Size-assortative Mating Research Articles

Size-assortative mating and arm loss in the wild shallow-water octopus Abdopus sp. (Cephalopoda: Octopodidae)

ABSTRACTThe ability to autotomise and regenerate body parts as a means of escaping predation or renewing damaged tissues has evolved in several animal groups. Some octopuses belonging to the genus Abdopus are known for a high frequency of arm loss and their capacity to regenerate arms. I investigated the mating behaviour and the incidence of arm loss in a shallow-water Abdopus sp. in the wild. Mating occurred at night and at low tide. The male octopus tracked and copulated by repeatedly stretching the hectocotylised arm into the female’s mantle cavity during mating bouts, mostly while the mated female actively moved to forage at the same time. Relatively smaller males adopted sneaking tactics in which they stayed around a mating pair and sometimes gained opportunistic copulations with the female. A significant positive relationship between the body weights of the male and female in each mating pair was detected. Moreover, the proportion of sneaker males that had lost at least one arm prior to mating and their number of lost arms per individual were significantly higher than those of paired males. These results suggest that male arm loss in Abdopus sp. may directly or indirectly influence male competition over females and mating success, resulting in a positive size association between members of a mating pair. This finding would be important when considering the life-history strategies of octopuses.

Reproductive biology and egg production of Hyalella longistila (Faxon, 1876) (Amphipoda: Hyalellidae), a freshwater amphipod in southeastern Brazil

The reproductive biology of Hyalella longistila (Faxon, 1876), a freshwater amphipod found in streams in southeastern Brazil, was investigated. We assessed body-size relationships between paired and unpaired males and females as well as the egg stage, fecundity, relationship between number of eggs and their stage, number of eggs and female size, and pairing and reproductive success. A positive Spearman rank correlation was observed between the cephalothorax length (CL) of paired males and females. The mean CL of paired males and females was positively related, indicating a size-assortative mating. The mean fecundity was 12.88 ± 2.00 eggs per female, with peaks in August 2012 and September 2012, with 16.82 and 15.08 eggs per female, respectively. The developmental stage and the number of eggs found on the brood pouch were negatively correlated, indicating that as the embryo developed, fewer eggs remained in the egg pouch (marsupium). Higher values for male pairing success were observed for size classes between 0.59 and 0.67 mm, and both sexes presented higher values for reproductive success between 0.63 and 0.68 mm. Our results could support conservation practices or even studies using such amphipods as model organisms.

Male mate preference and size-assortative mating in convict cichlids: A role for female aggression?

Many monogamous species demonstrate size-assortative mating patterns within natural populations. To better understand the role of intersexual selection in this process, we examined the effect of male preference for female body size in the convict cichlid (Amatitlania siquia). We provided males with a choice between females that differed in size, relative to each other and in relation to the focal male. Based on previous work, we expected males to prefer the largest available female mates across all treatments. Surprisingly, males spent more time near the smaller of two available females, but only when the other female was larger than the male. Additionally, males spent little time with either of two potential female mates when both females were larger than the male. We hypothesized that while males might prefer the largest of available females, female behavior might limit males from acting on this preference. To test this, males were force paired with a smaller or larger female. Pair formation only occurred when the female was smaller than the male, and females that were larger than their male counterparts showed significantly more aggression when compared to smaller females. Together, these data suggest that in the absence of intrasexual competition, male mate preference for large females in convict cichlids might be limited by female aggression.

Sampling scale can cause bias in positive assortative mating estimates: evidence from two intertidal snails

Assortative mating in the wild is commonly estimated by correlating between traits in mating pairs (e.g. the size of males and females). Unfortunately, such an approach may suffer from considerable sampling bias when the distribution of different expressions of a trait in the wild is nonrandom (e.g. when segregation of different size classes of individuals occurs in different microhabitats or areas). Consequently, any observed trait correlation in the wild can be an artefact of pooling heterogeneous samples of mating pairs from different microhabitats or areas rather than true nonrandom matings. This bias in estimating trait correlations as a result of sampling scale is termed the scale-of-choice effect (SCE). In the present study, we use two intertidal littorinid species from Hong Kong to show how the SCE can bias size-assortative mating estimates from mating pairs captured in the wild, empirically demonstrating the influence of this effect on measures of positive assortative mating. This finding cautions that studies overlooking the SCE may have misinterpreted the magnitude and the cause of assortative mating, and we provide a new analytical approach for protecting against this potential bias in future studies.

