Male Investment Research Articles

Lipid storage patterns in marine copepods: environmental, ecological, and intrinsic drivers

Abstract Seasonality of food supply is a major driver of physiological and ecological adaptations of marine zooplankton. High-latitude marine copepods accumulate lipids for maintenance and reproductive maturation during the food-depleted winter period. The relationship between latitude and lipid storage in copepods is well established, but it is influenced by many factors, such as trophic position, sex, and depth distribution. In this study, the influence of latitude and collection depth, trophic level, sex, and the presence or absence of dormancy on the relative amount and composition of lipids stored was assessed by analysing published data. Our analyses confirmed higher lipid contents (expressed as % dry weight) in high-latitude species, and in deep-dwelling tropical copepods compared to shallow-living ones. Contrary to our original hypothesis, carnivorous and herbivorous copepods had similar lipid levels. Copepod species that undergo dormancy had higher levels of wax ester and were more common at polar and temperate latitudes. Lastly, adult male and female copepods did not significantly differ in the amount of lipids they store, suggesting that the portion of male reproductive investment, which may depend on lipid stores, has been underestimated. Taken together, these results both confirm some previously reported trends and refute others.

Combined cues of male competition influence spermatozoal investment in a moth

Abstract Male animals usually raise their sperm allocation after detecting sperm competition risk. To date, only a few studies have investigated the cues used by males to sense and respond to rivals. Yet, it is still largely unknown whether males respond to single or combined cues and whether they can increase their lifetime spermatozoal investment after a perception of rival cue(s). Here we postulate that males increase ejaculation and production of sperm after detecting combined cues from rivals, but such response quickly diminishes after the cues are removed. We exposed newly emerged and virgin focal males of the moth Ephestia kuehniella to various rival cues and then permanently removed the cues. We introduced a virgin female to an exposed focal male and an unexposed focal male, respectively, per day and counted the number of sperm transferred by the focal males in each mating and recovered in their body after death. We demonstrate that males significantly increased their lifetime sperm allocation and production after premating detection of either single (acoustic or chemical) or combined cues (acoustic + chemical or acoustic + chemical + tactile) from their rivals with combined cues (acoustic + chemical + tactile) somewhat strengthening such response in eupyrene production. The number of sperm ejaculated by males significantly decreased over successive matings in their lifetime regardless of whether they were exposed to rival cues, but the decline was significantly faster in rival‐cue‐exposed males than in unexposed ones. This suggests that the increase in spermatogenesis cannot fully compensate for that of sperm expenditure in response to rival cues. We show that 10‐hr premating rival exposure was sufficient to maximize males' response in sperm ejaculation and production. The impact of the rival perception on sperm transfer persisted for most of males' reproductive life, suggesting that the moth males have a long‐term memory of sperm competition risk experienced in the early adulthood. A free Plain Language Summary can be found within the Supporting Information of this article.

Both small and large plants are likely to produce staminate (male) flowers in a hermaphrodite lily

Resource allocation to female and male function may vary among individual plants in species with variable sex expression. Size-dependent sex allocation has been proposed in hermaphrodites, in which female-biased allocation may increase with plant size. In many hermaphrodites with large floral displays, however, later-produced flowers tend to be functionally male. This paradoxical relationship between female and male function and plant size remains poorly understood. The subalpine lily Lilium lankongense has individuals of three sexual types: males with only staminate flowers, hermaphrodites with only perfect flowers, and andromonoecious plants with both perfect and staminate flowers. Here we tested theoretical predictions of size-dependent sex allocation in L. lankongense by measuring plant height and flower number of individuals of each sex at five field sites in the mountainous region of Shangri-La, southwestern China. To investigate variation in phenotypic gender, we identified sex expression of 457 individuals one year later. Our investigation showed that male plants, which usually produced one flower, were significantly smaller than andromonoecious and hermaphrodite plants. In addition, the total flower numbers of andromonoecious and hermaphrodite plants increased significantly with plant size. Large individuals were more likely to produce terminal staminate flowers, as there were more flowers in andromonoecious than in hermaphrodite individuals. Non-flowered plants were significantly smaller than flowering ones. Perfect flowers had significantly larger petals and pistils than staminate flowers, but they did not differ in dry weight of stamens. Our findings indicate that when plants are small, the less costly sex is favored, consistent with the ‘size-advantage hypothesis’. When plants are large, both female and male investments change isometrically, as later-produced flowers tend to be functionally male.

