Female Squirrels Research Articles

Mass-dependent reproduction or reproduction-dependent mass? A comment on body mass and first-time reproduction in female sciurids

We question the validity of Svendsen and White's conclusion that body mass affects the timing of primiparity in eastern chipmunks (Tamias striatus) (G.E. Svendsen and M.M. White. Can. J. Zool. 75: 1891-1895. 1997). Because the masses of reproductive and non-reproductive females were measured during the postpartum period only, the differences reported by Svendsen and White may be due to reproductive females gaining more mass than non-reproductive females between the premating and postpartum periods. We evaluated the plausibility of this alternative explanation by comparing the body mass of reproductive and non-reproductive female red squirrels (Tamiasciurus hudsonicus). Like Svendsen and White, we found that postpartum reproductive females were significantly heavier than non-reproductive females, but contrary to their interpretation, these differences did not exist during the premating period. We conclude that primiparity in female sciurids may not be mass-dependent, but rather that postpartum mass differences may occur because reproductive females augment their energy reserves to help sustain reproductive demands during late lactation.

Conflict and assessment in a predator–prey system: ground squirrels versus rattlesnakes

Adult California ground squirrels,Spermophilus beecheyi beecheyi, actively confront and harass northern Pacific rattlesnakes,Crotalus viridis oreganus, which are the principal predator of ground squirrel pups. In this report we examine the roles of risk (snake size) and context (location of encounter and squirrel reproductive category) in rattlesnake assessment by ground squirrels. In interpreting the results, we borrow heavily from the well-developed conceptual framework applied in the analogous case of intraspecific conflict. Large and small snakes were tethered near the home burrows of male ground squirrels, and maternal and nonmaternal female ground squirrels. Ground squirrels appeared to employ assessment strategies which served to mediate the level of risk associated with confronting larger snakes. The results suggest that ground squirrels exercise greater caution when dealing with large snakes and invest more in monitoring the snake from a safe distance. Maternal squirrels, which have more at stake reproductively, spent more time and effort in snake-directed activities than did squirrels from other reproductive categories. Mothers also differentiated more strongly between large and small snakes, perhaps reflecting the greater vulnerability of their pups to larger snakes. Finally, ground squirrels discriminated between snakes found close to their home burrow and those encountered further abroad. At the home burrow, squirrels monitored the snake from a closer distance, displayed a greater willingness to confront the snake, and escalated to more dangerous levels of harassment. This assessment strategy may reflect a higher payoff to squirrels that persist in driving snakes out of the home area, thereby reducing the risk of future ambush.

Reproductive Decisions in Female European Ground Squirrels: Factors Affecting Reproductive Output and Maternal Investment

Physiological and behavioural parameters associated with reproductive effort and success were investigated in female European ground squirrels Spermophilus citellus. The proportion of reproductive (lactating) females in the study population was over 90% and was not related to age. Timing of oestrus and ovulation was found to be affected by the female’s emergence date and condition. Females with low emergence mass showed delayed oestrus. Differences in ovulation dates were shown to affect reproductive output in terms of litter size and sex ratio. Early litters were larger and male biased. X‐ray techniques were used to determine intrauterine litter size in individual females. The results indicated that litter size and sex ratio were fixed prenatally. Lactation costs were reflected in the intensity of mass loss and duration of lactation. Mass loss varied with litter size, in that females with large litters showed a more rapid loss than others. The second parental investment parameter, lactation duration, varied among individual females and was dependent on the timing of reproduction and litter size (except yearlings). Early born litters, which were, in most cases, larger than later ones, were nursed longer. Prolonged lactation periods affected female condition in that they started prehibernation fattening later and entered hibernation with a lower mass than individuals that had shorter lactation periods. Yearling females probably could not afford the energetic costs of long lactation, independent of their offspring number. These results indicated that females with higher reproductive output and higher investment were unable to compensate these costs before hibernation. Consequences for these individuals could therefore be lower over‐winter survival or a delayed oestrus in the following season.

Survival of Captive-Born Cross-Fostered Juvenile Columbian Ground Squirrels in the Field

Effects of birth in captivity and cross-fostering of neonates on subsequent survival of female Columbian ground squirrels ( Spermophilus columbianus ) and their litters, after their immediate reintroduction into field sites, was evaluated for 116 litters over 2 years. Giving birth in captivity did not affect adversely survival of adult females to the following spring. Frequency of loss of entire litters was not greater for litters born in captivity than for those born in the field, and partial loss also appeared to be similar in the two groups based on litter sizes at emergence. Among captive-born litters, cross-fostering had no discernible adverse effect on juvenile survival relative to unmanipulated litters. Overall, fostered juveniles survived as well as non-fostered littermates to emergence and to the following spring, although in litters suffering partial loss, fostered juveniles had lower survival rates to emergence than non-fostered rearing mates. Of the elements of our protocol, only body weight of females significantly affected success in rearing the litter, and neither larger litters nor larger numbers of neonates fostered adversely affected success of the litter. Our results indicate that the protocol used is a robust method in situations requiring manipulation of litters or knowledge of litter size at birth.