Dispersal and mating in a size-dimorphic ant

Alternative reproductive strategies linked to size are often connected with differences in dispersal and mating behaviour. Temporal and spatial mating isolation of different sized individuals can lead to size-assortative mating, which may maintain intraspecific size polymorphism. In the size-dimorphic ant Myrmica ruginodis, newly produced sexuals of the larger macrogyne morph join nuptial flights to mate, but mating behaviour of the smaller microgyna females remains elusive and even less is known about males. In this study, we examined the differences in dispersal strategies of the two size morphs as a possible mechanism causing reproductive isolation between them. We developed a new method for sampling sexuals dispersing from their nests, and sampled also mating couples and random males from a nearby mating site. We studied propensity and timing of dispersal and assortative mating of the size morphs based on their expected and observed participation in the nuptial flight. Our study shows that predominantly, macrogynes participate in the nuptial flight, while microgynes stay close to their natal nest. However, dispersal is not associated with male size, and larger males, preferred by all females, are available for both morphs. Size-assortative mating, mainly caused by macrogynes mating with large males, exists at the nuptial flight and should promote mating isolation of the size morphs, but microgynes counterbalance this to a certain degree by mating with large males. Size-associated alternative dispersal strategies in size polymorphic species provide insight into the mechanisms enhancing genetic divergence, a potential early step in the speciation process. We studied the association between size and dispersal in the size-dimorphic ant Myrmica ruginodis, to evaluate potential premating isolation between the morphs. To date, there is a shortage of studies on mating and dispersal behaviour of ants in natural settings. We captured dispersing sexuals from their natal nests, separating those dispersing on foot or by flight—a technique never applied to dispersing ants before—and assessed mate selection at the mating swarm. We found that dispersal propensity depends on the size of the females (but not males) so that the larger macrogyne morph joins a nuptial flight and the smaller microgyne morph rarely does. Furthermore, macrogyna females actively choose to mate with large males causing size-assortative mating, creating partial mating isolation between the size morphs and opening a possibility for genetic divergence between the morphs.

Assortative mating in a fiddler crab

Non-random mating, particularly positive size-assortative mating, is common and can have major evolutionary consequences. The causes of size-assortative mating, however, are rarely examined. Here we explore the possibility of sexual selection, mate availability and mating constraints causing the strong correlation between male and female sizes in the fiddler crabUca mjoebergi. We show that the full size range of males is available to females throughout the mating period, so mate availability is unlikely to cause size-assortative mating in this species. We also show that mechanical constraints do not prevent females from entering the burrows or mating with the full size range of males. We suggest that the strong size assortative mating that we observed throughout the mating cycle is driven by sexual selection. Both males and females prefer large partners. The benefit to mating with large partners needs further investigation.

The effect of size-assortative mating on fertilization success of the common toad (Bufo bufo)

Although size-assortative mating (SAM) is a frequently studied phenomenon in anurans, its effect on fitness rarely has been evaluated. Using a controlled experiment, we assessed the presence of SAM in the common toad,Bufo bufo, and evaluated the effect on fertilization success of male-female size (snout-vent length, SVL) difference and ratio in mated pairs. Even though the toads paired non-randomly with respect to size, the difference and ratio of female and male SVL in pairs had no significant effect on fertilization success. Our findings and the majority of other studies suggest that the commonly accepted idea that SAM serves to maximize fitness may not be completely accurate. The statistical power and effect size in these studies are often poor, however, and the methods used are inconsistent. We conclude that more manipulative experiments with appropriate sample sizes are needed to fully understand this phenomenon.

Size-assortative copulation in the simultaneously hermaphroditic pond snail Radix auricularia (Gastropoda: Pulmonata)

To better understand the importance of body size in hermaphroditic mating, we studied the mating behaviour of the pond snail Radix auricularia. This simultaneous hermaphrodite exhibits a positive correlation between body size and egg production and provides weak evidence for size-assortative mating. In addition to this mating pattern, the partner performing the male role was significantly smaller than the mounted partner, performing the female role. In this study, two laboratory experiments were done to test whether body size has (1) an effect on mate choice of the sperm donor, and (2) poses mechanical constraints. These snails did not display physical limitation in mating with a much larger or smaller partner. In the mate choice experiment, we found no evidence for significant preferences for large partners. Therefore, mate choice of sperm donor and mechanical constraints are not likely to contribute to the formation of size-assortatively mating pairs in this species, which contributes to understanding about how these processes work in hermaphrodites.

