Individual identity is communicated through multiple pathways in male rock hyrax (<Emphasis Type="Italic">Procavia capensis</Emphasis>) songs

Communicating individual identity is essential for stable social systems. It is assumed that there are benefits for both senders and receivers to provide and discriminate identity cues. In this study, we investigate the possible routes senders use to acoustically broadcast their individual identity. Using discriminant function analysis of temporal and spectral acoustic measurements and analysis of song-element order, we explore the means male rock hyrax (Procavia capensis) singers utilize vocalization to express individual identity. Despite the fact that males use only three song elements, the pattern of acoustic characteristics, their temporal and frequency attributes vary according to the identity of singer. We show that in hyrax, individuality is expressed by highly variable, complex signals that are not condition dependent and are stable over years in singers that did not alter their spatial position. We also show that individuality signals are not linked to relatedness or to geographic location. The ability to discriminate individuals from vocal signatures needs to be further tested using controlled playback experiments.     

Seasonal variation in the duetting behaviour of rufous-and-white wrens (<Emphasis Type="Italic">Thryothorus rufalbus</Emphasis>)

Seasonal variation in animal signalling behaviour has been well documented and has contributed much to our understanding of male signals. In contrast, we know little about seasonal variation in female signals or signals produced jointly by males and females, such as the vocal duets of birds. Here, we examine how singing behaviour changes in relation to time of year and breeding stage in rufous-and-white wrens (Thryothorus rufalbus), neotropical songbirds where both males and females sing and where breeding partners coordinate songs to produce vocal duets. We recorded a colour-marked population of birds over an extended time period encompassing multiple breeding stages. Across all time frames and breeding stages, males sang at higher rates than females and male solos were more common than duets or female solos. Males and females showed divergent seasonal patterns of singing. Females sang more often early in the year, during the pre-breeding season, and female song tapered off as the breeding season progressed. Duetting followed a parallel pattern, which resulted from females showing less duet responsiveness to their partner’s songs later in the year. Male independent song rate peaked at the onset of the rainy season – a time when females become fertile – and males showed the highest level of duet responsiveness during this period. Our results suggest that early in the year, duets appear to be cooperative displays, functioning in joint territory defence and/or the coordination of breeding activities. When females are fertile, however, increased duet responsiveness by males is consistent with mate or paternity guarding.     

Cofoundress relatedness and group productivity in colonies of social <Emphasis Type="Italic">Dunatothrips</Emphasis> (Insecta: Thysanoptera) on Australian <Emphasis Type="Italic">Acacia</Emphasis>

Facultative joint colony founding by social insects provides opportunities to analyze the roles of genetic and ecological factors in the evolution of cooperation. Although cooperative nesting is observed in range of social insect taxa, the most detailed studies of this behavior have been conducted with Hymenoptera (ants, bees, and wasps). Here, we show that foundress associations in the haplodiploid social thrips Dunatothrips aneurae (Insecta: Thysanoptera) are most often comprised of close relatives (sisters), though groups with unrelated foundresses are also found. Associations among relatives appear to be facilitated by limited female dispersal, which results in viscous population structure. In addition, we found that per capita productivity declined with increasing group size, sex ratios were female-biased, and some female offspring apparently remained in their natal domicile for some time following eclosion. D. aneurae thus exhibits a suite of similarities with eusocial Hymenoptera, providing evidence for the convergent evolution of associated social and life-history traits in Hymenoptera and Thysanoptera.     

<Emphasis Type="Italic">Juniperus oxycedrus</Emphasis> ssp.<Emphasis Type="Italic">macrocarpa</Emphasis> in SW Spain: Ecology and conservation problems

Woodlands of the Mediterranean speciesJuniperus oxycedrus ssp.macrocarpa (maritime juniper) are both vulnerable and ecologically important. Their ecology and biological status along the SW coast of Spain are not well known; this, the first major study of these juniper populations is a basis for future research and restoration policies. These communities are subject to harsh conditions, the plant composition being controlled by several factors at different scales. On a large scale, climate and soil texture play an important role in controlling the soil water availability to plants, and in separating xerophytic from mesic communities. On a small scale, coastal physiography, and substrate composition are related to differences in the floristic composition. Coastal plantations modify environmental conditions, such as sand mobility and salt spray deposition, inducing important changes in plant communities. The population of maritime juniper on this coast was estimated in ca. 25 000 individuals, of which 93.6% are concentrated in three locations. Large proportions of young individuals were found in extensive and protected populations. Howerver, adult individuals dominated the smaller populations located under pine plantations. This limitation of recruitment may be imposed by several factors. A male biased ratio was detected on the southern coast of Cádiz, which I hypothesize is due to the lower cost of pollen production in a stressful habitat. Preservation of suitable habitats, the recovery of abandoned pine plantations, and the connection between juniper populations, seem to be important requisites for the conservation of maritime juniper in the southwestern coast of Spain.     