Sexual Differences in Reproductive Effort of Richardson's Ground Squirrels

Male and female Richardson's ground squirrels exhibit differences in a constellation of behavioral, ecological, and physiological characteristics related to a fundamental sexual difference in reproductive effort, viz. males primarily expend energy on mate acquisition whereas females primarily expend energy on parenting. Male Richardson's ground squirrels emerge from hibernation earlier than females, at a heavier body mass, and with larger fat reserves. Once females emerge, males reduce time foraging and they compete aggressively and intensively for access to estrous females with resultant physical wounding, loss of body mass, depletion of fat reserves, and high mortality. Females retain post-hibernation fat reserves during gestation and even increase in personal body mass. Although mass of the litter exceeds the mother's own mass by late lactation, mothers are largely able to meet the demands of milk production through foraging, and their body mass remains fairly constant through lactation. Adult males immerge into hibernation earlier than adult females, and are heavier and fatter on immergence. In their first active season, juvenile males complete growth to adult size, whereas juvenile females attain only 80% of adult size; females then complete growth during pregnancy the following spring. Juvenile males attain adult size by delaying entry into hibernation until 7-9 weeks later than juvenile females. During hibernation, both juvenile and adult male Richardson's ground squirrels spend substantially less time than females in the physiological state of torpor. In particular, males tenninate torpor> I week before emergence, during which time they subsist on seeds cached in the hibernaculum, replenish fat, and initiate spennatogenesis in preparation for the forthcoming mating season. In contrast, female Richardson's ground squirrels do not store food in the hibernaculum, they emerge from hibernation <2 days after termination of torpor, and they are impregnated 2-4 days after' emergence from hibernation when at their lowest annual body mass. On an annual basis, adult females have a higher survival rate than adult males, largely because females are more likely to survive through the reproductive period.

Entrainment of Activity Rhythms to Temperature Cycles in Diurnal Palm Squirrels

RAJARATNAM, S. M. W. AND J. R. REDMAN.Entrainment of activity rhythms to temperature cycles in diurnal palm squirrels.PHYSIOL BEHAV 63(2) 271–277, 1998.—Ambient temperature cycles entrain circadian rhythms of homeotherms. The phase that entrainment occurs at varies, particularly in diurnal species. We investigated whether ambient temperature cycles of 12 h warm (34 to 40°C) and 12 h cool (24 to 28°C) entrained locomotor activity rhythms of diurnal Indian palm squirrels (Funambulus pennanti) that were free-running in constant dim light (3.2 to 7.6 lux). Seven female squirrels were exposed to the temperature cycle for 21 days, after which a 5-h phase delay of the cycle was imposed. The cycle then continued for a further 50 days. Three of the seven squirrels showed entrainment to the temperature cycle. Of the three which entrained, one was warm-active and two were cool-active. Of the remaining squirrels, two showed entrainment or relative coordination of one component of the rhythm, and two did not show any entrainment. Positive and negative masking of activity by the warm and cool fractions were observed regardless of whether or not squirrels entrained. These results suggest that ambient temperature is an effective zeitgeber for F. pennanti. As has been reported for other diurnal species, interindividual differences exist in the phase of entrainment to temperature cycles.

Effects of continuous light, continuous darkness and pinealectomy on pineal-thyroid-gonadal axis of the female Indian palm squirrel, Funambulus pennanti.

Effects of continuous light (CL; 24L:0D), continuous darkness (CD; 24D:0L) and pinealectomy (Px) were tested on pineal-thyroid-gonadal axis of a tropical seasonally breeding rodent, Funambulus pennanti during its sexually active reproductive phase (February-March). CL had no effect on pineal, thyroid and ovarian functions as judged by weight, plasma level of thyroxine (T4) and estradiol (E2). However, CD reduced significantly thyroid and ovarian weight and plasma T4, and E2 levels. Px was stimulatory for thyroid and ovarian functions under normal photoperiod, and under CL, Px significantly increased thyroid and ovarian weights with no observable changes in their hormonal levels. Px under CD condition prevented the reduction in thyroid and ovarian weights but the hormonal levels were increased. These results suggest that CD could be a effective stimulator for the pineal function and increased pineal activity in this mammal. The inhibitory effect of CD on thyroid and ovarian functions could be due to the increased pineal activity in term of induced melatonin by CD itself.