Shifts in Male Reproductive Tactics over the Life Course in a Polygynandrous Mammal.

In polygynous and polygynandrous species, there is often intense male-male competition over access to females, high male reproductive skew, and more male investment in mating effort than parenting effort [1]. However, the benefits derived from mating effort and parenting effort may change over the course of males' lives. In many mammalian species, there is a ∩-shaped relationship between age, condition, and resource holding power as middle-aged males that are in prime physical condition outcompete older males [2-8] and sire more infants [9-12]. Thus, males might derive more benefits from parenting effort than mating effort as they age and their competitive abilities decline [13]. Alternatively, older males may invest more effort in making themselves attractive to females as mates [14]. One way that older males might do so isby developing relationships with females and providing care for their offspring [14, 15]. Savannah baboons provide an excellent opportunity to test these hypotheses. They form stable multi-male, multi-female groups, and males compete for high ranking positions. In yellow and chacma baboons (Papio cynocephalus and P.ursinus), there is a ∩-shaped relationship between male age and dominance rank [12], and high rank enhances paternity success [12,16]. Lactating female baboons form close ties ("primary associations" hereafter) with particular males [15-20], who support them and their infants in conflicts [15, 19] and buffer their infants from rough handling [20]. Females' primary associates are often, but not always, the sires of their current infants [16,20-22].

Immature mating as a tactic of polygynous male western widow spiders.

In polygynous species, males typically mate with more than one female, and male investment in any one mating should decrease if securing that mating reduces future mating opportunities. In contrast, monogynous males mate only once, so they should invest maximally in that single mating. In two monogynous, sexually cannibalistic Latrodectus spider species (L. geometricus, L. hasselti), males can mate and fertilize immature females. This recently described tactic is time-consuming because males must wait days for females to be capable of mating. It is also risky since immature females approached too early may kill males outright. However, if males typically find only one female in their lifetime, increasing the opportunity to mate may be worth these costs. We investigated whether this tactic is also practiced by a polygynous congener, L. hesperus, in which males typically mate more than once and may avoid time-consuming, risky encounters. In laboratory trials, we showed that males copulate with immature females. Moreover, males mounted immatures more rapidly, copulated for longer, and fathered more offspring than males that mated adults females. We concluded that monogyny is not a necessary condition for immature mating to be favored as an alternative reproductive tactic and suggest that it may be common in other spider taxa.

Males sacrifice their legs to pacify aggressive females in a sexually cannibalistic spider

Monogynous male mating strategies have repeatedly evolved in spiders along with female-biased sexual size dimorphism (SSD) and extreme male mating investment. As a manifestation of sexual conflict, male African golden-silk spiders, Trichonephila fenestrata, are regularly attacked by females during copulation, and sexual cannibalism is common. Curiously, attacked males actively cast off (autotomize) their front legs and copulation continues while the female is feeding on these legs. Since the loss of legs is costly in reducing males’ ability to mate guard, it should yield significant benefits. In a series of experiments, we investigated the behavioural mechanism of male leg ejection and tested three hypotheses. First, we performed feeding experiments to test whether conspecific male legs are particularly attractive for females and act as a sensory trap. Second, we conducted mating experiments with sibling and nonsibling pairings to test whether males preferentially invest in high-quality mates. Third, by offering male legs during copulations, we tested whether male leg ejection serves to distract and pacify females. In support of the female pacifier hypothesis, our results confirm a significantly reduced probability of attacks in females that had been offered a male leg, but we found no relationship between simulated leg ejection and male survival. While there was no evidence for special properties of male legs, females accepted male legs significantly more often than insect food. The degree of male leg sacrifice did not depend on male–female relatedness, but large males lost more legs than small males, and small males achieved more copulations. However, total copulation duration was unrelated to male size. Male leg sacrifice in T. fenestrata may represent a rare example of an evolutionary transition in which the antipredation behaviour of autotomizing body parts has changed its function into a sexual context, here to pacify females and to facilitate undisturbed copulations.

