Spatial organization of the arboreal carnivorous marsupial Phascogale tapoatafa

Phascogale tapoatafa is a squirrel-sized carnivorous marsupial (Dasyuridae) found in dry eucalyptus forests of Australia. Radio-tracking and live-trapping studies at three sites in Victoria indicated that population densities were typically low, with females occupying home ranges averaging 41 ha (minimum convex polygon method) that were intrasexually exclusive. Male home ranges (mean 106 ha) overlapped extensively with females and other males, and expanded during the short breeding season to an average length of 2.7 km. When presented with the faeces of unfamiliar conspecifics, both female and male P. tapoatafa investigated the faeces of females significantly longer than those of males, suggesting that the exclusive home ranges of females are reinforced by olfactory communication, and that males are less responsive to the presence of potential rivals than to potential mates. Both sexes foraged throughout their home range, but the core area of intensive use was less than one-third of the total area traversed. The spatial organization of P. tapoatafa differs markedly from that of the closely related species Antechinus stuartii , but is similar to that of the only other extensively studied carnivorous marsupial, Dasyurus geoffroii.     

Biparental care and obligate monogamy in the rock-haunting possum, Petropseudes dahli, from tropical Australia

Monogamy is rare among mammals, including marsupials. I studied the social organization of the little-known rock-haunting possum in Kakadu National Park in Northern Australia. Preliminary field observations revealed that the majority of possums live in cohesive groups consisting of a female-male pair and young, suggesting a monogamous mating system. I used radiotracking to determine home range patterns, and observations to measure the degree of symmetry between the sexes in maintaining the pair bond and initiating changes in group activity. I also measured the extent of maternal and paternal indirect and direct care. Nocturnal observations and radiotelemetric data from 3 years showed that six possum groups maintained nonoverlapping home ranges with long-term consorts and young sharing dens. Males contributed more than females to maintaining the pair bond but they contributed equally to parental care. For the first time, the parental behaviours of bridge formation, embracing, marshalling of young, sentinel behaviour and tail beating are reported in a marsupial. Males participated to a high degree in maintaining relationships with one mate and their offspring. Collectively, these results suggest that the mating system of this wild population of rock-haunting possums is obligate social monogamy. Copyright 2000 The Association for the Study of Animal Behaviour.     

保加利亚北部地区匈牙利小家鼠的空间组织结构

本文研究了保加利亚北部地区的匈牙利小家鼠在繁殖期的空间关系和分布。这种动物最普遍的空间关系是一雄一雌,也观察到两个或三个雄鼠与一个雌鼠、一个雄鼠与两个或三个雌鼠的组织形式。很少观察到雌性的社群小组。雌鼠之间的巢区没有发现重叠。一般雄鼠的巢区大于雌鼠。有一半雄鼠的巢区与一个雌鼠的巢区相重叠,其他的雄鼠与两个成年雌鼠的巢区相重叠。这些结果表明,匈牙利小家鼠具有多变的社群繁殖婚配体制,即单配制和多配制共存。比较普遍的一雄一雌的配对形式和雌性的独居特性,以及雌性之间巢区的相对独立性,表明单配制是该物种的主要婚配体制。     

The Round-Eared Sengi and the Evolution of Social Monogamy: Factors that Constrain Males to Live with a Single Female

Animal dispersion in space and time results from environmental pressures, and affects the outcome of a species' social organization. When females are solitary, males may either roam or be pair-living. We studied possible environmental influences affecting the social organization of the round-eared sengi ( Macroscelides proboscideus ) in a semi-desert in South Africa, using trapping and radio-tracking across 2.5 yr. Adult sex ratios did not deviate from 1:1 and we found no indication of sexual dimorphism in body mass. Females maintained exclusive areas, which had little overlap (<4%) with neighbouring females (NF), and males overlapped predominately only with the home range of single females. Generally, inter- and intra-sexual overlap with neighbouring individuals was low (3–6%) for both sexes, indicating territoriality and pair-living. Pairs were perennial and territories were maintained year-round. However, males generally maintained much larger areas than females, which were sensitive to population density. Male space use appeared to be primarily limited by the presence of neighbouring males. Female home ranges were smaller-sized despite changes in population density, possibly for energetic efficiency. Some paired males attempted to take over widowed females, but shifted back to their original home range following the intrusion of an un-paired male. We conclude that social monogamy is the predominant social organization in round-eared sengis in a semi-desert that may have resulted from females living solitarily in small exclusive territories, balanced sex ratios, and from a low variation of body mass between males.     