Alternative mating tactics in the Italian treefrog, <Emphasis Type="Italic">Hyla intermedia</Emphasis>

As in many lekking anurans, Italian treefrog males use two mating tactics: they can attract females by calling vigorously or be satellites, that is, they can remain silent in proximity of a calling male and try to intercept females attracted by their neighbour. We investigated the factors that affected the expression of this mating tactic. Consistent with the conditional mating tactic hypothesis, satellites were smaller than average and smaller than their parasitised calling males. They spent a larger-than-average number of nights at the breeding site, where most of them were also observed calling. Moreover, satellites showed lower call rates and lower mating success than those of males they parasitise but not lower than those of males they did not parasitise. Overall, these results, together with those derived from the analyses of the seasonal and spatial distribution of males, provide evidence for a non-random association between satellites and calling males and are consistent with the hypothesis that satellites have spectral and temporal acoustic preferences that parallel those of females. By adopting the less-successful satellite mating tactic, competitively inferior males can nevertheless maximise their potential reproductive fitness by sexually parasitising the most attractive chorusing males.     

Association networks in spider monkeys (<Emphasis Type="Italic">Ateles geoffroyi</Emphasis>)

We use two novel techniques to analyze association patterns in a group of wild spider monkeys (Ateles geoffroyi) studied continuously for 8 years. Permutation tests identified association rates higher or lower than chance expectation, indicating active processes of companionship and avoidance as opposed to passive aggregation. Network graphs represented individual adults as nodes and their association rates as weighted edges. Strength and eigenvector centrality (a measure of how strongly linked an individual is to other strongly linked individuals) were used to quantify the particular role of individuals in determining the network's structure. Female–female dyads showed higher association rates than any other type of dyad, but permutation tests revealed that these associations cannot be distinguished from random aggregation. Females formed tightly linked clusters that were stable over time, with the exception of immigrant females who showed little association with any adult in the group. Eigenvector centrality was higher for females than for males. Adult males were associated mostly among them, and although their strength of association with others was lower than that of females, their association rates revealed a process of active companionship. Female–male bonds were weaker than those between same-sex pairs, with the exception of those involving young male adults, who by virtue of their strong connections both with female and male adults, appear as temporary brokers between the female and male clusters of the network. This analytical framework can serve to develop a more complete explanation of social structure in species with high levels of fission–fusion dynamics. This contribution is part of the special issue “Social Networks: new perspectives” (Guest Editors: J. Krause, D. Lusseau and R. James)     

Auditory defence in the peacock butterfly (<Emphasis Type="Italic">Inachis io</Emphasis>) against mice (<Emphasis Type="Italic">Apodemus flavicollis</Emphasis> and <Emphasis Type="Italic">A. sylvaticus</Emphasis>)

Morphological and behavioural traits can serve as anti-predator defence either by reducing detection or recognition risks, or by thwarting initiated attacks. The latter defence is secondary and often involves a ‘startle display’ comprising a sudden release of signals targeting more than one sensory modality. A suggested candidate for employing a multimodal defence is the peacock butterfly, Inachis io, which, by wing-flicking suddenly, produces sonic and ultrasonic sounds and displays four large eyespots when attacked. The eyespots make small birds retreat, but whether the sounds produced thwart predator attacks is largely unknown. Peacocks hibernate as adults in dark wintering sites and employ their secondary defence upon encounter with small rodent predators during this period. In this study, we staged predator–prey encounters in complete darkness in the laboratory between wild mice, Apodemus flavicollis and Apodemus sylvaticus, and peacocks which had their sound production intact or disabled. Results show that mice were more likely to flee from sound-producing butterflies than from butterflies which had their sound production disabled. Our study presents experimental evidence that the peacock butterfly truly employs a multimodal defence with different traits targeting different predator groups; the eyespots target birds and the sound production targets small rodent predators.     