Disruption of male mating strategies in a chemically compromised environment

A leading source of endocrine-disrupting chemicals (EDCs) in the environment is run-off of veterinary pharmaceuticals used in agriculture, including hormonal growth promotants (HGPs). Despite being banned in various countries, HGP use is still common in beef production around the world. The androgenic steroid 17β-trenbolone (17β-TB) is a HGP that commonly enters surface waters via livestock effluent run-off. Here, we used a flow-through system to expose wild-caught adult male guppies (Poecilia reticulata) to an environmentally realistic level of 17β-TB (average measured concentration = 2 ng/L) for 21 days. We then compared the response of exposed and unexposed males to sequentially presented large and small stimulus (unexposed) females. Due to a positive size-fecundity relationship, larger females are generally expected to be preferred by males. While we found no evidence that the size of a previously encountered female affected the amount of courtship or coercive ‘sneak’ mating behaviour performed by males during the second presentation, males from both exposure treatments conducted more frequent courting events towards larger females during both presentations, suggesting an absolute preference for greater female size. Further, across both presentations, 17β-TB exposure caused a shift in male mating strategy towards increased coercive sneaking behaviour, although male sequential investment into mating effort was not impacted at the tested dosage. In combination, our findings demonstrate that exposure to a field-realistic level of a widespread agricultural pollutant alters male mating strategies in fish, and contribute to a growing understanding of sub-lethal impacts of chemical contaminants on complex behaviours in wildlife.

Women’s sexual strategies in pregnancy

Abstract Humans exhibit an unusual pattern of sexual behavior compared to other mammalian females. Women's extended sexuality has been hypothesized to be related to a variety of possible benefits, especially non-genetic reproductive benefits, such as securing male investment via reinforced pairbonds or paternity confusion. But sexual behavior also comes at a cost, particularly for pregnant women, in terms of energetic costs, potential disease, and possible harm to the fetus. We hypothesize, therefore, that sexual behavior in pregnant women should reflect adaptive strategies and that pregnant women will be particularly strategic about their sexual behavior in order to maximize potential benefits while minimizing potential costs. One hundred twelve pregnant women completed a survey of their partners' qualities and their sexual desires toward their primary partners and men other than their primary partners. Results showed that women's perceptions of relationship threat positively predicted sexual desire for primary partners, while their perceptions of their partner's investing qualities negatively predicted sexual desire for extra-pair mates. These qualities, as well as cues to partner's genetic quality and gestation age, also interacted in ways that suggest that pregnant women's sexual desires are sensitive to cues of future investment and relationship stability.

Impact of low sperm competition on male reproductive trait allometries in a bush-cricket

BackgroundStudying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection. In promiscuous mating systems, studies across several taxa suggest that testes allometry is usually positive, presumably due to strong selection on sperm numbers through intense sperm competition. Here, we investigated testes allometry in a bush-cricket species, Metaplastes ornatus, in which females mate promiscuously, but where sperm removal behaviour by males likely drastically reduces realised sperm competition level.ResultsAs hypothesised, we found evidence for negative testes allometry and hence a fundamentally different male investment strategy compared to species under intense sperm competition. In addition, the mean relative testes size of M. ornatus was small compared to other species of bush-crickets. Surprisingly, the spermatophore gland, a potential alternative trait that males could invest in instead of testes, also did not show positive allometry, but was approximately isometric. We further observed the expected pattern of negative allometry for the male morphological structure responsible for sperm removal in this species, the subgenital plate, supporting the one-size-fits-all hypothesis for intromittent genitalia.ConclusionOur findings suggest that the evolution of sperm removal behaviour in M. ornatus was a key adaptation for avoiding sperm competition, with important consequences for reproductive trait allometries. Nevertheless, they also imply that it does not pay for larger males to invest disproportionately in nuptial gift production in this species.

