Neville Pillay

INTRASPECIFIC VARIATION IN THE SPATIAL AND SOCIAL ORGANIZATION OF THE AFRICAN STRIPED MOUSE

Abstract Social flexibility, that is, the expression of different types of social systems within one species, has been reported in several mammalian taxa, including rodents. However, sociality in rodents has been studied mostly in captivity and the results are often regarded as laboratory artifacts. We present field data for 2 populations of the striped mouse (Rhabdomys pumilio), a diurnal muroid rodent from southern Africa. The 2 populations inhabit different environments and demonstrate differences in social organization. R. pumilio in the arid succulent karoo lives in social groups, comprising multiple adults of both sexes that share 1 nest and the same territory. Striped mice in the moist grasslands of South Africa are solitary. Females inhabit exclusive territories and territories of males overlap those of several females; association between the sexes appears to be restricted to mating. Home ranges of females in the grasslands were 6 times larger and home ranges of males were 10 times larger than their counterparts in the succulent karoo. Onset of reproductive activity occurred earlier and at a significantly lighter body weight in grasslands. In the succulent karoo, sexually mature offspring remain in their natal group without reproducing. We suggest that group living in the succulent karoo is in response to habitat saturation and the benefits of philopatry, whereas living alone in the grasslands may be a response to limiting resources, such as food.

The Striped Mouse (Rhabdomys pumilio) From the Succulent Karoo, South Africa: A Territorial Group-Living Solitary Forager With Communal Breeding and Helpers at the Nest.

The authors studied the striped mouse (Rhabdomys pumilio) in the semiarid succulent karoo of South Africa. Mice forage alone, but they live in groups that share a common nest. Groups consist of 1 to 4 breeding females, 1 to 2 breeding males, and their offspring of both sexes, which remain in their natal group even after reaching adulthood, participating in territorial defense and nest building without showing signs of reproductive activity. Interactions are typically amicable and take place inside or in front of the nest. In contrast, encounters with mice from other groups are aggressive. Group living in the succulent karoo is possibly due to ecological constraints imposed by habitat saturation because of a year-round stable food supply as well as associated benefits of philopatry.

Paternal Care in the Social and Diurnal Striped Mouse (Rhabdomys pumilio): Laboratory and Field Evidence

Paternal care is rare in mammals, occurring mainly in carnivores and neotropical primates, in which the difficulties of long generation time and large individuals lead to small sample sizes. Here, the authors show that paternal care can be easily studied in the four-striped mouse (Rhabdomys pumilio) because (a) captive males show all the patterns of parental care as do females, with the obvious exception of nursing; (b) in the field, wild males act amicably toward juveniles and retrieve pups experimentally presented to them; (c) the striped mouse facilitates experimental approaches in captivity because it has a short generation period and can be kept in large numbers; and (d) the striped mouse is diurnal, not only making observations in captivity easier but also enabling direct observations in the field.

Social flexibility and social evolution in mammals: a case study of the African striped mouse (Rhabdomys pumilio)

Environmental change poses challenges to many organisms. The resilience of a species to such change depends on its ability to respond adaptively. Social flexibility is such an adaptive response, whereby individuals of both sexes change their reproductive tactics facultatively in response to fluctuating environmental conditions, leading to changes in the social system. Social flexibility focuses on individual flexibility, and provides a unique opportunity to study both the ultimate and proximate causes of sociality by comparing between solitary and group‐living individuals of the same population: why do animals form groups and how is group‐living regulated by the environment and the neuro‐endocrine system? These key questions have been studied for the past ten years in the striped mouse Rhabdomys pumilio. High population density favours philopatry and group‐living, while reproductive competition favours dispersal and solitary‐living. Studies of genetic parentage reveal that relative fitness of alternative reproductive tactics depends on the prevailing environment. Tactics have different fitness under constrained ecological conditions, when competitive ability is important. Under conditions with relaxed ecological constraints, alternative tactics can yield equal fitness. Both male and female striped mice display alternative reproductive tactics based on a single strategy, i.e. all individuals follow the same decision rules. These changes are regulated by endocrine mechanisms. Social flexibility is regarded as an adaptation to unpredictably changing environments, selecting for high phenotypic flexibility based on a broad reaction norm, not on genetic polymorphism for specific tactics.

Reproductive competition favours solitary living while ecological constraints impose group-living in African striped mice

1. Social groups typically form due to delayed dispersal of adult offspring when no opportunities for independent breeding exist, or the costs of dispersal are higher than the costs of remaining philopatric. Ecological constraints are thought to be a main reason for group-living in animals. 2. Reproductive competition within groups can induce high costs of philopatry, and is thought to be a main reason for solitary living. 3. Experimental manipulation of reproductive competition is difficult. One solution is to compare sociality between periods with and without reproductive competition. 4. Here, we show empirically in a 8-year field study that striped mice (Rhabdomys pumilio) of both sexes were group-living during the breeding season when population density (PD) was high but solitary living when PD was low, supporting the ecological constraints model. 5. After the breeding season, in the absence of reproductive competition, the positive correlation between PD and percentage of group-living striped mice was absent. Almost all striped mice were group-living even under very low population densities. This supports the reproductive competition model. 6. Ambient temperature, food availability and predation pressure, did not influence sociality. 7. In captivity, the costs of reproductive competition in communal groups include female infanticide and aggression between females. 8. We conclude that group-living is favoured by constraints imposed through habitat saturation and by its benefits (improved thermoregulation by huddling, group-territoriality and predator avoidance), and that reproductive competition is a major force favouring solitary living in striped mice.

Testosterone levels in dominant sociable males are lower than in solitary roamers: physiological differences between three male reproductive tactics in a sociably flexible mammal.

