Reduced Mating Opportunities Research Articles

Environmental conditions lead to shifts in individual communication, which can cause cascading effects on soundscape composition.

Climate change is increasing aridity in grassland and desert habitats across the southwestern United States, reducing available resources and drastically changing the breeding habitat of many bird species. Increases in aridity reduce sound propagation distances, potentially impacting habitat soundscapes, and could lead to a breakdown of the avian soundscapes in the form of loss of vocal culture, reduced mating opportunities, and local population extinctions. We developed an agent‐based model to examine how changes in aridity will affect both sound propagation and the ability of territorial birds to audibly contact their neighbors. We simulated vocal signal attenuation under a variety of environmental scenarios for the south, central semi‐arid prairies of the United States, ranging from contemporary weather conditions to predicted droughts under climate change. We also simulated how changes in physiological conditions, mainly evaporative water loss (EWL), would affect singing behavior. Under contemporary and climate change‐induced drought conditions, we found that significantly fewer individuals successfully contacted all adjacent neighbors than did individuals in either the contemporary or predicted climate change conditions. We also found that at higher sound frequencies and higher EWL, fewer individuals were able to successfully contact all their neighbors, particularly in drought and climate change drought conditions. These results indicate that climate change‐mediated aridification may alter the avian soundscape, such that vocal communication no longer effectively functions for mate attraction or territorial defense. As climate change progresses, increased aridity in current grasslands may favor shifts toward low‐frequency songs, colonial resource use, and altered songbird community compositions.

Masculinized Sexual Ornaments in Female Lizards Correlate with Ornament-Enhancing Thermoregulatory Behavior.

SynopsisThe adaptive significance of colorful or exaggerated traits (i.e., ornaments) expressed in females is often unclear. Competing hypotheses suggest that expression of female ornaments arises from maladaptive (or neutral) genetic inheritance from males along with incomplete epigenetic regulation, or from positive selection for ornaments in females under social competition. Whether costly or advantageous, the visibility of such traits can sometimes be behaviorally modulated in order to maximize fitness. Female eastern fence lizards express blue badges that are variable in size and color saturation. These are rudimentary compared to those seen in males and carry important costs such as reduced mating opportunities. Body temperature is a well-established enhancer of badge color, and thus thermoregulation may be one way these animals modulate badge visibility. We quantified realized body temperatures of female lizards paired in laboratory trials and observed that females with larger badges attained higher body temperatures when freely allowed to thermoregulate, sometimes beyond physiological optima. In this association between phenotype and behavior, females with larger badges exhibited thermoregulatory patterns that increase their badges’ visibility. This signal-enhancing behavior is difficult to reconcile with the widely held view that female ornaments are maladaptive, suggesting they may carry context-dependent social benefits.

Tick parasitism impairs contest behavior in the western fence lizard (Sceloporus occidentalis)

Parasites may impair host behavior in ways that reduce host fitness, especially when access to territories or mates becomes disrupted. Western fence lizards (Sceloporus occidentalis) are a key host to western blacklegged ticks (Ixodes pacificus). Males are highly aggressive during the mating season, competing with rivals through displays of color badges, pushups, and other behaviors. We hypothesized that experimental tick infestation diminishes the performance of male western fence lizards in intrasexual contests, via either blood loss, damage to sensory structures, or both. We infested adult males with larval ticks for 7 to 10 days, staged contests between infested and quality-matched control males, and measured their behavior in enclosure arenas. Infested lizards were less aggressive and exhibited decreased hematocrit, compared to non-infested animals. We found no relationships between aggression and either body size or blue ventral badge color traits, for either infested or uninfested males. There was also no effect of tick attachment location and hemoparasite infection on host contest behavior. This is the first demonstration of the impact of I. pacificus parasitism on intraspecific interactions of western fence lizards and suggests that tick infestation has substantial impacts on lizard fitness. Because I. pacificus rely heavily on these lizards for blood meals and dispersal, these impacts could also influence the abundance of ticks and the pathogens they vector. Parasitism frequently impairs host behavior. We investigated the effect of blacklegged tick parasitism on western fence lizard contest behavior using a lab-based manipulation experiment. We demonstrated that tick infestation reduced lizard aggressiveness, which may lead to territory loss and reduced mating opportunities in the wild. We found no evidence that host body size or coloration are correlated with aggression. This study adds to a growing body of evidence that tick parasitism is costly for lizards, while demonstrating the usefulness of manipulation experiments to understand host-parasite interactions.

