Is sex-biased maternal care limited by total maternal expenditure in polygynous ungulates?

We examined data on sex-specific differences in neonatal weight, litter size and adult female body weight in 32 populations of polygynous ungulates of 18 different species to test for the existence of a trade-off between sex-biased maternal care and the total amount of maternal expenditure. This corresponds to an extension of the hypothesis of Byers and Moodie (1990) that sex-biased maternal care is limited by a high level of maternal expenditure. We did not find any relationship between sex-biased care and two measures of total maternal expenditure. We highlighted high intraspecific variability in sex-biased care and very low intraspecific variability in total maternal expenditure. Even when this between-population variability in sex-biased care was accounted for, no relationship between sex-biased maternal care and maternal expenditure was detected. Apart from difficulties in finding suitable measures for both variables, two other reasons may account for the lack of a relationship between sex-biased maternal care and total maternal expenditure. Firstly, male offspring seem to be more affected than female offspring by harsh environmental conditions. This may lead to the variation observed in the extent of sex-specific differences in birth weight within a single species. If we assume that for a given maternal expenditure reproductive costs incurred by mothers are highest during harsh conditions, this could indicate the existence of a trade-off between sex-biased maternal care and maternal expenditure at the intra-specific level, thereby supporting the Byers and Moodie hypothesis. Secondly, polygyny is only a poor predictor of sex-biased care and factors such as compensatory growth or extended periods of growth may be expected to modify predictions for different species. Thus, environmental conditions and relative effects of maternal care on male and female lifetime reproductive success are better predictors of sex-biased care than total maternal expenditure.Communicated by P.M. Kappeler     

Reproductive effort in relation to maternal social rank in reindeer (<Emphasis Type="Italic">Rangifer tarandus</Emphasis>)

In polygynous mammals, high-quality females may increase their fitness by providing superior care to their offspring. Based on the agonistic interactions of female reindeer in an experimental herd during two consecutive years (1997 and 1998), we tested whether maternal social rank influenced: (1) winter body-mass change of females, (2) preparturition reproductive effort (measured as fecundity, the birth mass and the birth date of their calves), (3) preweaning maternal effort (measured as calves preweaning mortality, early preweaning and late preweaning growth rate and September body mass of calves), and (4) postweaning maternal effort (measured as calves body-mass change during their first winter). In the models, we included September females body mass as a covariate to separate the effects of maternal rank and body mass. We also tested whether the effect of social rank on maternal efforts was dependent on offspring sex. High-ranked females gained body mass whereas low-ranked females lost weight during the winter. Fecundity was higher and date of birth was earlier in high-ranked females than in subordinates, whereas no effect of females rank on birth mass of calves was found. Early preweaning growth rate and September body mass of calves increased with increasing females social rank, whereas late preweaning daily growth rate of calves was not influenced by females rank. Calves preweaning mortality was only influenced by year, which also explained most of the variance in the winter body-mass change of calves. The effects of females rank on the reproductive-efforts parameters studied were not specific to offspring sex. These findings suggest that females rank influences reproductive effort during the preparturition, as well as the preweaning, period, the effect being sex independent.Communicated by R. Gibson     

Competition,fluctuating asymmetry and sperm transfer in male gryllid crickets (<Emphasis Type="Italic">Gryllus bimaculatus</Emphasis> and <Emphasis Type="Italic">Gryllodes sigillatus</Emphasis>)

Sperm competition theory predicts that ejaculate expenditure by males should increase with the risk of competition from rivals. The tendency to modulate expenditure, however, may reflect various constraints acting on different individuals. We test this idea here by looking at the effects of competition and fluctuating asymmetry (FA) in two species of gryllid cricket. Male Gryllus bimaculatus and Gryllodes sigillatus modulated the sperm content of their spermatophores in relation to both apparent competition and FA in limb size. Males of both species increased the amount of sperm transferred when an apparent male competitor was present, sperm number increasing regardless of the species of apparent competitor in G. bimaculatus, but significantly more with a conspecific competitor in G. sigillatus. In addition, the relative size of the spermatophylax in G. sigillatus increased in the presence of apparent competition, while spermatophore transfer time decreased in G. bimaculatus. Sperm number showed opposite relationships with limb asymmetry in the two species, decreasing with increasing asymmetry in males in G. sigillatus, but increasing with asymmetry in both males and females in G. bimaculatus. G. bimaculatus, but not G. sigillatus, also transferred more sperm when paired with a larger female. Relationships between reproductive measures and limb asymmetry were significant only when competitors were present in G. bimaculatus, but were significant regardless of competition in G. sigillatus. The differences may reflect the additional burden of producing a spermatophylax in G. sigillatus.     