Sex differences in the influence of mothers on the sociosexual preferences of their offspring

The extent to which "nurture" as opposed to "nature" determines behavior and sociosexual preferences in mammalian species is controversial although most recent interest has focused on genetic determinants. We report here that if sheep and goats are cross-fostered at birth, but raised in mixed-species groups, their play and grooming behavior resembles that of their foster rather than genetic species. There are no sex differences in effects on these behaviors, and other species-specific behavior patterns such as aggression, browsing, climbing, and vocalizations are unaffected. In adulthood, cross-fostered males strongly prefer to socialize and mate with females of their foster mother's species, even if raised with a conspecific of their own species. Castration within 2 days of birth slightly reduces the level of this altered social preference but mating preference following short-term testosterone treatment is the same as for gonadally intact animals. Cross-fostered females also show significant preference for socializing with females and mating with males of their foster mother's species, although this effect is weaker than that in both gonadally intact and castrated males. When cross-fostered animals are placed in flocks containing members of only their genetic species for 3 years, male social and mating preferences for females of their mother's species remain virtually unaffected. Females change to display an exclusive mating preference for members of their genetic species in 1-2 years although they still retain some social interest in female members of their foster species. Thus, there are clear sex differences in the impact of the emotional bond between a mother and her offspring in these mammals. Effects on males are strongest and irreversibly maintained even after altering their social environment, whereas those on females are weaker and mating preferences are clearly adaptable in the face of altered social priorities. These sex differences are presumably caused by pre-, or early postnatal, organizational effects of sex hormones on the brain.     

Spatiotemporal distribution of individuals as an indicator for the social system of Lepilemur sahamalaza

Primate social systems are highly diverse, complicating the classification of particularly elusive species that are difficult to observe. The spatial distribution of individuals over time is a critica lindicator for the social organization and long‐term studies are important to establish patterns of social interactions. In recent years, species of the cryptic, nocturnal sportive lemurs of the genus Lepilemur were found to live in pairs in which a single male and a single female share and defend a mutual home range. The present study aimed to forward research into this underrepresented genus by determining the social organization and structure of the Sahamalaza sportive lemur, L. sahamalaza. We collected 773.15 hr of behavioral and GPS data during a period of 10 months (between 2015 and 2016) on 14 individuals: eight females and six males. There was no evidence of pair‐specific home range use as intra‐ and intersexual home range overlap was high. No pattern of social interactions between focal individuals could be distinguished despite high range overlap. Individuals met and interacted infrequently, resulting in an interaction rate of 0.32 interactions/hr. Sleeping associations between adult individuals were never observed. While both sexes had access to multiple potential mating partners, range sizes or ranging distances did not increase in mating periods. Overall, the social system of Sahamalaza sportive lemurs exhibits aspects of a solitary social organization and structure with potential for the polygamous mating system. These findings underline the importance of detailed social ecology studies that can provide the basis for understanding potential environmental influences on social system variability of closely related species.     

