Larger Dominant Males Research Articles

Untangling the influence of biotic and abiotic factors on habitat selection by a tropical rodent

Understanding how animal populations respond to environmental factors is critical because large-scale environmental processes (e.g., habitat fragmentation, climate change) are impacting ecosystems at unprecedented rates. On an overgrazed floodplain in north-western Australia, a native rodent (Pale Field Rat, Rattus tunneyi) constructs its burrows primarily beneath an invasive tree (Chinee Apple, Ziziphus mauritiana) rather than native trees. The dense thorny foliage of the Chinee Apple may allow high rat densities either because of abiotic effects (shade, in a very hot environment) or biotic processes (protection from trampling and soil compaction by feral horses, and/or predation). To distinguish between these hypotheses, we manipulated Chinee Apple foliage to modify biotic factors (access to horses and predators) but not shade levels. We surveyed the rat population with Elliott traps under treatment and control trees and in the open woodland, in two seasons (the breeding season—January, and the nesting season—May). In the breeding season, we ran giving-up density experiments (GUD) with food trays, to assess the perceived risk of predation by rats across our three treatments. Selective trimming of foliage did not affect thermal regimes underneath the trees but did allow ingress of horses and we observed two collapsed burrows as a consequence (although long term impacts of horses were not measured). The perceived predation risk also increased (GUD values at food trays increased) and was highest in the open woodland. Our manipulation resulted in a shift in rat sex ratios (indicating female preference for breeding under control but not foliage-trimmed trees) and influenced rat behaviour (giving-up densities increased; large dominant males inhabited the control but not treatment trees). Our data suggest that the primary benefit of the Chinee Apple tree to native rodents lies in physical protection from predators and (potentially) feral horses, rather than in providing cooler microhabitat.

Ecological and evolutionary consequences of alternative sex-change pathways in fish

Sequentially hermaphroditic fish change sex from male to female (protandry) or vice versa (protogyny), increasing their fitness by becoming highly fecund females or large dominant males, respectively. These life-history strategies present different social organizations and reproductive modes, from near-random mating in protandry, to aggregate- and harem-spawning in protogyny. Using a combination of theoretical and molecular approaches, we compared variance in reproductive success (Vk*) and effective population sizes (Ne) in several species of sex-changing fish. We observed that, regardless of the direction of sex change, individuals conform to the same overall strategy, producing more offspring and exhibiting greater Vk* in the second sex. However, protogynous species show greater Vk*, especially pronounced in haremic species, resulting in an overall reduction of Ne compared to protandrous species. Collectively and independently, our results demonstrate that the direction of sex change is a pivotal variable in predicting demographic changes and resilience in sex-changing fish, many of which sustain highly valued and vulnerable fisheries worldwide.

All-female monosex culture in the freshwater prawn Macrobrachium rosenbergii – A comparative large-scale field study

In crustacean aquaculture, size dimorphism between males and females is the main key factor determining the advantage of monosex aquaculture over that of mixed populations. This factor is particularly relevant for the freshwater prawn, Macrobrachium rosenbergii, for which intensification of cultures is complicated by a complex social structure in which large dominant males are territorial and inhibit the growth performance of smaller males and females. It has therefore been suggested that all-female mono-culture could be the practice of choice, since females are less aggressive and less territorial and are believed to exhibit a relatively homogenous growth pattern. Here we report the first large-scale comparative field study of all-female and mixed populations under extensive and intensive stocking conditions in earthen ponds. The study was facilitated by application of our novel biotechnology based on a single injection of suspended hypertrophied androgenic gland cells. Under both our intensive and extensive conditions, the all-female cultures showed better performance than the mixed cultures in most key aquaculture parameters including survival rate and yield per hectare. Also, the intensively stocked all-female ponds showed better feed conversion ratio than mixed ponds. Furthermore, while the mean size of the animals did not differ significantly between the two treatments, the all-female populations exhibited significantly higher size uniformity. Our study suggests that for M. rosenbergii, female monosex aquaculture is a sustainable method to yield a homogenous crop.