Fire millipedes obey the female sooner norm in cross mating Centrobolus (Myriapoda)

Sexual mediation of copulation duration is reviewed in arthropods. Copulation duration and ejaculate volume were recorded in cross mating Centrobolus to test species' mediation differences. Conspecific C. inscriptus copulation durations were long (170.0 ± 49.3 min, n = 46), C. annulatus copulation durations were short (39.4 ± 18.6 min, n = 8), while mating between heterospecifics (C. inscriptus X C. annulatus) had the shortest copulation durations (12.4 ± 10.6 min, n = 6); when re-mating interval, sexual size dimorphism, sex ratio and ejaculate volume were controlled. ⺠(the female sooner norm) suggests size-assortative mating without a size based preference in these mate-guarding millipedes.

Fire millipedes obey the female sooner norm in cross mating <em>Centrobolus</em> (Myriapoda)

Sexual mediation of copulation duration is reviewed in arthropods. Copulation duration and ejaculate volume were recorded in cross mating Centrobolus to test species' mediation differences. Conspecific C. inscriptus copulation durations were long (170.0 ± 49.3 min, n = 46), C. annulatus copulation durations were short (39.4 ± 18.6 min, n = 8), while mating between heterospecifics (C. inscriptus X C. annulatus) had the shortest copulation durations (12.4 ± 10.6 min, n = 6); when re-mating interval, sexual size dimorphism, sex ratio and ejaculate volume were controlled. ⺠(the female sooner norm) suggests size-assortative mating without a size based preference in these mate-guarding millipedes.

Low reproductive isolation and highly variable levels of gene flow reveal limited progress towards speciation between European river and brook lampreys.

Ecologically based divergent selection is a factor that could drive reproductive isolation even in the presence of gene flow. Population pairs arrayed along a continuum of divergence provide a good opportunity to address this issue. Here, we used a combination of mating trials, experimental crosses and population genetic analyses to investigate the evolution of reproductive isolation between two closely related species of lampreys with distinct life histories. We used microsatellite markers to genotype over 1000 individuals of the migratory parasitic river lamprey (Lampetra fluviatilis) and freshwater-resident nonparasitic brook lamprey (Lampetra planeri) distributed in 10 sympatric and parapatric population pairs in France. Mating trials, parentage analyses and artificial fertilizations demonstrated a low level of reproductive isolation between species even though size-assortative mating may contribute to isolation. Most parapatric population pairs were strongly differentiated due to the joint effects of geographic distance and barriers to migration. In contrast, we found variable levels of gene flow between sympatric populations ranging from panmixia to moderate differentiation, which indicates a gradient of divergence with some population pairs that may correspond to alternative morphs or ecotypes of a single species and others that remain partially isolated. Ecologically based divergent selection may explain these variable levels of divergence among sympatric population pairs, but incomplete genome swamping following secondary contact could have also played a role. Overall, this study illustrates how highly differentiated phenotypes can be maintained despite high levels of gene flow that limit the progress towards speciation.

Size-assortative mating in simultaneous hermaphrodites: an experimental test and a meta-analysis

Assortative mating by size has been argued to be widespread in the animal kingdom. However, the strength of size-assortative mating is known to vary considerably between species and the underlying mechanisms promoting this inter-specific variation remain largely unexplored. Size-assortative mating has been proposed to be particularly strong in simultaneous hermaphrodites, i.e. organisms that produce male and female gametes at the same time. Here, we build on this hypothesis by arguing that size-assortative mating mediated by sexual selection is generally stronger in reciprocally mating hermaphrodites compared with unilaterally mating species and separate-sexed organisms. We report a series of empirical tests suggesting that size-assortative mating in the unilaterally copulating freshwater snail Physa acuta is caused by spatial clustering of similar-sized individuals and not by mate choice. In addition, we present a meta-analysis testing, for the first time, the hypothesis that sexual selection-mediated size-assortative mating is stronger in reciprocally copulating simultaneous hermaphrodites. Overall, we found significant size-assortative mating across 18 tested species and substantial inter-specific variation. Importantly, part of this variation can be explained by mating type, providing support for the hypothesis that size-assortative mating is stronger in reciprocally mating hermaphrodites compared with unilaterally mating species. We highlight potential pitfalls when testing for sexual selection-mediated size-assortative mating and discuss the need for more experimental and comparative approaches in order to resolve the observed variation in the strength of size-assortative mating among species.