Genetic analysis of tuna populations,<Emphasis Type="Italic"> Thunnus thynnus thynnus</Emphasis> and<Emphasis Type="Italic"> T</Emphasis>.<Emphasis Type="Italic"> alalunga</Emphasis>

The genetic population structures of Atlantic northern bluefin tuna ( Thunnus thynnus thynnus) and albacore ( T. alalunga) were examined using allozyme analysis. A total of 822 Atlantic northern bluefin tuna from 18 different samples (16 Mediterranean, 1 East Atlantic, 1 West Atlantic) and 188 albacore from 5 samples (4 Mediterranean, 1 East Atlantic) were surveyed for genetic variation in 37 loci. Polymorphism and heterozygosity reveal a moderate level of genetic variability, with only two highly polymorphic loci in both Atlantic northern bluefin tuna ( FH* and SOD- 1*) and albacore ( GPI- 3* and XDH*). The level of population differentiation found for Atlantic northern bluefin tuna and albacore fits the pattern that has generally been observed in tunas, with genetic differences on a broad rather than a more local scale. For Atlantic northern bluefin tuna, no spatial or temporal genetic heterogeneity was observed within the Mediterranean Sea or between the East Atlantic and Mediterranean, indicating the existence of a single genetic grouping on the eastern side of the Atlantic Ocean. Very limited genetic differentiation was found between West Atlantic and East Atlantic/Mediterranean northern bluefin tuna, mainly due to an inversion of SOD- 1* allele frequencies. Regarding albacore, no genetic heterogeneity was observed within the Mediterranean Sea or between Mediterranean and Azores samples, suggesting the existence of a single gene pool in this area.     

Predation on egg capsules of <Emphasis Type="Italic">Zidona</Emphasis><Emphasis Type="Italic">dufresnei</Emphasis> (Volutidae): ecological implications

Among the diverse patterns of energy allocation to the offspring of gastropods, the presence of egg capsules to protect embryos is common. Females of the edible snail Zidona dufresnei attach egg capsules to hard substrates in shallow Argentine Patagonian waters (40°45′S, 64°56′W) during spring-summer. Embryonic development takes about 30 days at 22°C. In this study, three likely capsule predator species and the marks left by each on egg capsule walls were identified in laboratory experiments in February 2010. Abundances of predators and egg capsules with evidence of predation were assessed in the field in the summers of 2010 and 2011. Under laboratory conditions (N = 10 replicates per treatment and control), the predation rate by the chiton Chaetopleura isabellei was the highest (up to 90%), followed by the gastropod Tegula patagonica and the crab Neohelice granulata (~20% each). Nearly 60% of 41 capsules found in the field showed signs of predation. According to the marks identified in the laboratory, C. isabellei was responsible for 79% of this predation, and T. patagonica for the rest. Predation appears to be important during the encapsulated early life and could be an agent for selecting for resistant capsule walls and a relatively shorter development time.     

Conflict-involvement of male guinea pigs (<Emphasis Type="Italic">Cavia aperea</Emphasis> f. <Emphasis Type="Italic">porcellus</Emphasis>) as a criterion for partner preference

Polygynous mating systems can inflict considerable costs on males, often causing the emergence of alternative mating strategies to ensure reproductive success. These strategies can lead to different morphs of color, size, or behavior. The present work was done on guinea pigs, a polygynous rodent species. Some males can show a reduced readiness for conflict, perhaps employing an alternative mating strategy. To test this hypothesis, a two-stage experiment was set up, with males (N = 16) and females (N = 16) initially living in isosexual groups. Visual and olfactory contact was possible through a wire mesh. Male agonistic behavior was observed for 15 days, confirming the existence of less-conflict-involved (LCI, N = 7) and more-conflict-involved males (MCI, N = 9). Significant differences were found for a conditional parameter, body mass, and a morphometric one, testis width: LCI consistently surpassed MCI. Hormonally, cortisol was comparable, while testosterone was distinctly higher in MCI. Next, males and females were joined and observed for further 24 days. Males initially lost weight but reached original mass towards the end of the experiment. Testis width patterns were similar. Again, LCI had higher body mass. Cortisol was comparable, but testosterone release 3 days after merging was significantly higher in LCI. Behaviorally, LCI exhibited significantly less male–male aggression and more socio-sexual behavior than MCI. The former were more successful, with 57% capable of accessing females, in contrast to 11% of MCI. The existence of two distinct behavioral phenotypes in guinea pig males suggests that different reproductive strategies may exist in this species.     

