The assessment of female reproductive state during courtship and scramble competition in the fiddler crab, Uca paradussumieri

Male fiddler crabs, Uca paradussumieri, mate underground during a 4- to 7-day period each full and new moon. As soon as the tide recedes, males enter the burrows of females that will ovulate the following day ('pre-ovigerous' females). Males copulate with and guard these females until they ovulate. When interrupted by an intruding male, the first male to reach the female is usually able to defend her and successfully mate with her. In fiddler crabs, females mate multiply and there is last male sperm precedence. Before each semi-lunar mating period, male U. paradussumieri were more likely to court females with whom they would later mate than other nearby females with whom they did not mate. This suggests that males collect information on female reproductive state prior to the females becoming ovigerous. In this species, aggression was common between males that courted the same female. When previously courted females were approached by other males, the initial courter attempted to forcefully disrupt the courtship. This behavior may allow males the exclusive use of information on female reproductive condition. It also suggests a type of scramble competition between males over females. Furthermore, it indicates that males are able to locate receptive females prior to their becoming ovigerous. The shorter guarding period observed in this species, as compared with other fiddler crabs, is caused by females rejecting longer guarding periods. Male ability to assess female reproductive status may therefore be advantageous because it increases male mating success within a scramble type of competitive polygyny.     

Role of surface glycoproteins in mate-guarding of the marine harpacticoid Tigriopus japonicus

Surface glycoproteins serve as mate- and gamete-recognition molecules in some marine animals such as rotifers and sea urchins. We investigated the role of contact chemoreception of surface glycoproteins in mate recognition, mate-guarding, and spermatophore transfer in the marine harpacticoid Tigriopus japonicus Mori. Adult males accurately distinguished immature females from a pool of mixed-sex juveniles, and their behavior towards both live and dead females is described. A survey of the binding sites of twelve lectins to the surface of juvenile and adult females, and adult males demonstrated localized lectin-binding at sites considered important in mating behavior of each sex/age class. Treating adult males with the Triticum vulgaris lectin sharply inhibited normal mate recognition and guarding behavior, as did the monosaccharide glucosamine. Treatment of males with protease and detergents destroyed their mate-guarding behavior without affecting swimming behavior. These results suggest that protein receptors on the antennules of the males detect glycoprotein signals on the surface of females, recognizing carbohydrate moieties of glycoproteins to identify appropriate partners for guarding or mating. Surface glycoproteins may function as a common mechanism of sexual communication in many marine zooplankters. Received: 6 May 1997 / Accepted: 14 September 1997     

Costs of courtship and mating in a sexually cannibalistic orb-web spider: female mating strategies and their consequences for males

The costs of courtship and mating may include increased risks of predation, the transmission of pathogens, and a loss of foraging opportunities. Thus, a female's decision to tolerate a courting male will depend upon how these costs offset the benefits of mating, which will depend on her reproductive and nutritional status. While these costs may be similar for mated and unmated females, the benefits of mating will be less for mated than virgin females. However, the cost of lost foraging opportunities may be higher for females with fewer nutritional reserves necessary for forming eggs. We examined how these costs and benefits influence the courtship and mating behaviour of male and female orb-web spiders, Argiope keyserlingi. In the field, females on webs that also contained a courting male intercepted fewer prey items per hour than females on webs without a male. In the laboratory, the presence of a courting male at the hub also attracted mantid predators to the web, increasing the risk of predation for both male and female. Staged mating experiments in the laboratory revealed that the frequency of female attacks and pre-copulatory cannibalism was greater among mated than virgin females. Feeding history did not affect aggression in virgin females but, among mated females, food-deprived spiders attacked and cannibalized males more frequently than sated females and only the latter ever remated. These differences in female behaviour influenced male mating strategies. Choice experiments demonstrated that males preferred to venture onto the silk threads of virgin rather than those of mated females. Similar patterns of mate selectivity were observed in the field; females with narrow abdomens attracted more males to the webs than females with broad abdomens, and copulations were observed more frequently among females with narrow abdomens. These smaller females are likely to be virgins that have recently molted. Males that preferentially mate with virgin females will not only avoid potentially fatal attacks but also obtain, on average, a higher fertilization success.     

