Cooperative breeding in mammals

Cooperative breeding in mammals covers a diversity of breeding systems. In all cases, however, Individuals assist in the rearing of offspring other than their own. Recent research has highlighted some of the factors responsible for variation both within and between species. While it is possible to generalize about the selective pressures leading to cooperative breeding, doing so may obscure important contrasts between taxa. Of course, inclusive-fitness models explain the generalities of cooperative breeding, but differences in ecology, physiology and life history may result in distinctive processes operating in different taxa-data only likely to emerge from long-term field studies.     

Seasonal thermoregulatory responses in mammals

This study examined the proportional seasonal winter adjustments of total and mass-specific basal power (watts and watts g^–1, respectively), thermal conductance (watts g^–1 °C^–1), non-shivering thermogenesis capacity (ratio of NST/basal power), body temperature (°C), and body mass (g) of mammals. The responses are best summarized for three different body size classes; small mammals (<100 g), intermediate-sized mammals (0.1–10 kg), and large mammals (>10 kg). The principal adjustments of the small mammals center on energy conservation, especially the Dehnel Effect, the winter reduction in body size of as much as 50%, accompanied by reductions in mass-specific basal power. On average, these reductions reduce the total basal power approximately in direct proportion to the mass reductions. Reductions in mass-specific basal power are matched by concomitant reductions in conductance to maintain the setpoint body temperature during winter. The overall thermoregulatory adjustments in small mammals serve to (a) lower overall winter power consumption, (b) maintain the setpoint body temperature, and (c) lower the lower critical limit of thermoneutrality and hence thermoregulatory costs. In intermediate-size mammals, the seasonal response is centered more on increasing thermogenic capacity by increasing basal power and NST capacity, accompanied by predictable and large reductions in conductance. The Dehnel effect is negligible. Very large mammals undergo the largest reductions in total and mass-specific basal power and conductance. However, there are too few data to resolve whether the reductions in total basal power can be attributed to the Dehnel effect, because the moderate decreases in body mass may also be caused by nutritional stress. Apart from the seasonal changes in basal power, these observations are consistent with the predictions of Heldmaiers seasonal acclimatization model.     

Seasonal glucocorticoid responses to capture in wild free-living mammals

We determined baseline and capture-induced glucocorticoid concentrations during two different seasons in three species of wild free-living rodents: brown lemmings (Lemmus trimucronatus), golden-mantled ground squirrels (Spermophilus saturatus), and yellow-pine chipmunks (Tamias amoenus). Initial blood samples were obtained within 3 min of capture, so that initial glucocorticoid levels reflect baseline titers of undisturbed animals. Animals were held for an additional 30 min, when a second blood sample was taken to measure stress-induced glucocorticoid titers. The primary glucocorticoid differed in each species. Lemmings secreted extremely large amounts of corticosterone (as high as 8,000 ng/ml). These high concentrations were accompanied by high corticosterone-binding globulin capacity and resistance to negative feedback. Squirrels and chipmunks secreted a mixture of cortisol and corticosterone (10-400 ng/ml). In males of all three species and female squirrels and chipmunks, glucocorticoid levels were significantly elevated 30 min after capture. Baseline and 30-min glucocorticoid levels differed seasonally in each species. Levels were higher during summer (with no snow cover) than in spring (with approximately 60% snow cover) in female lemmings, higher during breeding than before hibernation in squirrels, and higher postreproductively than during breeding in chipmunks. Together, these data indicate that glucocorticoid responses to stress in these free-living species are similar to those in laboratory species, but the magnitude of the response appears to depend on life-history features specific to each species.     

Life histories and the evolution of cooperative breeding in mammals

While the evolution of cooperative breeding systems (where non-breeding helpers participate in rearing young produced by dominant females) has been restricted to lineages with socially monogamous mating systems where coefficients of relatedness between group members are usually high, not all monogamous lineages have produced species with cooperative breeding systems, suggesting that other factors constrain the evolution of cooperative breeding. Previous studies have suggested that life-history parameters, including longevity, may constrain the evolution of cooperative breeding. Here, we show that transitions to cooperative breeding across the mammalian phylogeny have been restricted to lineages where females produce multiple offspring per birth. We find no support for effects of longevity or of other life-history parameters. We suggest that the evolution of cooperative breeding has been restricted to monogamous lineages where helpers have the potential to increase the reproductive output of breeders.     