Life histories of anadromous salmon males reveal a trade-off between primary and secondary sexual traits

Abstract The life-history trade-off between investment in somatic growth and gonadal tissue is caused by individual energy limitations and results in individuals that adopt specific tactics to achieve reproduction. Allocation in primary and secondary sexual traits in Atlantic salmon males was studied by assessing life history traits (smolt size, sea age, growth rate) based on back-calculation of scales, ejaculate energy content (sperm ATP content, mass and density) and the size of secondary sexual traits. We found that males investing less in secondary sexual traits produce ejaculates with a higher energy content. Differences were found in the investment into primary and secondary sexual traits between fish that spent one year in the sea before returning to their spawning grounds (grilse) and multi-sea-winter adults, suggesting that different energy allocation patterns in reproductive effort reflect alternative developmental pathways. These findings are consistent with the pattern where multi-sea-winter male ejaculate investment relies principally on the resource acquisition in the ocean, whereas grilse ejaculate investment relies chiefly on the resource allocation of available surplus energy, thus representing alternative male reproductive tactics.

Plumage iridescence is associated with distinct feather microbiota in a tropical passerine

Birds present a stunning diversity of plumage colors that have long fascinated evolutionary ecologists. Although plumage coloration is often linked to sexual selection, it may impact a number of physiological processes, including microbial resistance. At present, the degree to which differences between pigment-based vs. structural plumage coloration may affect the feather microbiota remains unanswered. Using quantitative PCR and DGGE profiling, we investigated feather microbial load, diversity and community structure among two allopatric subspecies of White-shouldered Fairywren, Malurus alboscapulatus that vary in expression of melanin-based vs. structural plumage coloration. We found that microbial load tended to be lower and feather microbial diversity was significantly higher in the plumage of black iridescent males, compared to black matte females and brown individuals. Moreover, black iridescent males had distinct feather microbial communities compared to black matte females and brown individuals. We suggest that distinctive nanostructure properties of iridescent male feathers or different investment in preening influence feather microbiota community composition and load. This study is the first to point to structural plumage coloration as a factor that may significantly regulate feather microbiota. Future work might explore fitness consequences and the role of microorganisms in the evolution of avian sexual dichromatism, with particular reference to iridescence.

The effects of fathers on daughters' expectations for men.

Guided by paternal investment theory (PIT), the current research examines the effects of fathers on daughters' expectations for men in adulthood, and the role of these expectations in mediating women's short-term (casual or uncommitted) sexual behavior. Using a genetically informed differential sibling-exposure design (N = 223 sister pairs from divorced/separated families), we found that developmental exposure to low-quality paternal behavior (but not paternal absence per se) predicted adult women's expectations for men as partners. For older sisters, who spent much of their childhoods living with their fathers, lower-quality paternal behavior predicted lower expectations for male investment in their relationships as adults. Moreover, lower expectations for men as partners predicted higher numbers of sexual partners (past and anticipated) among these women. By contrast, for younger sisters, who spent relatively little time coresiding with their fathers, no such effects of quality of paternal behavior were observed. The current work provides evidence that exposure to low-quality paternal behavior during development may help calibrate daughters' expectations for the behavior of male relationship partners, and these expectations may shape their sexual behavior in adulthood. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Compete then Care: Shifts in Male Reproductive Strategies Over the Life Course in a Polygynous Mammal

In polygynous species, there is often intense intra-sexual competition over access to females and high reproductive skew, and males invest more in mating effort than parenting effort. However, the benefits of investments in mating effort and parenting effort may change systematically over the course of males’ lives. In many polygynous mammalian species, there is an ∩-shaped relationship between age, condition, and resource holding power as middle-aged males that are in prime physical condition outcompete older and younger males. As males age, and their competitive abilities decline, males might benefit from reallocating investment from mating effort to parenting effort. We examined the effects of male age and paternity on the likelihood that male olive baboons, Papio anubis, would invest in parenting effort. Male baboons compete for high ranking positions in the group, and there is an ∩-shaped relationship between male age and dominance rank. High ranking males sire more infants than lower ranking males. Males and lactating females often form close ties (primary associates), and females’ primary associates are likely to be the sires of their infants. Males are attentive to their primary associates’ distress, support them in conflicts, and buffer their infants from rough handling and harassment. Here, we show that males are more likely to become the primary associates of lactating females as they age and their competitive abilities decline. These results suggest that fitness benefits derived from male parental investment may favor an extended post-reproductive lifespan in species in which males can identify their own offspring and positively affect their welfare.