Mouse lemurs in space and time: a test of the socioecological model

Socioecological theory predicts that the distribution of fertile females in space and time is the major determinant of male spacing behavior and mating strategies. Using a small nocturnal Malagasy primate, the gray mouse lemur (Microcebus murinus), we determined the spatiotemporal distribution of estrous females during the brief annual mating season to examine the predictive power of the socioecological model for male mating strategies. Mouse lemurs are particularly interesting in this respect because this polygynous species is characterized by seasonal reproduction, seasonally reversed sexual dimorphism, and relatively large testes. All resident animals in our 8-ha study area, a total of 30 adult males and 27 adult females, were individually marked and regularly recaptured to determine female reproductive status and to obtain home range data. We found that the mating season is limited to 4 weeks following female emergence from hibernation. Only 3-9 females could have synchronized estruses during a given week, indicating a moderately high male monopolization potential. However, receptive females were not spatially clumped and male ranges overlapped with those of many other rivals. Therefore, we suggest that individual powerful males may be unable to defend exclusive permanent access to receptive females because of prohibitive costs of range defense resulting from the strongly male-biased operational sex ratio and the corresponding intruder pressure. Our general conclusions are (1) that the socioecological model provides a useful heuristic framework for the study of mating systems, but that (2) it does not specify the degree of spatiotemporal clumping of receptive females at which male mating strategies switch among mate guarding, spatial exclusion of rivals, and roaming, and that (3) the operational sex ratio can have profound effects on male mating strategies as well.     

Repeatable female preferences, mating order and mating success in the poeciliid fish, Heterandria formosa

Sexual selection via female choice can afford preferred males comparably higher mating success than those males that lack preferred traits. In addition, many models of sexual selection assume that both male traits and female preferences are heritable. In this study we test whether females of the poeciliid fish, Heterandria formosa, have repeatable pre-copulatory preferences for larger males. We also test whether female pre-copulatory preferences are always reliable indicators of male mating success. When given a choice between a large and a small male, females prefer larger males, and the repeatability of this preference is high. Although there are no overall differences in male mating success between large and small males, large males have a higher mating success when they are the first to mate than when they are the second to mate. Likewise, preferred males also have higher mating success when they are the first to mate than when they mate second. Therefore, the repeatable female preferences observed in this study only predict male mating success when the preferred male mates first. These results illustrate that even significantly repeatable female preferences do not translate into male mating success, which is an assumption of many examinations of the importance of female choice in sexual selection.     

Abundance, growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the Oyashio region, western subarctic Pacific

Abundance and life-cycle features of the mesopelagic hyperiid amphipod Primno abyssalis (formerly P. macropa) in the Oyashio region, western subarctic Pacific, were investigated using samples collected between July 1996 and July 1998. P. abyssalis was collected throughout the entire survey period, with abundance peaks occurring in spring to autumn. While all maturity stages of males and females were observed throughout the study period, the peak reproduction season was in summer. Instar analysis based on the segment number of the pleopod rami indicated that hatched juveniles molted 10 times before becoming adult males and 13 times before becoming adult females. Judging from the dry and ash-free dry weights of each instar, males and females continued to feed throughout the final instar stage. Based on cohort analysis of seasonal samples and laboratory observations on molting frequencies, growth in body length of P. abyssalis was linear with time, and estimated generation lengths were 2.3-3.8 years for females and 1.4-1.9 years for males. Brood size of females ranged from 66 to 337 and increased with increasing female body length. Lifetime fecundity, calculated as the sum of six successive broods, was 1,004. Compared with P. abyssalis in the southern Sea of Japan, those in the Oyashio region have a larger number of adult instars (six versus five for females, three 3 vs one for males), a lower growth rate (0.014 mm day^-1 vs 0.021 mm day^-1), and mature earlier (instar 13 vs instar 15 for females; instar 10 vs instar 11 for males). These characteristics are considered to be advantageous life-history traits to counteract higher niche competition within the mesopelagic community and higher predation pressure by mesopelagic fishes in the Oyashio region than in the Sea of Japan.     

