High Male Density Research Articles

Burying in lake sediments: A potential tactic used by female northern map turtles to avoid male harassment

AbstractHow often males and females need to mate to maximize their fitness is a source of sexual conflict in animals. Sexual conflict over mating frequency can lead to antagonistic coevolution in which males employ tactics to coerce females into mating, while females resist or evade mating attempts by males. Here, we report on a novel burying behavior observed in female northern map turtles (Graptemys geographica) in Opinicon Lake, Ontario, Canada that appears to function as a tactic to avoid male detection during the mating season. Underwater videos indicated that females are heavily solicited during the mating season with over half the females being actively pursued by males. Biologgers indicated that females are less active and remain deeper than males during the mating season. Our data strongly suggest that female northern map turtles avoid intense solicitation and potential harassment by males by burying themselves in lake sediments. This behavior appears to be a low‐cost solution for females to reduce the costs of resistance and mating while they are constrained to habitats with high male densities for overwintering.

Reproductive behavior of Erythrodiplax abjecta (Rambur, 1842) from Andean Mountains

Research about the behavior of Neotropical species is crucial to understand how the rapid environmental changes in the Neotropics affect the reproduction of various organisms. The reproductive behavior of insects in tropical ecosystems, such as those belonging to the order Odonata, is as yet scarcely known. In this article, the reproductive behavior of Erythrodiplax abjecta is described from several localities in the Colombian Andean Mountains. Playing out between 10:00 and 14:30 h, male individuals of this species may exhibit either territorial or satellite behavior. Sexually mature males and females engage in tandem and copulation behaviors, which are brief and may occur either perched or in flight. Females of E. abjecta oviposit directly in the water at various times, alternating with periods of rest. During oviposition, males stand guard over females, perched or hovering, in a quest to protect them against other males. Conspecific males have been observed to engage in sieges of perching or ovipositing females, as well as mating pairs. Following a siege, either the female or the original pair flees, or the besieging male will succeed in outcompeting the original male, take the female in tandem, copulate with her, and then guard her while she oviposits. If the female flees, she may perch elsewhere, continue ovipositing, or may be taken by another male. The intense male guarding behavior during oviposition could represent an adaptation to temporarily high male densities at oviposition sites and a high level of competition during the short period in which active copulation takes place.

Evolution in Islands: contrasting morph frequencies in damselfly populations of the Balearic Islands

Abstract Colour polymorphism is an example of visible phenotypic variability that is often associated with ecological factors and may produce local adaptations. Small populations, particularly in islands, offer opportunities for evolutionary novelties, and are therefore of particular interest to the study of polymorphisms. Here we study the dynamics of female colour morphs in the damselfly Ischnura elegans in the Balearic Islands. We found that insular populations are small, show low density, low mating activity, and low androchrome frequency. Our surveys suggest that male harassment is a powerful force in the dynamics of this female-limited polymorphism, because high male densities result in lower presence of mature females around the water, where copulation takes place. Non-male-like (infuscans) females have higher mating frequency. Androchromes were rare (15%) in all populations, but the frequency of the two non-male like females (infuscans and aurantiaca) was reversed between islands, despite their geographical proximity. We found a possible novel morph, suggesting that insular conditions allow innovations. Fitness differences between the two non-male-like females of I. elegans are still understudied, because most previous research has concentrated on the maintenance of androchromes, and are therefore a priority for future research.

Condition-dependent survival and movement behavior in an endangered endemic damselfly

