Gail R. Michener

Maternal Traits and Reproduction in Richardson's Ground Squirrels

Differences among conspecifics in body mass result from underlying differ- ences in structural size and physiological condition. To determine whether the structural or physiological component of body mass has a stronger influence on reproductive traits at parturition, we studied the body composition (lean dry mass and fat content), structural size (1st principal component scores computed from 10 skeletal measurements), and body condition (residuals from regression of body mass on structural size) of yearling and older female Richardsons ground squirrels (Spermophilus richardsonii). At parturition, differ- ences among yearlings in body mass primarily reflected differences in structural size as- sociated with continuing structural growth. Older females appeared to reach a deterministic adult structural size, and body mass of older females was most strongly associated with body condition. Structural size of yearling females and body condition of older females had significant positive effects on litter mass. For older females, date of parturition had a significant negative influence on litter size. Both yearling and older females exhibited significant positive effects of parturition date and body condition on neonate mass. In older females, a trade-off resulted in smaller litters of heavier neonates as the breeding season progressed, perhaps reflecting the need for rapid growth of offspring before hibernation. Maternal size, maternal condition, and seasonal timing were important aspects of repro- duction in Richardsons ground squirrels, but these characteristics are seldom considered concurrently in studies of life histories.

DIFFERENTIAL COSTS OF REPRODUCTIVE EFFORT FOR MALE AND FEMALE RICHARDSON'S GROUND SQUIRRELS'

The costs of reproductive effort for adult male and female Richardsons ground squirrels were compared to determine whether these costs differed in timing or magnitude in a manner related to sexual differences in mating and parental effort. Repro- ductive effort was assessed from 1982 to 1986 for a population of Richardsons ground squirrels in southern Alberta, Canada, by monitoring seasonal mortality schedules, fecun- dity, and changes in body mass and fat content. Although the adult sex ratio was female- biased, the operational sex ratio was male-biased. Males lost mass and sustained injuries as a result of male-male conflict during the mating season. From 50 to 79% of males disappeared annually during reproduction (between spring emergence and the end of the mating season), whereas < 20% of females disappeared during reproduction (between spring emergence and litter emergence). Overwinter mortality rates were similar for adult males and females. Active seasons of adult males and females were of similar durations, but because reproductive effort finished earlier for males, they had a longer period in which to prepare for hibernation, and they entered hibernation (immerged) with larger fat stores. Juvenile males had a longer active season and attained a greater proportion of adult structural size than juvenile females (90 vs. 75%). Although males emerged from hiber- nation with more residual fat than females, fat stores were rapidly depleted during mating 2-4 wk postemergence, whereas females did not deplete residual fat until late lactation 7- 8 wk postemergence. Survival rates of females that did and did not wean litters did not differ significantly, and survival rates of mothers were not significantly or negatively cor- related with litter size or litter mass. Although litter size tended to regress toward the mean in consecutive years of reproduction, neither size nor mass of litter weaned negatively influenced litter size or mass in the subsequent year. Depletion of larger stored fat reserves and lower survival rate during reproduction indicate that, from both energetic and survival perspectives, mating effort is more costly for male Richardsons ground squirrels than parental effort is for females.

Sexual differences in over-winter torpor patterns of Richardson's ground squirrels in natural hibernacula

SummaryOver-winter torpor patterns of Richardsons ground squirrels hibernating in southern Alberta were monitored with temperature-sensitive radiocollars to determine if these patterns differed between males and females in a manner related to the greater costs of mating effort by males than females. The hibernation season (from immergence to emergence) was composed of three periods: post-immergence euthermy, heterothermy, and pre-emergence euthermy. The hibernation season was shorter for juveniles than adults both among males (< 150 versus 234 days) and females (185 versus 231 days), a reflection of the later immergence into hibernation by juveniles. However, regardless of the absolute duration of hibernation, heterothermy accounted for a smaller proportion of the hibernation season of males (93±5%) than females (98±1%) and, within the heterothermal period, males had shorter torpor bouts and longer inter-torpor arousals. Overall, males spent a smaller proportion of the hibernation season in torpor (85±6%) than females (92±1%). This sexual difference was largely attributable to the longer duration of preemergence euthermy for males than females. Males terminated torpor in January and February, when hibernacula were at their coldest, then remained euthermic for 8.8 days (range 0.5–25.0 days) before emergence. In contrast, females terminated torpor in March, when hibernaculum temperatures were increasing, then remained euthermic for only 1.1 days (range 0.5–2.0 days) before emergence. Males lost less mass per euthermic day during hibernation than females (7.0 versus 9.3 g/day). Males and females hibernated at similar depths (56 cm), but males had larger chambers than females (18 versus 16 cm3/g). Many males, but no females, cached seeds in the hibernaculum. Males met the costs of thermogenesis and euthermy from a combination of fat reserves and food caches, whereas females relied solely on fat. Access to food caches permitted males to terminate torpor several weeks in advance of emergence, during which time they recouped mass and developed sperm in preparation for the forthcoming mating season.

