allocation increased along the density gradient but decreased along the predation-risk gradient, reflecting shifts in intrusion frequency. Parcel number sharply increased and then decreased more gradually along a gradient of increasing local density, initially responding to increased availability of alternative mates across low densities and then to diminishing clutch size toward higher densities. Parcel number decreased with predation risk as each mating episode became more dangerous. Conflict intensities were usually greatest at intermediate positions along the two environmental gradients, and each conflict ameliorated the intensity of the other. Overall, parceling and sex allocation may be good though imperfect proxies for intensities of within-pair and pair-intruder sexual conflicts among chalk bass.

Mate availability facilitates cannibalistic behaviour in a nest brooding fish: effects of timing during the brood cycle

AbstractFilial cannibalism, eating one's own viable offspring, is accepted as an adaptive response to trade-offs between current and future reproduction. Theoretical models predict that high mate availability may induce more filial cannibalism, but this prediction is rarely tested. To examine this prediction, we performed laboratory experiments using the nest breeding goby Rhinogobius flumineus. Subject males were allowed to mate with a gravid female and care for the broods. A separate gravid female housed in a small cage (stimulus-female) was shown to the subject males at one of three different points during the brood cycle: prior to spawning, within 1 day after spawning and 1 week after spawning. Empty cages were shown as a control. Males that were shown the stimulus-female before spawning cannibalised more eggs than control males. In contrast, males that were shown the stimulus-females after spawning cannibalised as few eggs as control males did. Additionally, males that were shown the stimulus-female prior to spawning did not court females more intensively than other males. Thus, we suggest that the presence of an additional mate, rather than energy expenditure associated with courtship directed toward an additional mate, can facilitate males to cannibalise their eggs.

The evolution of conditional dispersal and reproductive isolation along environmental gradients

Dispersal modulates gene flow throughout a population's spatial range. Gene flow affects adaptation at local spatial scales, and consequently impacts the evolution of reproductive isolation. A recent theoretical investigation has demonstrated that local adaptation along an environmental gradient, facilitated by the evolution of limited dispersal, can lead to parapatric speciation even in the absence of assortative mating. This and other studies assumed unconditional dispersal, so individuals start dispersing without regard to local environmental conditions. However, many species disperse conditionally; their propensity to disperse is contingent upon environmental cues, such as the degree of local crowding or the availability of suitable mates. Here, we use an individual-based model in continuous space to investigate by numerical simulation the relationship between the evolution of threshold-based conditional dispersal and parapatric speciation driven by frequency-dependent competition along environmental gradients. We find that, as with unconditional dispersal, parapatric speciation occurs under a broad range of conditions when reproduction is asexual, and under a more restricted range of conditions when reproduction is sexual. In both the asexual and sexual cases, the evolution of conditional dispersal is strongly influenced by the slope of the environmental gradient: shallow environmental gradients result in low dispersal thresholds and high dispersal distances, while steep environmental gradients result in high dispersal thresholds and low dispersal distances. The latter, however, remain higher than under unconditional dispersal, thus undermining isolation by distance, and hindering speciation in sexual populations. Consequently, the speciation of sexual populations under conditional dispersal is triggered by a steeper gradient than under unconditional dispersal. Enhancing the disruptiveness of frequency-dependent selection, more box-shaped competition kernels dramatically lower the speciation-enabling slope of the environmental gradient.

Ovarian Development in Predacious Orius pumilio: Relationship to Diet, Mating, and Juvenile Hormone

Abstract A female minute pirate bug, Orius pumilio (Champion), was found to require both food and mating as an adult to achieve egg development. Last-instar nymphs of O. pumilio were individually placed into 96-well microtiter plates and recovered 24–48 h later as virgin adults, fed only on sucrose or water. The procedure yielded low mortalities. Correct identification of the sex of individuals as last-instar nymphs was achieved with 96% accuracy. The availability of food (eggs of Ephestia kuehniella Zeller) and mates for these isolated females was controlled. Unfed adult females, whether mated or not, did not produce defectible yolk protein when assayed by enzyme-linked immunosorbent assay, nor did they show any follicle development when examined microscopically. Fed but unmated females produced a significant, defectible amount of yolk protein, and some oocyte development was observed, but they contained no fully mature eggs. Females that were both fed and mated fell into two categories: 44% produced mature eggs at a mean rate of 6.4 eggs per female, whereas 56% had ovaries similar to those of fed but unmated females. If treated in mixed sex groups, females that were fed and mated contained more yolk protein than those held in pairs, but increasing the sex ratio of groups above 0.5 (males:females) did not result in a significant increase in yolk protein content. Topical treatment of fed females with methoprene, a juvenile hormone analog, did not trigger an increase in yolk protein contents. We conclude that vitellogenesis is a two-stage process in adult female O. pumilio. Early vitellogenesis requires a nutritious adult diet, whereas full vitellogenesis and egg maturation also require mating.

