Does male harassment of females contribute to reproductive synchrony in the grey seal by affecting maternal performance?

We investigated the possibility that male harassment of lactating females differed in relation to time of birth in the grey seal, Halichoerus grypus, on Sable Island, Nova Scotia. This was done by comparing the frequency of male disturbances, maternal performance and pup growth for females that either gave birth during the peak of the pupping season or after the peak. Of the females, 58% gave birth in a 7-day period near the beginning of the pupping period, when the operational sex ratio was 2–4 females per male. Late in the pupping period the operational sex ratio reversed to about 1 female for every 2 males. The relative frequency of disturbances by males was significantly greater for late-pupping mothers than for peak-pupping ones (1.9 vs. 1.4 encounters/h). Females that gave birth late also were disturbed by males 3 times more often than females that gave birth during the peak (3.4 vs. 1.1 % of observation time). Late-pupping mothers spent 22% less time suckling (4.0 vs. 5.1 % of observation time), had 30% slower growing pups (1.7 vs. 2.4 kg/d), and weaned pups that were 16% lighter (45.6 vs. 54.0 kg). The effect of birth time on pup mass gain and weaning mass was not attributable to factors such as maternal mass, pup birth mass or pup sex. We conclude that the reduced maternal performance is likely the result of the increased male harassment. As reduced weaning mass can lead to reduced juvenile survival, male harassment may have contributed to the enhanced reproductive synchrony in this species.     

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.     

Maternal care in house mice

Summary This paper analyzes the flexibility of maternal care in wild house mice (Mus domesticus) under different reproductive conditions in the laboratory. All maternal activities were both qualitatively and quantitatively analyzed over a period of 28 days after birth of a litter. The standard behavior of a lactating house mouse with 7–8 young can be described as follows: During days 1–16 the offspring fully depend on the mother for nutrition. Due to rapid growth of the litter, the energetic demands of lactation reach a peak for the female during days 13–16. During days 17–22, the weaning period, the young begin to eat solid food. This period is characterized by behaviors that indicate different interests of the mother and offspring, and thus the existence of a parent-offspring conflict sensu Trivers (1974). Resting alone and remaining far from the litter indicate the female's interest in avoiding the offspring's demands, which are expressed in frequent attempts to initiate sucking. There is no aggression towards the young during weaning. House mice are weaned at 23 days. The relationship between mother and young appears free of conflict after weaning. Nursing is replaced by resting with body contact, but the offspring do not try to suck. The following results suggest that during the weaning period the offspring do not get more milk than corresponds to the maternal optimum—despite their frequent sucking attempts:(a) When the mother is simultaneously lactating and pregnant, offspring are smaller at weaning than under standard conditions. (b) Small litters are weaned earlier than large ones. Despite a longer nursing period, offspring from large litters are lighter at weaning than those from small ones. (c) Under high energy demand, as after postpartum mating and with large litters, females wean their young at a body weight which corresponds to the earliest physiologically possible state of independence.Parity of the female has no effect on maternal activities, nor has the presence of the father. In the latter case, however, offspring are less often left alone and unprotected.Females seem to adjust their investment according to the body weight of the progeny by delaying or advancing the date of weaning (Table 2). This behavior allows the production of the largest possible number of offspring that can be raised to a minimal physiological threshold corresponding to a body weight of approximately 9 g. Such flexibility in parental care may enhance maternal fitness under different and unpredictable environmental conditions.     

Weaning in the guinea pig (<Emphasis Type="Italic">Cavia aperea</Emphasis> f. <Emphasis Type="Italic">porcellus</Emphasis>): Who decides and by what measure?

