Xerus Inauris Research Articles

Assessing the effects of resource availability and parity on reproduction in female Cape ground squirrels: resources do not matter

AbstractThe reproductive output of iteroparous females is influenced by parity and resource availability. To gain a better understanding of the reproductive decisions made when animals are faced with potential trade‐offs between current and future reproduction; we investigated the relationship of parity and resources availability on reproductive intervals and success by comparing the performance of primiparous and multiparous female Cape ground squirrels Xerus inauris from two study sites that differed in resource availability. Our study included the correlation of reproductive behavioral data with fecal hormonal information to establish a normative endocrine profile for pregnancy and lactation. Parity had a major influence on juvenile emergence with multiparous females having more juveniles emerge successfully. Resource availability did not influence reproductive output, either because of an interaction among multiple resource components including quantity, predictability and length of time those resources were available or because other factors (such as predation or parasitism) have a greater impact on reproductive success.

Mapping the Landscape of Fear of the Cape Ground Squirrel (Xerus inauris)

The suitability of a habitat to an animal includes food availability, physical and climatic factors, population interactions, and safety from predators. Mappings of vegetation, soils, and microclimates across ecological landscapes have become standard and important tools for assessing an animal's habitat. More elusive to the researcher, yet of equal importance to the animal, has been the ability to map the predation risk perceived by an animal. Laundreet al., in developing the concept, defined the landscape of fear as the spatial map of the animal's predation cost of foraging. We mapped this landscape of predation costs by measuring the use of depletable food patches (yielding giving up densities (GUDs)) arranged as a grid across the landscape of interest. The landscapes of fear for 3 colonies of Cape ground squirrels (Xerus inauris) in Augrabies Falls National Park, South Africa, revealed large and distinct spatial variation in predation costs that appeared to be governed primarily by proximity to burrows and open sight lines. By converting the GUDs into quitting harvest rates (joules per minute), we believe we have translated the animals' perceptions of risk into a physical map whose contours across the landscape represent lines of equal foraging costs. Among the 3 colonies only 3-22% of the space resulted in low foraging costs (,2,500 J/min), whereas 31-92% of the sampled areas represented very high foraging costs (.5,000 J/min).

Structure and allometry of genitalia in males and females of a social African ground squirrel with high polygynandry

AbstractThe few studies that have looked at genital allometry in mammals have typically shown a positively allometric relationship with body size and high coefficients of variation. Cryptic female choice, sexual conflict or sperm competition are mechanisms underlying genital evolution and as these are not mutually exclusive, they are often difficult to disentangle. In addition, these mechanisms are affected by both male and female social structure and/or mating strategies and, as such, pre‐ and post‐copulatory behaviours have been shown to alter selection on genitalia. We examined genital traits and allometry in a polygynandrous and social ground squirrel Xerus inauris. We found that male testes are positively allometric and account for 1.5% of their body weight, one of the highest percentages known for sciurids. The penis, at 42.4% of head/body length, was isometric while the female reproductive tract, 22.4% head/body length, demonstrated no such relationship. Based on the allometric relationships of both males and females presented here, in conjunction with high levels of competition for females and lack of male aggression and territoriality, we suggest that sperm competition is the most likely mechanism for the evolution of the extremely large genitalia in this species.

Alternative reproductive tactics in male Cape ground squirrels Xerus inauris

In some animal societies, males vary in the strategies and tactics that they use for reproduction. Explanations for the evolution of alternative tactics have usually focussed on extrinsic factors such as social status, the environment or population density and have rarely examined proximate differences between individuals. Anecdotal evidence suggests that two alternative reproductive tactics occur in cooperatively breeding male Cape ground squirrels. Here we show that there is strong empirical support for physiological and behavioural differences to uphold this claim. ‘Dispersed’ males have higher resting metabolic rates and a heightened pituitary activity, compared with philopatric ‘natal’ males that have higher circulating cortisol levels. Dispersed males also spend more time moving and less time feeding than natal males. Additionally, lone males spend a greater proportion of their time vigilant and less of their time foraging than those that were in groups. The choice of whether to stay natal or become a disperser may depend on a number of factors such as age, natal group kin structure and reproductive suppression, and the likelihood of successful reproduction whilst remaining natal. Measuring proximate factors, such as behavioural and endocrine function, may provide valuable insights into mechanisms that underlie the evolution of alternative reproductive tactics.