Sex allocation plasticity on a transcriptome scale: Socially sensitive gene expression in a simultaneous hermaphrodite.

Phenotypic plasticity can enable organisms to produce optimal phenotypes in multiple environments. A crucial life history trait that is often highly plastic is sex allocation, which in simultaneous hermaphrodites describes the relative investment into the male versus female sex functions. Theory predicts-and morphological evidence supports-that greater investment into the male function is favoured with increasing group size, due to the increasing importance of sperm competition for male reproductive success. Here, we performed a genome-wide gene expression assay to test for such sex allocation plasticity in a model simultaneous hermaphrodite, the free-living flatworm Macrostomum lignano. Based on RNA-Seq data from 16 biological replicates spanning four different group size treatments, we demonstrate that at least 10% of the >75,000 investigated transcripts in M.lignano are differentially expressed according to the social environment, rising to >30% of putative gonad-specific transcripts (spermatogenesis and oogenesis candidates) and tail-specific transcripts (seminal fluid candidates). This transcriptional response closely corresponds to the expected shift away from female and towards male reproductive investment with increasing sperm competition level. Using whole-mount in situ hybridization, we then confirm that many plastic transcripts exhibit the expected organ-specific expression, and RNA interference of selected testis- and ovary-specific candidates establishes that these indeed function in gametogenesis pathways. We conclude that a large proportion of sex-specific transcripts in M.lignano are differentially expressed according to the prevailing ecological conditions and that these are functionally relevant to key reproductive phenotypes. Our study thus begins to bridge organismal and molecular perspectives on sex allocation plasticity.

Bodyweight-Related Polyandry in the Tobacco Cutworm Moth Spodoptera litura (Lepidoptera: Noctuidae)

Recent studies on polyandry evolution have revolutionized our understanding of sexual selection. Therefore, study on traits in females, such as sensory perception and remating strategies, is expected to disclose a greater role of sexual selection in female evolution. Here, we tested how bodyweights of both sexes affected female and male remating behavior in Spodoptera litura (Fabricius, 1775) (Lepidoptera: Noctuidae). Results show most (74.4%; unmated virgin females may be sexually immature) females and males mated the first time in the second scotophase after eclosion. In the subsequent scotophase, 41.9% once-mated females and 94.1% once-mated males mated the second time with a novel virgin mate. Analyses indicate that female remating probability significantly increased with the increase of her own and the second male’s bodyweight but decreased with the increase of the first male’s bodyweight, while bodyweights of both sexes have no significant effect on male remating probability. Considering the nature of the low level of polyandry (two times on average) and last male precedence in S. litura, we propose that 1) females are more choosy when remating, from which females will benefit indirectly by sequential mate comparison and remate to replace sperm from the previous mates with those from higher quality mates, and 2) males will generally like to remate with any possible females, from which males will gain direct benefit by siring more offspring. The outcome of polyandry rate is likely to be the result of male investment × female choice interactions.

Slower senescence in a wild insect population in years with a more female-biased sex ratio.

Life-history theories of senescence are based on the existence of a trade-off in resource allocation between body maintenance and reproduction. This putative trade-off means that environmental and demographic factors affecting the costs of reproduction should be associated with changes in patterns of senescence. In many species, competition among males is a major component of male reproductive investment, and hence variation in the sex ratio is expected to affect rates of senescence. We test this prediction using nine years of demographic and behavioural data from a wild population of the annual field cricket Gryllus campestris. Over these generations, the sex ratio at adulthood varied substantially, from years with an equal number of each sex to years with twice as many females as males. Consistent with the predictions of theory, we found that in years with a greater proportion of females, both sexes experienced a slower increase in mortality rate with age. Additionally, phenotypic senescence in males was slower in years when there were more females. Sex ratio did not affect the baseline mortality rate in males, but females suffered higher age-independent mortality rates when males were in short supply.