Effect of the mating plug on female chemical attractiveness and mating acceptance in a scorpion

After mating, females may experience a decline in sexual receptivity and attractiveness that may be associated with changes in the production and emission of sex pheromones. In some cases, these changes are produced by chemical substances or structures (e.g., mating plugs) produced by males as a strategy to avoid or reduce sperm competition. In scorpions, sex pheromones may be involved in finding potential mates and starting courtship. Here, we tested the hypothesis that the males of Urophonius brachycentrus, a species that produces a mating plug, use chemical communication (sex pheromones) to detect, localize, and discriminate females according to their mating status (virgin or inseminated), aided by chemical signaling. We also explored the effect of extracting of the mating plug on chemical communication and mating acceptance. We used Y‐maze olfactometers with different stimuli to analyze male choice and exploration time. To evaluate mating acceptance, we measured the attractiveness and receptivity of females of different mating status. We found that chemical communication occurs through volatile pheromones, but not contact pheromones. Males equally preferred sites with virgin or inseminated females with removed mating plug. In turn, females with these mating statuses were more attractive and receptive for males than inseminated females. This study suggests that the mating plug significantly affects female chemical attractiveness with an effect on volatile pheromones and decreasing sexual mating acceptance of females. The decline in the female's sexual receptivity is a complex process that may respond to several non‐exclusive mechanisms imposed by males and strategically modulated by females.     

The Breeding System of the Acorn Woodpecker in Central Coastal California

Adult spatial relationships and social organization in a marked population of alpine mammals, the pika (Ochotona princeps), were studied over a 3-year period in the Rocky Mountains of Colorado. Home range size, distances between centers of activity of dyads, and weighted overlaps of home ranges of dyads were used to define space use patterns. Disappearance and establishment of individuals reflected the temporal component of space use. Relative frequencies of foraging (haying and feeding), surveillance, and communication (short calling, long calling, cheek rubbing) behaviors were recorded. Social relationships among adults were defined by agonistic interactions (aggression) and affiliative behaviors (social tolerance, vocal duets, copulations). Males and females occupied individual home ranges of equal size on talus, their obligate habitat type. Adjacent home ranges were normally occupied by pikas of the opposite sex. Replacement of home ranges was always by a member of the same sex as the previous occupant. A high degree of affiliative behavior was expressed between spatially contiguous heterosexual dyads. Aggression was greater among intrasexual than heterosexual dyads, and spatial overlaps among intrasexual conspecifics were less than among heterosexual conspecifics. Ecological constraints, such as the distribution of food (primarily located in meadows adjacent to the talus) and the short summer reproductive season (placing a premium on early appropriately timed litter;;) have apparently led to a facultatively monogamous mating system. Males can neither monopolize essential resources sufficiently to attract several females, nor defend groups of females as social repulsion among females further increases their dispersion.     

Sociality in the gray mouse lemur (Microcebus murinus) in northwestern Madagascar

The mating system and social organization of the gray mouse lemur (Microcebus murinus) was investigated in two three-month field studies (covering a period before, during, and after the first mating season) in Ampijoroa, northwestern Madagascar. The spatial and temporal distribution of the sexes within a population was studied using mark/recapture techniques and radiotelemetry to assess possible contest or scramble competition between the males. Sociality was inferred from the occurrence and probability of nocturnal social encounters, the temporal stability of daily sleeping groups, and nocturnal ranging patterns of co-sleepers. Males and females were evenly distributed in the study area within a network of highly overlapping home ranges. No indications were found for the spatial monopolization of the females by certain dominant males. Males and females had spatial access to several potential mates; the mating system is therefore characterized as a multi-male/multi-female system. Male home range sizes increased during the first mating season, which was interpreted as an indicator for scramble competition between the males. Competitive mate searching, sperm competition, and temporary mate guarding as well as female mate choice are suggested as the most probable reproductive strategies. Over the course of the study the animals lived continually within the study area, and most females formed stable individualized sleeping associations. Females that slept together shared a higher percentage of their home range than did females that slept at different sites. It is suggested that this network of social relationships should be described as a dispersed but individualized neighborhood.     

