Article Summaries

AMER. ZOOL., 38:191-206 (1998)

Ecological Bases of Hormone-Behavior Interactions: The
“Emergency Life History Stage”1

JOHN C. WINGFIELD,2 DONNA L. MANEY, CREAGH W. BREUNER, JERRY D. JACOBS,
SHARON LYNN, MARILYN RAMENOFSKY, AND RALPH D. RICHARDSON*

Department of Zoology, University of Washington, Seattle, Washington 98195
•Department of Psychology, University of Washington, Seattle, Washington 98195

SYNOPSIS. Superimposed upon seasonal changes in morphology, physiology and
behavior, are facultative responses to unpredictable events known as labile (i.e.,
short-lived) perturbation factors (LPFs). These responses include behavioral and
physiological changes that enhance survival and collectively make up the “emer-
gency” life history stage. There is considerable evidence that glucocorticosteroids,
and other hormones in the hypothalamo-pituitary-adrenal (HPA) cascade, initiate
and orchestrate the emergency life history stage within minutes to hours. This stage
has a number of sub-stages that promote survival and avoid potential deleterious
effects of stress that may result from chronically elevated levels of circulating
glucocorticosteroids over days and weeks. These sub-stages may include: redirec-
tion of behavior from a normal life history stage to increased foraging, irruptive-
type migration during the day, enhanced restfulness at night, and elevated gluco-
neogenesis. Once the perturbation passes, glucocorticosteroids may also promote
recovery. Additional evidence from birds indicates that glucocorticosteroid re-
sponses to a standardized capture, handling and restraint protocol are modulated
both on seasonal and individual levels. Field work reveals that these changes in
responsiveness to LPFs have ecological bases, such as reproductive state, body
condition etc., that in turn indicate different hormonal control mechanisms in the
HPA cascade.

INTRODUCTION

Most of us interpret “emergency” re-
sponses of animals as the “fight-or-flight”
response—the massive release of catecho-
lamines by adrenal medullary cells (chro-
maffin) that increase heart rate, mobilize
glucose, etc., within seconds (e.g., Axelrod
and Reisine, 1984; Sapolsky, 1987; Johnson
et al., 1992). This response is triggered by
sudden threatening environmental events
such as attack by a predator or dominant
conspecific, and it serves to facilitate im-
mediate and extreme physical exertion to
escape. The fight-or-flight response is usu-
ally over within seconds (assuming suc-
cessful escape) and the individual returns to
normal activity within minutes. Over the
past twenty years accumulating evidence

1 From the Symposium Animal Behavior: Integra-
tion of Ultimate and Proximate Causation presented at
the Annual Meeting of the Society for Integrative and
Comparative Biology, 26-30 December 1996, at Al-
buquerque, New Mexico.

2 E-mail: [email protected]

suggests another “emergency” response
may exist that involves interruption of the
life history cycle and re-direction of behav-
ior and physiology towards survival. It is
distinct from the “fight-or-flight” response
in that it takes several minutes or even
hours to develop and results in a more long-
lived (hours or days, even weeks) interrup-
tion of normal activities such as breeding.
This “new” phenomenon also raises ques-
tions about proximate and ultimate causa-
tions. Why has the emergency response
evolved and how is it orchestrated?

Organisms have a characteristic series of
life history stages that makes up their life
cycle (Jacobs, 1996). A highly simplified
series of life history stages in birds is pre-
sented in Figure 1. The winter (non-breed-
ing) stage and breeding stage each have
unique sets of sub-stages. Transition from
stage to stage is regulated by hormone se-
cretions, as is the activation of sub-stages
within a stage. Progression of stages and
timing are determined by predictable

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192 J. C . WlNGFIELD ETAL.

Winter (non-
breeding stage)

Breeding
stage

TABLE 1. Labile perturbation factors.