Movement is essential for the maintenance of populations in their natural habitats, particularly for threatened species living in fluctuating environments. Empirical evidence suggests that the probability and distance of movement in territorial species are context-dependent, often depending on population density and sex. Here, we investigate the movement behavior of the spring cohort of an endangered endemic damselfly Calopteryx exul in a lotic habitat of Northeast Algeria using capture-mark-recapture (CMR) of adults. By sampling 10 gridded river stretches across a 2 km section of the watercourse, we were able to estimate the distance of movement throughout individual lifespans and estimate movement probability for both males and females. We used multistate models to examine whether individual density and sex ratio influence survival and movement probability. We found that males and females had similar movement kernels with most individuals moving short distances (83% performing movements of < 100 m and only 1% > 1000 m). Of the 547 marked individuals, 63% were residents, and 37% were movers (moved at least 50 m from one sampling occasion to another). Survival probability showed higher estimates for females and was slightly density-dependent (i.e., lower survival probabilities were associated with high male densities). Survival probability did not show a marked difference between residents and movers. Movement probability and distances were positively correlated with individual density, but were not or slightly correlated with sex ratio, respectively. These results are not in line with the hypotheses of sex-biased movement and survival costs of movement. Our results suggest that the species performs mostly short-distance movements that are dependent on intraspecific interactions.

Male residency of Sunda Clouded Leopard Neofelis diardi (Cuvier, 1823) (Mammalia: Carnivora: Felidae) in a peat swamp forest, Indonesian Borneo

The Sunda Clouded Leopard Neofelis diardi is the apex predator on the island of Borneo, yet little is known of its ecology. We document the length of residency of male Sunda Clouded Leopards in Central Kalimantan, Indonesian Borneo. Over 10 years, camera trap data have been obtained in the Sebangau peat swamp forest in a study area of ~ 105km2. We identified 11 individuals (eight males, one female, and two with unknown sex), from 152 notionally independent photographs. On average, males remained in the study area for 39.3 months (SE 8.3), or 3.3 years (SE 0.7), ranging from less than a month of residency up to 71 months. Females were infrequently recorded, possibly as a result of human disturbance and/or high male densities. Our results reveal that even a 10-year dataset is inadequate to answer some basic ecological questions, emphasising the importance of long-term monitoring of this species.

Exposure to high male density causes maternal stress and female-biased sex ratios in a mammal.

A shift from the traditional perspective that maternal stress is invariably costly has instigated recent interest into its adaptive role in offspring sex allocation. Stress generated by social instability has been linked to offspring sex ratio biases that favour the production of female offspring, which converges with the theoretical prediction that mothers in the poor condition are better off investing in daughters rather than sons. However, previous research has failed to disentangle two different processes: the passive consequence of maternal stress on sex-specific mortality and the adaptive effect of maternal stress at the time of conception. Here, I show that exposure to high male density social conditions leads to elevated stress hormone levels and female-biased in utero offspring sex ratios in house mice (Mus musculus domesticus), and identify that sex-specific offspring production-not sex-specific mortality-is the mechanism accounting for these sex ratio skews. This outcome reflects the optimal fitness scenario for mothers in a male-dominated environment: the production of daughters, who are guaranteed high mate availability, minimizes male-male competition for their sons. Overall, this study supports the idea that maternal stress has the potential to be adaptive and advances our understanding of how exposure to different social conditions can influence sex allocation in mammals.

Exposure to male-dominated environments during development influences sperm sex ratios at sexual maturity.

Different stages during development are important when it comes to phenotypic adjustments in response to external stimuli. Critical stages in mammals are the prenatal phase, where embryos are exposed to a milieu of sex steroid hormones, and the early‐postnatal phase, where littermates interact and experience their incipient social environment. Further, the postmaternal environment will influence the development of traits that are linked to reproductive success in adulthood. Accumulated evidence of male‐driven sex allocation establishes the currently untested hypothesis that the sperm sex ratio is a plastic trait that can be mediated to align with prevailing social conditions. Here, we used natural variation in the maternal environment and experimentally manipulated the postmaternal environment to identify the importance of these developmental phases on sperm sex ratio adjustments in wild house mice (Mus musculus domesticus). We found that male density in both environments was predictive of sperm sex ratios at sexual maturity: males from more male‐biased litters and males maturing under high male density produced elevated levels of Y‐chromosome‐bearing sperm. Our findings indicate that the sperm sex ratio is a variable phenotypic trait that responds to the external environment, and highlight the potential that these adjustments function as a mechanism of male‐driven sex allocation.