Paternal effort and its relation to mating success in the European ground squirrel

The necessity for parental care is a driving force for determining mating systems and social organization. The European ground squirrel, Spermophilus citellus, is a polygynous, gregarious species in which male parental behaviour would not be expected. We had observed males digging in litter burrows that were later occupied by females and their offspring. Males never stayed overnight within these burrows. To determine whether this was some kind of paternal effort we tested the following hypotheses: (1) that male burrowing behaviour was directed towards the male's own offspring or towards the pregnant or lactating mother of the male's offspring; (2) that this behaviour had costs in terms of condition, decreased survival or fecundity; and (3) that it benefited offspring condition or survival. All three assumptions were met. Males worked on the litter burrow of their copulatory partners. Thus, this behaviour was directed towards the male's potential offspring. Male burrowing costs were seen in decreased foraging time and increased body mass loss. Offspring benefits were evident in increased mass at natal emergence. We conclude that male digging at litter burrows can be considered as paternal effort. Lastly, we considered the effects of polygyny on this male parental effort by comparing mating effort, mating success and paternal effort. High mating success was associated with high mating effort and low paternal effort. Moderate to low mating success was associated with lower mating effort and higher paternal effort, indicating a trade-off between the two.     

Female reproductive strategies, paternity and community structure in wild West African chimpanzees

Although the variability and complexity of chimpanzee behaviour frustrates generalization, it is widely believed that social evolution in this species occurs in the context of the recognizable social group or community. We used a combination of field observations and noninvasive genotyping to study the genetic structure of a habituated community of 55 wild chimpanzees, Pan troglodytes verus, in the Ta? Forest, C?te d'Ivoire. Pedigree relationships in that community show that female mate choice strategies are more variable than previously supposed and that the observed social groups are not the exclusive reproductive units. Genetic evidence based on nuclear microsatellite markers and behavioural obser-vations reveal that females in the Ta? forest actively seek mating partners outside their social unit; noncommunity males accounted for half the paternities over 5 years. This female mating strategy increases male gene flow between communities despite male philopatry, and negates the predicted higher relatedness among community males. Kin selection seems unlikely to explain the frequent cooperation and sharing observed among group males in this population. Similarly, inbreeding avoidance is probably not the sole cause of permanent adolescent female dispersal as a combination of extragroup mating and avoidance of incest with home group males would allow females to avoid inbreeding without the hazards associated with immigration into a new community. Extragroup mating as part of chimpanzee females' reproductive strategy may allow them to choose from a wider variety and number of males, without losing the resources and support provided by their male social group partners. Copyright 1999 The Association for the Study of Animal Behaviour.     

The Race for Females: The Mating System of the Red Mason Bee, Osmia rufa (L.) (Hymenoptera: Megachilidae)

The ecology of a species strongly influences the strategies with which males and females maximize their lifetime reproductive success. When males and females do not invest equally in offspring, the sex with the higher parental investment becomes a rare resource for the other. The spatial and temporal distribution of the limiting sex forms the basis of the mating system. In nest-constructing Aculeata such as the red mason bee, Osmia rufa, females perform intensive brood care, whereas males do not invest in their offspring but instead compete for access to mates. Receptive females of this species are widely distributed and do not assemble at certain places. Therefore, territorial behavior is not an advantageous mating tactic for males, which instead search for females within individual home ranges usually centered around food plants. The unmarked or defended home ranges of different males may completely overlap. Competitive searching leads to a random distribution of matings among males that is largely independent of body size. The mating system of O. rufa can be described as scramble competition polygyny.     

Does female mortality drive male semelparity in dasyurid marsupials?

In some members of the marsupial families Didelphidae and Dasyuridae, males are semelparous, that is, they live for only one mating season. Semelparity is proposed to be the result of the high energy demands of competing for matings with many females during a short breeding season. We argue that high adult female mortality rates between mating and weaning of the offspring selects for a 'bethedging' mating strategy in males. We tested this hypothesis in a well-studied field population of Antechinus agilis by estimating the number of females a male needs to mate with in order to have a high chance of siring at least one offspring that survives to the next breeding season. Our hypothesis predicts that species in which males are semelparous should have higher female mortality rates than species in which males are iteroparous. The limited available data for dasyurid marsupials support this prediction.     

