Personality differentially affects individual mate choice decisions in female and male Western mosquitofish (Gambusia affinis)

Integrated behavioural and physiological responses of sand smelt larvae to the effects of warming and hypoxia as combined stressors

Forecasts indicate that rising temperatures towards the future and the expansion of dead zones will change environmental suitability for fish early stages. Therefore, we assessed the chronic effects of warming (26 °C), hypoxia (<2-2.5 mg L-1) or their combination on mortality rate, growth, behaviour, energy metabolism and oxidative stress using Atherina presbyter larvae as a model species. There were no differences between the treatments in terms of mortality rate. The combination of warming and hypoxia induced faster loss of body mass (+22.7%). Warming, hypoxia or their combination enhanced boldness (+14.7-25.4%), but decreased exploration (-95%-121%), increased the time in frozen state (+60.6-80.5%) and depleted swimming speed (-45.6-50.5%). Moreover, routine metabolic rate was depleted under hypoxia or under the combination of warming and hypoxia (-56.6 and 57.2%, respectively). Under hypoxia, increased catalase activity (+56.3%) indicates some level of antioxidant defence capacity, although increased DNA damage (+25.2%) has also been observed. Larvae also exhibited a great capacity to maintain the anaerobic metabolism stable in all situations, but the aerobic metabolism is enhanced (+19.3%) when exposed to the combination of both stressors. The integrative approach showed that changes in most target responses can be explained physiologically by oxidative stress responses. Increased oxidative damages (lipid peroxidation and DNA damage) and increased interaction between antioxidant enzymes (superoxide dismutase and catalase) are associated to increased time in frozen state and decreased swimming activity, growth rates and boldness. Under all stressful situations, larvae reduced energy-consuming behaviours (e.g. depleted exploration and swimming activity) likely to stabilize or compensate for the aerobic and anaerobic metabolisms. Despite being an active small pelagic fish, we concluded that the sensitive larval phase exhibited complex coping strategies to physiologically acclimate under thermal and hypoxic stress via behavioural responses.

Early life stage mechanisms of an active fish species to cope with ocean warming and hypoxia as interacting stressors

Ocean's characteristics are rapidly changing, modifying environmental suitability for early life stages of fish. We assessed whether the chronic effects of warming (24 °C) and hypoxia (<2-2.5 mg L-1) will be amplified by the combination of these stressors on mortality, growth, behaviour, metabolism and oxidative stress of early stages of the white seabream Diplodus sargus. Combined warming and hypoxia synergistically increased larval mortality by >51%. Warming induced faster growth in length and slower gains in weight when compared to other treatments. Boldness and exploration were not directly affected, but swimming activity increased under all test treatments. Under the combination of warming and hypoxia, routine metabolic rate (RMR) significantly decreases when compared to other treatments and shows a negative thermal dependence. Superoxide dismutase and catalase activities increased under warming and were maintained similar to control levels under hypoxia or under combined stressors. Under hypoxia, the enzymatic activities were not enough to prevent oxidative damages as lipid peroxidation and DNA damage increased above control levels. Hypoxia reduced electron transport system activity (cellular respiration) and isocitrate dehydrogenase activity (aerobic metabolism) below control levels. However, lactate dehydrogenase activity (anaerobic metabolism) did not differ among treatments. A Redundancy Analysis showed that ∼99% of the variability in mortality, growth, behaviour and RMR among treatments can be explained by molecular responses. Mortality and growth are highly influenced by oxidative stress and energy metabolism, exhibiting a positive relationship with reactive oxygen species and a negative relationship with aerobic metabolism, regardless of treatment. Under hypoxic condition, RMR, boldness and swimming activity have a positive relationship with anaerobic metabolism regardless of temperature. Thus, seabreams may use anaerobic reliance to counterbalance the effects of the stressors on RMR, activity and growth. The outcomes suggests that early life stages of white seabream overcame the single and combined effects of hypoxia and warming.

