are near their minimum annual body mass, as compared to
late fall /early winter, when larger temperature drops are re-
quired to stimulate torpor (Lehmer et al. 2001, Lehmer and
Van Horne 2001, Lehmer et al. 2003).
From heterothermy we may infer that energy conserva-
tion is important, but there is also abundant evidence that
energy availability influences population change. For ex-
ample, Van Horne et al. (1997) described near-zero sur-
vival of the juvenile cohort; and reduced survival of adults,
especially females, when a drought caused primary foods
to dessicate during what is normally the active season. Low
survival was associated with low body mass. This lower sur-
vival of females during periods of low food supply is likely
associated with the high energetic cost of reproduction.
A dramatic population decline in Northern Idaho ground
squirrels was attributed to inadequate food resources, espe-
cially seeds, resulting from drying of the habitat and changes
to plant species composition associated with incursion of
conifers and with grazing (Sherman and Runge 2002).
Columbian ground squirrels have been the subject of ex-
acting physiological and demographic studies that demon-
strate the effects of energy limitation on survival and repro-
duction. Bennett (1999) found that females that did not
produce litters had higher chances of surviving, and that fe-
males that lost their young during lactation showed higher
survival rates than those completing the lactation period.
Juvenile Columbian ground squirrels with larger body
masses survived better over winter. Body mass was, in turn,
related to the availability of digestible energy, with forbs
being a better source than grasses. Similarly, another study
showed that the proportion of Columbian ground squir-
rels breeding as yearlings increased as seasonal energy in-
take increased (Ritchie and Belovsky 1990). A snowstorm
early in the above-ground season was associated with mass
loss in females, and those losing more mass produced fewer
young. (Neuhas et al. 1999). The importance of food lim-
itation and body condition during reproduction for female
Columbian ground squirrels is further supported by the
work of Neuhaus (2000). Heavier females were more likely
to produce litters (the same pattern is evident in King
et al. 1991), and females that did not produce litters were
heavier going into hibernation and emerging the following
spring, and had higher survival. When young were added
to litters, the females seemed to adjust the litter size back
down, according to their ability to wean the young. Females
that did not raise litters had a shorter active and longer
hibernation season (Neuhaus 2000). Columbian ground
squirrel females that gained the most mass also weaned the
largest litters (Dobson et al. 1999).
This pattern of apparent energetic limitation is evident in
other ground squirrels. As in Columbian ground squirrels,
Morton and Sherman (1978) found that a spring snow-
storm substantially reduced survival of Belding’s ground
squirrels, and the same circumstance was observed in Piute
(formerly Townsend’s) ground squirrels (Van Horne et al.
1997). Because Belding’s ground squirrels have a short time
to store sufficient fat to survive over winter, maintaining
habitat quality in meadows (abundant grasses) is essential.
Where habitat quality declines, long winters may be asso-
ciated with low survival rates (Morhardt and Gates 1974).
Golden-mantled ground squirrels subjected to a shorter
than normal above-ground season reproduced at a lower
rate (Phillips 1984). Female European ground squirrels with
high reproductive output and investment exhibited lower
overwinter survival or delayed estrus the following season
(Millesi et al. 1999).
Drought can affect the seasonal energy balance by reduc-
ing the length of the above-ground season. Many ground-
dwelling sciurids satisfy their water needs through consump-
tion of succulent food. Lack of water stimulates animals in
dry environments to immerge into seasonal torpor (Blintz
1984; Harlow 1997; Van Horne et al. 1997). Piute ground
squirrels in Idaho entered torpor earlier in drought than
in nondrought years (Smith and Johnson 1985; Van Horne
et al. 1997).
Experiments involving adding individuals, and thus put-
ting pressure on resources, or adding or improving the
quality of food also indicate the importance of bottom-up
processes. Arctic ground squirrel populations to which in-
dividuals were added declined to match unmanipulated
low-density populations as a result of declines in weaning
rates and overwinter survival (Karels and Boonstra 2000).
Because overwinter survival was not a function of pre-
dation, it appears that bottom-up processes involving nu-
tritional status were responsible for the density-dependent
population declines. Forage improved in quality (water,
nitrogen) by point fertilization with urine was consumed
preferentially by Columbian ground squirrels. Females with
access to such forage gained more mass and produced
larger litters than those without high-quality forage (Boag
and Wiggett 1994a, 1994b). Dobson and Kjelgaard (1985)
observed a similar effect from enriched forage, including
larger litters, earlier female maturation, higher juvenile sur-
vival, and greater female body mass.
Food supplementation experiments generally support the
linkage between food availability and survival /fecundity.
Dobson and Oli (2001) found that food supplementation
produced both earlier age at maturity and increased fer-
tility rates in Columbian ground squirrels. Supplementa-
tion (Dobson and Kjelgaard 1985) also led to increased re-
production and a higher survival rate for resident young
animals.
Broad-scale experiments conducted by Byrom et al.
(2000) on populations of arctic ground squirrels in theConservation of Ground Squirrels 469