Potential Reproductive Success Research Articles

Successful reproduction of female loggerhead sea turtles with severe limb amputations

AbstractWe evaluate the frequency of flipper amputation and its impact on reproduction in the threatened loggerhead turtle (Caretta caretta) population of the Eastern Atlantic, through a long‐term study on several high‐density nesting beaches of Cabo Verde. 5982 adult females were individually identified, tagged and measured, and the presence of limb amputations was recorded. 918 of them had one or multiple amputations of the rear or front flippers. Most of these turtles (n = 778) had partial rather than complete flipper amputations. However, we found no difference in body condition indices (1.2 vs. 1.1), clutch size (82 vs. 83 eggs), or hatching success (79 vs. 80%) between amputated and non‐amputated females. While amputations may cause individual mortality or reduced fitness, our results indicate that some amputated turtles successfully nest and survive across multiple nesting seasons. Moreover, the amputations do not appear to be impacting the strong recovery of this population. This study provides evidence of the successful reproductive capacity of amputated loggerhead sea turtles and, by extension, the potential reproductive success of amputees released from recovery centers.

Relationship between the Social Structure and Potential Reproductive Success in Muroid Rodents (Rodentia, Myomorpha)

Relationship between the Social Structure and Potential Reproductive Success in Muroid Rodents (Rodentia, Myomorpha)

Female behavioral strategies during consortship in Tibetan macaques (Macaca thibetana).

Consortship has been defined as a temporary association between an adult male and an estrous/receptive female. It has been considered as male mating strategies to improve male mating success and potential reproductive success. However, the female roles have been more or less neglected, and thus, less is known about female behavioral strategies during the consortship periods. In this study, during the two consecutive mating seasons, we collected behavioral data of free‐ranging Tibetan macaques (Macaca thibetana) habituated in Mt. Huangshan, China, to investigate female behaviors when she was consorted by an adult male. The results showed that (a) females were more likely to approach and exhibit sexual solicitation to their consorting males during the consorted period, and females also exhibited less approach to their nonconsorting males; (b) females exhibited strong responses (either departed distantly or formed affiliative relationships with their consorting male partner) when their consorting males mated with rival females or showed sexual motivation toward rival females; (c) female preferences were positively correlated to the duration of consortships and the frequencies of ejaculation copulations, independent of the social ranks of their consorting male partners. Our results suggested that female strategies played much more important roles in forming and maintaining consortship than previously assumed. It provides new insight into understanding female adaptive strategies to male strategies by forming consortships in multimale–multifemale primate species when males could not identify female's fertile phase accurately.

Floral and pollination characteristics of Eriocaulon heleocharioides, an extinct species in the wild, for evidence-based conservation management.

Generally, floral characteristics and pollination are important factors enhancing the quality and quantity of reproductive output for regeneration in plant conservation. However, lack of evidence-based management could decrease fitness under ex-situ conservation. We investigated the capitulum and pollination characteristics of Eriocaulon heleocharioides Satake (Eriocaulaceae), which is extinct in the wild, to develop an evidence-based conservation management plan incorporating previously ignored reproductive characteristics. To evaluate the functional characteristics of capitula, pollen-ovule ratio, and reproductive status (maximum pollination success/florivory damage) were investigated along six flowering sequences of capitulum. To evaluate the effect of plant density on pollen transfer, high- and low-density plots were established. Total deposited pollen on stigma, insect visitation, and visit duration per capitulum were observed. A significantly lower pollen-ovule ratio was observed in the first of six capitula, reflecting higher female functionality. The highest pollination success was found in the second-fourth capitula, whereas florivory increased along the terminal capitula position. High plant density affected the pollen deposited on stigmas via insect visitation and low pollinator visit duration. Different capitula in E. heleocharioides could have different effects: different sexual functionality, enhancement of reproductive output both in quality and quantity through active pollen transfer, and escaping from florivores. High plant density could facilitate outcross-pollen transfer in E. heleocharioides. Multiple perspectives are important for determining potential reproductive success in ex-situ conservation. Thus, density management reflecting capitulum characteristics could improve the efficiency of conservation efforts for E. heleocharioides.