Behavioural discrimination of male mental gland secretions of the gopher tortoise (Gopherus polyphemus) by both sexes

Chemical communication is important for mate choice, especially at long distances in fragmented populations. The gopher tortoise is a social species that is threatened in the southeast U.S. due to habitat fragmentation and decline. One consequence of habitat loss is reduced mating opportunities, yet chemical signalling in gopher tortoises is relatively under-studied. Here, we investigated chemoreception of tortoise discrimination of chin secretions, or mental gland (MG) secretions. To assess conspecific recognition of male MG secretions, we conducted two paired-choice experiments: one with a neutral odorant control (NC; distilled water) and one with a pungent odorant control (PC; acetone) vs. male MG secretions. Behaviours were defined a priori, and their durations were quantified relative to treatments. Each sex spent significantly more time with MG secretions vs. acetone control during the PC study (p= 0.001). Each sex also sniffed MG swabs more frequently in both studies (PC study: p=0.0003; NC study: p=0.001). A principal components analysis of behavioural durations from the PC study identified one component with a significant treatment effect performed to MG secretions (p=0.0003), including the behaviours sniffing, head bobbing, biting, and eating near a swab. Our study provides the first chemical-behavioural bioassay of MG secretions from male gopher tortoises, suggesting MG secretions may be a source of pheromones.

Balancing risk and reward: mating opportunity influences thermal refuge use in fiddler crabs

Behavioural thermoregulation can ameliorate thermal stress but is costly. For species that court in a thermally stressful microhabitat, sexual selection via endurance rivalry favours individuals that are able and willing to endure harsh conditions in the courtship habitat, as retreats to a thermal refuge will reduce mating opportunities. The relative costs and benefits of refuge use versus continued courtship in the face of thermal risk, which vary across abiotic and biotic contexts, determine the optimal behavioural strategy. We examined the social and abiotic factors driving behavioural decisions related to thermoregulatory retreat in the fiddler crab Austruca mjoebergi. Male fiddler crabs perform a courtship display on the thermally stressful intertidal sediment surface. Time on the surface, and thus time available for display, was limited by high temperatures; as temperature increased, surface time decreased. Yet when presented with a stimulus female, males were more likely to perform the courtship display, displayed at a higher rate and increased time spent on the surface. These results demonstrate that behavioural decisions related to thermal retreat depend both on the abiotic conditions that influence the degree of thermal stress and on the social conditions that influence the reproductive prospects of the individual.

Edge effects and mating patterns in a bumblebee-pollinated plant.

Researchers have long assumed that plant spatial location influences plant reproductive success and pollinator foraging behaviour. For example, many flowering plant populations have small, linear or irregular shapes that increase the proportion of plants on the edge, which may reduce mating opportunities through both male and female function. Additionally, plants that rely on pollinators may be particularly vulnerable to edge effects if those pollinators exhibit restricted foraging and pollen carryover is limited. To explore the effects of spatial location (edge vs. interior) on siring success, seed production, pollinator foraging patterns and pollen-mediated gene dispersal, we established a square experimental array of 49 Mimulus ringens (monkeyflower) plants. We observed foraging patterns of pollinating bumblebees and used paternity analysis to quantify male and female reproductive success and mate diversity for plants on the edge versus interior. We found no significant differences between edge and interior plants in the number of seeds sired, mothered or the number of sires per fruit. However, we found strong differences in pollinator behaviour based on plant location, including 15 % lower per flower visitation rates and substantially longer interplant moves for edge plants. This translated into 40 % greater pollen-mediated gene dispersal for edge than for interior plants. Overall, our results suggest that edge effects are not as strong as is commonly assumed, and that different plant reproduction parameters respond to spatial location independently.