Body condition
Social status
Territory or
home range

Gonadal maturation
courtship,

territorial behavior
ovulation

parental phase

Transitory
emergency stage

Facultative
bohBvioml snd
physiological

FIG. 1. A highly simplified series of life history
stages in birds. The winter (non-breeding) stage and
breeding stage have unique sets of sub-stages. Transi-
tion from stage to the next is regulated by hormone
secretions as is the activation of sub stages within a
stage. Progression from stage to stage and timing of a
specific stage are determined by predictable changes
in the environment (e.g., photoperiod). However, the
emergency life history stage may be triggered at any
time by unpredictable events in the environment (see
labile perturbation factors in Table 1). This transitory
emergency stage has its own unique set of sub stages.
After the perturbation passes, the individual can return
to the original life history stage. If the perturbation was
long lived then the next, or an appropriate life history
stage for that time of year will be assumed. Modified
from Jacobs (1996) and Wingfield et al. (1997).

changes in the environment {e.g., photope-
riod). However, the emergency life history
stage may be triggered at any time by un-
predictable events in the environment (Ja-
cobs, 1996; Wingfield et al., 1997). This
transitory emergency stage has its own
unique set of sub-stages. After the pertur-
bation passes, the individual can return to
the original life history stage. If the pertur-
bation is long lived then the next, or an ap-
propriate, life history stage for that time of
year will be assumed.

The unpredictable environmental factors
that trigger an emergency life history stage
have been termed “labile perturbation fac-
tors” (LPFs, Jacobs, 1996). It is important
to understand that these factors are unpre-
dictable (can occur at any time of year), and
they are usually transitory (i.e., labile), al-
though in recent years human disturbance
and pollution may result in “permanent
perturbations.” There are two major types
of LPFs-direct and indirect (Table 1, see

Loss of eggs or
young to predator

Loss of eggs or
young to short se-
vere storm

Brief disturbance
(e.g., human)

Direct

Prolonged severe weather
Interspecific competition
Loss of mate
Pollution
Habitat change or loss
Prolonged disturbance (e.g.,

human)

Expanded from, Wingfield (1988, 1994).

Wingfield, 1988, 1994; Jacobs, 1996). In-
direct LPFs result in loss of a nest and
young, or temporary deterioration of the
habitat. The individuals involved may not
trigger an emergency life history stage, but
may initiate a fight or flight response. Such
unpredictable disturbances are over quickly
and the individual continues in its life his-
tory stage appropriate for that time of year.
Increased glucocorticosteroids may be, but
usually are not, involved (Wingfield, 1988).
Direct LPFs, on the other hand, affect the
individual directly by decreasing available
food resources, increasing energetic de-
mands (e.g., especially bad weather), or re-
stricting access to resources by disturbing
optimal habitat (see Wingfield, 1988). In-
creased interspecific competition may also
result in restricted access to resources, fol-
lowed by adjustment of home range and
habitat partitioning (Repasky and Schluter,
1994), or at least an increase in energy re-
quired to compete for those resources. In
these cases, the emergency life history
stage is triggered. Although in this paper
we will focus primarily on birds, the emer-
gency life history stage concept may be
widely applicable to all vertebrates—at
least at the level of behavioral responses to
unpredictable events (e.g., Clutton Brock,
1991).

THE EMERGENCY LIFE HISTORY STAGE

There are several clearly definable events
that make up the emergency life history
stage in response to LPFs. These have been
summarized by Wingfield and Ramenofsky
(1997) and are expanded here under four
major headings:

1. Deactivation of territorial behavior/dis-
integration of social hierarchies:

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THE EMERGENCY LIFE HISTORY STAGE 193

a) Reproduction and associated behavior,
seasonal migration or wintering strategies
are suppressed.

b) Social relationships may be suspended
temporarily.

2. Activation of emergency behavior:
a) Seek or remain in a refuge. If food

supply is not compromised, then the best
strategy may be to find shelter and “ride-
out” the LPF.

b) Move away from the source of per-
turbation. If food resources are compro-
mised in any way such that negative energy
balance is likely, then the best strategy
would be to leave and seek alternate habi-
tat.

c) Seek a refuge and try to ride out the
LPF at first, but then leave if conditions do
not improve. The time spent in a refuge be-
fore leaving may be a direct function of
stored energy reserves. Note that the indi-
vidual should leave while energy stores are
still sufficient to fuel a flight.