Adult playful individuals have more long- and short-term relationships.

Number of romantic/sexual relationships is suggested as a proxy of potential reproductive success. Cross-culturally, both sexes desire playful long-term mates and playfulness predicts relationship quality. It is yet to be tested, however, if playfulness is associated with number of long- and short-term relationships. We hypothesised that specific playfulness dimensions would correlate with the number of lifetime short- and long-term relationships. We expected that lighthearted playfulness would be associated with more short-term relationships, while other-directed playfulness would be associated with the number of long-term relationships. In total, 1191 Brazilian adults (mean age = 28.7 years, standard deviation = 10.2) responded to online sociodemographic questions and a playfulness inventory. Other-directed playfulness positively predicted the number of short-term and long-term partners in men and whimsical playfulness predicted the number of short-term relationships in women. This suggests that playfulness is used by both sexes to compete for access to more and better mates, but in slightly different ways. For the first time, we show that playful adults have more partners and that playfulness can be used as a part of mating strategies.

Climbing up the ladder: male reproductive behaviour changes with age in a long-lived fish

High reproductive performance is the key attribute of male fitness, especially due to the high reproductive skew among the males of most animal species. Males of long-lived iteroparous species have opportunities to improve upon their previous reproductive attempts with increasing age. We collected individual-specific reproductive behaviour and age data on a cyprinid fish, the asp (Leuciscus aspius), from 2015 to 2019. We tested whether males changed their performance over time using a unique dataset where individual performance was recorded yearly with passive telemetry. Individual fish behaviour was tracked from one to five reproductive seasons at least a year after the tagging. Fish were scored by measures of quality (first arrival time, number of visits and time spent in the reproductive grounds, and encountered proportion of males to all adult fish). In general, fish improved in the first three metrics with age, suggesting a shift towards behaviours likely to enhance reproductive success as individuals aged. A larger size at tagging was predictive of earlier fish arrival on the spawning ground in subsequent years. Our study therefore demonstrates the importance of age as a factor when considering the potential reproductive success of long-lived fish species.Significance statementHigh reproductive performance is the key attribute of male fitness. Males of long-lived species reproducing multiple times in their life have opportunities to improve upon their previous reproductive performance with increasing age. In this 5-year study, we tracked a large cyprinid fish with telemetry systems during their reproduction. We investigated the age-related behavioural changes in males and demonstrated the improvement of male reproductive timing and length of stay with potential repercussions for male’s reproductive output. We emphasize the importance of old and experienced individuals among the fish population, which are often targeted and selectively removed from the human-managed waters.

Relationship Between the Social Structure and Potential Reproductive Success in Muroid Rodents (Rodentia, Myomorpha)

In many systematic groups of mammalian species, the evolution of sociality leads to the formation of large social groups (group-size evolution). In rodents, however, the most social species live in family groups consisting, as a rule, of not numerous individuals. The family-group lifestyle of some ground-dwelling sciurids (Sciuridae) and voles (Microtus) results in fewer breeding adult females, a decreased litter size, and a longer time till first reproduction. Statistical analysis of published data has been conducted in the present study and has shown that transition to the family-group lifestyle leads to a decreased potential reproductive success in many muroid rodents: females produce fewer litters and, consequently, fewer pups during the reproduction season. However, the potential reproductive success in muroid rodents with a family-group lifestyle could be increased due to delayed offspring dispersal (who spend the severe winter period in family groups) and extensive cooperation in foraging, defending their territory and food stores, as well as in care-giving activities. One can suggest that females of these species produce fewer offspring, but of “better quality”. Ultimately, social rodent species may gain inclusive fitness benefits because of the higher fitness of their offspring as compared to solitary dwellers and gregarious species.