Mating has No Effect on Male Longevity in the MothAmorbia cuneana(Tortricidae)

In some insects, males invest heavily in each mating thus reducing the typical sexual imbalance in potential reproductive rates which, in turn, influences the mode and intensity of sexual selection. In Lepidoptera, males invest in mate searching, courtship, ejaculate production and, in some species, in genital spines shed inside the female. Despite this substantial investment, evidence regarding its effect on male survival is contradictory. The effect of successful mating (i.e. copulations resulting in transfer of an ejaculate and deciduous genital spines) on male longevity was experimentally estimated in the moth Amorbia cuneana (Tortricidae), a pest of avocado. Our results do not support the prediction that mating costs result in a reduction in male longevity: virgin (n = 16) and mated (n = 26) males lived a similar number of days. It is hypothesized that reduced mating opportunities have selected for virgin males that are always ready to transfer large ejaculates and thus against the diversion of resources from reproductive tissues to somatic maintenance in unmated males.

Reduced mate availability leads to evolution of self-fertilization and purging of inbreeding depression in a hermaphrodite.

Basic models of mating-system evolution predict that hermaphroditic organisms should mostly either cross-fertilize, or self-fertilize, due to self-reinforcing coevolution of inbreeding depression and outcrossing rates. However transitions between mating systems occur. A plausible scenario for such transitions assumes that a decrease in pollinator or mate availability temporarily constrains outcrossing populations to self-fertilize as a reproductive assurance strategy. This should trigger a purge of inbreeding depression, which in turn encourages individuals to self-fertilize more often and finally to reduce male allocation. We tested the predictions of this scenario using the freshwater snail Physa acuta, a self-compatible hermaphrodite that preferentially outcrosses and exhibits high inbreeding depression in natural populations. From an outbred population, we built two types of experimental evolution lines, controls (outcrossing every generation) and constrained lines (in which mates were often unavailable, forcing individuals to self-fertilize). After ca. 20 generations, individuals from constrained lines initiated self-fertilization earlier in life and had purged most of their inbreeding depression compared to controls. However, their male allocation remained unchanged. Our study suggests that the mating system can rapidly evolve as a response to reduced mating opportunities, supporting the reproductive assurance scenario of transitions from outcrossing to selfing.

Copulation rate declines with mating group size in dusky dolphins (Lagenorhynchus obscurus)

Males in polygamous species often engage in intrasexual competition for mates. If females actively evade mating attempts, it may benefit males to cooperate to restrict female movement, as has been found in some mammals. We tested if male dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) cooperate or compete during group mating chases. If they cooperate, then the per-male probability of copulating should increase with group size; if they compete, then the probability should decrease. We followed mating groups by boat during the breeding season (October 2013 – January 2014) off Kaikoura, New Zealand. The copulation rate per male decreased with increasing group size and with the number of noncopulating males in proximity to a copulating female. Male dusky dolphins have multiple mates and appear to use sperm and exploitative scramble competition. Males may remain in mating groups despite competition because there are alliances within the groups, they are unable to exclude rivals from joining a group, the time and energy costs of searching for unescorted females exceed the costs of reduced mating opportunities in a group, they receive other direct or indirect benefits that offset the costs of reduced mating opportunities, or they are in the group largely for social learning rather than procreation.