The effect of individual state on the strength of mate choice in females and males

Animals are thought to gain significant fitness benefits from choosing high-quality or compatible mates. However, there is large within-species variation in how choosy individuals are during mating. This may be because the costs and benefits of being choosy vary according to an individual's state. To test this, I systematically searched for published data relating the strength of animal mate choice in both sexes to individual age, attractiveness, body size, physical condition, mating status, and parasite load. I performed a meta-analysis of 108 studies and 78 animal species to quantify how the strength of mate choice varies according to individual state. In line with the predictions of sexual selection theory, I find that females are significantly choosier when they are large and have a low parasite load, thus supporting the premise that the expression of female mate choice is dependent on the costs and benefits of being choosy. However, female choice was not influenced by female age, attractiveness, physical condition, or mating status. Attractive males were significantly choosier than unattractive males, but male mate choice was not influenced by male age, body size, physical condition, mating status, or parasite load. However, this dataset was limited by a small sample size, and the overall correlation between individual state and the strength of mate choice was similar for both sexes. Nevertheless, in both males and females individual state explained only a small amount of variation in the strength of mate choice.

Sex Differences in Mate Choice Preference Characteristics of Aequidens rivulatus

Simple Summary

Generally, animals prefer mating with partners of the opposite sex with specific features, which suggests that animals tend to choose mates with particular phenotypic traits. However, there are some differences in mate choice behavior and criteria between males and females. This study analyzed these differences between males and females in Aequidens rivulatus by quantifying body size, behavioral intention, and appearance. The results showed that males paid more attention to preference degree and female attractiveness, whereas females focused on ability and physical strength displays. Consequently, males who chose to mate were primarily associated with body size, behavioral intention, and appearance, whereas the preferences of females were body size, appearance, and behavioral intention. Collectively, our initial findings revealed that males and females have different criteria for mate choice, which is vital in determining successful mating and improving artificial mating.

Abstract

The mating roles of males and females, to a certain extent, are dynamic and variable. Several factors influence the mate choice process. Nonetheless, the main preference features have not yet been fully understood in Aequidens rivulatus. In this study, because of its natural pairing characteristics, A. rivulatus was selected to explore the mate choice preferences of different sexes. Specifically, male and female behavioral performances were described and quantified through a “no-choice paradigm” during mate choice. A total of 12 behavioral performances were defined in male mate choice (experiment 1), whereas 14 behavioral performances were defined in female mate choice (experiment 2). According to the obtained results, unselected females did not display any proactive behaviors in experiment 1, whereas unselected males exhibited proactive behaviors in experiment 2, including quivering, nipping, tail beating, swimming up and down, and aggression. It was also found that both male and female individuals tend to express dislike rather than like. Those behaviors with higher frequencies (e.g., quivering) often mean less energy expenditure, thus easier repeatability. Moreover, principal component analysis (PCA) was employed to extract and identify mate choice preference features. Preliminary results indicated that male preferences for a mate were mainly associated with body size, behavioral intention, and appearance, whereas the intensity of female preferences was in the order of body size, appearance, and behavioral intention. In addition, sex hormone levels were associated with mate choices.

Male mate choice in mosquitofish: personality outweighs body size

Background

Despite its important implications in behavioural and evolutionary ecology, male mate choice has been poorly studied, and the relative contribution of personality and morphological traits remains largely unknown. We used standard two-choice mating trials to explore whether two personality traits (i.e., shyness and activity) and/or body size of both sexes affect mate choice in male mosquitofish Gambusia affinis. In the first set of trials involving 40 males, we tested whether males would prefer larger females and whether the preference would be affected by males’ body length and personality traits, and females’ activity level. In the second set of trials (using another 40 males), we tested whether males would prefer more active females and whether the preference would be affected by males’ body length and personality traits.

Results

Both shyness and activity in males were significantly repeatable and constituted a behavioural syndrome. No overall directional preference for large (or small) females with the same activity levels was detected because larger males preferred larger females and smaller males chose smaller females. Males’ strength of preference for larger females was also positively correlated with the activity level of larger females but negatively with the activity level of smaller females. Males spent more time associating with active females regardless of their body lengths, indicating males’ selection was more influenced by female activity level than body size. Males’ preference for inactive females was enhanced when females became active. There was no convincing evidence for the effect of males’ personality traits or body length on their preferences for females’ activity level.

Conclusions

Our study supports the importance of body size in male mate choice but highlights that personality traits may outweigh body size preferences when males choose mating partners.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00450-3.

A meta-analysis of sex differences in animal personality: no evidence for the greater male variability hypothesis

The notion that men are more variable than women has become embedded into scientific thinking. For mental traits like personality, greater male variability has been partly attributed to biology, underpinned by claims that there is generally greater variation among males than females in non-human animals due to stronger sexual selection on males. However, evidence for greater male variability is limited to morphological traits, and there is little information regarding sex differences in personality-like behaviours for non-human animals. Here, we meta-analysed sex differences in means and variances for over 2100 effects (204 studies) from 220 species (covering five broad taxonomic groups) across five personality traits: boldness, aggression, activity, sociality and exploration. We also tested if sexual size dimorphism, a proxy for sex-specific sexual selection, explains variation in the magnitude of sex differences in personality. We found no significant differences in personality between the sexes. In addition, sexual size dimorphism did not explain variation in the magnitude of the observed sex differences in the mean or variance in personality for any taxonomic group. In sum, we find no evidence for widespread sex differences in variability in non-human animal personality.