Demographic costs and benefits of herbicide‐based restoration to enhance habitat for an endangered butterfly and a threatened plant

Restoring habitat degraded by invasive species is often a primary focus of conservation strategies, yet few studies investigate the effects of invasive species control on multiple at‐risk taxa. Selective herbicides are increasingly used because they can selectively reduce aggressive invasive plant species with the aim of minimizing effects on other taxa within the habitat. We conducted a four‐year experiment to test how annual application of grass‐specific herbicide affected the demography on Fender's blue butterfly (Icaricia icarioides fenderi) and Kincaid's lupine (Lupinus oreganus), two federally protected species which persist in highly degraded prairie remnants in western Oregon, USA. Effects of herbicide application were transitory for the butterfly; reduction of invasive grasses increased fecundity and led to higher annual population growth (λ) at one of two conservation areas in the first season. There were no detectable differences in λ in subsequent seasons—suggesting that treatments caused neither extensive harm nor extensive benefit to the butterfly population. For the lupine, there were no detectable differences in leaf and flower abundance between control and herbicide treatments. However, greater seed production in herbicide plots in the first and third seasons suggests that lupines in herbicide‐treated plots have greater potential reproductive success. While treatments do not have a long‐term benefit to annual population growth for the butterfly, increasing reproductive success of the threatened plant may justify integrating this strategy into restoration plans. Considering the impact of restoration practices on the demography of multiple at‐risk taxa within a community is critical to effective recovery strategies.

Does winter oilseed rape grown from clothianidin-coated seeds affect experimental populations of mason bees and bumblebees? A semi-field and field study

Impacts of neonicotinoid-containing pesticides on pollinators have been heavily debated in recent years. While bees in the field get rarely exposed to lethal concentrations of neonicotinoids applied as seed coating, sublethal levels found in pollen and nectar may affect bee population development. We assessed a realistic and a worst-case scenario of clothianidin exposure to mason bees (Osmia bicornis) and bumblebees (Bombus terrestris) by conducting a small-scale field and semi-field experiment at sites planted with winter oilseed rape. Flight activity, mortality and population development (brood, colony strength and weight) of bumblebees and number of mason bee brood cells were assessed at three locations. We also analysed clothianidin residues in bee-collected pollen and nectar. We detected clothianidin at low concentrations in nectar and pollen; residues in pollen were higher than in nectar but did not exceed a maximum field concentration of 2.7 µg/kg for bumblebees and 4.7 µg/kg for mason bees. Exposure did not result in significant negative impacts on bumblebee colony development and potential reproductive success of mason bees in either semi-field or field setup. During exposure bumblebees in semi-field treatment tunnels were flying less actively than in control tunnels. Bumblebee colonies in treatment tunnels weighed more than control colonies at the end of the experiment. Our findings suggest that field-realistic exposure to oilseed rape grown from clothianidin-treated seeds (10 g clothianidin/kg seeds) poses little risks to mason bees, bumblebees and their population development. However, an impairment of flower visitation under a worst-case scenario may have implications for pollination services and crop production.

The influence of crop tiller density on the breeding performance of a cereal‐nesting specialist