Differential distribution of resources for females on a dioecious plant affects the small–scale distribution of male of an oligolectic bee

Females of the solitary digger bee Andrena florea Fabricius, 1793 (Hymenoptera, Andrenidae) nest in aggregations and collect pollen almost exclusively on dioecious plants of the genus Bryonia, making this species a good model to study the relationship between nest density, male density, male behaviour and female lecty. At a study site in the valley of the river Serio, in Italy, an aggregation of this bee showed low density of randomly distributed nests and was closely surrounded by B. dioica plants. Female nectar foraging and male feeding and mate–searching activity, confined to the host plants, peaked at similar hours across the day, while female pollen foraging peaked earlier. Males fed on plants of both sexes but seemed to perch waiting for females more frequently on male B. dioica leaves. Individual males more often visited only one of the male plants, for up to four days; here they did not interact aggressively with conspecifics, suggesting scramble competition in resource–based home ranges and not territoriality. These findings are preliminarily in accordance with the predicted resource–based rendez–vous sites at low nest density for oligolectic bees and the predicted occurrence of scramble competition in case of high male density. Additionally, males would maximize their mating opportunity by mainly perching on male plants, the only source of the most limited resource for females (pollen).

Sex before or after blood feeding: Mating activities of Aedes aegypti males under conditions of different densities and female blood feeding opportunities

Abstract Blood feeding and mating are critical behaviors that regulate both mosquito population maintenance and disease transmission. However, our understanding of mosquito mating systems remains incomplete. One of the most critical issues is a lack of understanding regarding how and where males and females encounter one another. This study was performed to investigate changes in key mating behaviors of Ae. aegypti relative to female blood feeding opportunities, taking into account male density. We compared courtship latency and copulation activity between single and pooled males in a range of assays performed in the presence or absence of a blood source and after blood feeding. The time taken by grouped males to initiate courtship in the presence of a host was much shorter than that in single males. There was no significant difference in courtship latency between pooled and single males in the absence of a blood source or after blood feeding. At low male density, the presence of the host and blood meal ingestion provided better conditions for copulation. At high male density, however, copulation activity was decreased after blood feeding, but remained high regardless of the presence or absence of the host. In addition to providing insight into the mating ecology of Aedes aegypti, this study indicated that the presence of a blood source influences how males encounter and copulate with females. The observation that copulation activity decreases after blood feeding when males are numerous provides new avenues for improving mass release programs of sterile mosquitoes.

Land tenure shapes black bear density and abundance on a multi-use landscape.

Global biodiversity is decreasing rapidly. Parks and protected lands, while designed to conserve wildlife, often cannot provide the habitat protection needed for wide‐ranging animals such as the American black bear (Ursus americanus). Conversely, private lands are often working landscapes (e.g., farming) that have high human footprints relative to protected lands. In southwestern Alberta, road densities are highest on private lands and black bears can be hunted year‐round. On protected lands, road densities are lowest, and hunting is prohibited. On public lands under the jurisdiction of the provincial government (Crown lands), seasonal hunting is permitted. Population estimates are needed to calculate sustainable harvest levels and to monitor population trends. In our study area, there has never been a robust estimate of black bear density and spatial drivers of black bear density are poorly understood. We used non‐invasive genetic sampling and indices of habitat productivity and human disturbance to estimate density and abundance for male and female black bears in 2013 and 2014 using two methods: spatially explicit capture–recapture (SECR) and resource‐selection functions (RSF). Land tenure best explained spatial variation in black bear density. Black bear densities for females and males were highest on parkland and lowest on Crown lands. Sex ratios were female‐biased on private lands, likely a result of lower harvests and movement of females out of areas with high male density. Synthesis and application: Both SECR and RSF methods clearly indicate spatial structuring of black bear density, with a strong influence based on how lands are managed. Land tenure influences the distribution of available foods and risk from humans. We emphasize the need for improved harvest reporting, particularly for non‐licensed hunting on private land, to estimate the extent of black bear harvest mortality.

High male density favors maintenance over reproduction in a butterfly

The social environment experienced by an individual may have striking effects on its trait expression. We show that an increasing number of conspecifics increased male body mass and the probability of success in aggressive interactions, but decreased courtship activity and tended to decrease sperm number in B. anynana butterflies. Thus, males kept at high densities favored maintenance whereas males kept at low-density favored reproduction, indicating prudent strategic investment.