Maternal investment in Antarctic fur seals: evidence for equality in the sexes?

Studies of the otariids (fur seals and sea lions), a highly sexually dimorphic group, have provided conflicting evidence of differential maternal expenditure in male and female offspring and, thus, suggestions that they conform to predictions of investment theory are equivocal. Since the mid-1970s, a diversity of research on Antarctic fur seals (Arctocephalus gazella) including studies of their reproductive ecology, lactation energetics, and foraging behaviour have been conducted at Bird Island, South Georgia that have resulted in one of the more complete and diverse data sets for any species of otariid. These long-term data were reviewed to determine whether there was any evidence to support that differential maternal expenditure occurred in Antarctic fur seals. Most of the data examined were collected during five consecutive austral summers from 1988 through 1992 and included years in which local food resources were abundant and scarce. We were unable to detect differences in the sex ratios of pups at birth or sex-biased differences in growth rates estimated from serial data, the number of foraging trips made, the duration of attendance ashore, diving behaviour, suckling behaviour, or milk consumption in any year and in the duration of foraging trips or age at weaning in 2 of 3 years. In addition, we found no evidence of greater reproductive costs between mothers with sons or daughters relative to their reproductive performance the following year. In contrast, sex-biased differences were only found in the duration of foraging trips in 1990, the age at weaning in 1988, and consistently in growth rates estimated from cross-sectional data. We suggest that differential maternal expenditure does not occur in Antarctic fur seals because male pups probably do not gain greater benefit from additional maternal expenditure than female pups. After weaning, males experience a period of rapid juvenile growth over 3–4 years during which time body mass nearly trebles. This growth will almost certainly be dependent upon available food resources then rather than on any maternal expenditure received over the first 4 months of life and, thus, the assumptions of the Trivers and Willard hypothesis are probably invalid for Antarctic fur seals. Received: 10 July 1996 / Accepted after revision: 3 March 1997     

Male-biased maternal expenditure and associated costs in fallow deer

This study tested whether fallow deer mothers, Dama dama, bias their investment towards sons and, thus, whether sons are more costly to produce than daughters. Young (2 years) and old (≥3 years) hinds were analysed separately. Old hinds who raised sons accumulated less body mass than those who raised daughters, during the period between late gestation and the end of lactation. This difference in body mass persisted to the following spring. Mothers who had raised sons gave birth later and their offspring's pre-winter mass was lower the following year than for mothers who had raised daughters. These results indicate higher expenditure for hinds who raise sons and support theories of male-biased maternal investment. However, young mothers with sons and those with daughters did not differ in reproductive performance the following year. One reason might be that young mothers are close to the maximum level of maternal expenditure, since they are still growing, and cannot invest any extra resources in sons. Received: 28 August 1997 / Accepted after revision: 5 April 1998     

Sex bias or equal opportunity? Patterns of maternal investment in bison

Summary In polygynous mammals, it may be adaptive for mothers to invest more in sons and/or to adjust the sex ratio of offspring in relation to body condition. Calving patterns were examined over an 8-year period (1982–1989) for a population of Bison bison in which barren females are not selectively culled. From these data, we tested predictions of the sex ratio adjustment hypothesis as well as two assumptions: (1) that offspring weight at the end of the period of parental investment (PI) is correlated with later condition, and (2) that maternal and offspring condition during the period of PI are correlated. In contrast to predictions, there was little evidence that mothers in better condition bear more sons. Short- and long-term measures of maternal condition (previous reproductive status, age, dominance status, pre-pubertal body weight, age at first reproduction, birth date, and the duration of the mother's own suckling period) were little related to offspring sex ratio, although the last calves of old females were nearly always female. Similarly, there was little evidence for sex-biased PI. Weights at about 7 months of age were greater for males than females; males also had somewhat later birth dates, suggesting either longer gestation or later conception. However, maternal reproductive costs, as measured by subsequent fecundity, weight loss, and interbirth intervals, did not vary with calf sex. Both assumptions of the model received some support. However, while maternal condition was correlated with offspring condition, there may be sex differences in investment patterns. Mothers appear better able to influence the condition of daughters than of sons. This sex difference may negate any benefit from male-biased investment.     