Summary In the pursuit of maximizing agricultural yields and profits, crop management under intensive agriculture has reduced the heterogeneity of crop sward structure within fields. Several farmland bird specialists that nest within arable crops have undergone marked population declines during recent periods of agricultural intensification. Understanding the mechanisms that link crop sward structure to demographic fitness and population change may be important for developing effective conservation solutions. We studied whole‐season breeding performance of a declining cereal crop specialist, the corn bunting Emberiza calandra, to test the relationships between crop sward structure and both nest site selection and reproductive output within intensively farmed landscapes. Experimental double‐drilling of cereal seed was used to manipulate crop sward structure and explain the observed within‐field nest distribution. The density of crop tillers (reproductive cereal stems) was a strong predictor of nest site selection and a disproportionate number of nests occurred close to (15–30 m) crop edges where regular seed sowing overlaps (operational double‐drilling) occurred during crop establishment. Nest survival was subject to a strong crop edge effect and re‐nesting rate increased with mean tiller density in the surrounding crop. Experimental double‐drilling that simulated seed sowing overlaps increased crop tiller density (by 25%) and corn bunting nest density (by 406%) compared to adjacent areas of operational double‐drilling. Corn buntings breeding in intensively managed cereal crops therefore place their nests in areas of high tiller density which under normal sowing practice tend to occur close to crop edges where nest survival is low. A simple population model indicated that productivity under conventional, intensive cereal management may be too low to sustain local populations in the absence of immigration. Modern cereal husbandry therefore creates an ecological trap for nesting corn buntings. Synthesis and applications. This study highlights a novel mechanism by which modern cereal crop management lowers the potential reproductive success of a conservation priority species nesting in intensively managed cereal crops. Unavoidable overlapping of cereal seed along crop headlands, close to crop edges, systematically produces patches of dense sward that attract nesting corn buntings. This creates an ecological trap because nests close to crop edges are subject to high predation rates, and this may be linked to the loss of arable weeds elsewhere within crops. When double‐drilled strips are intentionally provided adjacent to these areas, nesting preference switches to these strips, suggesting a potential conservation solution. Provision of mid‐field double‐drilled strips at least 100 m from crop edges, if similarly selected by nesting females, should raise nest survival rates and improve population trends. Such interventions are simple and cheap to implement, unlike most conservation measures in high‐value cropping systems.

Effect of Copulins on Rating of Female Attractiveness, Mate-Guarding, and Self-Perceived Sexual Desirability

Olfaction and chemical signaling play an important role in the mating behaviors of many taxa, yet there is minimal empirical research on human putative pheromones. A mixture of five volatile fatty acids secreted vaginally, identified and named “copulins,” significantly increase in concentration during the follicular phase and decrease in concentration during the luteal phase in nonpill using women. Men exposed to copulins exhibit an increase in testosterone, are inhibited in discriminating the attractiveness of women’s faces, and behave less cooperatively. According to Anisogamy, Sexual Selection and Parental Investment Theory, mammalian males, having low cost and high benefit from any copulatory interaction, may adaptively utilize any useful cues to identifying ovulating females and adjust their behavior accordingly in order to maximize their potential reproductive success. In the current study, we attempted a replication of Jütte and Grammer’s finding indicating copulins inhibit the ability of men to discriminate attractiveness of women’s faces, and we examined the role of copulins in self-reported mate-guarding behaviors and self-perceived sexual desirability. We utilized a randomized placebo-controlled design and as predicted, results indicated men exposed to copulins were more likely to rate themselves as sexually desirable to women and, on average, the copulin group rated women’s faces as more attractive than controls. There were no significant findings with mate guarding.

Biologia reprodutiva de L. leucocephala (Lam.) R. de Wit (Fabaceae: Mimosoideae): sucesso de uma espécie invasora

The species Leucaena leucocephala (Lam.) R. de Wit., known as leucaena, is originally from Central America. It has been considered a pioneer with high densities of individuals in its populations that could prevent the development of native species. The present study aims to understand the reproductive biology of L. leucocephala , synthetizing information about its reproductive success. The study was conducted in Trindade, central Brazil, from July to November 2012. All areas contained specimens in bloom. We selected four study areas containing populations of leucaena with at least six individuals with flowering. Observations were made of floral visitors and we did tests of first, second and third visit, and cross-pollination between plants, self-pollination and control group (without human intervention). There was a predominance of Apis mellifera and Trigona spinips as flower visitors in these areas. The flowers undergo senescence at the end of the day, however, there is a decrease in flower visiting by insects after 10 am. We found 342 visitors belonging to 12 insect species. After assessing the number of visits of pollinators and their influence on fruit production, we found that the number of visits of the main pollinator ( A. mellifera ) does not influence the production of fruits, showing the efficiency of this bee in its first visit to flowers. Leucaena leucocephala combines several attributes to be considered an invasive species, such as the efficient action of generalist pollinators as Apis mellifera , being able to produce considerable amounts of fruits and seeds that contribute to the potential dispersal and reproductive success of the plant. Keywords : pollination, Apis mellifera , reproductive success.