The relative plasticity hypothesis predicts that alternative tactics are associated with changes in steroid hormone levels. In species with alternative male reproductive tactics, the highest androgen levels have usually been reported in dominant males. However, in sociable species, dominant males show amicable behaviors to gain access to females, which might conflict with high testosterone levels. We compared testosterone, corticosterone, and resting metabolic rate in male striped mice (Rhabdomys pumilio) following a conditional strategy with three different reproductive tactics: (i) philopatric group‐living males, (ii) solitary‐living roamers, (iii) dominant but sociable group‐living territorial breeders. Philopatrics had the lowest testosterone but highest corticosterone levels, suggesting that they make the best of a bad job. Dominant territorial breeders had lower testosterone levels than roamers, which have a lower competitive status. Roamers had the highest testosterone levels, which might promote risky behavior, such as invading territories defended by territorial males. Roamers also had lower resting metabolic rates than either type of group‐living males. Our results suggest that dominant males’ testosterone levels reflect a trade‐off between low testosterone amicable behavior and high testosterone dominance behavior.

Female home range size is regulated by resource distribution and intraspecific competition: a long-term field study

The size of an individuals home range is an important feature, influencing reproduction and survival, but it can vary considerably among both populations and individuals. The factors accounting for such variation are still poorly understood, and comprehensive long-term field studies considering various environmental factors that influence home range size are rare. We investigated the effects of seasonality, availability of food, cover, number of direct neighbours and the relative individual body mass on home range sizes in 125 adult female striped mice, Rhabdomys pumilio, in South Africa from 2004 to 2008. We used radiotelemetry to estimate home range sizes, trapping to determine the number of direct neighbours, and plant surveys in every home range to determine availability of food and cover. Home ranges were smaller when food quantity was high, many territorial neighbours were present, females had a relatively small body mass and during the nonbreeding season. We conclude that the availability of food resources and intraspecific competition are the main factors influencing home range size in female striped mice. Females enlarged their home ranges when territorial neighbours were few, and there was a significant positive correlation between home range size and quantity of food plants. This indicates that home range size might not reflect the minimal trade-off between access to resources that allow for a females survival and lowest cost for defending and foraging in that area. Instead, we propose a hypothesis for future research that female striped mice occupy areas several times larger than needed to improve their fitness by providing resources for future offspring.

Female mate preference and reproductive isolation in populations of the striped mouse Rhabdomys pumilio

Laboratory experiments were conducted to study female mate preference in three allopatric populations of the striped mouse Rhabdomys pumilio , an African murid rodent. The three populations (Alice, Goegap, Irene) represent the extremes of the distribution range of the species in South Africa. Using two-way choice tests, virgin females were exposed to: (i) males of the same population (homotype) and those of another population (heterotype) in whole animal choice tests, or (ii) the odiferous bedding of homotype and heterotype males. Generally, females significantly preferred homotype stimulus males or their odour. This preference was only evident when females were in oestrus, suggesting that females were choosing potential mates. In addition, prior association with males is not needed for the establishment of female mate preference, since females were never exposed to adult males of any of the populations prior to testing. The only exception was receptive Goegap females which showed equal preference for homotype and Alice heterotype males. Mate preference in these two populations is asymmetric since Alice females showed strong preference for homotype males over Goegap males. The results indicate that divergence has occurred in allopatry, resulting in populationspecific communication signals and in particular olfactory cues, assortative mate choice, and pre-mating reproductive isolation.

Stereotypic behaviour is genetically transmitted in the African striped mouse Rhabdomys pumilio

Abstract The development of stereotypic behaviour was studied in captive striped mice, Rhabdomys pumilio. Based on the findings of previous studies, it was hypothesised that there is a genetic basis for stereotypy in striped mice. Breeding and cross-fostering experiments were used to test this hypothesis. Two treatments were performed per experiment, involving pairings of: (i) stereotypic females and non-stereotypic males; and (ii) non-stereotypic females and non-stereotypic males. In breeding experiments, the incidences of stereotypy were approximately four times more common in the offspring of stereotypic females than in those of non-stereotypic females. Cross-fostering of young between treatments (stereotypic and non-stereotypic females) or within treatments (control experiments) did not influence the patterns of transmission of stereotypy. The data indicate that the development of stereotypy in captive R. pumilio is strongly related to its occurrence in the biological mother. However, there was no congruence between the type of stereotypies displayed by mothers and offspring, with almost all stereotypic offspring displaying one type of stereotypy (cage-lid climbing and back-flipping). The results show that there is a strong genetic basis for the development of stereotypy in striped mice, while social influences appear to be minimal.

Behavioral strategies of the African ice rat Otomys sloggetti robertsi in the cold

Thermoregulation in cold environments imposes great energy demands on small mammals. For the ice rat Otomys sloggetti robertsi, a herbivore endemic to high altitude alpine regions of southern Africa, these demands might be high since it does not hibernate and is poorly-adapted physiologically to low temperatures. We investigated whether ice rats modify their behavior patterns seasonally by recording the duration of behaviors as well as sequential transitions among behaviors for free-living adult males and females. Contrary to expectations, gender and time of day did not influence ice rat behavior in both seasons. However, there were marked seasonal differences in the duration of behaviors, with ice rats spending significantly more time foraging and basking in winter whereas they remained belowground in their burrows for longer periods of time in summer. Hoarding behavior increased significantly from summer to winter and females showed higher levels of hoarding in summer than males. Sequential analyses revealed that behaviors were strongly influenced by season. There were more two-way transitions in winter than in summer, indicating the occurrence of behavioral routines which might reduce energy expenditure. Our data suggest that ice rats modify their behavior in terms of the duration and sequences of behaviors in response to prevailing environmental conditions.