3. Mobilization of stored energy reserves:
Since in 2b and c, negative energy bal-

ance is likely, then stores of fat should be
tapped. In many cases gluconeogenesis may
include mobilization of proteins as well.

4. Settlement in alternate habitat or return
to the original site—termination of the
emergency life history stage:

a) If the individual remains in its original
habitat, then the normal life history stage
can be assumed immediately after the LPF
has abated.

b) If the individual leaves, then suitable
habitat should be identified and the individ-
ual can then settle and resume the normal
series of life history stages.

c) In many cases, the individual may re-
turn to its original habitat once the LPF has
passed.

d) Recovery following an emergency life
history stage may be a critical component
of the whole process.

Evidence to date suggests that the behav-
ioral and physiological components of the
emergency life history stage are similar, if
not identical, at all times of year, and re-
gardless of the life history stage from which
it may have been triggered (Jacobs, 1996).

It is then logical to propose that the mech-
anisms by which this stage is initiated,
maintained and terminated may be the same
at all times of year and throughout the life
cycle of the individual. We propose that
neuropeptides associated with the hypoth-
alamo-pituitary-adrenal cortex (HPA) axis,
adrenocorticotropin (ACTH) and glucocor-
ticosteroids regulate the emergency life his-
tory stage (e.g., Wingfield, 1994), although
it is certain that other endocrine secretions
may also be involved. Many hormones
have been identified in classical responses
to stress in vertebrates, and since many as-
pects of the emergency life history stage are
superficially similar to stress, it is tempting
to draw parallels. However, evidence is ac-
cumulating that the emergency life history
stage is a mechanism by which individuals
avoid stress thus enhancing survival and
potentially lifetime reproductive success
(Wingfield et al, 1997).

The hypothalamo-pituitary-adrenal axis
It has been known for decades that a host

of obnoxious agents (stressors) activate the
hypothalamo-pituitary-adrenal axis result-
ing in marked elevation of glucocorticoste-
roid secretion. Although they orchestrate
many of the physiological, morphological
and behavioral responses to stress, other
hormones are also involved (e.g., Axelrod
and Reisine, 1984; Munck et al, 1984;
Johnson et al, 1992). The actions of glu-
cocorticosteroids during this so-called
“stress-response” attracted our attention at
first because of the apparent parallels of the
emergency life history stage and a classical
stress response. Owing to constraints of
space we will focus primarily on the actions
of opioids and glucocorticosteroids (Table
2). Because the measurement of plasma (3-
endorphin levels has proved technically dif-
ficult, there are few studies addressing its
action in free-living individuals. However,
it is known to influence reproductive be-
havior, analgesia, and feeding behavior,
making this peptide an ideal candidate for
involvement in the emergency life history
stage. We also include the peptide ACTH
because it is released during the initiation
of the emergency life history stage, is co-
released with p-endorphin (Guillemin et al.,

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194 J. C . WlNGRELD ETAL.

TABLE 2. Effect of Corticosterone in an Emergency
Life History Stage.

Rapid
{i.e., short term.

minutes to hours)

Suppress reproductive be-
havior

Regulate immune system
Increase gluconeogenesis
Increase foraging behav-

ior
Promote escape (irruptive)

behavior during day
Promote night restfulness

by lowering standard
metabolic rate

Promote recovery on re-
turn to normal life his-
tory stage

Chronic
(i.e.. long term.
days to weeks)

Inhibit reproductive sys-
tem

Suppress immune system
Promote severe protein

loss
Disrupt second messenger

systems
Neuronal cell death
Suppress growth and

metamorphosis

Modified and expanded from Wingfield (1994).

1977), and binds to opioid receptors (Ter-
enius, 1977).