Histological investigation of the reproductive cycles of the limpets <Emphasis Type="Italic">Patella </Emphasis><Emphasis Type="Italic">vulgata</Emphasis> and <Emphasis Type="Italic">Patella ulyssiponensis</Emphasis>

This study devised a staging system for, and monitored, the gonad development of the limpet species Patella vulgata and Patella ulyssiponensis on the South West coast of Ireland using histological techniques. Maturation began in the males of both species in January and in the females it began in March. There was no statistical difference in gonad development between sexes and between species. Spawning in the male P. vulgata occurred from September to December 2003 and in September and October 2004. In female P. vulgata spawning occurred from October to December 2003, no spawning of females was observed in 2004. In male P. ulyssiponensis spawning occurred in November and December 2003 and from September 2004 to December 2004. Spawning was observed from November 2003 to January 2004 and in September 2004 in female P. ulyssiponensis. Sex ratios also varied between the species and between months sampled. Nevertheless more males were observed in both species.     

Sperm competition and ejaculate investment in red squirrels (<Emphasis Type="Italic">Tamiasciurus hudsonicus</Emphasis>)

Sperm competition is widespread in mammals and occurs when ejaculates from two or more males compete within the female’s reproductive tract to fertilize the ova. Enlarged testes are associated with sperm competition because they produce sperm, but the accessory glands produce fluids and proteins that are also important for fertilization success. Sperm morphology can also have consequences for fertilization success because of its influence on sperm motility. Red squirrels engage in multiple mating, and thus sperm competition is likely. Here, we assess levels of multiple paternity in a natural population of red squirrels, test the prediction that testis size is correlated with size of the accessory glands and sperm morphometry, and test the prediction that ejaculate investment is condition-dependent. Five of six litters (83%) showed evidence of multiple paternity, indicating that sperm competition is likely to have occurred. Testis size was correlated with the size of all three accessory glands (prostate, seminal vesicle, epididymides), and there was a generally positive relationship between the size of the accessory glands and sperm length. Sperm morphology showed significant variation in size and shape among individual male squirrels. There was no evidence of condition dependence of testis size or the size of the accessory glands, but sperm midpiece length was negatively related to body condition. Further work should include determining the fitness consequence of variation in sperm morphometry, testis size, and accessory gland size, and determining the effects of variation in ejaculate investment on sperm motility.     

Comparative life-history study on sympatric hyperiid amphipods (<Emphasis Type="Italic">Themisto pacifica</Emphasis> and <Emphasis Type="Italic">T</Emphasis>. <Emphasis Type="Italic">japonica</Emphasis>) in the Oyashio region,western North Pacific

Life-history features of the sympatric amphipods Themisto pacifica and T. japonica in the western North Pacific were analyzed based on seasonal field samples collected from July 1996 through July 1998, and data from laboratory rearing experiments. T. pacfica occurred throughout the year, with populations peaking from spring to summer. In contrast, T. japonica were rare from autumn to early winter, but became abundant in late winter to spring. Mature T. pacifica females and juveniles occurred together throughout the year, indicating year-round reproduction. Mature T. japonica females were observed only in spring, and juveniles occurred irregularly in small numbers, suggesting limited, early-spring reproduction in this study area. Size composition analysis of T. pacifica identified a total of eight cohorts over the 2 years of the study. Due to the smaller sample size and rarity of mature females (>9.6 mm) and males (>7.1 mm), cohort analyses of T. japonica were not comparable. Laboratory rearing of specimens at 2°C, 5°C, 8°C and 12°C revealed that a linear equation best expressed body length growth by T. pacifica, while a logistic equation best expressed body length growth by T. japoncia. Combining these laboratory-derived growth patterns with maturity sizes of wild specimens, the minimum and maximum generation times of females at a temperature range of 2–12°C were computed as 32 days (12°C) and 224 days (2°C), respectively, for T. pacifica, and 66 days (12°C) and 358 days (2°C), respectively, for T. japonica. The numbers of eggs or juveniles in females marsupia increased with female body length and ranged from 23 to 64 for T. pacifica and from 152 to 601 for T. japonica. Taking into account the number of mature female instars, lifetime fecundities were estimated as 342 eggs for T. pacifica and 1195 eggs for T. japonica. Possible mechanisms for the coexistence of these two amphipods in the Oyashio region are also discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Heart rate reflects osmostic stress levels in two introduced colonial ascidians <Emphasis Type="Italic">Botryllus schlosseri</Emphasis> and <Emphasis Type="Italic">Botrylloides</Emphasis><Emphasis Type="Italic">violaceus</Emphasis>