THE LANDE-KIRKPATRICK MECHANISM IS THE NULL MODEL OF EVOLUTION BY INTERSEXUAL SELECTION: IMPLICATIONS FOR MEANING, HONESTY, AND DESIGN IN INTERSEXUAL SIGNALS

The Fisher-inspired, arbitrary intersexual selection models of Lande (1981) and Kirkpatrick (1982), including both stable and unstable equilibrium conditions, provide the appropriate null model for the evolution of traits and preferences by intersexual selection. Like the Hardy–Weinberg equilibrium, the Lande–Kirkpatrick (LK) mechanism arises as an intrinsic consequence of genetic variation in trait and preference in the absence of other evolutionary forces. The LK mechanism is equivalent to other intersexual selection mechanisms in the absence of additional selection on preference and with additional trait-viability and preference-viability correlations equal to zero. The LK null model predicts the evolution of arbitrary display traits that are neither honest nor dishonest, indicate nothing other than mating availability, and lack any meaning or design other than their potential to correspond to mating preferences. The current standard for demonstrating an arbitrary trait is impossible to meet because it requires proof of the null hypothesis. The LK null model makes distinct predictions about the evolvability of traits and preferences. Examples of recent intersexual selection research document the confirmationist pitfalls of lacking a null model. Incorporation of the LK null into intersexual selection will contribute to serious examination of the extent to which natural selection on preferences shapes signals.

Space use patterns of mountain hare (Lepus timidus) on the Alps

Populations on the limits of species’ distribution can show different behavioral adaptations to strong ecological pressure than in the central part of the range. We investigated space use patterns of alpine mountain hare (Lepus timidus) at two areas on the southern edge of the species’ range. We monitored 34 hares between 2005 and 2008, estimating home range size, overlap, and site fidelity, and compared our results with space use in Scottish and North-European populations. Home ranges of mountain hares did not differ between two study areas with different habitat types. Subadult animals used larger ranges than adults and both age groups reduced home range size in autumn, a period that might be critical for hares due to changes in diet and/or high energy expenditure during the previous breeding season. Home ranges in these alpine populations were smaller than in Scandinavian populations but within the range of populations in different habitat types in Scotland. Seasonal home ranges overlapped considerably, but differed among the sexes: male–female overlap was higher than same sex (male–male and female–female) spatial overlap. Seasonal shifts of home ranges were small, and site fidelity remained high over the seasons, suggesting that resource distribution remained constant throughout the year and that the knowledge of an intensively frequented area is an important element of habitat quality. We concluded that habitat structure and availability of mates interact in affecting mountain hare space use in alpine habitats.

Mate availability affects female choice in a fish with paternal care: female counterstrategies against male filial cannibalism

Theory predicts that individuals should adopt counterstrategies against intersexual conflict with their mating partners if the counterstrategies are effective and cost-efficient. In fishes, males with parental care often cannibalize their own offspring, which reduces the female’s fitness and creates intersexual conflicts. Males of the goby Rhinogobius flumineus cannibalize more eggs in the nest when they have access to additional females prior to spawning. Thus, it is predicted that females will strategically avoid spawning with males that have high mate availability. In the present study, we experimentally tested this prediction. When sexual pairs were placed in tanks, most females (control females; 21/22) successfully spawned inside the nest. In contrast, when a gravid female (stimulus female) that was housed in a small transparent cage was shown to the experiment pairs prior to spawning, only about half of the females (experiment females; 16/29) spawned inside the nest; the remaining females released unfertilized eggs outside of the nest. Moreover, experiment females infrequently accepted and followed males into nests, and delayed spawning more often than control females. R. flumineus females prefer males that court frequently. Indeed, experiment females that infrequently received courtship tended to spawn outside of the nest. However, infrequent courtship alone could not explain outside-nest spawning, delay in spawning, or the shorter stay of females in nests. These results imply that the presence of a stimulus female dampens female spawning with males. We suggest that R. flumineus females may strategically reject or hesitate to spawn with males that have high mate availability, and that this spawning avoidance may be a counterstrategy against male filial cannibalism.