Wingfield (1994) suggested that there
may be two distinct types of response to
glucocorticosteroids during a stress re-
sponse. By far the most well studied are
chronic effects induced by many days or
even weeks of exposure to continual high
circulating levels of glucocorticosteroids re-
sulting from prolonged exposure to stress.
These effects (Table 2) include total failure
of reproductive function, increased suscep-
tibility to disease owing to suppression of
the immune system, neuronal cell death
(particularly in the hippocampus), severe
protein loss (for gluconeogenesis), disrup-
tion of the arachidonic acid cascade, and
inhibition of growth and metamorphosis
(e.g., Axelrod and Reisine, 1984; Munck et
al, 1984; Johnson et al., 1992; Sapolsky
1987, 1996). Although these effects have
immense importance for medicine and ag-
riculture, it is difficult to imagine how any
one of these states would be adaptive for
an organism in the field. Indeed death
would be imminent in any of these states.
Thus it is unlikely that chronic effects of
high circulating levels of glucocorticoste-
roids have much biological significance
since survival by this time would be virtu-
ally zero (Wingfield et al., 1997). It is well
documented that severe environmental per-
turbations occasionally result in massive
mortality in natural populations (see Wing-

field et al., 1997), but presumably there
would be strong selection for mechanisms
by which such deleterious states are avoid-
ed in survivors. Therefore, the short term
effects of elevated glucocorticosteroids
(over minutes to hours) may be highly
adaptive in avoiding the severe stressed
state. These short term effects are also sum-
marized in Table 2. It is these effects that
may orchestrate the emergency life history
stage and avoid the clearly severe, and very
likely fatal, consequences of chronic high
levels of glucocorticosteroids and other hor-
mones of the HPA axis. The evidence for
short term effects of HPA hormones con-
sistent with the emergency life history stage
are as follows.

Suppression of reproductive behavior
One of the hallmarks of an emergency

life history stage is that individuals redirect
their activities from those typical of the nor-
mal life history stage, to others more con-
ducive to survival. There are many ac-
counts of abandonment of breeding terri-
tories and offspring in response to LPF-like
environmental events (e.g., Gessamen and
Worthen, 1982; Clutton Brock, 1991), sug-
gesting that redirection of behavior may be
widespread. At first this may appear mal-
adaptive because reproductive success be-
comes zero. However, temporary suspen-
sion of breeding activity may actually en-
hance lifetime reproductive success by al-
lowing an individual to survive the
perturbation in good condition so that it can
then breed again at the earliest opportunity.

Glucocorticosteroids.—In free-living
pied flycatchers, Ficedula hypoleuca, im-
plants of corticosterone reduced parental
behavior in both sexes (Silverin, 1986).
Nestlings were fed less, fewer fledglings re-
sulted, and young that did fledge weighed
less than fledglings from control implanted
birds. Another group that received implants
designed to give even higher circulating
levels of corticosterone resulted in complete
abandonment of nests with zero reproduc-
tive success (Silverin, 1986). In breeding
male song sparrows, Melospiza melodia,
similar implants of corticosterone resulted
in marked reduction of territorial aggres-
sion. Furthermore, plasma levels of testos-

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THE EMERGENCY LIFE HISTORY STAGE 195

terone were still in the range typical of this
period in the reproductive cycle, suggesting
that corticosterone may override the effects
of testosterone in activation of territorial ag-
gression (Wingfield and Silverin, 1986).
Similarly in side-blotched lizards, Uta
stansburiana, implants of corticosterone
significantly reduced home range size and
activity if control implanted lizards were
also present (DeNardo and Sinervo, 1994a).
However, if all individuals at a site were
implanted with corticosterone, there was no
decrease in home range size or activity,
suggesting that corticosterone may reduce
the effectiveness of males in retaining their
home ranges when in competition with nor-
mal males. In another experiment, it was
shown that if lizards were also implanted
with testosterone, then corticosterone treat-
ment still resulted in reduced home ranges
if control males were present (DeNardo and
Sinervo, 1994fc). These data suggest further
that the effects of corticosterone override
any effect of testosterone on spatial behav-
ior. The mechanisms underlying these be-
havioral responses remain unknown. Glu-
cocorticosteroids also may directly suppress
reproductive behavior. Subcutaneous injec-
tion of corticosterone profoundly inhibits
courtship behavior in male rough-skinned
newts, Taricha granulosa (Moore and Mil-
ler, 1984).

