Size-assortative Mating Research Articles

LACK OF SIZE-ASSORTATIVE MATING IN A WIDESPREAD TREEFROG: PSEUDACRIS CRUCIFER IN IOWA

The Spring Peeper (Pseudacris crucifer) is a small treefrog native to much of the Central and Eastern USA. Here, we tested if Spring Peepers exhibit size-assortative mating—non-random mate pairings based on body size—from a population in eastern Iowa. We captured 75 amplecting pairs of Spring Peepers during the 2021 breeding season, measured their body sizes, and marked them for future recognition. We found that while sexual-size dimorphism exists between males and females, there was no evidence for size-assortative mating among amplectant pairs. Our study contributes to other recent work indicating that size-assortative mating is rare among most anurans, and that previous findings of this phenomenon in frogs should be interpreted with caution.

Size-assortative mating of Buergeria buergeri (Anura: Rhacophoridae): breeding strategies for balancing fertilisation rate and swimming ability

Abstract Buergeria buergeri is a stream-breeding rhacophorid frog. The sexual dimorphism in body size is significant in this species, which might be advantageous for its movement within the stream in an amplected state. However, the fertilisation rate of egg masses decreases as the body size difference between both sexes in the amplectant pair increases because the distance between the cloacae increases. In this study, we examined the mating pattern of B. buergeri and the relationship between female/male SVL ratio in the amplectant pair and fertilisation rate, and the swimming ability within the stream. As a result, when the female/male SVL ratio was less than 1.45, the fertilisation rate reached a plateau (nearly 100%). However, the fertilisation rate decreased as the female/male SVL ratio increased when this ratio was greater than 1.45. Moreover, a significant positive correlation was observed between the female/male SVL ratio and swimming ability. These results suggest that there was a trade-off between fertilisation rate and swimming ability via the female/male SVL ratio. In the field, we observed a significant positive correlation between male and female SVLs in 16 amplectant pairs, with a mean female/male SVL ratio of 1.44. Our experimental results suggest that when the female/male SVL ratio is 1.44, the fertilisation rate is close to 100%, and the swimming ability can be maintained at an intermediate level. Therefore, B. buergeri adopts a size-assortative mating and maintains the female/male SVL ratio of approximately 1.44 to balance both fertilisation rate and swimming ability, thereby increasing reproductive success.

Rates of male rivalry and mate choice vary with density and determine whether size-assortative mating is absent, positive, or negative in the clown weevil, Eudiagogus pulcher

At four sites in Texas, the density of the clown weevil, Eudiagogus pulcher, in stands of rattlebox, Sesbania drummondii, increased with proximity to water. Size-assortative mating (SAM) was negative at high density, positive at intermediate density, and absent at low density. Only at intermediate density were relatively small potential mates frequently rejected. Females rejected males via resistance to attempts to mount while males rejected females by abandoning them soon after mounting. A laboratory experiment showed that mate rejection is more likely when individuals experience intermediate densities, but not low or high densities, prior to courtship. In the field, rival males displaced mounting males that were small relative to either the female or rival. Rates of rivalry increased with density. At high density, low rates of mate rejection and high rates of rivalry resulted in the formation of displacement-resistant pairs consisting of large males and small females. This left small males to vie for large females and resulted in negative SAM. At intermediate density, low rates of rivalry coupled with rejection of small individuals favored the formation of pairs consisting of similarly sized males and females, causing positive SAM. At low density, low rates of both mate rejection and rivalry resulted in random mating by size and the absence of SAM.