Plasticity in secondary sexual characteristics in male freshwater blennies (Salaria fluviatilis)

Alternative reproductive tactics (ARTs) driven by environmental factors are common among fish. However, the flexibility of fish to adopt distinct tactics in response to the characteristics of their environment has received little attention. The aim of the present work was to study phenotypic plasticity in the adoption of dominant behaviour (“bourgeois tactic”) by male freshwater blennies (Salaria fluviatilis (Asso, 1801)). For this purpose, two simultaneous experiments in aquaria were performed to examine the effect of social cues and nest abundance on the acquisition of secondary sexual characteristics (SSCs). Experiments were conducted with small (individuals without SSCs), medium-sized (1-year-old individuals), and large older dominant males (more than 2 years old), all collected in the wild. In experiment 1, the three sizes of males were combined to compare the development of SSCs depending on intrasexual context. In experiment 2, the effect of nest abundance (two nests vs. six nests) was tested for each size of male. Medium-sized males showed phenotypic plasticity in response to the environmental conditions simulated in the two experiments. The absence of larger dominant males was found to be the main factor enhancing SSCs and the onset of parental behaviour. Nest shortage also influenced the degree of cephalic crest development among medium-sized males. This knowledge helps to understand how the population of freshwater blennies still persists when it is reduced to young individuals during the summer droughts in Mediterranean streams.

Body size correlates with fertilization success but not gonad size in grass goby territorial males.

In fish species with alternative male mating tactics, sperm competition typically occurs when small males that are unsuccessful in direct contests steal fertilization opportunities from large dominant males. In the grass goby Zosterisessor ophiocephalus, large territorial males defend and court females from nest sites, while small sneaker males obtain matings by sneaking into nests. Parentage assignment of 688 eggs from 8 different nests sampled in the 2003–2004 breeding season revealed a high level of sperm competition. Fertilization success of territorial males was very high but in all nests sneakers also contributed to the progeny. In territorial males, fertilization success correlated positively with male body size. Gonadal investment was explored in a sample of 126 grass gobies collected during the period 1995–1996 in the same area (61 territorial males and 65 sneakers). Correlation between body weight and testis weight was positive and significant for sneaker males, while correlation was virtually equal to zero in territorial males. That body size in territorial males is correlated with fertilization success but not gonad size suggests that males allocate much more energy into growth and relatively little into sperm production once the needed size to become territorial is attained. The increased paternity of larger territorial males might be due to a more effective defense of the nest in comparison with smaller territorial males.

Visual Information Alone Changes Behavior and Physiology during Social Interactions in a Cichlid Fish (Astatotilapia burtoni)

Social behavior can influence physiological systems dramatically yet the sensory cues responsible are not well understood. Behavior of male African cichlid fish, Astatotilapia burtoni, in their natural habitat suggests that visual cues from conspecifics contribute significantly to regulation of social behavior. Using a novel paradigm, we asked whether visual cues alone from a larger conspecific male could influence behavior, reproductive physiology and the physiological stress response of a smaller male. Here we show that just seeing a larger, threatening male through a clear barrier can suppress dominant behavior of a smaller male for up to 7 days. Smaller dominant males being “attacked” visually by larger dominant males through a clear barrier also showed physiological changes for up to 3 days, including up-regulation of reproductive- and stress-related gene expression levels and lowered plasma 11-ketotestesterone concentrations as compared to control animals. The smaller males modified their appearance to match that of non-dominant males when exposed to a larger male but they maintained a physiological phenotype similar to that of a dominant male. After 7 days, reproductive- and stress- related gene expression, circulating hormone levels, and gonad size in the smaller males showed no difference from the control group suggesting that the smaller male habituated to the visual intruder. However, the smaller male continued to display subordinate behaviors and assumed the appearance of a subordinate male for a full week despite his dominant male physiology. These data suggest that seeing a larger male alone can regulate the behavior of a smaller male but that ongoing reproductive inhibition depends on additional sensory cues. Perhaps, while experiencing visual social stressors, the smaller male uses an opportunistic strategy, acting like a subordinate male while maintaining the physiology of a dominant male.

Strong Reproductive Skew Among Males in the Multiply Mated Swordtail Xiphophorus multilineatus (Teleostei)

Male swordtails in the genus Xiphophorus display a conspicuous ventral elongation of the caudal fin, the sword, which arose through sexual selection due to female preference. Females mate regularly and are able to store sperm for at least 6 months. If multiple mating is frequent, this would raise the intriguing question about the role of female choice and male-male competition in shaping the mating system of these fishes. Size-dependent alternate mating strategies occur in Xiphophorus; one such strategy is courtship with a sigmoid display by large dominant males, while the other is gonopodial thrusting, in which small subordinate males sneak copulations. Using microsatellite markers, we observed a frequency of multiple paternity in wild-caught Xiphophorus multilineatus in 28% of families analyzed, but the actual frequency of multiple mating suggested by the correction factor PrDM was 33%. The number of fathers contributing genetically to the brood ranged from one to three. Compared to other species in the family Poeciliidae, both frequency and degree of multiple paternity were low. Paternity was found to be highly skewed, with one male on average contributing more than 70% to the offspring. Hence in this Xiphophorus mating system, typically one male dominates and sneaker males do not appear to be particularly effective. Postcopulatory mechanisms, however, such as sperm competition, are also indicated by our data, using sex-linked phenotypes among the offspring.