$-endorphin.—The effects of opioids on
reproductive behavior are well known and
too extensive to cover in detail here. Ex-
periments with antagonists and agonists
have demonstrated an inhibitory role for
both central and circulating opioids. In the
rough-skinned newt, stress-induced inhibi-
tion of courtship can be reversed by treat-
ment with naloxone, an opioid antagonist
(Miller and Moore, 1982). In rats, central
infusion of P-endorphin causes a decrease
in mounting by males and an inhibition of
lordosis in females (Meyerson and Berg,
1977; Sirinathsinghji, 1984). Intraventricu-
lar infusion of corticotropin-releasing factor
(CRF) results in suppression of lordosis that
is reversible by (B-endorphin antagonists
(Sirinathsinghji et al., 1983a, b). In female
white-crowned sparrows, Zonotrichia leu-
cophrys gambelii, central infusion of P-en-
dorphin strongly inhibits copulation solici-

tation whereas naloxone enhances it (Ma-
ney and Wingfield 1998). The mechanism
of opioid-induced suppression of reproduc-
tive behavior is unknown, but there is evi-
dence that p-endorphin acts within the brain
to suppress gonadotropin-releasing hor-
mone (GnRH) neuronal systems (see Siri-
nathsinghji, 1984; Fan and Ottinger, 1996).

Promotion of gluconeogenesis
Glucocorticosteroids play a key role in

promoting gluconeogenesis, especially
from protein, in many vertebrate taxa
(Chester-Jones et al., 1972). In mammals,
glucocorticosteroids play a central role in
metabolic responses to stress by sustaining
gluconeogenesis by increasing the supply of
hepatic gluconeogenic precursors and by
maintaining glycogen availability in the liv-
er (e.g., Fujiwara et al., 1996). Acute in-
creases in glucocorticosteroids increase the
gluconeogenic conversion of alanine to glu-
cose by elevating uptake of alanine by the
liver, and may also be accompanied by a
transient decrease in insulin to further en-
hance gluconeogenesis (Goldstein et al.,
1992). Similar mechanisms may operate in
birds, although increased glucose (or gly-
cogen) may not be the only result. In song
sparrows and pied flycatchers, corticoste-
rone treatment results in apparent loss of
protein from flight muscles, but no change
in body weight because fat depots increased
markedly (Wingfield and Silverin, 1986;
Silverin, 1986). Similar effects were found
in captive dark-eyed juncos, Junco hyemal-
is, (Gray et al, 1990). Furthermore, al-
though adipose lipoprotein-lipase (LPL) ac-
tivity was unchanged, the concentration of
LPL in muscle increased significantly even
though muscle mass declined. These data
are consistent with the hypothesis that fly-
ing birds utilize fatty acids as a major fuel
for flight rather than glycogen.

Ward (1969) and others have suggested
that the pectoralis flight muscles of birds
may be important reservoirs of readily-
mobilizable protein for reproduction and
possibly flight. A morphological study by
Kendall et al. (1973) of flight muscles of
Quelea quelea, suggested that soluble pro-
teins may be stored in mitochondria and be-
tween myofibrillar bundles in sarcoplasm.

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196 J. C . WlNGFIELD ETAL.

However, such stores, if they exist, are dif-
ficult to quantify morphologically. Honey
(1990) devised a biochemical method to
separate soluble and structural (contractile)
proteins in avian muscle by extraction in
low or high salt phosphate buffers. When
corticosterone was implanted into captive
house sparrows, Passer domesticus, there
was a significant decline in body weight
and particularly in weight of pectoralis
muscles compared with controls. Further,
this loss of mass in muscle was due to a
significant decline in soluble protein frac-
tions. Structural (myofibrillar) fractions did
not differ between treatments. The influenc-
es of corticosterone, and other hormones,
on gluconeogenesis and utilization of pro-
tein require further study on wild birds in
different life history stages.