Can sex-specific consumption of prey be determined from DNA in predator scat?

Sex-biased predation, a predator’s bias for one prey sex over the other, can have important demographic impacts on prey species of conservation concern. Yet, it is difficult to measure in the wild. Molecular scatology has been used to indirectly determine the proportion of prey items consumed in the diet, and it may be possible to apply similar approaches to determine the sex-biased consumption of prey items. We developed a molecular method to indirectly examine sex-specific predation employing scat, focusing on predator–prey interactions between Chinook salmon (Oncorhynchus tshawytscha) and harbor seals (Phoca vitulina richardii). We established that the proportions of male and female Chinook DNA can be determined in a controlled sample by measuring a Y-linked marker, growth hormone pseudogene, using qPCR. We then applied the assay to harbor seal scat samples. Although the assay amplified in 83% of scat samples, 27% of samples quantified had an estimated male proportion > 1, which may have been due to a lack of robustness of the PCR assay in samples. Lastly, we constructed a biomass calibration curve to determine whether DNA measurements could estimate the proportions of male and female biomass consumed. The calibration curve was skewed by high male DNA density precluding our ability to quantify the relative amounts. We demonstrated that nuclear prey markers can be amplified in predator scat, however, contamination and extreme DNA density differences between the prey sexes may pose practical difficulties to estimate the relative amounts of male to female biomass consumed.

Multiple hypotheses explain variation in extra-pair paternity at different levels in a single bird family.

Extra-pair paternity (EPP), where offspring are sired by a male other than the social male, varies enormously both within and among species. Trying to explain this variation has proved difficult because the majority of the interspecific variation is phylogenetically based. Ideally, variation in EPP should be investigated in closely related species, but clades with sufficient variation are rare. We present a comprehensive multifactorial test to explain variation in EPP among individuals in 20 populations of nine species over 89years from a single bird family (Maluridae). Females had higher EPP in the presence of more helpers, more neighbours or if paired incestuously. Furthermore, higher EPP occurred in years with many incestuous pairs, populations with many helpers and species with high male density or in which males provide less care. Altogether, these variables accounted for 48% of the total and 89% of the interspecific and interpopulation variation in EPP. These findings indicate why consistent patterns in EPP have been so challenging to detect and suggest that a single predictor is unlikely to account for the enormous variation in EPP across levels of analysis. Nevertheless, it also shows that existing hypotheses can explain the variation in EPP well and that the density of males in particular is a good predictor to explain variation in EPP among species when a large part of the confounding effect of phylogeny is excluded.

Male density influences mate searching speed and copulation duration in millipedes (Polydesmida: Gigantowales chisholmi)

Density effects can have a strong influence over both the mating system of a species and the reproductive strategies of individuals. The way in which males respond to an increase in the density of other male competitors is generally explained by sperm competition theory. When the perceived risk of sperm competition is high, males increase reproductive effort to aspects of mate searching, copulation and mate-guarding that will ensure reproductive success. In species with little likelihood of female defence, male competition is largely restricted to a scramble for access to females and sperm competition. In such species, the ability to search quickly and locate females will be under strong selection. Millipedes (Arthropoda: Myriapoda: Diplopoda) are classic scramble competitors, although their utility in the study of scramble competition has been generally overlooked. Here we investigate the Australian Polydesmidan millipede Gigantowales chisholmi and describe their mating behaviour. We manipulated male density, exposing individual males to either high or low male density treatments, and compared aspects of mate searching and copulation. We found that males from high-density treatments searched at faster speeds and copulated for shorter durations, than those kept alone. We also found that larger males achieved higher mating success but copulated for shorter durations. Our data support the idea that for scramble competitors, males who are more likely to achieve mating success (in this case large males) will use mating tactics that emphasise increased efforts towards mate acquisition rather than investment in their current mates. In an extraordinary number of animal species, the males who are most successful are those who can efficiently locate females, mate with them and move on in search of additional mates. This is scramble competition and is relatively unstudied compared with other animal mating systems such as those where males aggressively fight for mates. Millipedes, also relatively unstudied, represent great systems for the study of scramble competition. Here we show that males of the Australian millipede, G. chisholmi, search for females at higher speeds and mate more quickly when they are kept at higher density. Larger males are more successful but mate for shorter durations. This suggests that for scramble competitors, those more likely to achieve mating success (in this case large males) use tactics that increase mate acquisition rather than investment in their current mates.