Reproductive suppression in female cooperatively breeding cichlids

Suppression by dominants of female subordinate reproduction has been found in many vertebrate social groups, but has rarely been shown experimentally. Here experimental evidence is provided for reproductive suppression in the group-living Lake Tanganyika cichlid Neolamprologus pulcher. Within groups of three unrelated females, suppression was due to medium- and small-sized females laying less frequently compared with large females, and compared with medium females in control pairs. Clutch size and average egg mass of all females depended on body size, but not on rank. In a second step, a large female was removed from the group and a very small female was added to keep the group size constant. The medium females immediately seized the dominant breeding position in the group and started to reproduce as frequently as control pairs, whereas clutch size and egg mass did not change. These results show that female subordinate cichlids are reproductively capable, but apparently suppressed with respect to egg laying. Nevertheless, some reproduction is tolerated, possibly to ensure continued alloparental care by subordinate females.     

小毛足鼠繁殖活动的季节变化

有关小毛足鼠 (Phodopusroborovskii)冬季活动和季节性繁殖模式的资料很少。九年来 ,我们在自然温度和光照条件下维持了一个实验室种群。这些自然条件说明其繁殖活动与野生条件下的很一致。甚至在零下 4 0度的冬季 ,罗氏仓鼠都不休眠。第一个繁殖高峰 (每 10对鼠繁殖 2 4窝 ,每窝平均 3 5只幼鼠 )在 4月份 ,第二个繁殖高峰 (每 10对鼠繁殖 2 9- 2 6窝 ,每窝平均 3 9- 3 6只幼鼠 )在 6 - 7月份。在秋季和初冬 ,繁殖强度降低到每 10对鼠产 0 1- 0 4窝 ,但繁殖并不完全终止。全年内 ,所有成年雄鼠可见下降的睾丸 ,雌鼠可见开放的阴道口。雄鼠的血液睾酮浓度在夏季最高 ,秋季和初冬降低。但是 ,独居鼠在最短的光周期和寒冷季节保持高的睾酮浓度。春季 ,睾酮平均水平显著升高 ,但未达到夏季的水平     

Higher reproductive skew among birds than mammals in cooperatively breeding species

While competition for limited breeding positions is a common feature of group life, species vary widely in the extent to which reproduction is shared among females (‘reproductive skew’). In recent years, there has been considerable debate over the mechanisms that generate variation in reproductive skew, with most evidence suggesting that subordinates breed when dominants are unable to prevent them from doing so. Here, we suggest that viviparity reduces the ability of dominant females to control subordinate reproduction and that, as a result, dominant female birds are more able than their mammal counterparts to prevent subordinates from breeding. Empirical data support this assertion. This perspective may increase our understanding of how cooperative groups form and are stabilized in nature.     

Developmental genetics of the female reproductive tract in mammals

The female reproductive tract receives the oocytes for fertilization, supports the development of the fetus and provides the passage for birth. Although abnormalities of this organ system can result in infertility and even death, until recently relatively little was known about the genetic processes that underlie its development. By drawing primarily on mouse mutagenesis studies and the analysis of human mutations we review the emerging genetic pathways that regulate female reproductive-tract formation in mammals and that are implicated in congenital abnormalities of this organ system. We also show that these pathways might be conserved between invertebrates and mammals.     

Seasonal changes in the brain and skull sizes of small mammals

A comparative study of seasonal-age dynamics of brain size in six small mammal species, Clethrionomys glareolus, C. rutilus, Microtus oeconomus, M. gregalis (Rodentia); Sorex araneus, S. minutis (Insectivora) has been carried out. The analysis of seasonal changes in brain weight confirms the existence of autumn-winter regression of brain weight, which takes place at the organism level. The regression is less pronounced in voles than in shrews. The decrease in brain weight both in voles and in shrews is accompanied by the decrease in height (capacity) of brain capsule of cranium. In spring the brain weight increase and reaches its maximal specific values in wintered voles in summer. In wintered shrews the brain weight never reaches the value, inherent in young animals before winter regression. The analysis of the data obtained and published data on variability of craniometric features allowed to conclude that seasonal changes in brain size, accompanied by the changes in capacity of cranium capsule might be considered as a general pattern for a large group of palearctic and nonarctic species of small mammals.     