Offspring should be selected to influence maternal effort to maximize their own fitness, whereas mothers are selected to limit investment in present progeny. In mammals, this leads to a conflict over the amount of milk provided and the timing of weaning. The intensity and time course of such conflict has so far mostly been investigated experimentally in altricial rodents. However, it is expected that offspring options for conflict will depend on developmental state. We therefore investigated in the highly precocial domestic guinea pig (Cavia aperea f. porcellus) who decides over nursing performance and weaning and how pup state influences these decisions. Specifically, we tested whether a threshold mass of pups predicts weaning time. By exchanging older litters against neonates and vice versa, we produced a situation in which females differed in lactational stage from the cross-fostered pups. Our results indicate that females decide about the timing of weaning, as cross-fostered younger pups were weaned at a much younger age than controls and older pups benefited from continuing lactation of foster mothers. Growth rates did not differ in the treatment groups, and different weaning ages resulted in differing weaning mass refuting the hypothesis that weaning is based on a threshold mass of offspring. This constitutes clear evidence that in a precocial rodent, the guinea pig, decisions about maternal care are primarily determined by maternal state and little influenced by pup state despite the extreme precociality of offspring. We suggest that precocial pups show little resistance to early weaning when food is abundant, as they reach sufficient nutritional independence by the middle of lactation to enable independent survival.     

Mixed sex allocation strategies in a polytocous mammal,the house mouse (<Emphasis Type="Italic">Mus musculus</Emphasis>)

The issue of adaptive adjustment of offspring sex ratio (proportion of male births) in polytocous mammals, producing several offspring per litter, is controversial because females of these species can maximize their fitness mainly by adjusting offspring number. To address this issue, we examined the effect of maternal condition at mating, experimentally decreased by pre-mating food restriction, on the sex ratio variation in 137 female mice. We tested two basic sex allocation hypotheses plausible for polytocous mammals: (1) the Myers hypothesis, predicting that cheaper sex should be favored in poor environmental conditions to maximize offspring number; and (2) the Williams hypothesis, predicting maximum fitness returns by adjusting size- and sex-specific composition of the litter according to the maternal condition. The food-restricted mothers produced larger litters with a higher proportion of cheaper daughters than the control mothers. By contrast, the control mothers optimized size and sex composition of the litter according to their weight at mating. In addition, the offspring of the food-restricted mothers suffered less from pre-weaning mortality than those of the control mothers. Therefore, when comparing the groups, the Myers hypothesis had a general significance while the Williams hypothesis was plausible only for the control mothers. Furthermore, some of the food-restricted mothers partly coped with the pre-mating food restriction and increased the proportion of sons in the litter with the increasing maternal weight loss (during the period of food restriction). The sex ratio variation was thus a result of three sex allocation strategies depending on the maternal condition at mating.     

Sex ratio manipulation and decreased growth of male offspring of undernourished golden hamsters (Mesocricetus auratus)

Summary The theory that female mammals in poor condition may increase individual fitness by skewing the sex ratio of their offspring toward daughters and by investing more resources in daughters than in sons was tested in hamsters. Newly mated experimental females were food-restricted during pregnancy and lactation (RR) or during lactation only (AR). Controls received food ad libitum. Maternal body weights were assessed daily from mating to 25 days postpartum. Litter survival (% litters with at least one pup surviving on any day), litter size, offspring sex ratios (=% males), and pup weights were monitored daily from birth (Day 1) to Day 25. All control and AR dams gave birth 16 days after mating. Gestation was extended by 1–3 days for 35.4% of RR dams. RR dams weighed significantly less at parturition than controls and AR females. During lactation, AR females showed the greatest weight loss and control females the least. AR weight loss exceeded that of RR females, possibly because the former maintained larger litters. Survival was highest for controls, intermediate for AR, and lowest for RR litters. Mean sex ratio at birth was significantly less for RR (40.7%) than for control (49.6%) and AR (48.8%) litters. RR sex ratio did not change significantly postnatally. Sex ratios of control and AR litters never differed statistically from 50%. Control male pups were significantly heavier than their sisters throughout the experiment. No significant gender differences were observed for AR pup weights after Day 2 postpartum. RR female offspring weighed more than their brothers throughout the experiment, and this difference was statistically significant immediately prior to the time that pups began to feed independently (Days 14–17). RR female pup weights were similar to, and sometimes significantly greater than, weights of control daughters during the period of postnatal maternal investment. Control males were always heavier than males from the other treatments. Patterns of weight gain by AR and RR males varied with age. We conclude that underfed female hamsters are able to adjust the sex ratio of offspring prenatally and parental investment postnatally to favor daughters.     