Monogamy as a Rule Rather than Exception in Nocturnal Lemurs: the Case of the Fat-tailed Dwarf Lemur, Cheirogaleus medius

According to present hypotheses on the evolution of life history traits and social systems in Malagasy lemurs, nocturnality and infant parking are associated with a solitary lifestyle and a polygynous mating system. However, theoretically extreme seasonality of reproduction could limit the number of females that can be monopolized by a given male and thus hinder the evolution of polygyny. The aim of this ongoing study is to test these contrasting expectations by looking at the social and mating system of the fat-tailed dwarf lemur Cheirogaleus medius . This species hibernates for up to 7 mo, so that time for breeding and raising offspring is extremely limited.
A mark-recapture study in western Madagascar was combined with observations and radio-tracking of 36 individuals during the rainy seasons from 1995 to 1998. According to these data, fat-tailed dwarf lemurs live in permanent sleeping groups consisting of a male and a female (n = 8) or one male and two females (n = 1). One or two offspring from the previous year were frequently observed to sleep together with an adult pair. Members of each sleeping group were the exclusive users of their nest holes and home ranges. During the birth season, males and females took turns at baby-sitting their offspring. Females without paternal help were unable to raise their offspring successfully. Since females did not exhibit oestrus synchrony, the ultimate selective factor favouring pair-living could be obligate paternal investment. The results, together with the lack of sexual size dimorphism and relatively small testis size, suggest that the fat-tailed dwarf lemur lives in family groups with a monogamous mating system. A review of the mating systems of nocturnal lemurs shows that monogamy appears to be the rule rather the exception.     

The role of temperature on clustering behavior and mating opportunity in Bonin flying foxes

Social thermoregulation is an important adaptation for animals living in cold environments, especially for those with high surface area-to-volume ratios. Huddling behavior can influence animal dispersion patterns. However, little attention has been paid to the relationship between huddling behavior and social organization, including mating systems. We studied aggregations of Bonin flying foxes Pteropus pselaphon on Chichijima Island in Japan, which is located at one of the northern limits of their geographical range. Bonin flying foxes aggregate in arboreal roosts, where they form temporary clusters within the colony during winter months. Specifically, we hypothesized that the occurrence and intensity of clustering behavior are affected by ambient temperature; therefore, we examined the relationship between cluster size and ambient temperature. We also investigated the age and sex compositions of clusters and mating behavior to gain insight into the mating system of this species. Field observations showed that P. pselaphon clusters were roughly separated into three groups: males, females, and subadults. Statistical analyses showed that ambient temperature had significant negative effects on the proportion of individuals that formed clusters for all groups. Cluster size decreased significantly with increasing ambient temperature in female and subadult groups. Some female clusters were composed of a single male and multiple females. Marked males were observed excluding other males from the periphery of female clusters, and they monopolized copulations with clustering females. Our findings suggest that clustering plays an important warming role in the species, and that this behavior is exploited by males resulting in a social system that is best described as a form of female-defense polygyny.     