Climate and maternal effects modify sex ratios in a weakly dimorphic marsupial

There is growing evidence that the sex ratios of wild vertebrate populations are determined by mechanisms that are directly influenced by environmental characteristics. The Trivers–Willard (TWH) and extrinsic modification (EMH) hypotheses postulate differing determinants of mammalian offspring sex ratios. TWH states that mothers allocate resources according to their current condition and sex-specific offspring costs. EMH states that environmental forces that affect maternal condition determine offspring sex ratios, independently of maternal tactics of sex-biased allocation. We statistically assessed support for each of these hypotheses using long-term life histories of the allied rock-wallaby, Petrogale assimilis; a continuously breeding, polygynous, weakly dimorphic marsupial. We showed that birth sex ratios were equal and independent of maternal and environmental conditions. However, secondary sex ratios were male-biased under good environmental conditions and for high quality mothers or mothers in good condition. Sex differences in offspring survival contributed to these biases: (1) environmental conditions strongly influenced survival to pouch emergence (in support of EMH) and (2) maternal quality affected survival to the end of maternal care (in support of TWH). Environmental effects on survival were more important than maternal factors over the entire period of maternal care and contributed most to male-biased sex ratios at pouch emergence. In contrast, maternal mass was the best predictor of sex ratios at the end of maternal care—the life history stage where offspring body mass differed between the sexes.     

Karyotype and sex determination in <Emphasis Type="Italic">Dinophilus gyrociliatus</Emphasis> (Polychaeta: Dinophilidae)

The study of the male and female karyotypes of the polychaete Dinophilus gyrociliatus was performed with the aim of ascertaining the diploid number of chromosomes and confirming the existence of karyological differences between the two sexes. Our results evidenced that the female karyotype of D. gyrociliatus consists of 2n=24 chromosomes, while the male karyotype has 2n=23 chromosomes. X chromosomes are subtelocentric and much bigger than the autosomes. The size of the D. gyrociliatus chromosomes, which in the female embryos vary in length from little more than 2.8 wm to less than 0.7 wm, are relatively small by comparison with those of other polychaetes. The existence of maternal and environmental factors able to influence the sex ratio of D. gyrociliatus and chromosomal differences between the sexes confirms that: (1) sex determination is chromosomal and syngamic of the XX-X0 type and (2) control of the sex ratio is progamic and depends on genetic and environmental factors.     

Extended alloparental care in the almost solitary bee Exoneurella eremophila (Hymenoptera: Apidae)

The possibility of extending brood care via the overlapping presence of relatively short lived adults could generate advantages that may have been among the selective forces at the origin of eusociality. In this paper we provide evidence for extended brood care through sib-rearing in the arid-zone allodapine bee, Exoneurella eremophila. Solitary females of the overwintered generation generally die before all their offspring have become independent. In a relatively high proportion of nests, a newly eclosed female invests in her siblings while producing her own offspring in the maternal nest. The sex ratio of the first offspring produced by the overwintered female is highly female biased, but the overall sex ratio of the brood is unbiased. This finding supports the prediction of Bull's 'insurance by protogyny' model of a female bias in the first-produced offspring as a strategy by the mother to ensure extended brood care.     

Sex-biased maternal expenditure in Rocky Mountain bighorn sheep

Summary We examined maternal energy expenditure in Rocky Mountain bighorn sheep to test the hypothesis (Trivers and Willard 1973) that individual sons in polygynous mammals should obtain a larger maternal subsidy. In accord with theory, males weighed more at birth (an index of relative prenatal expenditure), tended to suckle more between 40 and 100 days of age (an index of relative postnatal expenditure) and imposed greater delays on their mothers' return to estrus in the subsequent breeding season (an index of relative total preweaning expenditure). Mothers rather than offspring appeared to have primary control over postnatal expenditure. The effect of maternal reproductive effort on return to estrus was cumulative over a period of 3–5 years so that ewes making relatively large expenditures gave birth progressively later. Ewe and lamb mortality was associated with late conception in the previous rut. Thus, differential reproductive effort by ewes in this population appeared to entail a fitness cost involving reductions in both offspring and maternal survivorship. Correspondence to: J.T. Hogg at the present address     

Effects of maternal nutrition on post-weaning growth in two North American rodents