The influence of abiotic factors on the establishment and success of invasive species is often difficult to determine for most marine ecosystems. However, examining this relationship is critical for predicting the spread of invasive species and predicting which habitats will be most vulnerable to invasion. Here we examine the mortality and physiological sensitivity to salinity of adult colonies of the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. Adult colonies of each species were exposed to abrupt changes in salinity (5, 10, 15, 20, 25, 30 psu) in the laboratory. Salinity ranges used in the laboratory corresponded with those of the field distributions of B. violaceus and B. schlosseri in the Great Bay Estuary, NH. Heart rate was used as a proxy for health to assess the condition of individual colonies. Heart rates were monitored daily for approximately 2 weeks. Results revealed that both species experienced 100% mortality after 1 day at 5 psu and that their heart rates declined with decreasing salinity. Heart rates of B. schlosseri remained consistent between 15 and 30 psu and slowed at 10 psu. Heart rates of B. violaceus remained constant between 20 and 30 psu, but slowed at 15 psu. These laboratory results corresponded to the distribution of these species in estuaries, indicating salinity is a key factor in the distribution and dominance of B. schlosseri and B. violaceus among coastal and estuarine sites. Furthermore, physiological differences to salinity were found between colonies of B. schlosseri in the Venetian Lagoon and colonies in Portsmouth Harbor, suggesting adaptation to environmental variables.     

Advertisement-call modification,male competition,and female preference in the bird-voiced treefrog <Emphasis Type="Italic">Hyla avivoca</Emphasis>

Senders and receivers influence dynamic characteristics of the signals used for mate attraction over different time scales. On a moment-to-moment basis, interactions among senders competing for a mate influence dynamic characteristics, whereas the preferences of receivers of the opposite gender exert an influence over evolutionary time. We observed and recorded the calling patterns of the bird-voiced treefrog Hyla avivoca to assess how the dynamic characters of calls vary during interactions among groups of males in a chorus. This question was also addressed using playback experiments with males. Playback experiments with females showed how changes in dynamic call properties are likely to affect male mating success. Frogs calling in pairs, groups, or in response to playbacks produced longer calls than did isolated males. During call overlap, males often increased the duration of the silent interval (gaps) between the pulses of their calls so that the pulses of the calls of two neighbors interdigitated. This change resulted in increased variability of pulse rate, a traditionally static acoustic property; however, males also produced high proportions of non-overlapped calls in which variability in pulse rate was low and had species-typical values. Females preferred long calls to short- and average-duration calls, and non-overlapped calls to overlapped calls. Given a choice between pairs of overlapped calls, females preferred pairs in which the proportion of overlap was low and pairs in which the pulses of such calls interdigitated completely. The observed patterns of vocal competition thus reflect the preferences of conspecific females, which have influenced the evolution of the calling behavior of H. avivoca. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cryptic prey colouration increases search time in brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>): effects of learning and body size

Little is known about how cryptic colouration influences prey search in near-surface aquatic habitats, although such knowledge is critical for understanding the adaptive value of colour crypsis as well as the perceptive constraints influencing foraging behaviour in these environments. This study had two main aims: (1) to investigate how background colour matching by prey affects foraging efficiency by brown trout parr and (2) to investigate how foraging ability on cryptic and conspicuous prey is affected by fish size at age (reflecting dominance). We addressed these questions by training wild brown trout parr to forage individually on live brown-coloured maggots on a cryptic (brown) or conspicuous (green) background. A separate experiment confirmed the absence of trout preference for brown or green substrate. The results show that prey background colour matching increases search time in brown trout. Search time generally decreased by learning, but conspicuous prey remained an easier prey to find throughout the six training trials. Thus, perceptive constraints appear to limit search efficiency for cryptic prey, suggesting that cryptic colouration can confer survival benefits to prey in natural environments. Smaller fish generally found conspicuous prey faster than larger individuals, whereas search time for cryptic prey was not influenced by body size. This suggests that smaller individuals compensate for inferior competitive ability by increasing foraging activity rather than improving cognitive ability. The technique of varying cognitive demands in behavioural tasks could be used more in future studies aimed at distinguishing motivational effects from cognitive explanations for variation in behavioural performance.     