PATTERNS OF PARENTAL CARE AND PARENTAL INVESTMENT IN MARSUPIALS

I. Information on growth, development and care of young has been assembled for 62 species of marsupial. 2. During gestation, development of the marsupial embryo proceeds only so far as to allow the neonate to make its way from the urogenital opening to the mammary area on the abdomen of the female where it attaches to a teat. Specific structural adaptations keep the neonate firmly attached to the teat for at least the first month after birth. 3. Six types of pouch are distinguished ranging from lateral folds of skin, which do not cover the mammary area or enclose the developing young, to a fold of skin that covers the mammary area and forms a deep pouch, completely enclosing the developing young. 4. Although the young is very small at birth and birth is rapid, specific changes in the behaviour of females occur around the time of birth, and a specific birth position is adopted. 5. The time at which marsupial young leave the pouch cannot be equated with birth in eutherians, because of the considerable variations in the type of pouch and in patterns of parental care. From a consideration of the functional development of the young in the pouch, it is suggested that the nearest equivalent to eutherian birth is the time at which the marsupial young achieves homeostasis, when it is well furred and endothermic. 6. Maternal behaviour is influenced by the type of pouch. In all species, the mother keeps the young and the pouch clean by licking, especially when the young are wholly within the pouch or attached to the abdomen. In species with reduced pouches where young are left in a nest at an early stage of development, maternal behaviour includes nest building, defence, and retrieving and carrying the young. These functions are performed by the pouch itself in species with large deep pouches in which the young is carried for a much larger part of its development, and other specific maternal behaviours are infrequent. 7. The patterns of parental care are reviewed over all families of marsupial. Not all members of a family have the same pattern of parental care, which appears to be influenced by many factors including body size, type of pouch, diet, litter size and other aspects of life history strategy. 8. Three patterns of parental care are distinguished: (A) As soon as young begin to release the teat they are no longer carried by the mother, and are left in a nest when still barely furred, ectothermic and before the eyes open. This pattern is found in species with large litter size and a pouch reduced or absent, e.g. some Dasyuridae and some Didelphidae. (B) Young remain in the pouch after they begin to release the teat but are left in a nest, at a later stage of development than in A, when well furred, endothermic and with eyes open. After first pouch exit, there is generally a period when young return to the pouch from time to time. This pattern is found in species with well developed pouches and litters of I or > 1 e.g. Peramelidae, some Didelphidae. (C) Young remain in the pouch after they begin to release the teat. At first pouch exit, the young is well furred and endothermic, and leaves the pouch only for brief periods, gradually spending more time out until permanent pouch exit. It is not usually left in a nest. This pattern is found in species with well developed pouches and litters of one, e.g. Macropodidae. 9. Pattern A is seen particularly in the smaller species in any family, where large litter size means that by the time young release the teat, the litter is about 50% of maternal body weight and a considerable burden. In such species, young are left in a nest as soon as possible. In larger species with patterns B or C, litter size is smaller, and by the time they are no longer carried by the female, the litter is still only 20% of maternal weight. 10. Whatever the pattern of parental care, mortality from birth to permanent pouch exit is not unusually high in marsupials in comparison with eutherians. 11. I suggest that the presence of the pouch and the associated patterns of parental care are important determinants of social organization in marsupials. For much of the period of dependence, the young is small, attached to a teat or in a pouch. The male can make little contribution to parental care, and there is little room for improvement in the care of young in complex social groups. In most species, the female on her own is sufficient caretaker. The male is most likely to increase his own biological fitness by going off to mate again and leaving the female to raise his offspring. 