The Trivers-Willard hypothesis of sex-biased maternal investment in response to fluctuations in resource availability has provided a theoretical foundation for research on maternal investment for more than two decades. Their hypothesis holds that mothers in poor condition as a result of poor resource availability should bias parental investment towards offspring with the highest probability of reproducing. In the polygynous mating system of mammals where males compete for breeding access to females, the hypothesis predicts investment favoring females. Although data from many systems have supported this hypothesis, other systems do not follow the predicted patterns and have resulted in various alternative hypotheses. The present research was designed to test whether differences in body condition of young reared by nutritionally stressed dams relative to young reared by unstressed dams were maintained into adulthood, one of the fundamental assumptions underlying the Trivers-Willard hypothesis. Post-weaning growth in eastern woodrats (Neotoma floridana) and northern grasshopper mice (Onychomys leucogaster) was examined relative to maternal nutritional plane. Individuals from undernourished dams were lighter than their unrestricted counterparts at weaning but no difference was evident by the time they had reached adult size. Failure to maintain body condition differences into adulthood violates one of the assumptions essential for application of the Trivers-Willard hypothesis of maternal investment patterns. Although the Trivers-Willard model proposed that natural selection favors differential investment in the sexes over the entire course of parental investment, evidence from this and other studies suggests that the Trivers-Willard hypothesis might not be appropriate to address maternal investment questions in postnatally malnourished dams, but instead should be restricted to systems concerned with prenatal maternal condition or resource availability. Received: 22 February 1995/Accepted after revision: 30 December 1995     

Lactation,maternal behavior and infant growth in common marmoset monkeys (<Emphasis Type="Italic">Callithrix jacchus</Emphasis>): effects of maternal size and litter size

We examined the relationship between lactation performance and infant growth in a captive population of common marmosets (Callithrix jacchus) that varied in both maternal and litter size. Though common marmosets display a typical primate pattern of dilute milk and relatively slow infant growth rates (factors associated with low daily lactation investment and minimal maternal size effects), we hypothesized that the marmoset's small body size would make lactation investment more sensitive to maternal size than is true for larger-bodied primates. Smaller mothers rearing twins had lower milk fat, lower gross energy in milk samples collected in mid to late lactation and lower nursing-bout frequencies than did large mothers nursing twins. Lactation performance and maternal behavior did not differ between large and small mothers when rearing singletons, with a single exception: small mothers had a lower gross energy in mid-lactation milk samples. Relative growth rates in twins but not singletons were affected by maternal size, such that small mothers supported more growth per infant when rearing singletons while large mothers supported more growth per infant when rearing twins. Among the larger mothers, only, older mothers supported somewhat, though not significantly, less growth per infant, regardless of litter size. Twin infants of small mothers appeared to respond to below-optimal levels of milk yield by initiating maternal carrying less often. The relative energy intake of mothers was similar regardless of litter or maternal size. Small mothers rearing twins drew more heavily on reserves, reflected in a linear weight loss during lactation; however, the reserves drawn upon were inadequate to meet the lactation demand, resulting in lower milk energy output. In addition, small mothers rearing twins were more likely to be ill and less likely to be fertile in the year following lactation than were large mothers of twins or mothers of any size rearing singletons.     

Determinants of ejaculate size in a grasshopper (Chorthippus parallelus)

Models for the evolution of ejaculate expenditure predict that ejaculate size (ES) depends on male body condition, on female fecundity, or on the risk of sperm competition. These ideas were tested in the grasshopper Chorthippus parallelus. ES ranged over four orders of magnitude, around a mean of 442,000 sperm. ES was independent of male body size and female fecundity. None of the male condition parameters - age, degree of parasitism, macropterism score (a measurement of competitive energy allocation to other sources), and the mass-femur length residuals - explained variation in ES. As expected in species where females are cyclically non-receptive after mating, male-male encounters before copulation were a more reliable indicator of the risk of sperm competition than the number of males present during copulation. In particular, the summative number of male-male encounters rather than the male-male encounter frequency was a significant predictor of ES variation (24.9%).     