Suprachiasmatic nucleus: role in circannual body mass and hibernation rhythms of ground squirrels

Female golden-mantled ground squirrels that sustained complete ablation of the suprachiasmatic nucleus (SCNx) were housed pre- and post-operatively at 23°C and then at 6.5°C for 5–7 yr. SCNx and control animals held at the higher temperature manifested circannual rhythms (CARs) in body mass. In contrast, body mass CARs were not expressed in 50% of SCNx squirrels during cold exposure; rhythm amplitude was reduced to 25–40% of pre-operative values and the interval between successive peaks in body mass fell outside the circannual range. Unlike normal squirrels that hibernate for about 6 months during each circannual cycle, these SCNx squirrels expressed bouts of torpor nearly continuously throughout 2.5 yr of cold exposure. Body mass increases were often observed during hibernation—a phenomenon never observed in control animals. The remaining SCNx squirrels that did not hibernate continuously displayed CARs in body mass within the normal range. The effects of SCN ablation on body mass rhythms presumably are related to disrupted patterns of hibernation, food intake, and metabolism. The SCN, which sustains neural and metabolic activity at low tissue temperatures, may exert greater influence on thermoregulation and metabolism during the hibernation season than at other times of year, thereby accounting for the greater effect of SCN ablation in squirrels maintained at low ambient temperatures.

Effects of temporal and spatial variation in habitat quality on red squirrel dispersal behaviour

Immigration patterns and the frequency of breeding dispersal in tree squirrels are predicted to be related to the amount of temporal and spatial variation in tree seed crops, their primary food supply. We studied Eurasian red squirrel,Sciurus vulgarisdispersal patterns in a stable habitat with predictable food supply and a variable habitat with large temporal and spatial variations in food availability. In both habitats, we observed a male-biased immigration in spring and a female-biased immigration in autumn. However, there were more adults among immigrating red squirrels in the variable (66%) than the stable (31%) habitat and large differences in the extent of site fidelity between the two squirrel populations. Multivariate analyses indicate that food availability appeared to be the main factor affecting female dispersal behaviour. The data also suggest that female red squirrel dispersal patterns are an adaptive response to the predictability of food resources in space and time. Male dispersal behaviour seemed to be influenced by the distribution of the females. Their level of site fidelity was high in the stable habitat, whereas they appeared to track the movement of females in the variable habitat.

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

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

Effects of Administration Regime on the Psychotomimetic Properties of d-amphetamine in the Squirrel Monkey ( Saimiri sciureus )

Studies have reported that d-amphetamine can induce a schizophreniform psychosis in humans and can induce abnormal behaviour patterns in monkeys that resemble the psychotic symptoms observed in man. The purpose of the present study was to identify a drug administration regime that in squirrel monkeys reliably could induce such behaviours in order to use this as a model of schizophrenia. The behavioural effects of acute, subchronic and continuous administration of d-amphetamine were determined in male and female squirrel monkeys during short term separation from the colony and in the home cage. It was found that abnormal behaviours developed in both male and female subjects and that they were most evident in the home cage. The number of subjects responding was highest during continuous infusion followed by subchronic treatment. The study indicated that prolonged administration of high doses of d-amphetamine is necessary for the development of abnormal behaviours. These findings suggest that animal models of schizophrenia based on d-amphetamine should be based on chronic administration or continuous infusion of d-amphetamine instead of acute injections.

Effects of Hoard Manipulations on Life History and Reproductive Success of Female Red Squirrels (Tamiasciurus hudsonicus)

We conducted a series of hoard manipulations on the red squirrel ( Tamiasciurus hudsonicus ), a solitary species that constructs central food caches, to determine if the size and content of the hoard influenced the life-history parameters and reproductive success of females. Larderhoards of jack pine ( Pinns banksiana) cones were experimentally increased in autumn of 1989 and 1990, and decreased in autumn 1989. In both years we also supplied sunflower seeds (a food item with high energy content and low handling time) to individual females. Our experiments had no significant effect on the survival of females (prior to parturition or after weaning), prepartum or postpartum body mass, size of litters, number of offspring emerging from nests, or number of offspring weaned. Parturition date was significantly earlier ( P = 0.003) in females that received additions of seeds in the 1st year of the experiment. We also determined that control females harvested only a small fraction of the jack pine cones available on their territories. Our observations suggest that availability of food was not placing constraints on females during the years of our experiment. We also discuss possible reasons why dates of parturition (and estrus) were relatively more responsive to levels of food than our other measurements.