12. Patterns of energy expenditure on offspring by female marsupials were assessed throughout the development of young. Investment before birth was assessed by weight of the neonate, during development by growth rate and the time for which the young was carried (pouch life), and total investment by weight of young at weaning and time from birth to weaning. Regression of measures of investment against maternal body weight allowed comparison of investment in animals of different size. 13. Investment in young before birth is very small. Neonatal marsupials range in size from 0·01 to 1 g, and the largest is less than 0·2 % of the size of the mother. Larger mothers produce larger young which are smaller relative to the mother than are the young of smaller species. Individual young in the family Dasyuridae are particularly small. 14. Growth rates in g/d were calculated over the period from permanent pouch exit to weaning. There is a very close correlation between growth rate and maternal body weight - that of litters increases as the 0·78 power of body weight. During this period the growth rate of individuals is comparable with that of eutherian young during lactation, and in litters it is higher still, suggesting that the difference in patterns of growth are not due to the lower metabolic rate of marsupials. As in eutherians there is considerable individual variation in growth rate; it is very high in litters of small dasyurids, which have individual rates comparable to those of larger species. Young of the family Peramelidae grow and develop rapidly; those of the arboreal folivore Phascolarctos do both slowly. I 5. Pouch life, the period for which the young is carried by the mother, increases with body size; as expected, species with pattern A parental care have shorter pouch lives than species of the same size with patterns B or C, reflecting the early stage of development at which young are left in the nest in pattern A. 16. Time from birth to weaning is also longer in larger species. There is a close relationship of age at weaning with maternal weight, with some significant exceptions. For their size, the family Peramelidae have a very short time from birth to weaning, and the time from pouch exit to weaning is particularly short. Many arboreal species have longer periods of dependence than expected from their size. 17. The weight at weaning of individual young is closely related to MBW^0·71, but the weight of one young relative to maternal body weight shows no trend with size, and ranges from 25–61 %, with a mean of 42 %. 18. Parental Investment, as measured by the function Wt. of litter at weaning × 100/MBW, decreases with increasing size of mother as MBW^0·28. The highest levels of investment are found in very small species. In many small species of the family Dasyuridae, a litter at weaning is > 300% MBW. By contrast, investment in the family Peramelidae is low - at weaning a litter of three is about 50% MBW, comparable with investment in a single young of the family Macropodidae. 19. The evolution of patterns of parental care and investment appears to follow three main lines: (1) Species with large litter size, high levels of investment in litters and in individual young. Investment is directed to growth and not to carrying the young in the pouch, since young are left in a nest at an early stage. Typical of this group is the family Dasyuridae, in which many species make few reproductive attempts per year. (2) Species with litters of more than one, low levels of investment in litters and in individuals, but rapid growth and development of young. Because of the small relative size of young they are carried in the pouch for a large part of the period from birth to weaning. This pattern is shown by the family Peramelidae, and seen as an adaptation to rapid and repeated reproduction in an environment with an extended favourable season. (3) Species with small litter size, lower total investment, but investment in individual young is not low, and investment in carrying young to an advanced stage of development is high. Patterns of this type are found in the Diprotodonta, with extreme development in the Macropodidae. 20. Many of the measures of investment have been expressed as a power function of maternal body weight. The exponents of body weight in these functions are such as to suggest that an important underlying variable is metabolic rate. 21. It has been suggested elsewhere that the marsupial mode of reproduction evolved as an adaptation to environmental uncertainty, in that it allows a reproductive attempt to be abandoned at any time much more readily than in eutherians, thereby increasing the likelihood that a female will survive to reproduce again. I consider this suggestion in the light of patterns of parental investment. For small, short-lived species, any reproductive attempt represents a substantial part of its lifetime reproductive output. Investment in any one reproductive attempt is high, and the cost of replacing an abandoned attempt is so high that it seems unlikely that the desertion of offspring would be an important reproductive strategy in small ancestral marsupials, although it may be an important response to environmental uncertainty in certain large modern macropodids.     