Differential maturation of sexual traits: revealing sex while reducing male and female aggressiveness

Sexually mature young males of many dichromatic species begin breeding while expressing female-like or incomplete adult phenotypes, which contradicts sexual selection theory. It is thought that by delaying the acquisition of adult appearance, young males may avoid aggression from other males, but reduce mating opportunities, resulting in trade-offs between these 2 life-history traits. Thus, an optimal young male phenotype should allow mating while reducing aggressiveness. Differential character maturation (i.e., the acquisition of adult appearance does not occur in all traits at the same time) has been described in bird species, but has largely been neglected in the study of delayed plumage maturation. Using the common kestrel Falco tinnunculus, we tested the prediction that maturation of different adult traits depends on the aggressiveness elicited by both male and female individuals and that young male phenotypes should be a combination of traits triggering low levels of aggressiveness while still revealing sex. We explored the pattern of delayed plumage maturation observed in 4 different adult male traits (head, rump, tail, and back plumages) in young breeding males captured over 8 years. We then analyzed the intensity of agonistic responses against these traits in breeding males and females by presenting young male decoys showing these mature characters. We found that those characters eliciting high aggressiveness in males (head and rump) were partly expressed in young males and only 1 (tail) of the 2 traits (tail and back) eliciting high aggressiveness in females was poorly expressed. Our study suggests that conspecific aggressiveness of both males and females can play a significant role in determining patterns of delayed plumage maturation.

Sex, War, and Disease: The Role of Parasite Infection on Weapon Development and Mating Success in a Horned Beetle (Gnatocerus cornutus)

While parasites and immunity are widely believed to play important roles in the evolution of male ornaments, their potential influence on systems where male weaponry is the object of sexual selection is poorly understood. We experimentally infect larval broad-horned flour beetles with a tapeworm and study the consequent effects on: 1) adult male morphology 2) male-male contests for mating opportunities, and 3) induction of the innate immune system. We find that infection significantly reduces adult male size in ways that are expected to reduce mating opportunities in nature. The sum of our morphological, competition, and immunological data indicate that during a life history stage where no new resources are acquired, males allocate their finite resources in a way that increases future mating potential.

Non-adaptive speciation of snails by left-right reversal is facilitated on oceanic islands

The nearly neutral theory of molecular evolution predicts that small population size is essential for non-adaptive evolution. Evolution of whole-body left-right reversal in snails is generally a compelling example of non-adaptive speciation, because variants with reversed chirality would suffer from reduced mating opportunities within a population. Despite this reproductive disadvantage, sinistral snail species have repeatedly originated from dextral ancestors in terrestrial pulmonates. Here I show that snail speciation by reversal has been accelerated on oceanic islands. Analysing the global biogeography of 995 genera across 84 stylommatophoran families, I found that the proportion of sinistral snail genera was enhanced in genera endemic to oceanic islands. Oceanic islands are relatively small land masses offering highly fragmented habitats for snails. Thus, the upper limit of population size would probably have been small for a long time there. Oceanic islands may have facilitated the fixation of the nonadaptive allele for speciation by reversal, allowing subsequent ecological divergence of sibling species. This study illustrates the potential role of genetic drift in non-adaptive speciation on oceanic islands.

Doing the twist: A test of Darwin's cross-pollination hypothesis for pollinarium reconfiguration

Mating success in plants depends largely on the efficiency of pollen dispersal. For hermaphrodite plants, self-pollination, either within or among flowers, can reduce mating opportunities because of pollen and ovule discounting and inbreeding depression. Self-pollination may be particularly detrimental in plants such as orchids and asclepiads that package each flower's pollen into one or more pollinia which, together with accessory structures, comprise a pollinarium. Darwin proposed that physical reconfiguration of pollinaria serves as a mechanism for reducing the likelihood of self-pollination. To be effective, the time taken for pollinarium reconfiguration would need to exceed that spent by a pollinator on a plant. We investigated pollinarium reconfiguration (including pollinarium bending, pollinium shrinking and anther cap retention) in 19 species and found a strong positive relationship between reconfiguration time and the duration of pollinator visits. Reconfiguration times were also consistently longer than pollinator visit times. These results provide strong support for Darwin's idea that this mechanism promotes cross-pollination.