Post‐release behaviour, home range establishment and settlement success of reintroduced red squirrels

The behaviour of eight radiotagged red squirrels (Sciurus vulgaris Linnaeus, 1758) reintroduced at Parco Groane, northern Italy, was monitored for two months following their release. Both male and female squirrels moved over considerable distances the first two weeks after introduction, exploring the new habitat. Two types of exploring behaviour were observed: (i) gradually expanding the home range, moving only short distances each day; and (ii) making regular excursions of more than 1 km followed by short periods of activity in a small area. The latter behaviour resulted in very large home ranges during the exploring phase. After the initial exploring, home range size decreased and one male and two female squirrels changed their space use by increasing the number of activity centres (core‐areas) within their home range. Post‐settlement home ranges were spaced out over the available habitat and all squirrels used almost completely exclusive core‐areas. Only four red squirrels (two males and two females) survived long enough to be the potential founders of the new population that established itself at Parco Groane. We discuss methods to decrease post‐release mortality in red squirrels and consider genetic and demographic consequences of squirrel reintroductions.

Single and repeated intravenous toxicity studies of pamiteplase (genetical recombination) in rats and monkeys.

Single and repeated intravenous toxicity studies of Pamiteplase (genetical recombination) YM866, a novel recombinant human tissue-type plasminogen activator, were conducted. No animal died from toxic effects of YM866 after single administration to F344 rats, squirrel monkeys and cynomolgus monkeys. Male and female F344 rats were given YM866 intravenously for 4 weeks at doses of 0 (vehicle), 0.1, 0.3 and 1 mg/kg/day. An increase in platelet count, slight decreases in hemoglobin and hematocrit, increases in plasma phospholipids, total cholesterol and total protein, and liver weight were observed at 1.0 mg/kg. Histopathology revealed no changes in any organ except for hemorrhage at the injection sites. These changes recovered after 4 weeks of withdrawal. Male and female squirrel monkeys were given YM866 intravenously for 4 weeks at doses of 0 (saline), 0 (vehicle), 0.1, 0.3 and 1 mg/kg/day. Prolongation of coagulation time at the injection site was observed at 0.3 mg/kg or more. Subcutaneous hemorrhage and a transient decrease in locomotor activity were observed at 1 mg/kg. Prolongation of coagulation time at the injection site was considered to be related to the pharmacological action of YM866. The results show that the approximate single lethal dose of YM866 is more than 60 mg/kg in rats, and more than 10 mg/kg in squirrel monkeys and cynomolgus monkeys. The no-toxic-effect level of YM866 after repeated administration for 4 weeks in rats and squirrel monkeys is considered to be 0.3 mg/kg.

Chuck Vocalizations of Wild Female Squirrel Monkeys (Saimiri sciureus) Contain Information on Caller Identity and Foraging Activity

Analysis of the acoustic signal of the chuck vocalizations of adult female squirrel monkeys (Saimiri sciureus) in Parque Nacional de Manu, Peru, revealed consistent differences within and between individuals. We quantified four peak frequency parameters: (a) the peak frequency of single chucks, (b) the first and (c) the second peak frequencies of double chucks, and (d) the peak difference: the difference between the first and the second double chuck peaks. One-way ANOVAs and a posteriori comparisons of these variables revealed that each distinguished more than 70% of all possible pairs of females. When all double chuck measures were included in a discriminant analysis, 57% of double chucks were correctly assigned to the caller. Another category of information potentially encoded in the acoustic structure of chuck vocalizations is foraging activity. When the chucks of squirrel monkeys during foraging and nonforaging activities were compared, the single chuck peak frequency, and the first peak frequency and the peak difference of double chucks, were significantly reduced during foraging contexts. Previously Boinski and Mitchell (1992) concluded that chucks facilitate group cohesion among widely dispersed troop members by providing information of the location of callers; the rate of chucks produced by an adult female increases as she becomes more spatially and visually separated from other adult females. The additional information potentially conveyed by chucks on caller identity and foraging activity documented in these new analyses further emphasizes the role chucks serve to enhance group coordination and cohesion.