Seasonal variability in survivorship of a cooperatively breeding tropical passerine

Survival of tropical passerines is thought to be higher than those in northern temperate regions, but relatively few tropical studies have addressed this issue, particularly in tropical Asia. We examined factors that may have influenced the survival rate of a cooperatively breeding bird, the puff-throated bulbul (Alophoixus pallidus), in an evergreen forest in northeastern Thailand. These factors included year, season (breeding and non-breeding), sex, and presence of helper(s) in a family group. We present evidence of breeding season-dependent survival in a tropical passerine using an information theoretic approach based on both mark-recapture and resighting data collected during 6 years of study. Based on colour-banded adults the annual survival rate did not vary significantly among years (average = 0.85 ± 0.02 SE). The mean lifespan (MLS) for the population was 6.22 ± 4.38 SE years. Survivorship was lower during the breeding season (0.89 ± 0.02 SE) than during the non-breeding season (0.96 ± 0.02 SE). The MLS of males and females was 6.70 ± 7.73 SE and 5.87 ± 4.88 SE years, respectively. The annual survival rate we observed was high compared to the estimates of other tropical and temperate passerines, possibly due to the relatively stable climatic conditions in tropical latitudes and puff-throated bulbuls being generalists that exploit a wide range of food resources both in space and time.     

Seasonal changes in hippocampus size and spatial behaviour in mammals and birds

Hippocampus is involved in processing of environmental spatial information, and its size is known to correlate positively with spatial abilities in mammals and birds. Comparisons between species suggest that amount of spatial information processed (the mean area of home range in particular) is related with hippocampus size. Do seasonal and age changes in hippocampus size correlate with seasonal dynamics of spatial behaviour during ontogenesis? The data obtained through observational and experimental studies confirm the possibility that hippocampus size may be subjected to adaptive modifications along with cyclic changes in spatial behavior. In course of seasonal dynamics, strong positive correlation was found between hippocampus mass, home range size, and mobility of small mammals. Recently, first facts demonstrating seasonal changes of hippocampus and spatial behaviour (in connection with food-storing and brood parasitism) were found in birds. A lot of facts obtained for different taxonomical groups shows parallel seasonal changes in spatial behaviour and morphology of brain region functionally related to such behaviour. Thus, in adult birds and mammals, not only behaviour but also brain structure is phenotypically flexible in response to seasonally changing environment. Morphophysiological mechanisms of hippocampus seasonal changes are also discussed.     

The evolution of social philopatry and dispersal in female mammals

In most social mammals, some females disperse from their natal group while others remain and breed there throughout their lives but, in a few, females typically disperse after adolescence and few individuals remain and breed in their natal group. These contrasts in philopatry and dispersal have an important consequence on the kinship structure of groups which, in turn, affects forms of social relationships between females. As yet, there is still widespread disagreement over the reasons for the evolution of habitual female dispersal, partly as a result of contrasting definitions of dispersal. This paper reviews variation in the frequency with which females leave their natal group or range (social dispersal) and argues that both the avoidance of local competition for resources and breeding opportunities and the need to find unrelated partners play an important role in contrasts between and within species.     

Sexual attractivity,proceptivity, and receptivity in female mammals

Special definitions are proposed for three concepts representing characteristics of female mammals when they are in estrus. Attractivity refers to the female's stimulus value in evoking sexual responses by the male. Proceptivity connotes various reactions by the female toward the male which constitute her assumption of initiative in establishing or maintaining sexual interaction. Receptivity is defined in terms of female responses necessary and sufficient for the male's success in achieving intravaginal ejaculation. Attempts are made to measure each variable in the S-R paradigm and to identify the causal agents determining each aspect of the estrous female's behavioral characteristics.     

Seasonal breeding in rhesus monkeys: Influence of the behavioral environment

The accepted model of breeding seasonality in rhesus monkeys states that females become reproductively active in response to an environmental cue and that males become sexually active in response to ovulating females. This model must be modified to include direct responses of the male to the physical environment, endocrine responses of males to sexual activity, and responses of the female to the sexual activation of fellow group members. The complex set of social stimuli that influences the breeding readiness of both sexes may serve to delimit more precisely the annual periods of conception and birth than would be the case if each individual responded only to the changing physical environment.