Assortative mating in Gastropoda: A meta-analysis

The term ‘assortative mating’ denotes situations when the choice of a mating partner is nonrandom and the phenotypes across mate pairs are correlated, either positively or negatively. Assortative mating has been registered as a particular type of behaviour in many taxa of animals, including various vertebrate and invertebrate species. The aim of this study is to conduct a meta-analysis of published data concerning assortative mating in various taxa of Gastropoda. In total, we found 36 published peer-reviewed papers that consider the size-assortative mating in Gastropoda. 32 species belonging to different taxonomic groups of this class were studied, which provided 58 cases for further analysis. The range of estimates of the strength of assortment between individuals for species included into our meta-analysis (46 cases) is very wide: from –0.155 (Brephulopsis cylindrica) to +0.966 (Veronicella sloanii). Integrally, for the studied species of Gastropoda, the average weighted estimate of the strength of assortment between the sizes of copulating individuals was 0.381 ± 0.014. It revealed that virtually all the points representing individual studies form a funnel-shaped dispersion on a scatterplot that lies along the line representing the estimate of the generalized mean rgen = 0.343 and uniformly fill the funnel-shaped space between the lines of 95% confidence interval of the correlation coefficient for a given sample size adjusted for the overall mean. The distribution of the estimates of the correlation coefficient between copulating individuals among the various gastropod species has a shape close to the normal distribution (Kolmogorov-Smirnov's d = 0.061; P > 0.20). Moreover, most estimates are concentrated within a range from 0.2 to 0.6. We found that the probability of obtaining reliable estimates of the correlation coefficient between the sizes of copulating individuals is dependent upon the number of pairs used in the analysis (binary logistic regression: χ2 = 8.92; df = 1; P = 0.0028). It can be argued that the existence of the negative size-assortative mating in Gastropoda has not yet been proved. On the other hand, if only statistically significant cases of size-assortative mating are considered (37 cases out of 58, or 63.7%), the average weighted estimate of the strength of assortment between sizes of copulating mollusks is 0.439 ± 0.015 (95% confidence interval: 0.409–0.468). If we consider the mating system and the environment simultaneously, the positive size-assortativity is most pronounced among the gonochoristic snails living in the aquatic environment (0.448 ± 0.021) while among the aquatic hermaphroditic species it is the weakest (0.315 ± 0.028). Terrestrial hermaphrodites (land snails and slugs) take the middle position. The numerous examples of the assortative mating with respect to different morphological traits, either quantitative or qualitative, have been described in various species of aquatic and land snails.

Doğu Akdeniz, Türkiye'deki Su Kurbağalarının (Pelophylax ridibundus-complex) Üreme Ekolojisi ve Larval Gelişimi

Reproductive biology of Marsh frogs (Pelophylax ridibundus s.l.) was investigated in the Eastern Mediterranean Region of Türkiye in terms of reproductive phenology, fecundity, clutch size, reproductive effort, and mate choice. Frogs are active for approximately ten months throughout the year and hibernate between November and January. Breeding occurs from mid-January until early June and metamorphosis took 45-65 days, with juveniles observed from the end of March. The tadpole reaches its largest size at stage 38, with a mean total length of 60.61 mm and juveniles have an average snout-vent length of 20 mm at the end of metamorphosis. Secondary sexual characters can be observed on individuals reaching 45 mm snout-vent length and we could not detect any amplexed pairs below this length. The clutch size of spawned egg masses ranged between 144 – 645 and the mean egg size was calculated as 1.73 mm. The mean fecundity was 3853 (ranging between 940 and 6000) eggs and the reproductive effort was 0.094 in females. Females were approximately 16% larger than males and the snout–vent length of males and females of amplexed pairs is significantly different but not correlated. These findings signify random mating instead of size-assortative mating. However, the preference of 6.8% larger females than single ones to mate indicates a size-dependent mating for Western Mediterranean population.

Size-assortative mating in a long-lived monogamous seabird

Size-assortative mating in a long-lived monogamous seabird

Mate Choice in Molluscs of the Genus Littorina (Gastropoda: Littorinidae) from White Sea

We studied the reproductive behaviour associated with mate choice by size in three species of White Sea molluscs of the genus Littorina—L. saxatilis, L. obtusata and L. fabalis. Three behavioural patterns were revealed after a comparative analysis of the behaviour of the male snails. The males of L. obtusata, regardless of their size, could mate with all females but preferred to mate with larger ones. The males of L. fabalis chose females of their own size or larger. The males of L. saxatilis mated with females of approximately their own size. In the first case (L. obtusata), there is a free exchange of genes between all the size groups in the population. In the second case (L. fabalis), the gene transfer from small males to females of any size is free but the gene transfer from large males to small females is limited. In the third case (L. saxatilis), the gene flow is limited between groups of individuals of dramatically different size. Thus, in the case of L. saxatilis and L. fabalis, to varying degrees restriction of random mating associated with size assortativity in choosing a mating partner was expressed, which under certain circumstances (divergence of individuals in different econiches, the presence of ecotypes differing in size, lack of stronger natural selection in the other direction) can favour sympatric speciation. The males of all the three periwinkles’ species preferred larger females but sexual selection in favour of larger females was found only in L. obtusata. In the other two species, there were no statistically significant size differences between copulating and non-copulating females. This appears to be due to the occurrence of size-assortative mating in these species, which limits the ability of males to choose larger female partners.