Mate attraction by females in a sexually cannibalistic praying mantis

We studied mate attraction by females of the praying mantid, Tenodera aridifolia sinensis, testing honest signaling of mate availability versus deceptive signaling to attract males for sexual cannibalism. We experimentally varied female diet and mating history and measured the rate of attraction of a wild population of males to caged females. Honest signaling theory predicts that virgin females will attract males at the greatest rate whereas deceptive signaling predicts that hungry females (which are more likely to cannibalize males) will attract more males, particularly among non-virgin females. Our results show that hungry females did not attract more males than well-fed females. Indeed, the opposite was true: hungry females attracted significantly fewer males. Moreover, hungry females were no more likely than well-fed females to attract males subsequent to mating, and mated females attracted males at a lower rate than did virgin females. We also observed female T. aridifolia sinensis and male Mantis religiosa arriving at the caged females and we discuss the significance of these observations. The results refute the hypothesis of deceptive signaling and show that mate attraction signals of female T. aridifolia sinensis are honest indicators of female mate availability and a lower risk of sexual cannibalism.     

Interspecific attractiveness of structures built by courting male fiddler crabs: experimental evidence of a sensory trap

Male fiddler crabs Uca musica sometimes build sand hoods and male Uca beebei sometimes build mud pillars next to their burrows to which they attract females for mating. Mate-searching females preferentially approach these structures and subsequently mate with structure builders. Here we show that the preference for structures is not species-specific and argue that it may not have evolved for mate choice. When not near burrows, many species of fiddler crabs approach and temporarily hide near objects, suggesting that hoods and pillars may attract females because they elicit this general predator-avoidance behavior. To test this sensory trap hypothesis we individually released female U. musica, U. beebei and Uca stenodactylus, a non-builder, in the center of a circular array of empty burrows to which we added hoods and pillars and then moved a model predator toward the females. All species ran to structures to escape the predator and the two builders preferred hoods. Next, we put hood replicas on male U. beebei burrows and pillar replicas on male U. musica burrows. When courted, females of both species preferentially approached hoods as they did when chased with a predator. However, males of both species with hoods did not have higher mating rates than males with pillars perhaps because hoods block more of a male's visual field so he sees and courts fewer females. Sexual selection may often favor male signals that attract females because they facilitate general orientation or navigation mechanisms that reduce predation risk in many contexts, including during mate search.     

Reproductive biology of a wood-boring isopod, Sphaeroma terebrans, with extended parental care

The isopod Sphaeroma terebrans Bate, 1866 burrows in aerial roots of the red mangrove Rhizophora mangle L. The burrows serve as shelter and as a reproductive habitat, and females are known to host their offspring in their burrows. I examined the reproductive biology of S. terebrans in the Indian River Lagoon, a shallow lagoon stretching for ∼200 km along the Atlantic coast of Florida, USA. Reproductive isopods were found throughout the year, but reproductive activity was highest in the fall and during late spring/early summer. During the latter periods, large numbers of subadults established their own burrows in aerial roots. The average numbers of S. terebrans per root were high during the fall, but decreased during the winter and reached lowest levels at the end of the summer. Females reached maturity at a larger size than males, but also grew to larger sizes than the males. The average size of females varied between 8 and 10 mm, the average size of males between 6.5 and 8.5 mm. The number of embryos female⁻¹ was strongly correlated with female body length. No indication for embryo mortality during development was found. Parental females (i.e. with juveniles in their burrows) hosted on average between 5 and 20 juveniles in their burrows (range 1 to 59). Most juveniles found in female burrows were in the manca stage and 2 to 3 mm in body length. Juveniles did not increase in size while in the maternal burrow, and juveniles of similar sizes could also be found in their own burrows. Males did not participate in extended parental care, since most of them left the females after copulation. Many females that were born in the summer produced one brood in the fall and a second during winter/early spring. Females that were born in the fall produced one brood during spring/early summer, but then probably died. Extended parental care in S. terebrans is short compared to other peracarid crustaceans. It is concluded that this reproductive strategy in S. terebrans serves primarily to shelter small juveniles immediately after they emerge from the female body, when their exoskeleton is still hardening and their physiological capabilities are still developing. Thus, in S. terebrans, extended parental care probably aids to protect small juveniles from adverse physical conditions in their subtropical intertidal habitat. Received: 9 December 1998 / Accepted: 24 June 1999     