Reproductive ecology of Sichuan digging frogs (Microhylidae: Kaloula rugifera )

Abstract. We investigated the reproductive ecology of Sichuan digging frogs (Microhylidae: Kaloula rugifera ) in Mianyang, China during the wet season of 2013. Male Sichuan digging frogs first appear at temporary ponds following the first heavy rain of the wet season and initiate calling. Females arrive at ponds shortly after males. Male frogs chorus extensively throughout the wet season during the evenings and nights following rainstorms. Female frogs leave the pond after laying eggs, and likely only lay one clutch annually. Amplexus lasted up to three hours. Females were larger than males in terms of body size, but we found no evidence of size-assortative mating. Clutch size varied from 920 to 2200 eggs, with egg diameter ranging from 1.33 to 1.93 mm. Larger female frogs laid more eggs, and there was no correlation between egg number and egg size. Tadpoles hatched from eggs within 18-20 hours of oviposition, and grew for 30-40 days before complete metamorphosis occurred. The initial body length of tadpoles ranged from 3-5 mm snout-vent length. Growth was fastest immediately after hatching, and declined asymptotically with increasing tadpole body size. Overall, Sichuan digging frogs have a breeding biology characterized by strong male-male competition with prolonged breeding coinciding with the annual wet season. Key w ords . Breeding ecology; Kaloula rugifera ; life history; mating system

The effect of sex ratio on size-assortative mating in two explosively breeding anurans

Size-assortative mating (SAM) is a widespread phenomenon related to individual fitness. In our study, we examined: (i) the appearance of SAM, and (ii) the effect of sex ratio on intensity of SAM in wild populations of two explosively breeding anurans, common frogs,Rana temporaria, and common toads,Bufo bufo. Despite a higher male-biased operational sex ratio (OSR) in toads, the body lengths of the paired males and females were significantly correlated only in frogs. Increasing male-male competition, assessed via the OSR, resulted in a stronger correlation also in frogs only. Thus, great variability in the presence and intensity of SAM has been observed within both studied species.

Assortative mating with respect to size in the simultaneously hermaphroditic land snail Bradybaena pellucida

There is ample evidence that body size is tightly linked with quality and fecundity of individuals in simultaneously hermaphroditic species. Especially in simultaneous hermaphrodites with reciprocal intromission, it has been thought that assortative mating with respect to body size occurs. Contrary to this expectation, studies on size-assortative mating in such simultaneous hermaphrodites have shown inconsistent mating patterns. It is important to gain an understanding of mating patterns with regard to body size in various hermaphroditic species as a first step for considering the causes for such species differences. In the present study, we examined the relationship between body size and fecundity in the simultaneously hermaphroditic land snail Bradybaena pellucida. We also tested size-assortative mating patterns in this species under a laboratory condition. The results of the experiments showed that life-span fecundity was positively correlated with body size and that the snails tended to choose mating partners of similar body size. As we had predicted, size also affected the mating pattern and assortative mating in simultaneous hermaphrodites with reciprocal intromission. This size-assortative mating pattern seems to result from sexual selection processes, not from mechanical constraints and habitat heterogeneity between large and small individuals.

Female polyandry and size-assortative mating in isolated local populations of the Japanese common toadBufo japonicus

In anurans, female polyandry under male harassment is distributed across taxa because of external aquatic fertilization. According to the sexual selection theory, male–male competition for access to females is affected by the operational sex ratio (OSR) and population density. The Japanese common toad, Bufo japonicus, is widespread in mainland Japan, and like the European common toad, B. bufo, it engages in explosive breeding. In this study, we observed the breeding behaviour of B. japonicus in isolated local populations for over four years in two breeding ponds with different population sizes and densities: large-low (L-pond) and small-high (S-pond). We analysed the relative polyandry ratio in egg clutches laid by females and estimated the size-assortative mating pattern to be an indicator of male–male competition in the two ponds. Both ponds tended to exhibit a size-assortative mating pattern; however, the frequency of polyandry was different in the two ponds (L-pond = 20% and S-pond = 90%). Our results showed that polyandry could occur without multiple amplexus with a high population density, i.e. eggs were often fertilized by free-swimming sperm in the small shallow pond. We propose that high female polyandry ratios without continuous male harassment are generated because of a male-biased OSR and a high population density in the small pond. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 236–242.