Regulation of the immune system
It is now well known that unpredictable

events in the environment can stimulate re-
lease of cytokines and monokines. These
hormones of the immune system can inter-
act extensively with other components of
the endocrine system and in turn can mod-
ify behavior {e.g., Munck et al., 1984; Cun-
ningham and De Souza, 1993). Although
our knowledge of these effects in non-
mammalian vertebrates is sparse, it has
been demonstrated that when male Western
fence lizards, Sceloporus occidentalis, are
injected with human interleukin-1 fi (IL-1),
they show decreased activity (especially in
the morning hours) compared to saline in-
jected controls and untreated animals. This
suppression of activity is similar to that
seen in lizards infected with malaria (Dun-
lap and Church, 1996). The authors suggest
that IL-1 may mediate pathogen-induced
changes in activity. Whether these hor-
mones may also mediate other aspects of
activity in an emergency life history stage
in general awaits further study.

Increase in foraging behavior
Glucocorticosteroids.—As in mammals,

there is evidence that glucocorticosteroids,
along with other metabolic hormones, are
important in the regulation of food intake
{e.g., Richardson et al., 1995). Implants of
metyrapone (a blocker of 11 |3-hydroxylase,

an enzyme essential for the synthesis of
glucocorticosteroids) decreased foraging
behavior (a combination of searching,
scratching, pecking, and actual food intake)
in male white-crowned sparrows and re-
placement therapy with implants of corti-
costerone increased foraging (Wingfield et
al., 1990). However, implants of corticoste-
rone into otherwise untreated white-
crowned sparrows and song sparrows tend-
ed to increase foraging (Astheimer et al.,
1992), but this was not significant, and had
no effect in dark-eyed j uncos (Gray et al.,
1990). It is possible that corticosterone may
play a “permissive” role in the regulation
of food intake. Other factors acting central-
ly may also be important, as has been
shown in mammals (Leibowitz et al.,
1984).

$-endorphin.—Endogenous opioids are
well-known to affect feeding behavior, and
may initiate an increase in foraging during
the emergency life history stage. Intracere-
broventricular beta-endorphin has been
shown to increase food intake or feeding
behavior in a variety of vertebrates, includ-
ing rats (McKay et al., 1981), pigeons
(Deviche and Schepers, 1984), and white-
crowned sparrows (Maney and Wingfield,
1998). Food deprivation (see Morley et al.,
1983) causes beta-endorphin levels to de-
crease in the rat hypothalamus, suggesting
release of this peptide. Stress-induced feed-
ing can be reversed by naloxone, an opioid
antagonist (reviewed by Morley et al.,
1983). Intramuscular injection of naloxone
methobromide, an antagonist that does not
cross the blood-brain barrier, decreases
feeding in domestic fowl (Denbow and Mc-
Cormack, 1990), indicating that endoge-
nous opioids may also modulate feeding be-
havior at sites outside the CNS.

Promotion of diurnal escape (irruptivel
shelter) behavior

Corticosterone treatment of captive male
white-crowned sparrows resulted in a de-
cline of perch hopping activity over the day
(Astheimer et al., 1992), which is consistent
with “shelter” behavior related to “riding
out” the perturbation factor. This result is
particularly compelling since food was
available ad libitum and leaving may not

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THE EMERGENCY LIFE HISTORY STAGE 197

confer any advantage. However, if food was
removed for 24 hours (to simulate severe
storms that often reduce food resources),
then corticosterone-treated birds showed a
considerable increase in perch hopping ac-
tivity exceeding that of controls throughout
the day. These data suggest that under con-
ditions of reduced food availability, corti-
costerone may actually enhance activity,
possibly associated with leaving the source
of perturbation. Note that this activity is
during the day and not at night. In the
white-crowned sparrow normal spring and
autumn migratory behavior occurs at night
(e.g., Wingfield et al, 1990), suggesting
that corticosterone-induced activity is a dif-
ferent phenomenon consistent with the
emergency life history stage (Jacobs, 1996)
and not a normal life history stage (e.g.,
vernal or autumn migrations). Again, cen-
tral effects of hormones may be important
in distinguishing whether corticosterone has
an effect to decrease or increase activity.
This is currently under investigation. Note
also that in Western fence lizards, IL-1 de-
creased activity (Dunlap and Church,
1996). It is possible that such a mechanism
may also operate in white-crowned spar-
rows.