The influence of sex and sociality on parasite loads in an African ground squirrel

Male-biased parasite loads, which are common in vertebrates, could be a consequence of sexual selection, and for species that group, the costs of parasites could vary with group size or social structure. We examined sex-biased parasitism and the influence of group size on parasite loads in Cape ground squirrels (Xerus inauris), a highly social species that occurs in the arid regions of southern Africa. Group size did not affect numbers of ectoparasites or endoparasites. Males carried 3 times as many ectoparasites as females, but females harbored nearly 3 times as many endoparasites as males. Age class did not affect parasite loads in females, but in males, adults carried more ectoparasites than juveniles. Allogrooming was performed primarily by females, but no sex difference was found in autogrooming. Males in the subadult age class are becoming scrotal (indicating an increase in sex hormones) but typically remain in the natal group until adulthood, maintaining home range sizes comparable to adult females. Our results suggest that sexual selection does influence parasite loads in this species; increased androgen levels may reduce ectoparasite resistance in males, and smaller home ranges may increase females’ exposure to endoparasites. Allogrooming could reduce ectoparasite loads of the group and mitigate the costs of grouping. Key words: ectoparasite, endoparasite, fleas, group size, grooming, lice, sex-biased parasitism, sexual selection, ticks. [Behav Ecol]

Pituitary luteinizing hormone responses to single doses of exogenous GnRH in female social Cape ground squirrels exhibiting low reproductive skew

AbstractThe Cape ground squirrel Xerus inauris is unusual among social mammals as it exhibits a low reproductive skew, being a facultative plural breeder with not all females breeding within a group. We investigated pituitary function to assess whether there was reproductive inhibition at the level of the pituitary and potentially the hypothalamus in breeding and non‐breeding female Cape ground squirrels. We did so during the summer and winter periods by measuring luteinizing hormone (LH) responses to single doses of 2 g exogenous gonadotropin‐releasing hormone (GnRH) and physiological saline administered to 42 females from 11 colonies. Basal LH concentrations of females increased in response to the GnRH challenge. Basal plasma LH concentrations were greater during winter, when most oestrus events are observed. However, we found no differences in plasma LH concentrations between breeding and non‐breeding females. We showed that the anterior pituitary of non‐breeding female ground squirrels is no less sensitive to exogenously administered GnRH than that of breeding females. We therefore concluded that the pituitary is no more active in breeding than non‐breeding females. The lack of differentiation in response to GnRH suggests that either non‐breeding females have ovaries that are less sensitive to LH or that they refrain from sexual activity with males through an alternative mechanism of self‐restraint.

Energetic costs of parasitism in the Cape ground squirrel Xerus inauris.

Parasites have been suggested to influence many aspects of host behaviour. Some of these effects may be mediated via their impact on host energy budgets. This impact may include effects on both energy intake and absorption as well as components of expenditure, including resting metabolic rate (RMR) and activity (e.g. grooming). Despite their potential importance, the energy costs of parasitism have seldom been directly quantified in a field setting. Here we pharmacologically treated female Cape ground squirrels (Xerus inauris) with anti-parasite drugs and measured the change in body composition, the daily energy expenditure (DEE) using doubly labelled water, the RMR by respirometry and the proportions of time spent looking for food, feeding, moving and grooming. Post-treatment animals gained an average 19g of fat or approximately 25kJd-1. DEE averaged 382kJd-1 prior to and 375kJd-1 post treatment (p>0.05). RMR averaged 174kJd-1 prior to and 217kJd-1 post treatment (p<0.009). Post-treatment animals spent less time looking for food and grooming, but more time on feeding. A primary impact of infection by parasites could be suppression of feeding behaviour and, hence, total available energy resources. The significant elevation of RMR after treatment was unexpected. One explanation might be that parasites produce metabolic by-products that suppress RMR. Overall, these findings suggest that impacts of parasites on host energy budgets are complex and are not easily explained by simple effects such as stimulation of a costly immune response. There is currently no broadly generalizable framework available for predicting the energetic consequences of parasitic infection.

Interspecific associations of Cape ground squirrels with two mongoose species: benefit or cost?