Preliminary evidence regarding the hypothesis that the sex ratio at sexual maturity may affect longevity in men.

In human populations, variation in mate availability has been linked to various biological and social outcomes, but the possible effect of mate availability on health or survival has not been studied. Unbalanced sex ratios are a concern in many parts of the world, and their implications for the health and survival of the constituent individuals warrant careful investigation. We indexed mate availability with contextual sex ratios and investigated the hypothesis that the sex ratio at sexual maturity might be associated with long-term survival for men. Using two unique data sets of 7,683,462 and 4,183 men who were followed for more than 50 years, we found that men who reached their sexual maturity in an environment with higher sex ratios (i.e., higher proportions of reproductively ready men) appeared to suffer higher long-term mortality risks than those in an environment with lower sex ratios. Mate availability at sexual maturity may be linked via several biological and social mechanisms to long-term survival in men.

Small‐scale dispersal and survival in a long‐lived seabird, the wandering albatross

1. Dispersal is a fundamental but still poorly known process in population dynamics and several hypotheses have been proposed to explain its patterns. We studied natal and breeding dispersal and survival in a long-lived seabird, the wandering albatross (Diomedea exulans L.), and examined several hypotheses concerning dispersal patterns in birds. 2. We applied multi-state capture-recapture models to a 36-year data set (1969-2004) collected at three albatross colonies on Ile de Possession, Crozet Islands. Because the species has biennial reproduction, we introduced unobservable states in the model to account for the absence of individuals in those years. 3. Adults were highly faithful to their nesting colony but colony fidelity, as well as survival rate, differed slightly among colonies (fidelity ranged from 0.957 to 0.977). Breeding fidelity was highest in the colony where survival was lowest and individuals were not more likely to change colony following a failed breeding attempt than after a successful one. The colony that attracted most dispersers had the lowest density of nesting birds. 4. Philopatry (the probability that young return to breed at a birth site) was generally high but variable among colonies (ranging from 0.70 to 0.92), and survival of young differed little. Philopatry was highest in the largest colony, where the availability of potential mates was presumably greatest. However, among dispersing individuals, the colony that had the lowest density of nesting individuals, not the largest colony, attracted the most recruits. 5. Although size of the colony influenced the decision to stay or to leave in young, density was most influential in the selection of a new colony among both adult and young dispersers. Our results support the hypothesis that philopatry is the strategy favoured by most recruits and that conspecific attraction can explain variation in the level of philopatry among colonies but not settlement patterns among dispersing individuals.

New records of marine fishes illustrate the biogeographic importance of Christmas Island, Indian Ocean

Christmas Island is situated in the tropical eastern Indian Ocean on a biogeographic border where Indian and Pacific Ocean faunas meet. Detailed field studies in 2004, 2007 and 2008, of the island’s fish fauna revealed 30 new records from 15 families. For six families (Dasyatidae, Chanidae, Bramidae, Mugilidae, Siganidae, Molidae) this is the first time a species has been recorded at Christmas Island. Many of the newly recorded fishes appear to have recently colonised the island, and establishing populations will be dependent on the availability of suitable habitat and conspecific mates. These new records illustrate that Christmas Island is important for range expansion because it serves as a critical stepping-stone in the dispersal of Pacific Ocean species into the Indian Ocean and vice versa. Contact between Indian and Pacific Ocean sister species has also resulted in hybridisation at Christmas Island.

Bose–Einstein condensates form in heuristics learned by ciliates deciding to signal ‘social’ commitments