Parental investment in southern elephant seals,Mirounga leonina

Summary The southern elephant seal is among the most sexually dimorphic and polygynous of all mammals: males may be more than 10 times the weight of reproducing females and only the largest 2–3% of males are likely to breed. Current optimization theories of sexual selection predict that evolution would favor greater parental investment in individual males than in females. Because southern elephant seals represent an extreme of polygyny and sexual dimorphism, they might be expected to show a dramatic difference in parental investment in male and female pups. However, in a study of parental investment in elephant seals at South Georgia, using several different methods, we found no such difference after parturition. Mother-pup pairs were immobilized and weighed early in lactation, recaptured near the end of lactation and reweighed. A further 30 pups were weighed an average of five times during lactation to establish the shape of the growth curve and to serve as partial controls for the previous set of animals. Initial post-partum weight in females ranged from 346 to 803 kg (=506, SD=111, n=26). Pup birth weight was related to mothers' post-partum weight in female pups but small females often gave birth to large male pups. Male pups were significantly heavier at birth than females. However, this size difference did not persist. Male and female pups were suckled for the same period, grew at the same rate and were not significantly different in weight at weaning. Mothers lost weight at the same rate regardless of their pup's sex.     

Costs and benefits of communal rearing to female prairie voles (<Emphasis Type="Italic">Microtus ochrogaster</Emphasis>)

Communal rearing of offspring may help mothers maximize their investment in offspring at a reduced cost to their own bodily condition, thus maximizing their potential for reproductive success. The objective of this study was to quantify the costs and benefits of communal rearing to prairie vole (Microtus ochrogaster) pups and mothers. Mothers were assigned to one of three social units: solitary mothers, singularly breeding groups (i.e. one mother and one non-reproductive sister) and plurally breeding groups (i.e. two lactating sisters). For each type of social unit, some replicates were provided with food ad libitum, while others were provided with limited food. The body mass of focal mothers (i.e. the first mother to produce a litter) was a significant predictor of pup growth. Regardless of food availability, litters of focal mothers in plurally breeding groups gained more weight than litters reared by solitary mothers. Pups reared in singularly breeding groups were intermediate in weight gain, but did not gain significantly more weight than solitary offspring. There was no difference in the body mass of focal mothers from each type of social unit, regardless of food availability. Within plurally breeding groups, the weight gain of the two litters and body mass of focal and second mothers did not differ. However, focal mothers from plurally breeding groups nursed fewer pups than solitary mothers and also fewer pups than their nestmates when food was limited. Our results suggest that plural breeding results in greater fitness to mothers than solitary and singular breeding.Communicated by E. Korpimäki     

Sexual dimorphism in sea lion pups: differential maternal investment,or sex-specific differences in energy allocation?

Proximal mechanisms underlying a faster growth rate in male compared to female California sea lion pups were investigated. Males are significantly larger at birth than females. Specifically, we asked if differential maternal investment contributed to enhanced male growth via: (1) larger mothers having disproportionately more male pups, (2) more time and energy put into foraging by mothers of male pups, and (3) greater milk production in mothers of male pups. We also considered four aspects of differential energy utilization and acquisition by male and female pups: (1) male pups attempting to save energy for growth by changes in behavior, (2) longer suckling bouts with mother and more sneak suckling of non-filial females by male pups, (3) lower maintenance costs in males via a lowered resting metabolic rate, and (4) increased assimilation efficiency in males. Our study showed that there are no differences in the size of females or length of foraging trips for mothers of male and female pups. Male pups received more milk from their mothers, but the difference was no longer significant when the larger body size of males was considered. There were no differences in either the activity budgets or suckling behavior of male and female pups. Male pups, however, did have lower resting metabolic rates than females. We conclude that enhanced male perinatal growth is a consequence of a larger size at birth, proportionally more milk from mothers to support the greater demands of larger body size, and lower maintenance costs due to a lower resting metabolic rate. Received: 28 April 1995/Accepted after revision: 25 July 1995     