Mate choice and size-assortative mating in the climbing perch Anabas testudineus (Actinopteri: Anabantidae)

Non-random mating plays a key role in speciation and impacts the individual life history. Body size is one of the most important traits by which the mate choice is carried out. Various patterns of body size preference are known in fi sh, but this component of reproductive strategy has been studied in a limited number of species. Although the climbing perch (Anabas testudineus) is widespread and well-known, few descriptions of its spawning have been published, only in captivity and in most cases artifi cially induced with hormonal agents. This paper describes the main features of natural spawning of the climbing perch in aquarium compared to induced spawning. A positive size-assortative mating has been revealed. The mating system of these fish has been shown to be fl exible, presumably ranging from monogamy to polygamy. The results are discussed in the context of the general issue of the adequacy of fi sh behaviour research methods.

Body size as a magic trait in two plant-feeding insect species.

When gene flow accompanies speciation, recombination can decouple divergently selected loci and loci conferring reproductive isolation. This barrier to sympatric divergence disappears when assortative mating and disruptive selection involve the same "magic" trait. Although magic traits could be widespread, the relative importance of different types of magic traits to speciation remains unclear. Because body size frequently contributes to host adaptation and assortative mating in plant-feeding insects, we evaluated several magic trait predictions for this trait in a pair of sympatric Neodiprion sawfly species adapted to different pine hosts. A large morphological dataset revealed that sawfly adults from populations and species that use thicker-needled pines are consistently larger than those that use thinner-needled pines. Fitness data from recombinant backcross females revealed that egg size is under divergent selection between the preferred pines. Lastly, mating assays revealed strong size-assortative mating within and between species in three different crosses, with the strongest prezygotic isolation between populations that have the greatest interspecific size differences. Together, our data support body size as a magic trait in pine sawflies and possibly many other plant-feeding insects. Our work also demonstrates how intraspecific variation in morphology and ecology can cause geographic variation in the strength of prezygotic isolation.

Predation risk regulates prey assortative mating by reducing the expected reproductive value of mates.

Many animals exhibit size assortative mating (SAM), but how predation affects it remains largely unknown. We hypothesized that predation risk may turn prey less choosy, disrupting SAM, or reduce the expected reproductive value of mates, maintaining SAM but with different size ratio between mates. Using a manipulative field experiment, we found that desert isopods under risk of scorpion predation maintained SAM, but that males that choose and fight over females were on average smaller for a given female size. Less pairs were formed in risky sites, but there were no differences in female sizes and progeny number, size and age near and away from scorpion burrows. Our complementary behavioral experiments revealed that bigger males stayed longer near safe burrows, and won more male-male contests than smaller conspecifics. Our findings highlight that prey can anticipate future costs of predation and use this information to assess the expected reproductive value of mates.

Mate choice for body size leads to size assortative mating in the Ryukyu Scops Owl Otus elegans.

Understanding evolutionary phenomena that involve size assortative mating requires elucidating the generating mechanisms on which assortment is based. Although various mechanisms have been suggested, their relative importance may differ across taxonomic groups. Males selecting for large, fecund females combined with the dominance of large males in the competition for females has been suggested as a major mechanism in specific groups. However, raptors do not appear to conform to this, because the selection for smallness among males (assumed in a theory of reversed sexual size dimorphism) and the selection for largeness among males (assumed in the theory of size assortative mating) are in opposite directions. We studied the assortative mating during a long-term study of the Ryukyu Scops Owls Otus elegans interpositus. Significant assortative mating was found for culmen length (from the base to the tip of the bill) and wing length (from the bend of the wing to the tip of the longest primary). Statistical control of the spatial and temporal accessibility of potential mates did not affect the assortment. Males with short wings had slightly higher fitness components than those with long wings, and females settling early tended to have small wings. Considering that early-settling females can preferentially choose their mates, these results suggest that smaller females have an advantage when choosing smaller males with good reproductive performance. Improved flying and hunting ability of smaller individuals may be the background of choosing smaller individuals. We propose that, not passive process like similarity between individuals and their potential mates, but active mate choice for small individuals is an explanation for the assortative mating in this owl.