Detrimental effects of the isopod, <Emphasis Type="Italic">Edotia doellojuradoi,</Emphasis> on gill morphology and host condition of the mussel, <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis>

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.     

Coalitionary mate guarding by male chimpanzees at Ngogo, Kibale National Park, Uganda

Cooperative mate guarding by males is unusual in mammals and birds, largely because fertilizations are non-shareable. Chimpanzees live in fission-fusion communities that have cores of philopatric males who cooperate in inter-group aggression and in defending access to the females in their community. Male contest mating competition is restrained within communities, but single high-ranking males sometimes try to mate guard estrous females. Data from an unusually large chimpanzee commmunity at Ngogo, Kibale National Park, Uganda, that contains more males than any previously studied community show new variation in chimpanzee mate-guarding behavior. Contrary to expectation given the large number of males, mate guarding was as common as, or more common than, at other sites, and males other than the alpha male guarded more often. More strikingly, pairs or trios of top-ranking males sometimes engaged in cooperative aggression to prevent estrous females from mating with other males, but tolerated each other's mating activities. Both single males and coalitions mostly guarded periovulatory females. Mate-guarding coalitions were previously unknown in chimpanzees. Coalitions occurred in large mating parties, seemingly because these often contained too many males for single males to maintain exclusive access to estrous females. Coalition members gained higher shares of copulations than they could have expected from solo mate guarding, and suffered lower per capita costs of guarding (as inferred from aggression rates). Two males who most often participated in coalitions formed two-male coalitions at about the point where the number of males present made it unlikely that either could get 50% or more of total copulations on his own, and formed trios when this value dropped below 33%. Kin selection could be a factor in cooperation among male chimpanzees, but coalition members were not necessarily close relatives and the apparent structure of payoffs fit that of mutualism. Furthermore, reliance of male chimpanzees on support from allies to maintain high rank could have led to trading of mating exclusivity for support against mating competitors. Received: 28 May 1997 / Accepted after revision: 16 May 1998     

Mate guarding behavior in clam shrimp: the influence of mating system on intersexual conflict

Mate guarding is a male strategy to gain access to receptive females but often results in antagonistic interactions between the sexes because of different costs/benefits of guarding. In addition to social, morphological, and physiological parameters, the type of mating system should also affect the strength of the conflict and thus the guarding duration. Specifically, when compared to females, self-compatible hermaphrodites might have reduced benefits of outcrossing. We investigated mate guarding in dioecious (co-presence of females and males) and androdioecious (co-presence of hermaphrodites and males) branchiopod crustaceans. Both sexes in androdioecious systems should shift their guarding times to lower values relative to dioecious systems because (1) androdioecious males are present in lower percentages than dioecious males and thus encounter rates with receptive mates are relatively greater for them; and (2) hermaphrodites should have low incentive to incur high costs of mate guarding, having the alternative of self-fertilization, and thus should be highly eager to resist. While females preferred short guarding times, when allowed to control the guarding duration (males tethered), dioecious males did not increase their guarding duration when females (treated with muscular relaxant) could not resist, in contrast to what has previously been found for androdioecious males. This indicates that hermaphrodites are more willing to resist mate guarding than females. The among-species comparisons supported our hypotheses: compromised guarding times were significantly lower in androdioecious than in dioecious species. The introduction of a parameter (mating system) not previously investigated in mate guarding models resulted in a powerful test of mate guarding theory, adding a valuable contribution to our understanding of intersexual conflict.     