Mate Choice, Sex Roles and Sexual Cognition in Vertebrates: Mate Choice Turns Cognition or Cognition Turns Mate Choice?

The idea of “smart is sexy,” meaning superior cognition provides competitive benefits in mate choice and, therefore, evolutionary advantages in terms of reproductive fitness, is both exciting and captivating. Cognitively flexible individuals perceive and adapt more dynamically to (unpredictable) environmental changes. The sex roles that females and males adopt within their populations can vary greatly in response to the prevalent mating system. Based on how cognition determines these grossly divergent sex roles, different selection pressures could possibly shape the (progressive) evolution of cognitive abilities, suggesting the potential to induce sexual dimorphisms in superior cognitive abilities. Associations between an individual’s mating success, sexual traits and its cognitive abilities have been found consistently across vertebrate species and taxa, providing evidence that sexual selection may well shape the supporting cognitive prerequisites. Yet, while superior cognitive abilities provide benefits such as higher feeding success, improved antipredator behavior, or more favorable mate choice, they also claim costs such as higher energy levels and metabolic rates, which in turn may reduce fecundity, growth, or immune response. There is compelling evidence in a variety of vertebrate taxa that females appear to prefer skilled problem-solver males, i.e., they prefer those that appear to have better cognitive abilities. Consequently, cognition is also likely to have substantial effects on sexual selection processes. How the choosing sex assesses the cognitive abilities of potential mates has not been explored conclusively yet. Do cognitive skills guide an individual’s mate choice and does learning change an individual’s mate choice decisions? How and to which extent do individuals use their own cognitive skills to assess those of their conspecifics when choosing a mate? How does an individual’s role within a mating system influence the choice of the choosing sex in this context? Drawing on several examples from the vertebrate world, this review aims to elucidate various aspects associated with cognitive sex differences, the different roles of males and females in social and sexual interactions, and the potential influence of cognition on mate choice decisions. Finally, future perspectives aim to identify ways to answer the central question of how the triad of sex, cognition, and mate choice interacts.

Male size and reproductive performance in three species of livebearing fishes (Gambusia spp.): A systematic review and meta-analysis

The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this inconsistent body of evidence using pre-registered methods and hypotheses. We performed a systematic review and meta-analysis of summary and primary (raw) data combining both published (n = 19 studies, k = 106 effect sizes) and unpublished effect sizes (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand the sources of heterogeneity across effects. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r = 0.23, 95% confidence interval: 0.10-0.35, n = 36, k = 348, 4,514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), and was considerably larger for female choice (r = 0.43, 95% confidence interval: 0.28-0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status and environmental conditions. We found evidence of publication bias; however, its influence on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of open primary data for more accurate meta-analytic estimates. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

Size Selective Harvesting Does Not Result in Reproductive Isolation among Experimental Lines of Zebrafish, Danio rerio: Implications for Managing Harvest-Induced Evolution

Simple Summary

Mortality in fish populations is commonly size-selective. In fisheries, larger fish are preferentially caught while natural predators preferentially consume smaller fish. Removal of certain sized fish from populations and elevated fishing mortality constitute a selection pressure which may change life-history, behaviour and reduce adult body-size. Because behaviour and body-size are related and influence mating preferences and reproductive output, size-selective mortality may favour subpopulations that less readily mate with each other. Our aim is to test this possibility using three experimental lines of zebrafish (Danio rerio) generated in laboratory by removing large-sized, small-sized and random-sized fish for five generations. We tested mating preferences among males and females and tested if they spawned together. We found males and females of all subpopulations to reproduce among themselves. Females generally preferred large-sized males. Females of all lines spawned with males, and males of all lines fertilised eggs of females independent of the subpopulation origin. Our study shows that size-selective mortality typical of fisheries or in populations facing heavy predation does not result in evolution of reproductive barriers. Thus, when populations adapted to fishing pressure come in contact with populations unexposed to such pressures, interbreeding may happen thereby helping exploited populations recover from harvest-induced evolution.