Fringe quantum biology theories often adopt the concept of Bose–Einstein condensation when explaining how consciousness, emotion, perception, learning, and reasoning emerge from operations of intact animal nervous systems and other computational media. However, controversial empirical evidence and mathematical formalism concerning decoherence rates of bioprocesses keep these frameworks from satisfactorily accounting for the physical nature of cognitive-like events. This study, inspired by the discovery that preferential attachment rules computed by complex technological networks obey Bose–Einstein statistics, is the first rigorous attempt to examine whether analogues of Bose–Einstein condensation precipitate learned decision making in live biological systems as bioenergetics optimization predicts. By exploiting the ciliate Spirostomum ambiguum's capacity to learn and store behavioral strategies advertising mating availability into heuristics of topologically invariant computational networks, three distinct phases of strategy use were found to map onto statistical distributions described by Bose–Einstein, Fermi–Dirac, and classical Maxwell–Boltzmann behavior. Ciliates that sensitized or habituated signaling patterns to emit brief periods of either deceptive ‘harder-to-get’ or altruistic ‘easier-to-get’ serial escape reactions began testing condensed on initially perceived fittest ‘courting’ solutions. When these ciliates switched from their first strategy choices, Bose–Einstein condensation of strategy use abruptly dissipated into a Maxwell–Boltzmann computational phase no longer dominated by a single fittest strategy. Recursive trial-and-error strategy searches annealed strategy use back into a condensed phase consistent with performance optimization. ‘Social’ decisions performed by ciliates showing no nonassociative learning were largely governed by Fermi–Dirac statistics, resulting in degenerate distributions of strategy choices. These findings corroborate previous work demonstrating ciliates with improving expertise search grouped ‘courting’ assurances at quantum efficiencies and verify efficient processing by primitive ‘social’ intelligences involves network forms of Bose–Einstein condensation coupled to preceding thermodynamic-sensitive computational phases.

Evolution towards self‐compatibility when mates are limited

Theory of plant mating system evolution predicts the spread of self-compatibility (SC) in a predominantly self-incompatible population when inbreeding depression (ID; the decline in fitness because of selfing) is small and when compatible mates are limited. I tested these two predictions by measuring the occurrence of SC in 13 natural populations of Ranunculus reptans L. that varied in ID and frequency of cross-incompatible mates. Enforced selfing experiments were conducted in 2 years. In the first year, self-pollination was applied at two flower ages to investigate the occurrence of delayed SC. I found that SC was not uncommon across all populations, but self-compatible plants usually produced few seeds. There was no evidence for delayed SC. The occurrence of SC was not associated with population-level ID, but populations with more limited availability of compatible mates had a significantly higher frequency of plants that were at least partially self-compatible. The results indicate that, in R. reptans, a shortage of available mates in small populations may cause the evolution of partial SC and mixed mating.

Life‐history variation following habitat degradation associated with differing fine‐scale spatial genetic structure in a rainforest cycad

AbstractHabitat degradation can result in drastic environmental changes potentially affecting the life‐history of populations and aspects of the reproductive biology and the genetic structure within and among populations. Here, we explore how life‐history differences between subpopulations from contrasting habitats may affect mating availability, which in turn will indirectly affect the strength of spatial genetic structure within populations of a tropical rainforest cycad (Zamia fairchildiana). Subpopulations exposed to higher light availability in degraded‐forest habitats had male individuals that grew faster, reproduced earlier, and invested more in reproduction than in native‐forest habitat subpopulations. These differences in life history resulted in degraded‐habitat subpopulations showing a higher proportion of reproductive adults and greater mate availability in a reproductive season. Subpopulations in the degraded habitat showed weaker SGS, i.e., a smaller slope in the linear regression of genetic relatedness on linear distance. Environmentally induced changes in life history and subsequent changes in the strength of the SGS after habitat degradation may have important consequences for population viability and should be of concern in conservation.

Mate availability contributes to maintain the mixed‐mating system in a scolytid beetle

We investigated the mating system and population genetic structure of the beetle, Coccotrypes dactyliperda, with life history characteristics that suggest the presence of a stable mixed-mating system. We examined the genetic structure of seven populations in Israel and found significant departures from the Hardy-Weinberg equilibrium and an excess of homozygosity. Inbreeding coefficients were highly variable across populations, suggesting that low levels of outbreeding occur in nature. Experiments were conducted to determine whether the observed high inbreeding in these populations is the result of a reproductive assurance strategy. Females reared in the laboratory took longer to mate with males from the same population (inbreeding) than with males from a different population (outbreeding). These results suggest that females delayed inbreeding, and were more inclined to outbreed when possible. Thus inbreeding, which predominates in most populations, may be due to a shortage of mates for outbreeding rather than a preference for inbreeding. We conclude that C. dactyliperda has a mixed-mating system that may be maintained by a reproductive assurance strategy.