Plant Pollinator Networks along a Gradient of Urbanisation

BackgroundHabitat loss is one of the principal causes of the current pollinator decline. With agricultural intensification, increasing urbanisation is among the main drivers of habitat loss. Consequently studies focusing on pollinator community structure along urbanisation gradients have increased in recent years. However, few studies have investigated how urbanisation affects plant-pollinator interaction networks. Here we assessed modifications of plant-pollinator interactions along an urbanisation gradient based on the study of their morphological relationships.Methodology/Principal FindingsAlong an urbanisation gradient comprising four types of landscape contexts (semi-natural, agricultural, suburban, urban), we set up experimental plant communities containing two plant functional groups differing in their morphological traits (“open flowers” and “tubular flowers”). Insect visitations on these communities were recorded to build plant-pollinator networks. A total of 17 857 interactions were recorded between experimental plant communities and flower-visitors. The number of interactions performed by flower-visitors was significantly lower in urban landscape context than in semi-natural and agricultural ones. In particular, insects such as Syrphidae and solitary bees that mostly visited the open flower functional group were significantly impacted by urbanisation, which was not the case for bumblebees. Urbanisation also impacted the generalism of flower-visitors and we detected higher interaction evenness in urban landscape context than in agricultural and suburban ones. Finally, in urban context, these modifications lowered the potential reproductive success of the open flowers functional group.Conclusions/SignificanceOur findings show that open flower plant species and their specific flower-visitors are especially sensitive to increasing urbanisation. These results provide new clues to improve conservation measures within urbanised areas in favour of specialist flower-visitors. To complete this functional approach, studies using networks resolved to the species level along urbanised gradients would be required.

Change in offspring sex ratio over a very short season in Lincoln's Sparrows: the potential role of bill development

The sex ratios of offspring are targets of natural selection that can affect parental energy expenditure and fitness, adult sex ratios, and population dynamics. Parents may manipulate offspring sex ratios based on sex differences in potential reproductive success. In Lincoln’s Sparrows (Melospiza lincolnii), male bill shape is associated with the quality of songs, and song quality predicts female preferences in a reproductive context.Males and females that hatch later relative to brood mates or later in the breeding season tend to develop bill shapes that are, for males, associated with low-quality song. Because females do not sing and do not experience this selection pressure, we predicted that the sex of offspring produced late relative to their brood mates or relative to the season should be biased toward females. Using a molecular technique to sex nestlings, we found no effects of hatching order or any interaction between date of clutch initiation and hatching order on offspring sex. However, we found a seasonal decline in the proportion of male offspring, from approximately 0.8 at the beginning to 0.4 at the end of a clutch initiation season only 19 d in duration. To our knowledge, this is the shortest period over which the offspring sex ratio has been shown to change in a bird population. Moreover, these findings are consistent with the hypothesis that sex differences in the potential attractiveness of offspring ultimately influence offspring sex ratios.

Food availability and sex reversal in Mytella charruana, an introduced bivalve in the southeastern United States