Abstract

Size-selective mortality is common in fish stocks. Positive size-selection happens in fisheries where larger size classes are preferentially targeted while gape-limited natural predation may cause negative size-selection for smaller size classes. As body size and correlated behavioural traits are sexually selected, harvest-induced trait changes may promote prezygotic reproductive barriers among selection lines experiencing differential size-selective mortality. To investigate this, we used three experimental lines of zebrafish (Danio rerio) exposed to positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations. We tested prezygotic preferences through choice tests and spawning trials. In the preference tests without controlling for body size, we found that females of all lines preferred males of the generally larger small-harvested line. When the body size of stimulus fish was statistically controlled, this preference disappeared and a weak evidence of line-assortative preference emerged, but only among large-harvested line fish. In subsequent spawning trials, we did not find evidence for line-assortative reproductive allocation in any of the lines. Our study suggests that size-selection due to fisheries or natural predation does not result in reproductive isolation. Gene flow between wild-populations and populations adapted to size-selected mortality may happen during secondary contact which can speed up trait recovery.

Environmental complexity during early life shapes average behavior in adulthood

Personality has been identified in a range of animal taxa during the last few decades, with important ecological and evolutionary implications. Investigating the effects of environmental factors during early life can provide important insights into the ontogeny of animal personality. We reared newborn mosquitofish, Gambusia affinis, in tanks of different structural complexities, and measured their behavioral traits (i.e., shyness, exploration, and sociability) when they reached sexual maturity. Univariate linear mixed-effects models were fitted to test the effects of environmental complexity and sex on population-average behavior, whereas multivariate models were fitted to quantify behavioral repeatability (i.e., personality) and among-individual correlations (i.e., behavioral syndromes). On average, females were shyer and more social than males, and the fish reared in complex environments were shyer, less explorative, and more social than those reared in open environments. Among-individual differences were consistently large across trials for all behaviors, indicating that personality variation was present in mosquitofish of both sexes reared in different environments. Repeatability did not differ among behaviors, and there were no differences in repeatability in any behavior between sexes or among environments. A negative correlation between shyness and exploration was found in mosquitofish from all treatments at both phenotypic and among-individual levels, with the latter indicating a strong shyness–exploration behavioral syndrome. Our study provides robust evidence that average levels of personality might vary when mosquitofish are raised in different levels of structural complexity during early life.

Repeatability analysis improves the reliability of behavioral data

Reliability of data has become a major concern in the course of the reproducibility crisis. Especially when studying animal behavior, confounding factors such as novelty of the test apparatus can lead to a wide variability of data which may mask treatment effects and consequently lead to misinterpretation. Habituation to the test situation is a common practice to circumvent novelty induced increases in variance and to improve the reliability of the respective measurements. However, there is a lack of published empirical knowledge regarding reasonable habituation procedures and a method validation seems to be overdue. This study aimed at setting up a simple strategy to increase reliability of behavioral data measured in a familiar test apparatus. Therefore, exemplary data from mice tested in an Open Field (OF) arena were used to elucidate the potential of habituation and how reliability of measures can be confirmed by means of a repeatability analysis using the software R. On seven consecutive days, male C57BL/6J, BALB/cJ and 129S1/SvImJ mice were tested in an OF arena once daily and individual mouse behavior was recorded. A repeatability analysis was conducted with regard to repeated trials of habituation. Our data analysis revealed that monitoring animal behavior during habituation is important to determine when individual differences of the measurements are stable. Repeatability values from distance travelled and average activity increased over the habituation period, revealing that around 60% of the variance of the data can be explained by individual differences between mice. The first day of habituation was significantly different from the following 6 days. A three-day habituation period appeared to be sufficient in this study. Overall, these results emphasize the importance of habituation and in depth analysis of habituation data to define the correct starting point of the experiment for improving the reliability and reproducibility of experimental data.