Spring emergence schedules and vernal behavior of Richardson's ground squirrels: why do males emerge from hibernation before females?

SummarySchedules of emergence from hibernation were recorded for adult (≥1-year old) Richardsons ground squirrels each spring from 1979 to 1983 in southern Alberta, Canada. In all years and regardless of the calendar dates of emergence, males emerged from hibernation, on average, 8–16 days in advance of females. On the median date of emergence by males, 87–100% of females still remained in hibernation. On the day the last male appeared, the proportion of females still in hibernation ranged from 27% in 1979 to 89% in 1980. As females typically bred 1–4 days after resuming aboveground activity, late emerging males found that females that had been active several days had already been impregnated by earlier emerging males. Thus, late emergence by males resulted in reduced breeding opportunities.Male Richardsons ground squirrels differed from females in a constellation of behaviors related to male intrasexual competition for breeding opportunities. In the breeding season, males moved longer distances, occupied larger ranges, expended less time on burrow maintenance, engaged in more injury-producing fights, and lost more body weight than did females. Early emergence from hibernation by males relative to females is viewed as one of several behaviors exhibited by males to promote individual reproductive success by maximizing the number of potential mates available.The sex difference in time of vernal emergence by Richardsons ground squirrels is interpreted as an outcome of intersexual selection and is an example of the phenological strategy of the mate-limited sex maximizing reproductive success by being on the breeding ground by the time the mate-limiting sex appears.

Reproductive effort during gestation and lactation by Richardson's ground squirrels

SummaryReproductive effort by yearling and older female Richardsons ground squirrels was studied over a 4-year period in southern Alberta by obtaining serial weight records from marked individuals to compare the mothers mass at critical points in the annual cycle (emergence from hibernation, estrus, parturition, and litter emergence) with her litters mass at birth and weaning. Yearlings weighed only 80% of older adults at emergence from hibernation, but they mated at the same time as older females, attained adult mass coincident with pregnancy, and weaned litters that were not significantly smaller in size or mass than those of older females. Age and maternal mass were weak predictors of litter size and litter mass. Of the net increase in mass of the combined mother-litter unit during gestation, over half (60% of 139 g for yearlings; 52% of 127 g for older females) was attributable to an increase in the mothers own mass, whereas during lactation almost all of the net increase (93% of 545 g for yearlings; 96% of 567 g for older females) was attributable to an increase in the litters mass. On a daily basis, deposition of mass in the litter was 6 times greater during lactation than gestation. On average, neonates weighed 2.3% (6.5 g) of maternal mass at birth and 23.1% (81 g) at emergence from the natal burrow; offspring masses at birth and at emergence were significantly negatively correlated with litter size. On average, litters weighed 16.3% (48 g) of maternal mass at birth and 157.5% (578 g) at emergence from the natal burrow. Compared with other hibernating sciurids, Richardsons ground squirrels have a similar offspring mass relative to maternal mass both at birth and at emergence from the natal burrow. However, because of the large litter size (typically 6–8), absolute reproductive effort, measured either as litter mass at birth or at natal emergence, is large for the body size of the species.

Population Structure and Dispersal in Richardson's Ground Squirrels

Population structure and dispersal were studied over a 4-yr period in a population of Richardsons ground squirrels (Spermophilus richardsonii) on a 135-ha tract of natural prairie in southern Saskatchewan, Canada. Overall, fewer than 30% of juveniles and 50% of adults were recap- tured in a subsequent year. More 6 6 than Y Y disappeared from the population. The 1: I juvenile sex ratio was readjusted to ~0.3 d d/ Y in the adult population. Females generally remained within 100 m of their initial capture point, and fidelity to the original area lasted at least 3 yr. There was no substantive evidence to account for the loss of Y Y between successive years on the basis of dispersal. Males generally moved further than Y Y. About 90% of the observed between-year movements made by 6 6 could be explained on the basis of movements during the breeding season. Within their first year of life, 6 6 moved further in the spring when mature than in the previous year as juveniles. Some dispersal by juvenile 6 6 occurred, predominantly in July. In each year the population fit the Hardy-Weinberg equilibrium with respect to the transferrin locus, and there was no evidence of differential loss of genotypes between generations, between years, between sexes, or between marked and unmarked adults. The potential effect of spring dispersal by yearlings as opposed to fall dispersal by juveniles on population dynamics is considered. The roles of dispersal, predation, and overwinter mortality in accounting for losses of squirrels are examined.