Male reproductive tactics to increase paternity in the polygynandrous Columbian ground squirrel (<Emphasis Type="Italic">Urocitellus columbianus</Emphasis>)

In polyandrous and polygynandrous species where females mate with multiple partners, males are expected to maximize their fitness by exhibiting an array of reproductive behaviors to ensure fertilization success, such as competing for the best mating order within a mating sequence, optimizing their investment in copulation, and mate guarding. Though there is genetic evidence of a first-male precedence in siring success for many mammalian species, the causes of this effect are poorly understood. We studied influences on first-male precedence in Columbian ground squirrels (Urocitellus columbianus). We found that the time a male spent consorting and mate guarding declined with his mating order (both the highest for the first male to mate). Mate guarding by the first male significantly reduced, but did not exclude, the number of additional males a female accepted. Later mating males reduced the time spent consorting, suggesting a perceived decreased chance of fertilization success. Consortship and mate guarding durations were positively related to the male’s siring success and to each other, suggesting that males adjusted these behaviors strategically to increase their chances of fertilization success. Our results suggest that besides being the first male to consort, first-male sperm precedence is further enhanced through longer mating bouts and by suppressing the chances and/or efforts of later mating males.     

The influence of salinity and temperature on the survival and behaviour of the isopod Sphaeroma rugicauda from a salt-marsh habitat

The survival of the salt-marsh isopod Sphaeroma rugicauda (Leach) was investigated under varying conditions of salinity and temperature following thermal acclimation. It was found that the isopods showed increased resistance to high temperatures following warm-acclimation, and that this increase was greater in adults than in juveniles. S. rugicauda were able to withstand salinity changes event at high experimental temperatures, although this was more pronounced in the older individuals. Behavioural responses in a temperature gradient showed that the isopods could avoid potentially lethal temperatures.     

The presence of eggs in the nest and female choice in common freshwater gobies (Rhinogobius brunneus)

In many species of fishes with paternal care, females prefer to spawn with males who are already guarding eggs. We studied the effect of egg presence on female mate choice in common freshwater gobies, Rhinogobius brunneus sp. OR. In our tests, females did not prefer males with eggs, suggesting egg presence per se may not act as a cue to attract females. We also examined the effects of brood size on paternal care and offspring survival to look for possible benefits females could obtain when choosing males with eggs. Both fanning by egg-guarding males and egg survival increased with brood size. The presence of neighboring males did have a significantly negative effect on males' parental activity, which subsequently results in a lower level of egg survival than in solitary egg-guarding males. This result provides a partial explanation for the result of eggs not attracting females to mate.     

Sex ratio response to conspecifics in a parasitoid wasp: test of a prediction of local mate competition theory and alternative hypotheses

Maternal manipulation of offspring sex ratio in response to conspecifics is considered in relation to sex ratio theory using the parasitoid wasp Spalangia endius. Females produced a greater proportion of sons in response to mated but not virgin females. This is the first demonstration of a differential sex ratio response to virgin versus mated females and provides support for local mate competition theory. More recent sex ratio models that predict sex ratio responses to conspecifics, specifically constrained, perturbation, and crowding models, were not supported. An increased proportion of sons in response to another mated female occurred on the second day of oviposition but not on the first, and the day effect resulted from experience not age. When females oviposited alone after 2 days' exposure to another female, they still produced a greater proportion of sons than if they had always been alone, but only if the other female was mated, not if she was virgin. Females do not seem to assess the presence of virgin versus mated females indirectly by using a low density of males or a long latency to mate as an indicator for virgin females: neither affected offspring sex ratio. That mated females adjusted their sex ratios in response to other mated females, but not virgin females or males, may be due proximally to mated females not often encountering the latter. Virgin females and males are not located as deep in the oviposition substrate as mated females.     