Females sample more males at high nesting densities, but ultimately obtain less attractive mates.

BackgroundSexual selection is largely driven by the availability of mates. Theory predicts that male competition and female choice should be density-dependent, with males competing more intensely at relatively high density, and females becoming increasingly discriminating when there are more males from whom to choose. Evidence for flexible mating decisions is growing, but we do not understand how environmental variation is incorporated into mate sampling strategies. We mimicked threespine stickleback (Gasterosteus aculeatus) breeding conditions in pools with high and low densities of nesting males and allowed females to search for mates to determine whether 1) mate search strategies change with the density of breeding males and 2) pre-copulatory components of mate choice (signalling, competition, search patterns, and mating decisions) are modified in parallel.ResultsWhile females sampled more males at high male density, suggesting greater opportunity for sexual selection, the expanded search did not result in females choosing males with more attractive sexual signals. This is likely because red throat colouration was twice as great when half as many males competed. Instead, females chose similarly at high and low male density, using a relative strategy to compare male traits amongst potential suitors. Reduced throat colour could reflect a trade-off with costly male competition. However, we did not observe more intense competition at higher relative density. Density-dependent signalling appears largely responsible for females associating with males who have more attractive signals at low density. If we lacked knowledge of plasticity in signalling, we might have concluded that females are more discriminating at low male density.ConclusionsTo understand interactions between mate choice and population dynamics, we should consider how components of mate choice that precede the mating decision interact.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0481-3) contains supplementary material, which is available to authorized users.

Effects of Density on Spatial Aggregation and Habitat Associations of the Glass FrogEspadarana (Centrolene) prosoblepon

Abstract Social behaviors often are context dependent; behaviors at high density may change or disappear at lower population densities. We analyzed spatial dispersion patterns and habitat associations at high and low population densities of male Espadarana (Centrolene) prosoblepon that declined following the invasion of a pathogenic fungus, Batrachochytrium dendrobatidis (Bd). We mapped the spatial location of male frogs on four permanent 200-m stream transects over 4 pre- and 7 postdecline yr. We calculated distances between nearest neighbors and between hot spots—defined as areas of high male density—and compared habitat associations of frogs at high and low densities. Average predecline density across transects was 0.034 ± 0.02 males m−1 (mean ± SE) and dropped to 0.019 ± 0.02 males m−1 postdecline. Nearest neighbor and between cluster distances did not change significantly between pre- and post-decline yrs. Nine of 21 hot spots persisted in the same location across 12 yrs, whereas 8 hot spots disappea...

Factors Affecting the Spatial Distribution of Oviposition Sites for Tandem Black Saddlebags Dragonflies (Odonata: Libellulidae)

Oviposition site location may be affected by (1) factors influencing the costs and benefits to the offspring (e.g., resource availability, competition, predation risk) and (2) factors influencing the costs and benefits to the female (e.g., predation risk or mate harassment). In cases in which both the male and female are involved in locating a site, costs and benefits may differ for each parent and the resulting oviposition site location may represent the outcome of selection pressures on one or both of them. We studied oviposition behavior in the black saddlebags dragonfly (Tramea lacerata Hagen), a species in which the male and female typically remain together (i.e., in tandem) while traveling among potential oviposition locations. Oviposition sites tended to be away from pond shoreline at the outer edge of the vegetation on the water’s surface. We found that tandems distributed their oviposition locations widely around the pond, and interactions with other dragonflies (typically other T. lacerata, either territorial males or tandems) led to a larger distance between consecutive oviposition locations. Interestingly, for 10% of the tandems, the female became separated from the male and oviposited solitarily multiple times. These solitary females spent significantly less time and traveled significantly smaller distances between successive oviposition sites than when in tandem. Our results indicate that while some aspects of oviposition behavior and site selection may be consistent between the male and female (e.g., the characteristics that make a site suitable), other aspects, such as the distribution of sites, may be a result of a differing benefits and costs for the two sexes, perhaps as a consequence of potential sperm competition.