Implications of female body-size variation for the reproductive ecology of an anuran amphibian

Animal body size has profound ecological and evolutionary implications for lifespan, access to resources and reproduction. Body size, though, is phenotypically plastic and may be strongly correlated with both fecundity and mate choice. Therefore, temporal variation in body size within populations may correspondingly alter a population’s reproductive ecology over time and, if not taken into account, bias our perception of it. Male Fowler’s Toads (Anaxyrus fowleri) at Long Point, Ontario, have previously been shown to vary in average body size by as much as 18% over a period of 23 years. Here I demonstrate that the females also fluctuate in body size in correlation with their abundance and in synchrony with the males. Clutch size, which averaged 4443 eggs per clutch among 34 females, varied with individual female body size irrespective of temporal variation in average body size or abundance. Body size measurements of females and males in 75 mating pairs over 24 years were, overall, significantly correlated. However, when year-to-year variations in mean body size were accounted for statistically, this apparent evidence of size-assortative mating was negated. These results indicate that abundance, body size and fecundity likely are interrelated aspects of population regulation and that many purported observations of size-assortative mating in animals may need to be re-evaluated.

Size-Dependent Male Mate Preference and its Association with Size-Assortative Mating in a Mangrove Snail,Littoraria ardouiniana

In many animals, body size plays a crucial role in mating success in the context of competition and preference for mates. Increasing evidence has shown that male mate preference can be size-dependent and, therefore, an important driver of size-assortative mating. To test this theory, mate choice experiments were performed during the three consecutive stages of mating behaviour, namely trail following, shell mounting and copulation, in the dioecious mangrove snail, Littoraria ardouiniana. These experiments identified two possible forms of size-dependent male mate preference which could contribute to the formation of size-assortative mating in these snails. Firstly, whereas small males were unselective, large males were selective and preferred to follow mucus trails laid by large females. Alternatively, the results can also be interpreted as all males were selective and adopted a mating strategy of selecting females similar to, or larger than, their own sizes. Both small and large males also copulated for longer with large than with small females, and this was more pronounced in large males. When two males encountered a female, they engaged in physical aggression, with the larger male excluding the smaller male from copulating with the female. This study, therefore, demonstrated that size-dependent male mate preference may, along with male–male competition, play an important role in driving size-assortative mating in these mangrove snails, and this may also be the case in other species that exhibit male mate choice.

Do Cory’s Shearwaters Calonectris borealis choose mates based on size?

Many bird species rely on visual cues for mate choice, including those provided by body size or by the size of a body appendage. Mate choice based on size may lead to size-assortative mating, which may in turn have consequences for reproductive performance. In this study, we examined whether body size influences mate choice decisions of Cory's Shearwater Calonectris borealis ,a seabird with nocturnal activity on land. We found evidence of negative assortative mating for bill morphology (nostril height and bill length), but found no assortative mating according to body mass, tarsus length, and wing length. In addition, we investigated the influence of negative assor- tative mating and individual body size-related traits on reproductive performance, i.e., laying date and breeding success. We found that laying date and breeding success were not correlated with the extent of assortative mating, i.e., the assortatively mating breeders did not lay earlier and were not more likely to breed successfully. However, we found that heavier females (body mass measured during pre-laying) laid later in the season. We discuss these results in the light of possible effects of acoustic signals and for- aging strategies on the mating pattern. Furthermore, we suggest that some female attributes may be crucial for breeding success in Cory's Shearwaters.

‘Prudent habitat choice’: a novel mechanism of size‐assortative mating

Assortative mating, an ubiquitous form of nonrandom mating, strongly impacts Darwinian fitness and can drive biological diversification. Despite its ecological and evolutionary importance, the behavioural processes underlying assortative mating are often unknown, and in particular, mechanisms not involving mate choice have been largely ignored so far. Here, we propose that assortative mating can arise from 'prudent habitat choice', a general mechanism that acts under natural selection, and that it can occur despite a complete mixing of phenotypes. We show that in the cichlid Eretmodus cyanostictus size-assortative mating ensues, because individuals of weaker competitive ability ignore high-quality but strongly competed habitat patches. Previous studies showed that in E.cyanostictus, size-based mate preferences are absent. By field and laboratory experiments, here we showed that (i) habitat quality and body size are correlated in this species; (ii) territories with more stone cover are preferred by both sexes in the absence of competition; and (iii) smaller fish prudently occupy vacant territories of worse quality than do larger fish. Prudent habitat choice is likely to be a widespread mechanism of assortative mating, as both preferences for and dominance-based access to high-quality habitats are generic phenomena in animals.