Spatial movements of grey partridge Perdix perdix: male-biased spring dispersal and effect of habitat quality

The Grey Partridge Perdix perdix is a sedentary species with a surplus of males in spring populations. It experienced a dramatic decline in Europe primarily due to habitat loss that may have forced the species to modify its dispersal behaviour, a less transparent source of populations threats. We analysed the dispersal patterns of Grey Partridges during pre- and post-breeding periods in two landscapes with different habitat quality in the Czech Republic. Grey Partridges tended to pair off quickly, with most pre-breeding movements executed in pairs. The birds usually dispersed over a few hundred metres indicating that the habitat conditions in a landscape with a carrying capacity of 2–5 pairs/1 km2 was not below the threshold initiating strong dispersal, as observed in extreme habitats and at the northern limits of the species´ distribution range. A few single males that had remained unpaired until April also finally bred. However, the high level of sedentariness of males in a good landscape suggested that males prefer to stay near wintering sites unless habitat sources become limited. Unlike the males, the females dispersed in a wide range of distances with an extensive overlap in both landscapes. Autumn movements did not differ between the sexes, but were longer than the pre-breeding movements and increased in a poor landscape. We conclude that both reduced habitat availability and reduced mating opportunities initialise primarily male-biased breeding dispersal in Grey Partridge populations.

Spatial genetic structure of Sorbus torminalis: The extent of clonal reproduction in natural stands of a rare tree species with a scattered distribution

Distinct spatial genetic structure, as the result of various evolutionary and ecological processes, is a common feature of tree populations. The rare pioneer forest tree Sorbus torminalis occurs in scattered populations of low density and exhibits both clonal propagation and gametophytic self-incompatibility. Clonal reproduction can promote considerable spatial genetic structure and, together with a self-incompatibility system, may substantially reduce mating opportunities within S. torminalis populations, i.e. an Allee-effect owing to mate limitation. All 10 S. torminalis stands mapped in northern Switzerland and analysed with allozymes showed a considerable degree of clonal reproduction, but they were also characterised by large numbers of genotypes that occurred only once. However, spatial autocorrelation analysis revealed significant spatial genetic structure at distances between 15 and 30 m as the result of clonal reproduction. Once the effect of clonal propagation was removed from the analysis, the stands no longer exhibited significant spatial autocorrelation. This implies that seed dispersal was not locally restricted. The degree of clonal reproduction was neither correlated with population size, nor did smaller populations exhibit less genetic diversity. Because clonal patches were rather small and interspersed with other genetically unique and unrelated individuals, clonal reproduction seemed to have no negative impact on the species’ sexual reproduction. It is thus likely that the combination of an effective self-incompatibility system and high interstand gene flow helps to maintain genetic diversity in S. torminalis stands, while clonal propagation preserves the genetic diversity over time even if environmental conditions become less favourable during the course of succession.

Raiding parties of male spider monkeys: Insights into human warfare?

Raids into neighboring territories may occur for different reasons, including the increase of foraging and mating opportunities directly or indirectly through the killing of neighboring rivals. Lethal raids have been mainly observed in humans and chimpanzees, with raiding males being reported to search purposefully for neighbors. Here we report on the first cases ever witnessed of raiding parties of male spider monkeys, a species expected to show such a behavioral tendency, given its similarity with humans and chimpanzees in critical socio-ecological characteristics, such as fission-fusion social dynamics and male-male bonding. Despite the high degree of arboreality of spider monkeys, all seven witnessed raids involved the males progressing single file on the ground in unusual silence. This is remarkably similar to the behavior of chimpanzees. The circumstances around the raids suggest that factors such as reduced mating opportunities, number of males relative to that in the neighboring community, and the strength of bonds among males could play a role in the timing of such actions. The raids did not appear to be aimed at finding food, whereas there is some indication that they may directly or indirectly increase reproductive opportunities. Although no killing was observed, we cannot exclude the possibility that spider monkey raids may be aimed at harming rivals if a vulnerable individual were encountered. The similarity of spider monkey raids with those of chimpanzees and humans supports the notion that lethal raiding is a convergent response to similar socio-ecological conditions.