Mixed-species associations have been described in many vertebrate species, but few behavioral studies have investigated associations between species from different mammalian orders. Cape ground squirrels (Xerus inauris) are highly social rodents that inhabit burrows with two species of mongoose, but the benefits of these interspecific associations to ground squirrels remain unresolved. We compared the behavior of squirrels while solitary, with conspecifics, and in the presence of suricates (Suricata suricatta) and yellow mongooses (Cynictis pencillatus). Squirrels spent less time alert and more time feeding when suricates were present, but increased vigilance in the presence of yellow mongooses. In a series of mobbing trials with a puff adder (Bitis arietans), a common predator of all three species, Cape ground squirrels were the most active in mobbing the snake. Our results suggest that Cape ground squirrels benefit from associating with suricates, but not necessarily with yellow mongooses. Both mongoose species benefit from the burrowing activities of the squirrels for thermoregulation and escape from predators, and a suite of other organisms may similarly benefit from the habitat modifications by Cape ground squirrels, suggesting they could be considered ecosystem engineers of the arid and semi-arid regions of southern Africa. Thus, the association between Cape ground squirrels and suricates appears mutually beneficial, whereas yellow mongooses may merely be commensals of the squirrels.

Do Cape ground squirrels (Xerus inauris) discriminate between olfactory cues in the faeces of predators versus non-predators?

One way to avoid potential predators is to be sensitive to odour cues, particularly those in faeces and urine, left by predators. This sensitivity has been demonstrated in many solitary, nocturnal, small mammals which may fall victim to ambush predators.We tested the response of Cape ground squirrels, a diurnal, group-living small mammal, to the presence of predator (black-backed jackal) faeces and non-predator (black wildebeest) dung in baited traps, and also predator faeces and non-predator dung outside their burrows. The squirrels showed a significantly higher avoidance of traps scented with predator faeces than both control and non-predator dung-scented traps. They also took significantly longer to emerge from burrows that had predator faeces outside compared with control burrows and burrows with non-predator dung outside and showed a trend for higher vigilance once they emerged from burrows with predator faeces outside. We argue that as diurnal group-living reduces a reliance on olfactory cues, this species is more likely to rely on visual cues and the vigilance of other individuals than nocturnal solitary species that have been the focus of most studies up till now. As a result, squirrels very quickly return to normal behaviour after exposure to a predator cue. Level of sociality is likely to influence responses to olfactory cues of predators and should be the focus of further studies.

Do Cape ground squirrels (Xerus inauris) discriminate between olfactory cues in the faeces of predatorsversusnon-predators?

One way to avoid potential predators is to be sensitive to odour cues, particularly those in faeces and urine, left by predators. This sensitivity has been demonstrated in many solitary, nocturnal, small mammals which may fall victim to ambush predators. We tested the response of Cape ground squirrels, a diurnal, group-living small mammal, to the presence of predator (black-backed jackal) faeces and non-predator (black wildebeest) dung in baited traps, and also predator faeces and non-predator dung outside their burrows. The squirrels showed a significantly higher avoidance of traps scented with predator faeces than both control and non-predator dung-scented traps. They also took significantly longer to emerge from burrows that had predator faeces outside compared with control burrows and burrows with non-predator dung outside and showed a trend for higher vigilance once they emerged from burrows with predator faeces outside. We argue that as diurnal group-living reduces a reliance on olfactory cues, this species is more likely to rely on visual cues and the vigilance of other individuals than nocturnal solitary species that have been the focus of most studies up till now. As a result, squirrels very quickly return to normal behaviour after exposure to a predator cue. Level of sociality is likely to influence responses to olfactory cues of predators and should be the focus of further studies.

Effects of storage treatment on fecal steroid hormone concentrations of a rodent, the Cape ground squirrel ( Xerus inauris)

Fecal steroid analysis is an increasingly common non-invasive technique used in both captive and field studies to measure an animal’s approximate hormonal levels and corresponding physiological state. Fecal collection in the field necessitates storage and transportation methods that will prevent the degradation of hormonal metabolites by fecal bacteria. To determine the most stable and therefore preferred method of storage, 48 fecal samples were collected from six captive female Cape ground squirrels ( Xerus inauris). Each sample was randomly divided into three sub-samples to be processed for storage through freezing, drying, or preservation in ethanol. Frozen samples were stored at −20 °C, dried-treated samples were desiccated in a conventional oven at 40 °C for 4 h, and alcohol-treated samples were preserved in 3 ml of 95% ethanol. Samples were stored for 330 days followed by enzyme immunoassay analysis (EIA) to determine their progestogen and estrone conjugate (E 1C) concentrations. Validations were performed to establish that the progestogen and E 1C assays accurately measure fecal progestogen and estrone conjugate concentrations and were sensitive enough to detect biologically meaningful differences in these steroid metabolite concentrations in female X. inauris. Validation results showed a significant difference in progestogen concentrations of gravid females compared to sub-adults and non-gravid females. There was also a significant difference in estrone conjugates between sub-adult and adult females. Duration of storage time did not affect progestogen or estrone metabolite concentrations after being frozen for 3 months. Storage treatment results showed no significant difference between frozen and dried samples, but a significant difference was found between frozen and ethanol samples in both progestogen and estrone conjugate concentrations demonstrating that drying feces provides a reliable method for long-term preservation of fecal steroid concentrations and is the better alternative when freezing is not a viable option.