HUNTING TECHNIQUES AND TOOL USE BY NORTH AMERICAN BADGERS PREYING ON RICHARDSON'S GROUND SQUIRRELS

Abstract Techniques used by North American badgers (Taxidea taxus) when hunting Richardsons ground squirrels (Spermophilus richardsonii) were assessed over a 15-year period in southern Alberta to determine the relationship between activity of prey and methods used to capture prey. Badgers frequently hunted hibernating squirrels in autumn, sometimes hunted infants in spring, and rarely hunted active squirrels in summer. Badgers always captured hibernating squirrels and infants underground, usually captured active squirrels underground, and sometimes intercepted fleeing squirrels aboveground. Regardless of season or year, the most common hunting technique used by badgers was excavation of burrow systems, but plugging of openings into ground-squirrel tunnels accounted for 5–23% of hunting actions in 4 consecutive years. Plugging occurred predominantly in mid-June to late July before most ground squirrels hibernated and in late August to late October when juvenile males were active but other squirrels were in hibernation. Badgers usually used soil from around the tunnel opening or soil dragged 30–270 cm from a nearby mound (72% and 22% of 391 plugged tunnels, respectively) to plug tunnels. The least common (6%), but most novel, form of plugging used by 1 badger involved movement of 37 objects from distances of 20–105 cm to plug openings into 23 ground-squirrel tunnels on 14 nights. Aimed movement of objects to plug openings into burrow systems occupied by ground squirrels qualified this badger as a tool user.

Chronology of reproductive events for female Richardson's ground squirrels

Timing of reproductive events for adult (≥ 1-year old) female Spermophilus rich-ardsonii in southern Alberta was determined from 1979 to 1984 by daily monitoring of squirrels following emergence from hibernation until litters emerged from the natal burrow. Females typically mated 3 to 5 days after emergence from hibernation and gave birth 22 to 23 days later. Juveniles were usually 28 to 30 days old when they first left the natal burrow. Thus most litters (130/175) appeared 53 to 58 days after the mother had emerged from hibernation. The reproductive chronologies of yearling and older females were virtually identical. The 22.5-day gestation period of S. richardsonii is the shortest documented for the genus. Interspecific comparison of reproductive chronologies of ground squirrels indicated that at least five species in the subgenus Spermophilus typically wean litters ca. 8 weeks after the mothers emergence from hibernation, whereas for several species in other subgenera the mother emergence-to-litter emergence interval exceeds 10 weeks.

The Circannual Cycle of Richardson's Ground Squirrels in Southern Alberta

Times of spring emergence and autumn immergence were determined for Richardsons ground squirrels ( Spermophilus richardsonii ) in southern Alberta from 1975 to 1978. Spring emergence of females was synchronous and was not affected by their age, reproductive success in the previous year, or time of autumn immergence. Time of spring emergence correlated with climatic conditions, in particular with warming of the shallow layers of soil. Time of autumn immergence was asynchronous, even within the same age and sex class, and was affected by the condition of the animal. Factors affecting fattening, and hence immergence, included age and sex of the squirrel, its reproductive success that year, and the availability of food. Synchrony was reestablished in the following spring, suggesting that emergence is the point in the annual cycle at which entrainment occurs.

Over-Winter Weight Loss by Richardson's Ground Squirrels in Relation to Sexual Differences in Mating Effort

Duration of hibernation and over-winter changes in mass and body composition were studied in Richardsons ground squirrels ( Spermophilus richardsonii ) hibernating under natural conditions in southern Alberta, Canada. Adult males stored more fat, hibernated for slightly fewer days, and lost more mass over winter than adult females. Juvenile males grew larger, fattened later, immerged later, spent 2–3 months less time in hibernation, and lost less mass over winter than juvenile females. Despite differences in prehibernatory body mass and composition, immergence dates, duration of hibernation, and magnitude of over-winter weight loss, yearling and older males emerged from hibernation at the same time and had similar posthibernatory body mass and body composition. Males emerged before females and with twice as much residual fat as females. Sexual differences in proportion of adult size achieved before hibernation by juveniles and in size of posthibernatory fat stores are explained in terms of sexual differences in the energetic costs of acquiring mates in spring.