Experience does not alter alternative mating tactics in the burying beetle Nicrophorus vespilloides

Alternative male mating tactics are widespread, but the cues that determine which tactic is adopted remain unclear. Size is commonly associated with alternative mating tactics, but it is not known how individuals gauge their size effectively, especially given that size is relative and frequency dependent. One possibility is that interactions with conspecifics are used to assess size, relative to potential competitors, and thus fine-tune tactics. Success in mating might also influence mating tactics given that this should indicate the potential availability of mates in the population. We tested these ideas in the burying beetle Nicrophorus vespilloides, examining whether individuals use the outcome of larval or adult interactions as cues to adjust the tactics used to acquire mates. Male N. vespilloides employ 2 tactics; search for a carcass, a resource required for reproduction, or release a pheromone (call) to attract a mate. Males are plastic in the amount of time they invest in each tactic, and in a related species (Nicrophorus orbicollis), male size influences the tactic adopted. We examine the potential effects of parental care, sibling competition, relative size within a brood, and adult experience of agonistic interactions and mating on tactic adoption. Absolute size was consistently the best predictor of calling rate, with smaller males calling more often than larger males. We suggest that the lack of a response to adult cues may reflect unpredictability in the occurrence of social interactions or stable size distributions in this population. Key words: alternative mating tactic, behavioral plasticity, conditional strategy, mate attraction, pheromone. [Behav Ecol]

Mate availability and unmarried parent relationships.

Theoretically, a shortage of males in a local marriage market may influence the formation, quality, and trajectory of unmarried parent relationships. To test these hypotheses, I combine city-level sex ratio data from the U.S. census with microdata on unmarried couples who recently had a child from the Fragile Families and Child Wellbeing study. A shortage of men in a marriage market is associated with lower relationship quality for unmarried parents. Male shortages are associated with lower rates of marriage following a nonmarital birth, in part because of the mediating influence of relationship quality. A shortage of men is not significantly related to the economic quality of male, nonmarital childbearing partners.

Prior encounters with the opposite sex affect male and female mating behavior in a wolf spider (Araneae, Lycosidae)

Mate availability can vary widely in nature depending upon population density and sex ratio and can affect the ability of individuals to be selective in mate choice. We tested the effects of prior encounters with the opposite sex (i.e., exposure to the opposite sex either with or without mating) on subsequent mating behavior in two experiments that manipulated mate availability for both males and females in the wolf spider, Hogna helluo. The probability of mating in the experimental trial depended upon whether the prior encounter involved mating or not, and males and females responded in opposite directions. Exposure without mating resulted in a higher subsequent frequency of mating for females and a lower subsequent frequency of mating for males, while prior mating experience resulted in a lower frequency of female remating and a higher frequency of male remating. Prior exposure without mating did not affect female aggression. However, mated females engaged in precopulatory cannibalism more frequently than virgins. Mated males escaped postcopulatory cannibalism more frequently than virgins. Our results show that males respond to exposure without mating in the expected manner. However, prior mating (1 week earlier) had unexpected effects on males, which may be due to mated males being of higher quality. There were little or no effects of the size of the prior exposure individual or mate on subsequent mating behaviors. Further research is needed to determine why different species use different degrees of prior information in mate choice.

Supply and demand: causes and consequences of assortative mating in common terns Sterna hirundo

Although assortative mating is widespread among long-lived monogamous birds, the underlying mechanisms still remain unclear in many species. In this study, we analysed assortative mating by age and body mass in the common tern Sterna hirundo with special regard to mate choice decisions of newly paired birds. To assess whether assortative mating by age is an active decision involving either homotypic or directional preferences or rather a passive process due to restricted availability of potential mates, we analysed the influence of age-dependent arrival date and cohort size. Furthermore, we looked for direct benefits in terms of reproductive success. Common terns mated assortatively by age and arrival date but not by body mass at arrival. Assortative mating by age was age dependent and was detected mainly in younger birds, whereas birds older than 8 years rather re-paired with younger ones. The availability of same-aged mates was restricted by cohort size and arrival date. We found no general evidence for better reproductive performance of same-aged pairs. Instead, relative reproductive success was related to own age and the relative age of mate: It should be advantageous for any bird to acquire an old mate; hence, assortative mating by age seems to be beneficial only for old terns but not for young ones. Age-assortative mating in common terns occurred by both passive and active processes, which are not mutually exclusive. Our results do not indicate a homotypic but a directional preference and support the theory that high-quality (older/experienced) individuals tend to mate assortatively if same-aged mates are available, which leaves low-quality (young/inexperienced) individuals to mate among themselves.