The evolution of female copulation calls in primates: a review and a new model

Female copulation calls are mating-associated vocalizations that occur in some species of Old World monkeys and apes. We argue that copulation calls have two immediate functions: to encourage mating attempts by other males and to increase mate guarding by the consort male. We hypothesize that female copulation calls have evolved under the selective pressures of risk of infanticide and sperm competition. When male mate guarding is effective, copulation calls allow females to concentrate paternity in dominant males and benefit from their protection against the risk of infanticide. When mate guarding is ineffective, copulation calls may bring genetic benefits to females through facilitation of sperm competition. We present a quantitative model in which interspecific variation in females' promiscuity predicts their tendency to use copulation calls in conjunction with mating. The model predicts that in species with little female promiscuity, copulation calls should be rare and exhibited only in association with mating with dominant males. In species in which females are highly promiscuous, copulation calls should be frequent and unrelated to male dominance rank. The limited data available to test the model support its main predictions as well as the predicted relation between copulation calls and male dominance rank.

Factors influencing sexual cannibalism and its benefit to fecundity and offspring survival in the wolf spider Pardosa pseudoannulata (Araneae: Lycosidae)

Sexual cannibalism is hypothesized to have evolved as a way to obtain a high-quality meal, as an extreme mate choice or as a consequence of female aggressive spillover. Here, we examined underlying factors likely to influence sexual cannibalism in the wolf spider Pardosa pseudoannulata (Bösenberg & Strand, 1906) from China, including mating status, female egg-laid status, female hunger level, female adult age and mate size dimorphism. The results showed that about 10 % of P. pseudoannulata virgin females cannibalized the approaching males before mating and that 28 % of P. pseudoannulata virgin females immediately cannibalized the males after mating. No incidents of sexual cannibalism during copulation were observed. Before mating, previously mated females and starved females tended to engage in significantly higher rates of attacks compared to virgin and well-fed females. Females that had laid egg sacs tended to engage in a significantly higher rate of attacks and sexual cannibalism than virgin females before mating. Regardless of pre- or post-mating, there was a strong positive relationship between mate size dimorphism and the occurrence of sexual cannibalism. We also tested the effects of sexual cannibalism on the fecundity of cannibalistic females and the survival of their offspring. Our results indicated that sexual cannibalism affected positively the offspring survival of cannibalistic females, but not fecundity. Our findings support the hypothesis that sexual cannibalism has evolved as an adaptive component of female foraging strategy and that it benefits offspring survival as a result of paternal investment.     

A female preference for experienced males in the almond moth, Cadra cautella

Male mating status can affect female reproductive output if male ejaculate investment declines over consecutive matings. Accordingly, females are predicted to mate preferentially with virgin males. In mildly polyandrous lepidopterans, female fitness is less affected by reduced male investment than in more polyandrous species, and so the predictions for female mating preferences are less clear. We examined female mating preferences in the mildly polyandrous almond moth, Cadra cautella, in which ejaculate size does not affect female reproductive output. First, we allowed females to mate with virgin or once-mated males, in which the males were presented individually or simultaneously. We recorded the latency to mating and, in the case of the simultaneously presented trials, the identity of the successful, copulating male. We found that females mated more frequently with mated males (when simultaneously presented with both males), yet females did not differ in the time taken to initiate copulation with any male. We further examined if this mated male advantage was due to differential mate detection or locomotory behaviour of the male treatments. We tested the ability of virgin and mated males to locate a receptive female within a wind tunnel using long-distance pheromone cues and recorded their activity budget. We found no difference in the ability of mated or virgin males to locate or approach a receptive female, or in their activity levels. These data suggest a female preference for mated males in this species, a preference that may minimise other potential costs of mating.     

Biology and population dynamics of the intertidal isopod Cirolana harfordi

The distribution of Cirolana harfordi (Lockington) populations is determined largely by the availability of loose boulders on sandy beaches. The isopods swim out from under the rocks at high tide to feed. Their diet consists primarily of minute polychaetes and crustaceans. In addition, the isopods locate and utilize any available dead animal matter in the surf zone. Breeding occurs throughout most of the year, except for a brief lull in the fall. Females produce 1 or 2 broods of 18 to 68 young during their 2-year life-span. Marsupial incubation lasts 3 to 4 months. The population size and structure remained relatively constant from one year to the next. Mortality rates estimated for juveniles, males and females showed that newly emergent young and post-reproductive females suffer the greatest losses (up to 75% mortality per month).