Doing the twist: a test of Darwin's cross-pollination hypothesis for pollinarium reconfiguration

Mating success in plants depends largely on the efficiency of pollen dispersal. For hermaphrodite plants, self-pollination, either within or among flowers, can reduce mating opportunities because of pollen and ovule discounting and inbreeding depression. Self-pollination may be particularly detrimental in plants such as orchids and asclepiads that package each flower's pollen into one or more pollinia which, together with accessory structures, comprise a pollinarium. Darwin proposed that physical reconfiguration of pollinaria serves as a mechanism for reducing the likelihood of self-pollination. To be effective, the time taken for pollinarium reconfiguration would need to exceed that spent by a pollinator on a plant. We investigated pollinarium reconfiguration (including pollinarium bending, pollinium shrinking and anther cap retention) in 19 species and found a strong positive relationship between reconfiguration time and the duration of pollinator visits. Reconfiguration times were also consistently longer than pollinator visit times. These results provide strong support for Darwin's idea that this mechanism promotes cross-pollination.

Segregation of migration by feeding ground origin in North Atlantic humpback whales (Megaptera novaeangliae)

AbstractResults from a large‐scale, capture—recapture study of humpback whales Megaptera novaeangliae in the North Atlantic show that migration timing is influenced by feeding ground origin. No significant differences were observed in the number of individuals from any feeding area that were re‐sighted in the common breeding area in the West Indies. However, there was a relationship between the proportion (logit transformed) of West Indies sightings and longitude (r2=0.97, F1,3=98.27, P=0.0022) suggesting that individuals feeding farther to the east are less likely to winter in the West Indies. A relationship was also detected between sighting date in the West Indies and feeding area. Mean sighting dates in the West Indies for individuals identified in the Gulf of Maine and eastern Canada were significantly earlier than those for animals identified in Greenland, Iceland and Norway (9.97 days, t179=3.53, P=0.00054). There was also evidence for sexual segregation in migration; males were seen earlier on the breeding ground than were females (6.63 days, t105=1.98, P=0.050). This pattern was consistently observed for animals from all feeding areas; a combined model showed a significant effect for both sex (F1=5.942, P=0.017) and feeding area (F3=4.756, P=0.0038). The temporal difference in occupancy of the West Indies between individuals from different feeding areas, coupled with sexual differences in migratory patterns, presents the possibility that there are reduced mating opportunities between individuals from different high latitude areas.

Sexual interference of the floral kind.

Floral hermaphroditism results in conflicts and compromise in the parental roles of plants during pollination and mating. A potential cost of hermaphroditism is sexual interference between maternal and paternal functions resulting in gamete wastage and reduced fitness. Sexual interference may or may not be associated with self-pollination. In cases where self-pollination occurs, ovule or pollen discounting may reduce mating opportunities. Here I describe forms of sexual interference in flowering plants, distinguishing whether physical or biochemical interactions are involved and whether fitness costs associated with gamete wastage arise from intra-floral versus inter-floral processes. I review the limited experimental evidence for interference between sex functions and evaluate the hypothesis that some floral adaptations usually interpreted as anti-selfing mechanisms may serve an alternative function in reducing mating costs arising from this form of sexual conflict.

Modeling Folsom Mobility, Mating Strategies, And Technological Organization In The Northern Plains

Lithic and contemporary forager data facilitate the understanding of northern Plains Folsom mobility, technological organization, and mating strategies. Northern Plains Folsom foragers emphasized local and regional travel, with only occ sional direct acquisition of stone in exotic locales. The infrequency of such movements is reflected in the low percentages of exotic stone in northern Plains Folsom sites. Intercultural and intraculturalforager mating distance data supportthe hypothesis that population density is a significant control on distance traveled to find a mate. Individuals in lower density populations travel farther to locate mates than those in more highly populated groups. No ethnographic evidence supports the hypothesis that early Paleoindians turned to in breeding in the face of reduced mating opportunities. Rather, given low population densities of Folsom foragers in the northern Plains, some individuals likely sought mates during long-distance movements.