A histological study of reproduction in the serpulids<Emphasis Type="Italic"> Pomatoceros triqueter</Emphasis> and<Emphasis Type="Italic"> Pomatoceros lamarckii</Emphasis> (Annelida: Polychaeta)

The reproductive cycles of the serpulid polychaetes Pomatoceros lamarckii (Quatrefages, 1865, as Vermilia) and P. triqueter (Linnaeus, 1767, as Serpula triquetra) were studied by histological examination during 1999 and 2000 at Bantry Bay, south-west Ireland. Gametogenesis, sex ratios and hermaphroditism were also investigated. The male/female ratio in P. lamarckii was approximately equal, but in P. triqueter it deviated significantly in favour of females. There was no significant variation in the sex ratios throughout the year in either species. Results confirm that the species are protandric hermaphrodites. In P. lamarckii, but not in P. triqueter, sexual dimorphism was exhibited in body size, the females being larger than the males of the same species. In both species, the proportion of females increased with increasing body size. Simultaneous hermaphroditism was recorded in P. lamarckii, but not in P. triqueter. There was no clearly defined annual gametogenic cycle in either species. Both species appear to have an extended reproductive season, with numerous small-scale peaks in reproductive maturity that can vary annually. Spawning was broadly synchronous between sexes. Visual observation alone (i.e. without histology) was insufficient to accurately assess reproductive condition in individuals not in, or close to, a ripe state.     

Sociogenetic structure in a free-living nocturnal primate population: sex-specific differences in the grey mouse lemur (Microcebus murinus)

The grey mouse lemur (Microcebus murinus) has a dispersed social structure, within which female sleeping associations are common. These sleeping associations have been hypothesised to confer anti-predatory and thermoregulatory benefits, especially when rearing offspring. The genetic composition of these associations was determined using microsatellite markers to test predictions derived from kin selection theory. 161 (99 males, 62 females) individual M. murinus belonging to a free-living population in Ampijoroa, north-western Madagascar, were genotyped and observed over a total of 13 months distributed over the dry seasons of 3 successive years (1995-1997). Kin selection theory predicts that these female associations should consist of closely related members, and that female philopatry and male natal dispersal should characterise the dispersal pattern within this species. These predictions were confirmed by the data. Five out of six female sleeping groups consisted of one or more closely related dyads. Females that slept alone did not have close female kin in the vicinity or within the population at all. Closely related female dyads lived in significantly closer proximity than closely related male dyads and closely related male-female dyads showed intermediate proximity. In combination with the result that females possessed significantly more relatives within the population than males, these findings support the behavioural hypotheses of female philopatry and male natal dispersal. Matrilinear grouping patterns and sex-biased dispersal are therefore genetically established in a dispersed primate social organisation for the first time. The results further indicate that several generations of mouse lemurs live together within a given area, implying both an effective mechanism of kin recognition to avoid father-daughter incest and the potential for social learning to ensure individual recognition.     

Social structures,genetic structures and dispersal strategies in Australian rabbit (<Emphasis Type="Italic">Oryctolagus cuniculus</Emphasis>) populations

In this study, the pattern of movement of young male and female rabbits and the genetic structures present in adult male and female populations in four habitats was examined. The level of philopatry in young animals was found to vary between 18-90% for males and 32-95% for females in different populations. It was skewed, with more males dispersing than females in some but not all populations. Analysis of allozyme data using spatial autocorrelation showed that adult females from the same social group, unlike males, were significantly related in four of the five populations studied. Changes in genetic structure and rate of dispersal were measured before and during the recovery of a population that was artificially reduced in size. There were changes in the rate and distance of dispersal with density and sex. Subadults of both sexes moved further in the first year post crash (low density) than in the following years. While the level of dispersal for females was lower than that of the males for the first 3 years, thereafter (high density) both sexes showed similar, low levels of dispersal (20%). The density at which young animals switch behaviour between dispersal and philopatry differed for males and females. The level of genetic structuring in adult females was high in the precrash population, reduced in the first year post crash and undetectable in the second year. Dispersal behaviour of rabbits both affects the genetic structure of the population and changes with conditions. Over a wide range of levels of philopatry, genetic structuring is present in the adult female , but not the male population. Consequently, though genetic structuring is present, it does not lead to inbreeding. More long-distance movements are found in low-density populations, even though vacant warrens are available near birth warrens. The distances moved decreased as density increased. Calculation of the effective population size (N_e) shows that changes in dispersal distance offset changes in density, so that N_e remains constant.     