Female mate choice plasticity is affected by the interaction between male density and female age in a field cricket

Female mate choices influence the strength and direction of evolution by sexual selection. Such choices are often plastic, changing in response to changes in environmental (e.g. predator abundance or conspecific density) and internal (e.g. female age) conditions. Although these factors can interact naturally, the effects of such interactions are only beginning to be studied. We used female variable field crickets, Gryllus lineaticeps, to examine how female age and perceived male density interact to affect the plasticity of an aspect of female mate choice: female choosiness. Female choosiness is a measure of female responses to male traits that deviate from the female's preferred male trait; the more choosy a female is, the weaker her response will be to male traits that deviate from the preferred value. We exposed females to either a high or a low male-calling density environment, then tested them at one of three ages. We assessed female choosiness by measuring the female's response to low-quality male song: females spending less time near the low-quality song were deemed more choosy. We found that the interaction between female age and perceived male density significantly affected female responses. Young females were the most plastic, showing significantly more choosiness in the high male density environment compared to the low male density environment. Intermediate and old females showed less plasticity and intermediate choosiness. Our results suggest that in populations in which most females are young, sexual selection may be strongest at high male densities.

Female Reproductive Fitness Declines with Increasing Male Density in the Polyandrous Weevil Sitophilus oryzae

The effect of male density on female reproductive fitness is still poorly understood in many insect species. Here we examined how male density influenced female mating behaviour, fecundity, longevity and damage to the genital tract in Sitophilus oryzae (L.) (Coleoptera: Curculionidae), an important pest of whole grains worldwide. We show that with increasing male density, (1) both male mate competition and female copulation duration significantly increased and (2) female fecundity, longevity and feeding time significantly decreased. Dissection indicates that the backward pointing spines on the reversed internal sac of male genitalia may grip and damage the female genital tracts. These results suggest that longer copulation and more frequent matings under higher male densities reduce the time for feeding and oviposition and increase the damage to female genital tracts. We conclude that the decreased female reproductive fitness under high male densities is most likely caused by a combination of the decreased feeding and oviposition time and increased damage to female genital tracts.

Legacy tree retention in young aspen forest improves nesting habitat quality for Golden-winged Warbler (Vermivora chrysoptera)

Residual canopy trees as biological legacies in harvested aspen stands may mimic characteristics of naturally disturbed forests. We investigated the effects of legacy tree retention in young aspen (Populus spp.) forest stands on the quality of nesting habitat for the Golden-winged Warbler (Vermivora chrysoptera), a species of conservation concern that is dependent upon recently disturbed forest and shrub habitats. Habitat quality was assessed by evaluating male density, male pairing success, percent of successful nests, daily nest survival, and productivity in young aspen stands (4–7years post-harvest) with retained conifer legacy trees (n=3), with retained hardwood legacy trees (n=3), and without legacy trees (also referred to as clearcuts; n=3). Male pairing success was higher in stands with legacy trees (68% in stands with conifer legacy trees, 71% in stands with hardwood legacy trees) than in clearcuts (10%). Only one nest was found in clearcuts. The percent of successful nests, daily nest survival rate, and productivity did not vary between stands with conifer legacy trees and stands with hardwood legacy trees. Based on high pairing success (resulting in high levels of nesting activity), retention of legacy trees in young aspen stands provided higher quality nesting habitat than clearcuts. Male density was an excellent indicator of pairing success (pseudo R2=0.976). Aspen stands harvested for nesting habitat should support a minimum density of 0.2males/ha to have approximately 75% of males successfully paired. High male densities (>0.2males/ha) were achieved by retaining at least 13 legacy trees/ha with at least nine of these comprising hardwood species with a mean diameter at breast height ≥ 16cm.