胚胎期和哺乳期全氟辛烷磺酸(PFOS)暴露致大鼠学习记忆能力下降

为了探讨全氟辛烷磺酸(PFOS)的发育神经毒性,寻找PFOS发育神经毒性作用的敏感期,利用水迷宫和组织病理切片技术,研究了胚胎期和哺乳期暴露于PFOS后新生大鼠发育情况、学习记忆能力、抓力以及海马组织病理学改变。结果显示：PFOS导致仔鼠发育迟缓,睁眼期延迟。仔鼠出生后体重与对照组相比出现显著性降低。同一PFOS暴露浓度下,胚胎期暴露组体重低于哺乳期暴露组,抓力差异不显著。水迷宫实验结果显示,TT15(胚胎期和哺乳期均暴露于15 mg·L^-1 PFOS)和TC15(仅胚胎期暴露于15 mg·L^-1 PFOS)暴露组仔鼠逃避潜伏期显著高于对照组,且TC15暴露组仔鼠逃避潜伏期显著性高于CT15(仅哺乳期暴露于15 mg·L^-1 PFOS)暴露组。空间探索实验中,TT15暴露组仔鼠在目标象限的游泳时间显著性低于对照组,其他组无显著性差异。组织病理切片结果显示暴露组海马组织细胞数量减少,出现细胞凋亡现象。结果表明,PFOS造成仔鼠的发育延迟以及学习记忆能力下降的关键作用时期可能是胚胎期。     

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     

海狗油脂肪乳对大鼠围产期毒性试验的研究

观察了海狗油脂肪乳对大鼠妊娠晚期、分娩期、哺乳期及胚胎和胎仔出生后生长发育、学习能力以及生殖能力的影响.于大鼠妊娠D15至哺乳D21,连续皮下注射给予250、500和1000mg·kg-1剂量的海狗油脂肪乳,同时设0.9%氯化钠注射液为对照组,观察各组大鼠和胎仔的生长、发育、生殖能力等项指标.结果表明:给药后F0代母鼠体重增长明显减慢,摄食量显著减少,未见生殖、胚胎毒性反应;F1代仔鼠的出生体重和哺乳期体重增长减慢,生理达标、新生反射等发育迟缓;停药后F1代大鼠的体重增长、摄食量、脏器系数、同笼交配率、妊娠率及胚胎发育全过程未见明显生殖、发育等毒性变化.由此结论,海狗油脂肪乳可使母鼠和F1代仔鼠体重增长缓慢,生理发育迟缓,但不影响F1代仔鼠器官发育、生殖能力以及停药后体重增长和摄食量.     

Parental investment and the secondary sex ratio in northern elephant seals

Summary Data on northern elephant seals, Mirounga angustirostris, bearing on sex ratio theory were collected at Año Nuevo, California, and other Californian and Mexican Islands, during the period 1967 to 1988. The mass of males exceeded that of females by 7–8% at birth and at weaning. The sex ratio was biased to males at birth (51.2%) and was near unity at weaning (49.6% males). The sex ratio did not vary as a function of maternal age or maternal mass except in 6-year-old females, who produced significantly more males. Although sons cost more to rear in energetic terms than daughters, and mothers were more successful weaning the latter, the sex of the pup reared exerted no significant effect on the mother's reproductive performance the following year or on her subsequent survival. These data suggest that parents invest equally in sons and daughters when investment is measured in terms of future reproduction (Fisher 1930) and provide no support for the theory of adaptive shifts in sex ratio (Trivers and Willard 1973). The small sex difference in mass due to maternal effort reflects the fact that females fast during lactation and all energy transferred is from limited body stores. Because of these circumstances, selection for superior condition at the end of the period of parental investment may act more strongly on pups, who have the opportunity to steal milk, than on their mothers.     

Communal suckling in the cavy Galea musteloides

We quantified the extent of communal suckling in the cavy Galea musteloides. Six groups of animals were held in large indoor enclosures and suckling behavior was recorded over 113 h of observation. The groups contained 2–6 lactating females and 3–14 sucking pups. Due to the relative synchronization of births, 73% of the pups present in each group during lactation were non-offspring. Each of the 22 lactating females in the six groups suckled non-offspring in addition to her own offspring. On average, females suckled 86% of non-offspring present in their groups. Thus, 98% of all pups (n = 47) received milk from non-mothers. Although suckling frequencies were significantly higher for mothers with their own individual offspring than with non-offspring individuals, females invested more total time suckling all non-offspring than did suckling just their own; this was possible because for each mother many more non-offspring than offspring pups were present during lactation. Suckling bouts were significantly longer for mothers with their own individual offspring than with non-offspring individuals. The proportion of non-offspring suckling of mothers correlated negatively with the proportion of own young among the pups of a group. Non-offspring suckling did not affect future reproduction of females. Our observations demonstrate extensive practice of communal suckling in G. musteloides under laboratory conditions. Probably because all mothers of a group participated more or less equally in communal suckling behavior, the obvious cost of giving energetically expensive milk to non-offspring did not result in reduced (future) reproductive success. Potential benefits directly involved with communal suckling are unclear. More indirectly, communal suckling as well as birth synchrony might contribute to the formation of advantageous multi-litter kindergardens.     