ARE CAPE GROUND SQUIRRELS (XERUS INAURIS) INDUCED OR SPONTANEOUS OVULATORS?

Little is known about the ovulation patterns in ground squirrels, even though the timing of ovulation could influence mate-order effects and sperm competition, which have been documented in a number of species. The Cape ground squirrel (Xerus inauris) is a social, nonhibernating, semifossorial rodent with aseasonal reproduction. Evidence suggests that sperm competition is an important aspect of male reproductive success in this species. We examined whether the Cape ground squirrel is an induced or spontaneous ovulator. Fifteen reproductive adult females were removed from natal colonies and housed individually. Females were subjected to 1 of 3 trials: no male contact, chemical and visual contact but no physical contact with intact males, or direct contact with epididymectomized males. Females from each of the 3 experimental treatments exhibited similar urinary progesterone metabolite concentrations and qualitative ovarian histology. Neither the presence of males nor copulation appear necessary for ovulation to occur, and ovulation in this social African ground squirrel appears to be spontaneous.

Phylogeny and historical biogeography of African ground squirrels: the role of climate change in the evolution of Xerus

We used phylogenetic and phylogeographical methods to infer relationships among African ground squirrels of the genus Xerus. Using Bayesian, maximum-parsimony, nested clade and coalescent analyses of cytochrome b sequences, we inferred interspecific relationships, evaluated the specific distinctness of Cape (Xerus inauris) and mountain (Xerus princeps) ground squirrels, and tested hypotheses for historical patterns of gene flow within X. inauris. The inferred phylogeny supports the hypothesized existence of an 'arid corridor' from the Horn of Africa to the Cape region. Although doubts have been raised regarding the specific distinctness of X. inauris and X. princeps, our analyses show that each represents a distinct well-supported, monophyletic lineage. Xerus inauris includes three major clades, two of which are geographically restricted. The distributions of X. inauris populations are concordant with divergences within and disjunctions between other taxa, which have been interpreted as results of Plio-Pleistocene climate cycles. Nested clade analysis, coalescent analyses, and analyses of genetic structure support allopatric fragmentation as the cause of the deep divergences within this species.

Delayed maturity, group fission and the limits of group size in female Cape ground squirrels (Sciuridae: Xerus inauris)

AbstractFemale Cape ground squirrels Xerus inauris live in matrilineal kin groups in which the number of reproductive positions is limited to three females. Field observations on a population in Namibia revealed that the mechanisms by which group size was limited were reproductive suppression and group fission. Sub‐adult females attained sexual maturity at 8 months of age when only a single adult female was present in their social group, whereas sub‐adult females in groups with more than one adult female matured at 12 months of age. Social groups with more than three breeding females subsequently split into smaller groups. Several hypotheses for limiting group size were considered. There was no evidence of higher ectoparasite loads on females in larger groups, nor was survival of adult females affected by group size. Although adult females have lower body masses and higher mortality during the winter when food is more scarce, feeding competition alone does not seem to limit group size. Groups that split continued to overlap in their feeding ranges and there was no seasonal difference in juvenile survival. However, juvenile survival was affected by group size. The greatest cost of being in a larger group was lower juvenile survival. Larger groups use larger feeding ranges, and because squirrels return to their sleeping burrow when threatened, not just to the nearest refuge, slower juveniles have greater exposure to predators. Thus, although competition for food resources may be important, predation seems to be a major constraint on female group size as well as the primary selective force leading to female group formation.

The effect of drought on the social structure and use of space in Cape ground squirrels, Xerus inauris

Through changes in resource abundance, both spatial and temporal variability in environmental conditions can influence the type of sociality and life-history exhibited in a species. We documented the behavior and use of space of Cape ground squirrels, Xerus inauris, both before and during a prolonged (2-year) drought. This southern African species occurs in semiarid regions where drought is unpredictable, but not uncommon. As expected, during the drought we observed larger home ranges, lower population density and reduced breeding. However, body mass and sex ratios did not differ between conditions, and despite an obvious reduction in resources, female social groups did not disband or decrease in size. Instead, both before and during the drought, Cape ground squirrels remained in tightly cohesive social groups. We conclude that the resilient social organization in the face of dramatic changes in resource abundance reflects the great benefit of grouping as an anti-predator strategy for this species.