Linking male and female morphology to reproductive success in captive southern calamary (Sepioteuthis australis)

Sexual selection theory predicts that mating success influences the evolution of traits. Previous behavioural observations of male Sepioteuthis australis have revealed two main mating strategies in the field: (1) large dominant males pair and mate with females, which they defend from (2) smaller males attempting to mate using ‘sneaker’ tactics. The current study examined whether fertilisation of squid eggs laid by polyandrous females within a mesocosm reflected this field-observed size-based mating system. Polymorphic microsatellite loci were used to identify mothers and fathers of embryos, which in turn allowed us to determine the effect of body size, age, and nine other morphological traits on reproductive success. Parentage of 112 embryos was assigned among 14 possible females and 20 candidate males of varying sizes. The mating behaviour observed in captivity was consistent with the size-based strategies seen in the field yet large males did not sire proportionally more offspring; instead nearly all males, regardless of size, sired some offspring. Regression analyses indicated that females did not select sperm based on male size, shape or age. Surprisingly, female contributions were skewed, with younger females contributing more eggs than older females. The possibility of male mate-choice was explored.

Male body size and male‐male competition: Interspecific variation in poeciliid fishes

Abstract Poeciliidae are a family of viviparous fishes characterized by reversed sexual size dimorphism. Although in all species males are smaller than females, there is a large variability in size dimorphism both between and within species. It has been suggested that variation in the degree of sexual size dimorphism across the family is related to differences in intensity and direction of both intra‐ and intersexual selection. Here the relationship between male body size and in‐trasexual competition, and its effect on male mating success, have been studied in four poeciliid species. Wide interspecific variation was found in the intensity of male‐male competition and in the importance of body size to achieve dominance. In Belonesox belizanus and Girardinus falcatus, larger dominant males almost completely excluded other males from access to the females. Males of Phalloceros caudimaculatus are less aggressive and dominance influences their access to females to a lesser degree. In Heterandria formosa, aggre...

Dynamic relationships between the dominance of male dark chub, Zacco temmincki, and their acquisition of females

The mating system of the dark chub is a form of male-dominance polygyny. The principal spawning ground in this study was at the shallow end of pools where males fought to obtain females. Mating was promiscuous with little female choice, and dominant males commonly took females from other males. Spawning was performed by a female and a single large dominant male, while other individuals dashed to the spawning pair to release sperm or eat eggs. At sites of low fish density, spawning was infrequent but was generally accomplished successfully because of the scarcity of satellite individuals, and the males' dominance hierarchy was stable throughout the reproductive season. Conversely, at sites of high density, although many females spawned, spawning was frequently interfered with. Furthermore, the dominance hierarchy was unstable and the most dominant male tended to become exhausted after frequent reproductive activities. In addition to his body size and dominance, how long he spent around the spawning grounds, his skill at spawning and the selection of a spawning site affected a male's reproductive success. While the larger dominant males mainly used the most frequently used spawning grounds where many females spawned, the smaller sub-dominants tended to use the less frequently used spawning grounds.

Female choice and the mating cost of peripheral males

Males in many animals use alternative mating tactics to parasitize the mating effort of large dominant males. The primary cost of ‘peripheral’ males has been assumed to lie in stolen fertilizations and thereby reduced paternity of the dominant males whose mating effort is parasitized. Here, it is shown that for two nesting labrid fishes, Symphodus tinca and S. ocellatus, the loss of mating opportunities due to the presence of peripheral males exceeds the cost of paternity reduction by at least three and a half times. Experimentally reduced numbers of peripheral males in the wild revealed that when the majority of these were removed from nests, females of both species increased their spawning rate five-to eight-fold within minutes. Involvement in matings by peripheral males entailed no significant cost to females in fertilization rate, egg mortality or in the quality of subsequent parental care. The only hypothesis for this strong discrimination against peripheral males that cannot presently be refuted is that females choose mates based on age, defensive ability or size as an indicator of their genetic quality.