Seasonal variation in the degree of heterospecific association of two syntopic hyraxes (<Emphasis Type="Italic">Heterohyrax</Emphasis><Emphasis Type="Italic">brucei </Emphasis>and <Emphasis Type="Italic">Procavia capensis</Emphasis>) exhibiting synchronous parturition

Adult rock hyrax (Procavia capensis) and bush hyrax (Heterohyrax brucei) allocate much of their surface activity to being vigilant for vertebrate predators. Individuals of the two species associate for thermoregulatory basking on rock outcrops (koppies) and, in the Matobo National Park, Zimbabwe, frequently constitute heterospecific groups that produce offspring synchronously. Exposure to predators during basking is thought to put a high premium on vigilance, particularly during the presence of dependent offspring in nurseries. We predicted, therefore, an increase in the association of the two species of hyraxes when offspring were present because this would lead to larger groups, thus enhancing the ability to detect predators. Koppie aggregations of hyraxes were classified as homospecific or heterospecific according to basking group composition. Heterospecific association increased when offspring were present, and heterospecific groups were larger than homospecific ones. Fifty-one percent of nurseries were heterospecific for adults/subadults. During the pre-weaning phase (<2 months after birth), 85% of P. capensis and 65% of H. brucei juveniles were in nurseries of heterospecific basking groups. Heterospecific groups had more offspring than homospecific groups. Juvenile mortality is most evident after young are weaned and heterospecific associations break down. Heterospecific aggregation, through increased group size, may result in a higher probability of avoiding predators, thereby reducing the vulnerability of young.     

Lactation performance of guinea pigs (Cavia porcellus) does not respond to experimental manipulation of pup demands

To ensure proper development of young, mothers should react to offspring signals of need. Studies of such parent-offspring interaction often manipulated litter size to measure effects of changed offspring food demand. We used the extreme precociality of guinea pig (Cavia porcellus) pups to increase offspring demand without changing any other litter characteristic. As pups contribute to their own energy demands from birth by independent feeding, their food demand can be increased by withholding solid food early in lactation. We studied whether mothers reacted to increased food demand of offspring by enhanced parental care, especially by changes in the pattern of milk production and nursing. Pups deprived of solid food early in lactation grew more slowly and were in poorer body condition than pups in control litters, even after the former had access to solid food in late lactation. Mothers of deprived young reacted to offspring long-term need by maintaining nursing behaviour for longer than control mothers. However, this change in behaviour did not occur early in lactation when pup short-term need was greatest nor did it result in increased milk transfer at any time. Energy allocation of mothers measured by changes in their food intake, maternal body reserves, and milk production stayed the same whether offspring had early access to solid food or not. Thus mothers did not increase energy allocation to pups even though they apparently had information about the pups' poor state.     

The relationship between ant-tending and maternal care in the treehopper Publilia modesta

We examined the relationship between maternal care and ant-tending in the treehopper species, Publilia modesta. Ant-tending increased the number of nymphs surviving to adulthood by approximately fivefold. Maternal care increased the number of nymphs surviving to adulthood by approximately 50% in the presence of ants. Maternal care had either no effect or the opposite effect in the absence of ants; when ants were not present survivorship was 30% lower in the presence of mothers. Maternal care increased the average number of ants tending nymph aggregations approximately threefold early in the summer; consequently, we suggest that the primary benefit of maternal care is to attract ants while nymphs are small and few in number. Because there was no benefit of maternal care in the absence of ants, we conclude that ant-tending provides an ecological context that favors maternal care in this treehopper species.     

Circatidal activity rhythms in ovigerous blue crabs, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>: implications for ebb-tide transport during the spawning migration

Female blue crabs (Callinectes sapidus Rathbun) with mature embryos have a spawning migration in which they: (1) undergo ebb-tide transport for movement seaward from estuaries, (2) release their larvae, and (3) reverse direction by undergoing flood-tide transport for up-estuary movement. The study determined whether ebb-tide transport during the spawning migration is based upon an endogenous rhythm in vertical migration. Under constant conditions in a rectangular container, which limited horizontal and vertical movements, females with young and mature embryos had circatidal rhythms (periods=12.11-12.95 h) in migratory restlessness (swimming activity) and egg maintenance behavior (abdominal pumping). However, the rhythms were out of phase, as migratory restlessness occurred during the expected time of ebb tide in the field, and egg maintenance behavior, during the time of flood tide. Under constant conditions in vertical columns (1.32 m high), crabs with mature embryos had a circatidal rhythm (periods=12.2-13.7 h) in which they had frequent bouts of swimming to the surface of the column during the expected time of ebb tide in the field and remained on the bottom during the time of flood tide. This rhythm was not present in crabs with young embryos and disappeared after larval release. Thus, an endogenous rhythm in vertical migration does underlie the ebb-tide transport behavior of ovigerous blue crabs with mature embryos during their spawning migration.