Asymmetric Male Mating Success in Lek-Breeding Rhinella arenarum.

Mate choice is the attempt of an individual to gain higher reproductive fitness by preferring to mate with some individuals and not with others. We studied the role of mate choice in the mating system of the neotropical toad Rhinella arenarum by assessing male reproductive tactics for mate acquisition and the contribution of female choice for pair formation. In a shallow pond in central Argentina, we estimated male mating success and the corresponding reproductive tactics by focal observation. The variation of phenotypic and genotypic traits (size and shape, longevity, vocalization features, heterozygosity) was related to the observed mating success in 110 males. The phonotactic response of 21 reproductive females to conspecific advertisement call features was tested in arena experiments. Mating success was limited to 32 males, pair formation was size-assortative. The dominant reproductive tactics were advertising from call positions near suitable breeding sites and pre-mating fights with intruding males, whereas the interception of amplectant pairs and the displacement of mated males were never observed. Female phonotaxis was directed to conspecific choruses but complex and simple call structures were not distinguished. We conclude that the mating system is a lek combining pre-mating fights among males and female choice of slightly smaller males. Fights interfere with female choice, undermining size-assortative mating. This is a unique system in the R. marina species group, in which interception behavior dominates reproduction.

Sexual size monomorphism may evolve in lizards with a body size maximizing reproductive performance for both sexes.

We used Takydromus septentrionalis, a sexually size-monomorphic lacertid lizard, as a model system to test the hypothesis that sexual size monomorphism may evolve in lizards where reproductive performance is maximized at a similar body size for both sexes. We allowed lizards housed in laboratory enclosures to lay as many clutches (for females) as they could or to mate as many times (for males) as they could in a breeding season. Size-assortative mating was weak but evident in T. septentrionalis, as revealed by the fact that male and female snout-vent lengths (SVLs) in mating pairs were significantly and positively correlated. Mating frequency (indicative of male reproductive performance) varied from 1 to 8 per breeding season, generally increasing as SVL increased in adult males smaller than 67.4 mm SVL. Clutch frequency varied from 1 to 7 per breeding season, with female reproductive performance (determined by clutch frequency, annual fecundity, and annual reproductive output) maximized in females with a SVL of 68.0 mm. Accordingly to our hypothesis, the reproductive performance was maximized in the intermediate sized rather than the largest individuals in both sexes, and the body size maximizing reproductive performance was similar for both sexes. Future work could usefully investigate other lineages of lizards with sexually monomorphic species in a phylogenetic context to corroborate the hypothesis of this study.

Phylogenetic multilevel meta‐analysis: A simulation study on the importance of modelling the phylogeny

Abstract Meta‐analyses in ecology and evolution require special attention due to certain study characteristics in these fields. First, the primary articles in these fields usually report results that are observed from studies conducted with different species, and the phylogeny among the species violates the independence assumption. Second, articles frequently allow the computation of multiple effect sizes which cannot be accounted for by conventional meta‐analytic models. While both issues can be dealt with by utilizing a multilevel model that accounts for phylogeny, the performance of such a model has not been examined extensively. In this article, we investigate the performance of this model in comparison with some simpler models. We conducted an extensive simulation study where data with different hierarchical structures (in terms of study and species levels) were generated and then various models were fitted to examine their performance. The models we used include the conventional random effects and multilevel random‐effects models along with more complex multilevel models that account for species‐level variance with different variance components. Furthermore, we present an illustrative application of these models based on the data from a meta‐analysis on sizeassortative mating and comment on the results in light of the findings from the simulation study. Our simulation results show that, when the phylogenetic relationships among the species are at least moderately strong, only the most complex model that decomposes the species‐level variance into nonphylogenetic and phylogenetic components provides approximately unbiased estimates of the overall mean and variance components and yields confidence intervals with an approximately nominal coverage rate. Contrarily, removing the phylogenetic or non‐phylogenetic component leads to biased variance component estimates and an increased risk for incorrect inferences about the overall mean. These findings are supported by the results derived from the illustrative application. Based on our results, we suggest that meta‐analyses in ecology and evolution should use the model that accounts for both the nonphylogenetic and phylogenetic species‐level variance in addition to the multilevel structure of the data. Any attempts to simplify this model, such as using only the phylogenetic variance component, may lead to erroneous inferences from the data.