Feeding selectivities of the marine cladocerans<Emphasis Type="Italic"> Penilia avirostris</Emphasis>,<Emphasis Type="Italic"> Podon intermedius</Emphasis> and<Emphasis Type="Italic"> Evadne nordmanni</Emphasis>

We conducted grazing experiments with the three marine cladoceran genera Penilia, Podon and Evadne, with Penilia avirostris feeding on plankton communities from Blanes Bay (NW Mediterranean, Spain), covering a wide range of food concentrations (0.02–8.8 mm³ l^–1, plankton assemblages grown in mesocosms at different nutrient levels), and with Podon intermedius and Evadne nordmanni feeding on the plankton community found in summer in Hopavågen Fjord (NE Atlantic, Norway, 0.4 mm³ l^–1). P. avirostris and P. intermedius showed bell-shaped grazing spectra. Both species reached highest grazing coefficients at similar food sizes, i.e. when the food organisms ranged between 15 and 70 µm and between 7.5 and 70 µm at their longest linear extensions, respectively. E. nordmanni preferred organisms of around 125 µm, but also showed high grazing coefficients for particles of around 10 µm, while grazing coefficients for intermediate food sizes were low. Lower size limits were >2.5 µm, for all cladocerans. P. avirostris showed upper food size limits of 100 µm length (longest linear extension) and of 37.5 µm particle width. Upper size limits for P. intermedius were 135 µm long and 60 µm wide; those for E. nordmanni were 210 µm long and 60 µm wide. Effective food concentration (EFC) followed a domed curve with increasing nutrient enrichment for P. avirostris; maximum values were at intermediate enrichment levels. The EFC was significantly higher for P. intermedius than for E. nordmanni. With increasing food concentrations, the clearance rates of P. avirostris showed a curvilinear response, with a narrow modal range; ingestion rates indicated a rectilinear functional response. Mean clearance rates of P. avirostris, P. intermedius and E. nordmanni were 25.5, 18.0 and 19.3 ml ind.^–1 day^–1, respectively. Ingestion rates at similar food concentrations (0.4 mm³ l^–1) were 0.6, 0.8 and 0.9 g C ind.^–1 day^–1.Communicated by O. Kinne, Oldendorf/Luhe     

Patterns of gamete incompatibility between the blue mussels<Emphasis Type="Italic"> Mytilus edulis</Emphasis> and<Emphasis Type="Italic"> M</Emphasis>.<Emphasis Type="Italic"> trossulus</Emphasis>

Previous research on gametic incompatibility in marine invertebrates suggests that for highly dispersive marine invertebrate species, barriers to fertilization among closely related taxa are often incomplete and sometimes asymmetric. The nature of these barriers can dramatically affect the patterns of gene flow and genetic differentiation between species, and thus speciation. Blue mussels, in the genus Mytilus, are genetically distinct in allopatry yet hybrids are present wherever any two species within the group co-occur. The present study sampled M. edulis (L.) and M. trossulus (Gould) in May and June 2001 from the East Bay section of Cobscook Bay, Maine, USA (latitude 44°56′30″N; longitude 67°07′50″W), where the two species are sympatric. Gamete incompatibility was investigated in a series of laboratory fertilizations carried out in July 2001. The proportion of fertilized eggs typically exceeded 80% at sperm concentrations of 10³–10⁴ ml^?1 among intraspecific matings (n=18), but was <30% even at sperm concentrations in excess of 10⁵–10⁶ ml^?1 for interspecific matings (n=13). Further analysis indicated that approximately 100- to 700-fold higher sperm concentrations were required to achieve 20% fertilization in interspecific matings relative to intraspecific matings, indicating strong barriers to interspecific fertilization. The proportion of fertilized eggs did not follow this general pattern in all matings, however. The eggs from two (out of five) M. edulis females were almost as receptive to M. trossulus sperm as they were to M. edulis sperm. In contrast, the eggs from all M. trossulus females (n=3) were unreceptive to M. edulis sperm, suggesting that fertilization barriers between these species may be asymmetric. Given the experimental design employed in this study, the results are also consistent with a strong maternal or egg effect on the level of interspecific gamete compatibility in M. edulis.