Promotion of nocturnal restfulness
It was originally suggested that since cor-

ticosterone may have marked effects on ac-
tivity of birds during an emergency life his-
tory stage, then we might predict that this
glucocorticosteroid may also increase met-
abolic rate. In contrast, implants of corti-
costerone actually reduced extended meta-
bolic rate in captive white-crowned spar-
rows (as measured by oxygen consumption)
over night compared with controls (Butte-
mer et al, 1991). Control treated birds, as
well as birds sampled before treatment,
showed episodes of oxygen consumption
over a 60 min sampling period at night.
Corticosterone treatment did not reduce
standard metabolic rate, but eliminated ep-
isodes of increased oxygen consumption
with a net savings of energy over night.
Similar effects were obtained in American
goldfinches, Carduelis tristis, pine siskins,
C. pinus, and red crossbills, Loxia curvi-
rostra (Buttemer et al., 1991). The authors

interpreted these results as enhanced “night
restfulness” in an emergency life history
stage. Note also that this effect is not con-
sistent with nocturnal migratory activity in
normal life history stages of vernal and au-
tumnal migrations, and further supports the
concept of a distinct emergency life history
stage with its own suite of hormonal control
mechanisms.

Promotion of recovery on return to
normal life history stage

Implants of corticosterone into captive
song sparrows had little effect on foraging-
like behavior when food was removed for
24 hours, but did greatly enhance food in-
take when food was returned. Similar, but
less marked, effects were seen in male
white-crowned sparrows that were treated
with corticosterone and had food withheld
for 24 hours and then refed (Astheimer et
al., 1992). These data suggest an additional
role for corticosterone in the recovery phase
after a perturbation ceases. Because of its
well known role in analgesia, P-endorphin
must also be considered here. This aspect
of the emergency life history stage deserves
further study.

Overview
Experimental evidence to date thus sup-

ports the concept of an emergency life history
stage that can be triggered by increased cir-
culating levels of corticosterone. Although
other hormones are undoubtedly involved, it
seems clear that the transitory …

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$26
The price is based on these factors:
Academic level
Number of pages
Urgency
Basic features
  • Free title page and bibliography
  • Unlimited revisions
  • Plagiarism-free guarantee
  • Money-back guarantee
  • 24/7 support
On-demand options
  • Writer’s samples
  • Part-by-part delivery
  • Overnight delivery
  • Copies of used sources
  • Expert Proofreading
Paper format
  • 275 words per page
  • 12 pt Arial/Times New Roman
  • Double line spacing
  • Any citation style (APA, MLA, Chicago/Turabian, Harvard)

Our guarantees

Delivering a high-quality product at a reasonable price is not enough anymore.
That’s why we have developed 5 beneficial guarantees that will make your experience with our service enjoyable, easy, and safe.

Money-back guarantee

You have to be 100% sure of the quality of your product to give a money-back guarantee. This describes us perfectly. Make sure that this guarantee is totally transparent.

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Zero-plagiarism guarantee

Each paper is composed from scratch, according to your instructions. It is then checked by our plagiarism-detection software. There is no gap where plagiarism could squeeze in.

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Free-revision policy

Thanks to our free revisions, there is no way for you to be unsatisfied. We will work on your paper until you are completely happy with the result.

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Privacy policy

Your email is safe, as we store it according to international data protection rules. Your bank details are secure, as we use only reliable payment systems.

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Fair-cooperation guarantee

By sending us your money, you buy the service we provide. Check out our terms and conditions if you prefer business talks to be laid out in official language.

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Feel free to ask questions, clarifications, or discounts available when placing an order.

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