Reproductive Patterns Drive the Gene Flow and Spatial Dispersal of Euschistus heros (Hemiptera: Pentatomidae).

Euschistus heros (Hemiptera: Pentatomidae) has two allopatric strains with a hybrid zone in central Brazil. Asymmetric dispersal and gene flow between these strains of E. heros have been observed, where the South strain (SS) moves more quickly to the northern regions of the country than the North strain (NS) to the southern areas. In addition, SS generally has a bigger body size and presents dark brown coloration, and NS is usually smaller in size and presents light brown coloration. Here, we studied the reproductive behavior and tested for the presence of assortative mating and reproductive barriers between the two allopatric strains of E. heros. Nonrandom mating was observed in the SS strain based on mating choice trials and the reproductive isolation indexes. SS females and males prefer to mate with their co-specific (same strain) partner, while NS insects showed no mating preference. The insect's pronotum width was positively associated with the mating choice suggesting size-assortative mating in E. heros. Reciprocal crosses between strains yielded similar reproductive outputs when compared with pure strain crosses, suggesting similar fitness of hybrid pure strains. The asymmetric gene flow in the hybridization zone that favors SS seems to be associated with the reproductive behavior of the species, which favors the typical phenotype found in the SS populations.

Mixed Mating in a Multi-Origin Population Suggests High Potential for Genetic Rescue in North Island Brown Kiwi, Apteryx mantelli

Reinforcement translocations are increasingly utilised in conservation with the goal of achieving genetic rescue. However, concerns regarding undesirable results, such as genetic homogenisation or replacement, are widespread. One factor influencing translocation outcomes is the rate at which the resident and the introduced individuals interbreed. Consequently, post-release mate choice is a key behaviour to consider in conservation planning. Here we studied mating, and its consequences for genomic admixture, in the North Island brown kiwi Apteryx mantelli population on Ponui Island which was founded by two translocation events over 50 years ago. The two source populations used are now recognised as belonging to two separate management units between which birds differ in size and are genetically differentiated. We examined the correlation between male and female morphometrics for 17 known pairs and quantified the relatedness of 20 pairs from this admixed population. In addition, we compared the genetic similarity and makeup of 106 Ponui Island birds, including 23 known pairs, to birds representing the source populations for the original translocations. We found no evidence for size-assortative mating. On the contrary, genomic SNP data suggested that kiwi of one feather did not flock together, meaning that mate choice resulted in pairing between individuals that were less related than expected by random chance. Furthermore, the birds in the current Ponui Island population were found to fall along a gradient of genomic composition consistent with non-clustered representation of the two parental genomes. These findings indicate potential for successful genetic rescue in future Apteryx reinforcement translocations, a potential that is currently under utilised due to restrictive translocation policies. In light of our findings, we suggest that reconsideration of these policies could render great benefits for the future diversity of this iconic genus in New Zealand.

Size-assortative mating in explosive breeders: a case study of adaptive male mate choice in anurans

Abstract Exploration of size-assortative mating (SAM) in animals has led to a near consensus that it arises through constraints in choice, such as preference for large females combined with a large male advantage during intrasexual competition. Although such ‘apparent’ SAM is well explored, whether SAM arises because of specific preferences for size-matched mates has been less thoroughly considered. We tested for ‘preference-based’ SAM in an explosively breeding frog (Rana sylvatica), quantifying how male and female sizes affected fertilization and if males preferred size-matched females. We found that size mismatch severely reduced fertilization. Furthermore, males preferred size-matched, not larger, females in mate choice trials. Because males that mated with much larger females fertilized fewer eggs overall than they would have with size-matched females, male preference for size-matched females may be adaptive. Our results expand understanding of the mechanisms underlying SAM, suggesting that multiple mechanisms may simultaneously cause size-assortative mating patterns to emerge.