We studied the reproductive biology of Mytella charruana to determine the potential reproductive success of this newly introduced bivalve species from Central/South America. We analyzed gonad morphology, gametogenesis, and the sex ratios of introduced populations throughout a 12 month period. In the non-native habitat M. charruana shows the same strategy of gametogenesis that had been observed in its native environment, which is an opportunistic type of gonadal cycle with gametes produced throughout the year. Instead, the spawning period of M. charruana along the southeastern US coast is extended compared to that found in the native environment. We determined the minimum size (shell length) of sexually reproductive mussels to be 1.25 cm. Interestingly, throughout the year the population samples were typically composed of a higher proportion of females. The female to male sex ratio varied within a wide range from 1:0 to 1:3.3. Upon this discovery we tested the effects of food availability on the gametogenesis of adult animals. The sex ratio of mussels collected from different locations and maintained in the laboratory with or without food changed toward a male-bias under starvation conditions within a month. This is the first study directly showing that food availability can trigger sex reversal in an adult bivalve. According to our data this mussel species will likely continue to spread along the east coast of the US. Moreover, M. charruana may prove to be a model organism in the study of alternative sexuality in bivalves.

Maternal characteristics and the production and recruitment of sons in the eastern kingbird (Tyrannus tyrannus)

Sex allocation theory predicts that if variance in reproductive success differs between the sexes, females who are able to produce high-quality young should bias offspring sex ratio towards the sex with the higher potential reproductive success. We tested the hypothesis that high-quality (i.e., heavy) female eastern kingbirds (Tyrannus tyrannus) that bred early in the breeding season would produce male-biased clutches. A significant opportunity for sexual selection also exists in this socially monogamous but cryptically polygamous species, and we predicted that successful extra-pair (EP) sires would be associated with an excess of male offspring. Although population brood sex ratio did not differ from parity, it increased significantly with female body mass and declined with female breeding date, but was independent of the morphology and display (song) behavior (correlates of reproductive success) of social males and EP sires. Male offspring were significantly heavier than female offspring at fledging. Moreover, the probability that male offspring were resighted in subsequent years declined with breeding date, and was greater in replacement clutches, but lower when clutch size was large. Probability of resighting female offspring varied annually, but was independent of all other variables. Given that variance in reproductive success of male kingbirds is much greater than that of females, and that male offspring are more expensive to produce and have a higher probability of recruitment if fledged early in the season, our results support predictions of sex allocation theory: high-quality (heavy) females breeding when conditions were optimal for male recruitment produced an excess of sons.

A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires

BackgroundNatural selection should favour the ability of mothers to adjust the sex ratio of offspring in relation to the offspring's potential reproductive success. In polygynous species, mothers in good condition would be advantaged by giving birth to more sons. While studies on mammals in general provide support for the hypothesis, studies on humans provide particularly inconsistent results, possibly because the assumptions of the model do not apply.Methodology/Principal FindingsHere, we take a subset of humans in very good condition: the Forbe's billionaire list. First, we test if the assumptions of the model apply, and show that mothers leave more grandchildren through their sons than through their daughters. We then show that billionaires have 60% sons, which is significantly different from the general population, consistent with our hypothesis. However, women who themselves are billionaires have fewer sons than women having children with billionaires, suggesting that maternal testosterone does not explain the observed variation. Furthermore, paternal masculinity as indexed by achievement, could not explain the variation, since there was no variation in sex ratio between self-made or inherited billionaires.Conclusions/SignificanceHumans in the highest economic bracket leave more grandchildren through sons than through daughters. Therefore, adaptive variation in sex ratios is expected, and human mothers in the highest economic bracket do give birth to more sons, suggesting similar sex ratio manipulation as seen in other mammals.

Perpetrators of Sexual Assault Continuing to Have Sex With Their Victims Following the Initial Assault

Among a sample of college students, roughly 30% of the women and 12% of the men reported having been the victim of a sexual assault sometime in their lives. Of the assault victims, approximately 23% of both sexes stated that they had sexual intercourse with their assaulters on at least one subsequent occasion. Female victims of a completed sexual assault were significantly more likely to continue being sexually active with their assailants than were female victims who managed to block the assault, while no such difference was found for male victims. This would imply that some men are using assaultive tactics to secure sex partners beyond a single sexual episode, thereby enhancing their potential reproductive success in evolutionary terms. Also, men who committed sexual assault reported having had more lifetime sex partners than did sexually experienced men with no sexual assault history. Overall, the idea that sexual assault is part of an evolved reproductive strategy is consistent with findings from this study.