Transmission of food preferences in spiny mice (Acomys cahirinus) via nose-mouth interaction between mothers and weanlings

Summary Two laboratory experiments assessed the role of nose-to-head and nose-mouth investigation in the transmission of food preferences from Acomys cahirinus mothers to their offspring. Pups spent more time nosing and licking the nose-mouth area of mothers who ate a novel food (fresh potato or carrot) than of mothers who ate no novel food. A greater amount of the less-preferred novel food (potato) was subsequently eaten by pups whose mothers had eaten potato than pups whose mothers had eaten either carrot or familiar laboratory chow only. Mothers maintained a stereotypic posture during nose-mouth investigation, holding still with mouth slightly open, providing both a source of specific food cues and a context in which pups could become familiar with them. These data suggest an active role by Acomys mothers in the development of offspring food preferences.     

Evidence for obligate monogamy in the Djungarian hamster,Phodopus campbelli: pup survival under different parenting conditions

Summary Two recent studies have described pregnancy blocking responses in female Djungarian hamsters, Phodopus campbelli, under conditions where the potential for paternal investment by the male is low (Wynne-Edwards and Lisk 1984; Wynne-Edwards et al. 1987). If this pattern of pregnancy block is adaptive, solitary females should have a low probability of successfully rearing a litter without investment by their mate. The present study was designed to compare the relative reproductive success of solitary females with that of paired females. Weights and survivorship were determined daily from birth through weaning (day 18 after birth) for litters of Djungarian hamster pups raised by solitary females (Solitary females), male-female pairs (Pairs), or female-female pairs consisting of the experimental female and her unmated littermate sister (Sisters). Unmated littermate sisters were included in the design to differentiate between the contribution of the mate and the contribution of another conspecific adult to the reproductive success of the experimental female. As predicted by the hypothesis that the pattern of incidence of pregnancy block was adaptive (Wynne-Edwards et al. 1987), the presence of the mate significantly affected the reproductive success of the female. Pairs were very successful, raising 95% of pups and 100% of litters to weaning. Solitary females were significantly less successful, raising only 47% of pups and 77% of litters with 16% of litters intact on day 18. A littermate sister did not compensate for the absence of the mate. Sisters raised only 61% of pups and 73% of litters with 36% of litters intact on day 18. Pup survival and weights were independent of sex. However, for pups that were still alive on day 18, weight distributions did not differ significantly across the three parenting conditions. Likewise, the proportion of best pups alive on day 18 (with weights exceeding a threshold of 11.5 g) did not differ significantly across groups. Differences between parenting conditions lay in the survivorship of pups rather than in the quality of the pups that survived. This study provides evidence that the presence of the mate is essential for high female reproductive success even under the ideal climate and nutritional conditions of the laboratory. Thus, the Djungarian hamster is a strong candidate for a small mammalian species with obligate monogamy as the optimal mating system.     

Parent–offspring and sibling conflict in Galápagos fur seals and sea lions

Parent–offspring conflict theory is well supported by theoretical arguments. However, empirical observations are often difficult to interpret and have contradicted one of its most appealing predictions that parent and offspring should disagree over killing of nest or littermates. We present the first examples of deadly conflict between siblings of different cohorts. In Galápagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki), mothers often wean their single offspring at 2 years. This leads to a situation where up to 23% of all pups are born while the older sibling is still being nursed. Younger siblings are disadvantaged by being born lighter than neonates without older still dependent siblings. Pups born while an older sib is still dependent grow less in early life (fur seal) and suffer increased early mortality (both species) through direct aggression or scramble competition with the older sibling. This effect is much stronger in years of high sea surface temperature (El Niño) indicating low marine productivity and if the older offspring is a male. In both species, mothers interfere aggressively in this conflict by defending the younger offspring. In years of El Niño, intense resistance to maternal aggression by the older offspring happens frequently in the fur seal. Such resistance against weaning can induce maternal neglect of the newborn. Given substantial year to year variation in offspring growth, maternal aggression forces weaning in the older sibling only if it has reached sufficient size to support itself by foraging. In Galápagos fur seals, pups with older siblings can either represent insurance against loss of older offspring or extra reproductive value.     