Does plant allometry predict biased sex allocation in Triadica sebifera (L.) small (Euphorbiaceae)?

Early seed production by non-native plants may be important to their successful establishment and spread. Understanding the mechanisms underlying age- or size-biased seed production in introduced plants may also identify factors that contribute to propagule pressure and their success as invaders. Here, we asked whether differences in male and female reproductive allocation among individuals of the invasive woody plant species Triadica sebifera (L.) Small (Euphorbiaceae), a monoecious prolific seed producer, were consistent with a life-history trade-off involving sex allocation. We also asked if the quality of seeds produced by trees of different size differed in terms of seed size and germinability. We found no evidence that biomass allocation to either male or female function was related to plant size. Trees varied in ratio of female:male investment from 0.6 to 15.8, but investment ratio was also unrelated to plant size. We also found no relationship between plant size and seed size or mean germination time, and a weak negative relationship between percent seed germination and plant size. Overall, the results do not support the idea that patterns of reproductive sex allocation are the result of a life-history trade-off and are instead most consistent with local resource tracking by individual trees.

Males from populations with higher competitive mating success produce sons with lower fitness.

Female mate choice can result in direct benefits to the female or indirect benefits through her offspring. Females can increase their fitness by mating with males whose genes encode increased survivorship and reproductive output. Alternatively, male investment in enhanced mating success may come at the cost of reduced investment in offspring fitness. Here, we measure male mating success in a mating arena that allows for male-male, male-female and female-female interactions in Drosophila melanogaster. We then use isofemale line population measurements to correlate male mating success with sperm competitive ability, the number of offspring produced and the indirect benefits of the number of offspring produced by daughters and sons. We find that males from populations that gain more copulations do not increase female fitness through increased offspring production, nor do these males fare better in sperm competition. Instead, we find that these populations have a reduced reproductive output of sons, indicating a potential reproductive trade-off between male mating success and offspring quality.

Male Investment in Schooling with Frictional Labour and Marriage Markets

We present an equilibrium model with inter-linked frictional labour and marriage markets. Women’s flow value of being single is treated as given, and it captures returns from employment. Single unemployed men conduct a so-called constrained sequential job search, and can choose to improve their labour market returns as well as their marriage prospects by undertaking a costly ex-ante investment in schooling. We establish the existence of market equilibria where a fraction of men get educated, and show that this fraction decreases if women’s labour market returns increase. We also examine the robustness of such equilibria.

An experimental test of the effect of male attractiveness on maternal investment in Dark-eyed Juncos

AbstractWhile social mate choice is an obvious point where sexual selection acts on males, cryptic selection can occur based on how much females invest in a male’s offspring. I tested 4 hypotheses about the relationship between female investment and social mate attractiveness by manipulating a sexually selected trait, white tail feathers (“tail white”), in male Dark-eyed Juncos (Junco hyemalis thurberi): (1) the differential allocation hypothesis predicts that females optimize lifetime reproductive success by maximizing parental investment in the offspring of ornamented mates and minimizing investment when mated to less attractive males; (2) instead of positive differential allocation, females could show negative differential allocation by investing more in less attractive males to compensate for their lower quality; (3) female care could also be influenced directly by paternal investment, in particular what appears to be positive differential allocation could be compensation for reduced care by attractive males; or (4) females could reduce care when paired with socially attractive males that are “good parents”. To evaluate these hypotheses, I examined the impact of tail white manipulation on clutch size, female incubation and brooding, and female and male provisioning. Females did not differentially allocate in relation to social mate attractiveness, nor were females responsive to patterns of male investment, even although artificially attractive males provisioned offspring significantly more than did controls. Naturally bright males, who were also the largest males, did not provision at higher rates, highlighting the need to experimentally separate the role of specific traits from other correlated factors. Tail white-manipulated males were involved in rare cases of polygyny and at times were actively guarded by their mates. Together, these results suggest that tail white influences reproductive behaviors in this junco population, but because of the potential for extrapair paternity, additional data are needed to accurately investigate the trait’s role in female investment.