Natal Philopatry, Communal Nesting, and Kinship in Fox Squirrels and Gray Squirrels

Tree squirrels (Sciuridae: Sciurini) are considered to be asocial; however, communal nesting is reported for most species. I examined the sociality of uniquely marked fox squirrels ( Sciurus niger ) and gray squirrels ( S. carolinensis ) to investigate interspecific patterns of natal philopatry, communal nesting, and kinship. Natal philopatry was female-biased in gray squirrels, resulting in the formation of kin clusters, but no fox squirrels remained in their natal areas. Adult male and female squirrels interacted less than expected, suggesting that the social systems of the sexes are separate. Communal nesting occurred in all seasons, but was most frequent in winter. Unrelated adult males commonly nested together in both species. However, adult female fox squirrels rarely nested in groups, although communal nests were common among female gray squirrels. Amicable behaviors were directed only toward closely related females ( r = 0.5) within kin groups of gray squirrels; aggression was directed primarily at unrelated females. The female-female bond was important in the formation of groups in gray squirrels.

Reproductive Sharing in Different Types of Kin Associations

Reproductive Sharing in Different Types of Kin Associations

Ablation of suprachiasmatic nucleus alters timing of hibernation in ground squirrels.

Hibernation patterns were monitored continuously for 2.5 years in female squirrels that were neurologically intact or in which the hypothalamic suprachiasmatic nucleus (SCN) was completely ablated (SCNx). The number of hibernation bouts in SCNx squirrels increased by 159%, total hibernation time increased by 58%, and periodic arousals from hibernation were 47% longer in SCNx than in control squirrels; the duration of individual torpor bouts was 2 days shorter and far more variable in SCNx than in control animals. Some SCNx squirrels cycled through bouts of torpor continuously for nearly 2 years. The SCN appears to be part of the mechanism that controls the duration of the hibernation season and the temporal structure of individual torpor bouts.

Harderian gland function of Indian tropical palm squirrel, Funambulus pennanti.

The Harderian gland (HG) of the Indian palm squirrel, F. pennanti, is composed of acini of a single type of simple columnar cells with uniform-sized lipid droplets and porphyrin (P) in the lumen. Morphologically it presented no sexual dimorphism except for the HG weight which revealed that males are acyclic. Circadian study of Harderian gland porphyrin (HG-P), plasma melatonin, (aMT) and testosterone showed a characteristic two peak cyclicity. In females, HG, HG-P and pineal gland weight, and plasma aMT presented an annual inverse relation. Circadian study in females only exhibited a two peak cyclicity of HG-P, plasma aMT, and estradiol. Pinealectomy (PX) and harderianectomy (HGX) revealed increased HG weight and gonad weight in males. Gonadectomy (GX), on the other hand, had no effect on HG in males. PX in females brought almost a similar effect as noted for males, but HGX had no effect on ovarian weight. GX, interestingly, reduced HG weight and P concentration. Daily evening (4:30-5:00) administration of aMT and 5-methoxytryptamine (5-MT) in males reduced HG weight and HG-P content only in aMT-treated male and female squirrels, thereby suggesting that HG-P is perhaps negatively regulated by pineal gland production and vice versa. Injections of gonadotropin and steroids during the sexually inactive phase showed no effect on HG-P content in both sexes. Short photoperiod (SP) in both sexes stimulated pineal weight without affecting HG weight, while long photoperiod (LP) increased HG-P but reduced the plasma aMT level again without affecting HG-P content. Continuous dark (CD) decreased HG-P, whereas continuous light was ineffective without effecting HG weight in both sexes. In conclusion, HG in this rodent is functionally an important gland having diverse physiological effect in both sexes sometimes with a very clear HG-pineal-gonad relationship.

Plasma triglyceride levels correlate with body weight, age and gender influence body weight changes in Spermophilus lateralis during prehibernation and hibernation

Our experimental groups consisted of a total 16 male (M) and 15 female (F), juvenile and adult Spermophilus lateralis. This study evaluated weight gain rate (WGR) and mass-specific rate of gain (MSRG) during prehibernation, and weight loss rate (WLR), mass-specific rate of loss (MSRL) plus changes in the levels of plasma triglyceride (TG) during hibernation in these squirrels (SQ). Between January and March, the plasma samples were obtained from 26 SQ. Plasma TG was determined enzymatically, and it averaged 189 mg% with no significant difference in gender. Correlations were observed between plasma TG and sacrificed body weight ( r = 0.441, p < 0.05), and between plasma TG and MSRL ( r = 0.409, p < 0.05), although plasma TG was not correlated with WLR during hibernation. Juvenile female SQ had a higher WGR than adult female SQ, but no significant difference was found between juvenile and adult males. Both juvenile males and females showed a significantly higher MSRG than adult male and female SQ. Conversely, adult male SQ had a higher WLR and MSRL than juvenile male SQ, but no significant difference was observed between adult and juvenile females. Moreover, juvenile male SQ showed a significantly lower WLR but not MSRL than juvenile females. For these animals, our results suggest that the age factor influences on WGR and MSRG during prehibernation, and also influences on WLR and MSRL during torpor.