Multi-year lactation and its consequences in Bornean orangutans (Pongo pygmaeus wurmbii)

In most mammals, females pay for reproduction by dramatically increasing net energy intake from conception to mid- or late lactation. To do this, they time their reproductive events in relation to environmental cycles so that periods of peak food availability coincide with peak demand or are used to build energy stores. This timing is not possible in species with slow development in which lactation is prolonged over a multi-year period with fluctuating food availability. Here, mothers are expected to sustain a stable but generally lower level of nutrient transfer. In a sample of over 1,050 complete follow days of eight mother–infant pairs collected over 7 years, we document maternal effort for wild Bornean orangutans (Pongo pygmaeus wurmbii) over their average 6.5-year lactation period. As predicted, maternal feeding time was independent of the age of her growing offspring, indicating a stable sustained “plateau” effort of ≤ 25 % above baseline level, instead of a short peak lactation as seen in seasonal breeders. Infant orangutans started to regularly supplement milk with self-harvested food when they were 1–1.5 years old, indicating milk intake was insufficient from this age onwards, even though maternal effort did not decrease. We expect the same regulation of sustained maternal effort in other large and large-brained mammals with slow infant development. We also predict that mother–infant conflict over suckling may show another peak at the onset of the milk?+?solid food phase, in addition to the well-known conflict around the endpoint of lactation (weaning), which is reached after a long and gradual increase in solid food intake by the infant.     

Female-biased sex allocation of offspring by an <Emphasis Type="Italic">Apodemus</Emphasis> mouse in an unstable environment

We investigated the effects of population fluctuation on the offspring’s sex allocation by a weakly polygynous mouse, Apodemus argenteus, for 3 years. In acorn-poor seasons, heavier mothers invested more in sons, and lighter mothers invested more in daughters. In acorn-rich seasons, heavier mothers invested more in daughters, and lighter mothers invested more in sons. Maternal body condition and litter size affected the sex allocation. Furthermore, there was a maternal investment trade-off between a son’s birth mass and the number of daughters. Based upon the effect of population fluctuation on the lifetime reproductive success of each sex, we proposed the new “safe bet hypothesis”. This hypothesis predicts that frequent and unpredictable change in female distribution, which is often caused by abrupt fall in food condition, favors female-biased maternal investment to offspring by polygynous mammals and is applicable to many small mammals inhabiting in unstable environments.     

Why do female Belding's ground squirrels disperse away from food resources?

We examined the effects of food provisioning on the natal dispersal behavior of Belding's ground squirrels (Spermophilus beldingi). We provided extra food to adult and yearling females in their maternal territories during pregnancy and lactation, and to offspring of these females in their natal areas for 6 weeks after weaning. We used unprovisioned young of unprovisioned mothers as controls. Provisioning influenced the probability of dispersal from the natal area by female but not male S. beldingi. All surviving male S.␣beldingi dispersed by 55 weeks of age, regardless of whether they and their mothers received extra food. By contrast, we observed a significant trend, beginning 3 weeks after weaning and continuing through the yearling year, for a greater proportion of provisioned than control female S. beldingi to emigrate from the natal area. Competition for food did not appear to influence natal dispersal of females. However, overall population density, density of females weaning litters, and rates of aggression and vigilance among these females, were higher in provisioned than control areas, suggesting that competition for non-food resources was unusually intense in provisioned areas. We propose that juvenile female, but not juvenile male, S. beldingi may emigrate from the natal site to increase access to areas with low densities of conspecifics. Together with findings of earlier workers, our results suggest that spatial and temporal distributions of environmental resources are important influences on the dispersal behavior of female ground squirrels. Received: 28 February 1996 / Accepted after revision: 16 October 1996