Reproductive ecology and territorial behavior of Boana goiana (Anura: Hylidae), a gladiator frog from the Brazilian Cerrado

Anuran males and females adopt different reproductive and behavioral strategies in different contexts. We investigated the reproductive ecology and territorial behavior of the treefrog Boana goiana (B. Lutz, 1968) from the Brazilian Cerrado. We hypothesized that competitor density/proximity would increase the behavioral responses of B. goiana males, and that mating would be assortative. We also tested if the number of eggs correlates with female size and if there is a trade-off between clutch size and egg size. We conducted two territoriality experiments to test the effects of male size, competitor proximity and competitor density. Larger males called more in the presence of a second male. In the second experiment, the largest males emitted more calls and the distance to the nearest male increased as resident males called more. In both experiments, the number of calls was influenced by either male size or spacing between males. Some males behaved as satellites, probably to avoid fights. Our analyses indicate that females choose males with similar sizes to their own, corroborating our hypothesis of size-assortative mating. We found no relationships between female size and clutch size/volume, and between egg size and number of eggs per clutch. We also report multiple spawning for this species. The low incidence of physical combats and the spacing pattern indicate that this species relies almost solely on calls to resolve contests, which could be explained by low motivation, or simply because males avoid combats to decrease injury risks. Thus, acoustic or even multimodal communication seems crucial for social interactions of B. goiana.

Mating system and protandrous sex change in “Magochi” Platycephalus sp. 2 (Platycephalidae)

In this study we investigated sex change and the mating system of a platycephalid fish, Platycephalus sp. 2 (Japanese name: Magochi), found in Japanese waters. Among 202 specimens from Tateyama, Chiba, Japan, obtained in 2014 and 2015, histological observation of the gonads distinguished 103 females, 97 males and two hermaphrodites (individual with both ovarian and testicular parts). Females were significantly larger than males. In a rearing experiment conducted with six males and three females, only the smallest male changed sex to female, thereby revealing protandry in this species. In field observations, seven males and eight females appeared throughout the study period. During pairing, a male would follow a female on the sandy bottom. The mating system of this species has been regarded as random mating based on temporal pair bonding, no territoriality, and no size-assortative mating, which correspond well with the size-advantage model. In a previous study Platycephalus sp. 2 was regarded as a gonochore, with a higher growth rate for females than males. Thus, females should be larger than males. In the present study, there was no significant difference in age between the sexes, as estimated from the number of opaque zones in sagitta otoliths. Overall, these findings indicate that there are primary females and males without sex change. Thus, protandrous and gonochoristic individuals may co-exist in populations of Platycephalus sp. 2.

An integrated approach to infer the mechanisms of mate choice for size

Size-assortative mating and sexual selection on size are common across species. Since both may be a result of mate choice, mate choice based on size should also be a widespread process. This behaviour is, however, rarely studied directly and thus the biological causes that determine size-based mate choice are poorly understood. To address this, we studied the size-based mate choice in an intertidal snail, Echinolittorina malaccana, that has been used as a model to understand this process. Previous studies, assuming a quantitative Gaussian mating preference function, have inferred that mate choice in this snail is caused by a size similarity mechanism (males prefer to mate with females slightly larger than themselves). To further test and quantify this proposed mechanism, we conducted mate choice experiments with alternative designs (single, male and multiple choice) in the laboratory and compared the results to mate choice data observed in natural populations. This integrated approach allowed us to elucidate the mechanism of mate choice by evaluating alternative mating models that best fitted the observed data of various designs. Results confirmed the similarity-based mechanism but showed deviations at extreme size classes. The single choice design indicated that mate choice was exercised during one-on-one male–female interactions, but the strength of mate choice increased with the presence of additional individuals (males in the male choice design, and both males and females in the multiple-choice design). Multiple-choice experiments are, therefore, the most valuable and useful design to infer how males choose mates in the wild, as they best mimic the natural scenario and the results are the most similar to those observed in natural populations. To elucidate the mechanisms causing this male choice for particular female sizes, the next steps are to identify the genetic basis as well as potential physiological benefits associated with choosing slightly larger females.