Mating tactics of male Cape ground squirrels,Xerus inauris: consequences of year-round breeding

The Cape ground squirrel is a highly social, tropical ground squirrel that does not hibernate, suggesting that female receptivity could be scattered throughout the year. Males in this species are very social, living in all-male bands. I studied the mating tactics of male Cape ground squirrels to examine the effects of year-round female receptivity and male grouping on these tactics and on the degree of intrasexual competition. Female breeding was highly irregular and unpredictable, and the average operational sex ratio during oestrus was 10:1 (males to females). However, the predicted high rates of aggression were not observed. Instead, competition among males was manifested by competitive searching, repeated copulations and disruptions of copulations. Dominant males had more copulations with more females, first access to females and were more likely to copulate below-ground where disruptions were unlikely to occur. I conclude that the mating patterns of males in this species are a product of the asynchrony of female oestrus, year-round breeding and the survival benefits of grouping.

Why do male Cape ground squirrels live in groups?

The Cape ground squirrel,Xerus inauris, is a social species of ground squirrel inhabiting the arid areas of southern Africa. The high degree of sociality in this species is due to the occurrence of all-male social groups. This study used scan and all-occurrence sampling to investigate male grouping in this species. Several hypotheses concerning the cause and function of all-male groups were addressed: aggregations around oestrous females, assessment of competitors, enhanced thermoregulation, reproductive alliances, satellite males, information exchange and enhanced predator avoidance. Males were able to detect the onset of oestrus in females. Although oestrus influenced male group size, males continued to group beyond the breeding period. These groups persisted throughout the year, regardless of ambient temperature. Older males were better at finding oestrous females than younger males, but younger males did not preferentially associate with older males to follow them to females. Results best support the hypothesis that males benefit from enhanced predator detection and deterrence. Aggregations of males roaming between groups of asynchronously breeding females may persist beyond the breeding period because the benefits to survival outweigh the relatively minor costs of reproductive competition.

Reproductive Sharing in Different Types of Kin Associations

Reproductive Sharing in Different Types of Kin Associations

Reproductive Biology of a Tropical, Non-Hibernating Ground Squirrel

To examine the effects of climate and hibernation on reproductive patterns, I compared tropical and temperate species of ground and tree squirrels. I studied the reproductive biology of a tropical species, the Cape ground squirrel ( Xerus inauris ), by observing marked individuals in the Kalahari-bushveld region in Namibia. Breeding occurred throughout the year, although there was some seasonality. Interestrous intervals were 76.5–146 days, gestation averaged 47.7 days, lactation averaged 52 days, there were one to three litters per year, with litters of one or two young. Subadult males reached reproductive maturity at 8 months, while females had their first estrus at 10 months. Comparisons of these reproductive traits with other species in the family Sciuridae suggest that reproduction in the Cape ground squirrel is influenced by a long active season in an unpredictable habitat. Of the potential factors affecting reproduction in squirrels, I conclude that climate is the most critical, and nutrition, predation, and allometry are less important.

The Social Organization of the Cape Ground Squirrel (Xerus inauris; Rodentia: Sciuridae)

AbstractThe Cape ground squirrel (Xerus inauris) of southern Africa is a tropical species that does not hibernate. Field observations using scan and all‐occurrence sampling revealed that this species was highly social. Female Cape ground squirrels formed social units of related females and their subadult young, as is typical for other ground squirrels. Female social groups were usually composed of 2–3 adult females and 2–3 subadults of either sex. Members of these female social groups shared sleeping burrows and feeding ranges. Female social groups did not cooperatively defend their feeding ranges from adjacent groups in other burrow clusters. Interactions within female social groups were highly amicable, and no dominance hierarchy was evident. Males in this species also lived in groups. These all‐male bands of up to 19 individuals lived almost independently from female groups. The entire male band shared one home range, although ephemeral sub‐bands were formed daily. The composition and size of these sub‐bands changed daily. Interactions among males, which were largely amicable, included allogrooming and sleeping together. Analysis of interactions within the band indicated a stable, linear, dominance hierarchy among males. Dispersal in this species appeared to be male biased as is typical of other ground‐dwelling squirrels, with males dispersing at reproductive maturity. Males joining male bands were thus dispersers and were not likely to be closely related. Sociality in the Cape ground squirrel may be summarized as highly social female kin clusters and associated social non‐kin bands of males.