Assessing the potential for post-ejaculatory female choice in a polyandrous beach-spawning fish

In species with limited opportunities for pre-ejaculatory sexual selection (behavioural components), post-ejaculatory mechanisms may provide opportunities for mate choice after gametes have been released. Recent evidence from a range of taxa has revealed that cryptic female choice (i.e., female-mediated differential fertilization bias), through chemical cues released with or from eggs, can differentially regulate the swimming characteristics of sperm from various males and ultimately determine male fertilization success under sperm competition. We assessed the potential role that such female-modulated chemical cues play in influencing sperm swimming characteristics in beach-spawning capelin (Mallotus villosus), an externally fertilizing fish that mates as couples (one male and one female) or threesomes (two males and one female) with presumably limited opportunities for pre-ejaculatory sexual selection. We assayed sperm swimming characteristics under varying doses and donor origins of egg cues and also examined the possibility of assortative mating based on body size. We found mating groups were not associated by size, larger males did not produce better quality ejaculates, and egg cues (regardless of dosage or donor identity) did not influence sperm swimming characteristics. Our findings suggest that intersexual pre-ejaculatory sexual selection and cryptic female choice mediated by female chemical cues are poorly developed in capelin, possibly due to unique natural selection constraints on reproduction.

Geographical and temporal variation of multiple paternity in invasive mosquitofish (Gambusia holbrooki, Gambusia affinis)

Multiple paternity (MP) increases offspring's genetic variability, which could be linked to invasive species' evolvability in novel distribution ranges. Shifts in MP can be adaptive, with greater MP in harsher/colder environments or towards the end of the reproductive season, but climate could also affect MP indirectly via its effect on reproductive life histories. We tested these hypotheses by genotyping N = 2,903 offspring from N = 306 broods of two closely related livebearing fishes, Gambusia holbrooki and Gambusia affinis. We sampled pregnant females across latitudinal gradients in their invasive ranges in Europe and China, and found more sires per brood and a greater reproductive skew towards northern sampling sites. Moreover, examining monthly sampling from two G. affinis populations, we found MP rates to vary across the reproductive season in a northern Chinese, but not in a southern Chinese population. While our results confirm an increase of MP in harsher/more unpredictable environments, path analysis indicated that, in both cases, the effects of climate are likely to be indirect, mediated by altered life histories. In both species, which rank amongst the 100 most invasive species worldwide, higher MP at the northern edge of their distribution probably increases their invasive potential and favours range expansions, especially in light of the predicted temperature increases due to global climate changes.

Behavioural and life-history responses of mosquitofish to biologically inspired and interactive robotic predators

Invasive alien species threaten biodiversity worldwide and contribute to biotic homogenization, especially in freshwaters, where the ability of native animals to disperse is limited. Robotics may offer a promising tool to address this compelling problem, but whether and how invasive species can be negatively affected by robotic stimuli is an open question. Here, we explore the possibility of modulating behavioural and life-history responses of mosquitofish by varying the degree of biomimicry of a robotic predator, whose appearance and locomotion are inspired by natural mosquitofish predators. Our results support the prediction that real-time interactions at varying swimming speeds evoke a more robust antipredator response in mosquitofish than simpler movement patterns by the robot, especially in individuals with better body conditions that are less prone to take risks. Through an information-theoretic analysis of animal-robot interactions, we offer evidence in favour of a causal link between the motion of the robotic predator and a fish antipredator response. Remarkably, we observe that even a brief exposure to the robotic predator of 15 min per week is sufficient to erode energy reserves and compromise the body condition of mosquitofish, opening the door for future endeavours to control mosquitofish in the wild.

Predator-induced changes of male and female mating preferences: innate and learned components

While many mating preferences have a genetic basis, the question remains as to whether and how learning/experience can modify individual mate choice decisions. We used wild-caught (predator-experienced) and F₁ laboratory-reared (predator-naïve) invasive Western mosquitofish Gambusia affinis from China to test whether mating preferences (assessed in a first mate choice test) would change under immediate predation threat. The same individuals were tested in a second mate choice test during which 1 of 3 types of animated predators was presented: 1) a co-occurring predator, 2) a co-evolved but not currently co-occurring predator, and 3) a non-piscivorous species as control. We compared preference scores derived from both mate choice tests to separate innate from experiential effects of predation. We also asked whether predator-induced changes in mating preferences would differ between sexes or depend on the choosing individual’s personality type and/or body size. Wild-caught fish altered their mate choice decisions most when exposed to the co-occurring predator whereas laboratory-reared individuals responded most to the co-evolved predator, suggesting that both innate mechanisms and learning effects are involved. This behavior likely reduces individuals’ risk of falling victim to predation by temporarily moving away from high-quality (i.e., conspicuous) mating partners. Accordingly, effects were stronger in bolder than shyer, large- compared with small-bodied, and female compared with male focal individuals, likely because those phenotypes face an increased predation risk overall. Our study adds to the growing body of literature appreciating the complexity of the mate choice process, where an array of intrinsic and extrinsic factors interacts during decision-making.