A model for evolution of male parental care and female multiple mating

In most animals, males gain a fitness benefit by mating with many females, whereas the number of progeny per female is unlikely to increase as a function of additional mates. Furthermore, males of internally fertilizing species run the risk of investing in offspring of other males if they provide parental care. Nevertheless, males of many avian species and a minority of mammalian species provide parental care, and females of various species mate with multiple males. I investigate a two-locus genetic model for evolution of male parental care and female multiple mating in which females gain a direct benefit by multiple mating from the paternal care they thereby elicit for their offspring. The model suggests that, first, male parental care can evolve when it strongly enhances offspring survival and the direct costs of female multiple mating (e.g., loss of energy, risk of injury, exposure to infectious diseases) are greater than its indirect benefit (e.g., acquisition of good genes, increased genetic diversity among offspring); second, female multiple mating can evolve when paternal care is important for offspring survival or the indirect benefit of multiple mating is larger than its direct cost; and, finally, male parental care and female multiple mating can co-occur.     

Experimental field studies of Asian ape social systems

The Asian apes, orangutans and gibbons, possess unusual social systems among anthropoid primates. Social groups of gibbons consist of mated adult pairs and their offspring; mature orangutans are primarily solitary. Recent experimental field research has begun to yield insights into the behavioral mechanisms employed by these animals to maintain their characteristic patterns of social dispersion. While spatial separation between female orangutans appears to be maintained passively, aggression, which is manifest during direct encounters and long-distance vocal interactions, mediates male asociality. Male-male aggression is the result of intense intrasexual competition occurring between animals for mating access to females. To reduce intrasexual competition, male orangutans have adopted alternative mating tactics. In contrast to female orangutans, female gibbons show marked agonistic tendencies toward conspecifics. Female territoriality contributes to preventing males from becoming polygynous. Male gibbons, restricted to monogamous relationships, attempt to ensure their paternity through intrasexual aggression. These observations suggest that the spatial dispersion of females constrains male mating options in both species. However, variations between orangutan and gibbon social systems can be understood as consequences of the temporal dispersion of sexually receptive females. The temporal clumping of females, due to relatively high operational sex ratios, limits the ability of male gibbons to acquire multiple mates. Conversely, an extremely low operational sex ratio in orangutans creates a strong selection pressure for intrasexual competition and polygamous mating. These considerations provide a novel framework for interpreting the social systems of the African apes.     

Testes weight, body weight and mating systems in marsupials and monotremes

Relationships between testes weight, body weight and mating systems were examined in 40 marsupial species and in the extant monotremes. Relationships between relative testes weight and mating systems in marsupials resemble those previously described for primates. Thus relative testes weights are greatest in those marsupials where females mate with multiple males during the fertile period, i.e. polyandrous species (e.g. Antechinus ftavipes, Isoodon obesulus, Perameles nasuta, Potorous tridactylus, Macropus eugenii and M. agilis) and smallest in monandrous forms (e.g. Petauroides volans and Petaurus breviceps ) where females usually mate with a single male. These findings are consistent with effects of sperm competition upon the evolution of relative testes sizes in marsupials. Where field studies on marsupial mating systems are lacking, we make predictions based upon examination of their relative testes weights. Tarsipes rostratus, Acrobates pygmaeus, Macropus rufogriseus and Sarcophilus harrisii are predicted to engage in multiple matings and sperm competition. Conversely, Lasiorhinus latifrons, Cercatetus concinnus and Pseudoantechinus macdonnellensis are predicted to be monandrous in their mating behaviour. The monotremes ( Ornithorhynchus anatinus, Tachyglossus aculeatus and Zaglossus bruijnii ) are characterized by possession of very large testes; monotremes are shown to have significantly greater relative testes weights than marsupials, primates or avian species. This taxonomic difference is unlikely to be related lo the occurrence of oviparity or to the abdominal position of the testes in the Monotremata. Their mating systems are not known in detail, but some evidence for multiple matings (and hence for sperm competition) exists for Tachyglossus aculeatus so that their large testes may be adaptive in this context.     

The evolution of avian parental care

A stage model traces key behavioural tactics and life-history traits that are involved in the transition from promiscuity with no parental care, the mating system that typifies reptiles, to that typical of most birds, social monogamy with biparental care. In stage I, females assumed increasing parental investment in precocial young, female choice of mates increased, female-biased mating dispersal evolved and population sex ratios became male biased. In stage II, consortships between mating partners allowed males to attract rare social mates, provided a mechanism for paternity assessment and increased female ability to assess mate quality. In stage III, relative female scarcity enabled females to demand parental investment contributions from males having some paternity certainty. This innovation was facilitated by the nature of avian parental care; i.e. most care-giving activities can be adopted in small units. Moreover, the initial cost of care giving to males was small compared with its benefit to females. Males, however, tended to decline to assume non-partitionable, risky, or relatively costly parental activities. In stage IV, altriciality coevolved with increasing biparental care, resulting in social monogamy. Approaches for testing behavioural hypotheses are suggested.