Nordic rattle: the hoarse vocalization and the inflatable laryngeal air sac of reindeer (Rangifer tarandus).

Laryngeal air sacs have evolved convergently in diverse mammalian lineages including insectivores, bats, rodents, pinnipeds, ungulates and primates, but their precise function has remained elusive. Among cervids, the vocal tract of reindeer has evolved an unpaired inflatable ventrorostral laryngeal air sac. This air sac is not present at birth but emerges during ontogenetic development. It protrudes from the laryngeal vestibulum via a short duct between the epiglottis and the thyroid cartilage. In the female the growth of the air sac stops at the age of 2-3 years, whereas in males it continues to grow up to the age of about 6 years, leading to a pronounced sexual dimorphism of the air sac. In adult females it is of moderate size (about 100 cm3), whereas in adult males it is large (3000-4000 cm3) and becomes asymmetric extending either to the left or to the right side of the neck. In both adult females and males the ventral air sac walls touch the integument. In the adult male the air sac is laterally covered by the mandibular portion of the sternocephalic muscle and the skin. Both sexes of reindeer have a double stylohyoid muscle and a thyroepiglottic muscle. Possibly these muscles assist in inflation of the air sac. Head-and-neck specimens were subjected to macroscopic anatomical dissection, computer tomographic analysis and skeletonization. In addition, isolated larynges were studied for comparison. Acoustic recordings were made during an autumn round-up of semi-domestic reindeer in Finland and in a small zoo herd. Male reindeer adopt a specific posture when emitting their serial hoarse rutting calls. Head and neck are kept low and the throat region is extended. In the ventral neck region, roughly corresponding to the position of the large air sac, there is a mane of longer hairs. Neck swelling and mane spreading during vocalization may act as an optical signal to other males and females. The air sac, as a side branch of the vocal tract, can be considered as an additional acoustic filter. Individual acoustic recognition may have been the primary function in the evolution of a size-variable air sac, and this function is retained in mother-young communication. In males sexual selection seems to have favoured a considerable size increase of the air sac and a switch to call series instead of single calls. Vocalization became restricted to the rutting period serving the attraction of females. We propose two possibilities for the acoustic function of the air sac in vocalization that do not exclude each other. The first assumes a coupling between air sac and the environment, resulting in an acoustic output that is a combination of the vocal tract resonance frequencies emitted via mouth and nostrils and the resonance frequencies of the air sac transmitted via the neck skin. The second assumes a weak coupling so that resonance frequencies of the air sac are lost to surrounding tissues by dissipation. In this case the resonance frequencies of the air sac solely influence the signal that is further filtered by the remaining vocal tract. According to our results one acoustic effect of the air sac in adult reindeer might be to mask formants of the vocal tract proper. In other cervid species, however, formants of rutting calls convey essential information on the quality of the sender, related to its potential reproductive success, to conspecifics. Further studies are required to solve this inconsistency.

MALE-BIASED OFFSPRING SEX RATIO IN THE HOUSE WREN

Abstract The sex-ratio adjustment hypothesis predicts that females should modify offspring sex ratios according to the potential reproductive success of their offspring under existing ecological conditions. We tested this hypothesis in the polygynous, sexually size-monomorphic House Wren (Troglodytes aedon). Typically, nestling House Wrens that hatch in the first half of the three-month breeding season are heavier and have higher rates of survival and recruitment than those that hatch in the second half of the season. Therefore, the sex-ratio adjustment hypothesis predicts that early broods should be more male biased than later broods, because males in good condition have higher reproductive value than females. As conditions deteriorate, broods should become less male biased because female offspring have higher reproductive value than males in poor condition. Contrary to expectation, there was no seasonal decrease in offspring condition, and the sex ratio of broods was consistently male biased throughout ...