For example, reproductive suppression of social subordinates is generally associated with group living, but suppression may also occur in solitary species if the behavioral and physiolog
Trang 1Behavioral and endocrine correlates of reproductive failure
F Dalerum1, S Creel2& S B Hall3
1 Department of Zoology, Stockholm University, Sweden
2 Department of Ecology, Montana State University, Bozeman, MT, USA
3 Division of Recovery, Endangered Species Division, US Fish and Wildlife Service, Portland, OR, USA
Keywords
social stress; reproductive success;
sociobiology; carnivore; endocrinology.
Correspondence
Fredrik Dalerum, Department of Zoology,
Stockholm University, SE-106 91
Stockholm, Sweden Tel: +46-8-16-40 01;
Fax: +46-8-16-77-15
Email: fredrik.dalerum@zoologi.su.se
Received 19 April 2005; accepted
1 November 2005
doi:10.1111/j.1469-7998.2006.00116.x
Abstract
Sociality in mammals is often viewed as a dichotomy, with sociality contrasted against solitariness However, variation within these broad categories may have strong effects on individual fitness For example, reproductive suppression of social subordinates is generally associated with group living, but suppression may also occur in solitary species if the behavioral and physiological processes involved can be modulated by the demographic environment To investigate whether behavioral and physiological traits that normally are associated with group living might be latent even in a solitary species, we explored the level of sociality and investigated causes and mechanisms of reproductive failure in female wolverines Gulo gulothat experienced a highly aggregated social environment in captivity Behaviorally, females showed low levels of aggression and intermediate levels of social interactions Reproductive failure seemed to have been related to low social rank and to have occurred between ovulation and implantation in 13 out of
15 breeding attempts However, three of eight females observed to mate produced offspring, indicating that no individual female fully managed to monopolize breeding Reproductive failure was not related to elevated levels of glucocorticoid stress hormones Rather, elevated glucocorticoid levels during the mating season were associated with successful reproduction We suggest that social tendencies and physiological mechanisms mediating reproductive suppression may be viewed
as reaction norms to the social environment We further suggest that the social flexibility of solitary carnivores might be greater than is commonly observed, due
to ecological constraints that may limit aggregation
Introduction
Sociality in mammals is often viewed as a dichotomy, with
various forms of sociality contrasted to solitary living
However, even individuals of solitary species engage in
social interactions, although their interactions are less
com-mon and elaborate than in species that form cohesive social
bonds (Leyhausen, 1965) Hence, some authors favor an
alternative approach, which begins by describing the spatial
structures of individuals, and then treating social
interac-tions as a function of these spatial structures (Sandell, 1989)
There are many examples of intraspecific variation of the
spatial organization of individuals In these species, the
spatial organization within populations is affected by many
variables, such as the amount and spatial distribution of
resources, competitors and predators (Moehlman, 1989;
Johnsson, Macdonald & Dickman, 2000) Such social
flex-ibility may exist even in species where it is not observed, if
sociality is constrained by ecological factors Many of the
behavioral and physiological traits found in complex social
societies might then be found in species with solitary social structures, perhaps in less well-developed forms
Reproductive suppression of socially subordinate indivi-duals is commonly found among group-living mammal females (Wasser & Barash, 1983; Jennions & Macdonald, 1994) Such suppression of subordinates can be directly caused by dominant individuals killing offspring (Rood, 1990) or inhibiting mating opportunities (Creel et al., 1992; Clutton-Brock et al., 1998a; Ebensberger, 1998) Suppres-sion can also be caused by physiological mechanisms that disrupt the endocrine events that control ovulation and implantation (Creel, 1996; Faulkes & Abbott, 1997) or by mechanisms that terminate established pregnancies during gestation (Hackl ¨ander, M ¨ostl & Arnold, 2003) We will here use the term ‘physiological suppression’ for these latter two cases, that is when a subordinate is physiologically pre-vented from reproducing, either pre- or post-mating, due to the presence of dominants Physiological suppression is, however, rarely observed in solitary societies, and is in some cases assumed to be absent (e.g Creel & Macdonald, 1995)
Trang 2However, if individuals of solitary species experience
aggre-gated social situations, it is possible to test if the
physiologi-cal mechanism that mediates suppression involves latent
behavioral or physiological traits that can be modulated by
the social environment
From a proximate standpoint, glucocorticoids (GCs) have
been suggested as a mediator of physiological suppression
(Schoech, Mumme & Moore, 1991; Blanchard et al., 1995)
GCs are steroid hormones produced by the adrenal cortex
and are released in response to physical or physiological
stressors (Sapolsky, 2002) Among other effects, long-term
elevation in GC levels can suppress reproduction (Dobson &
Smith, 2000) However, long-term elevations in GCs also
have a wide range of negative effects, such as suppressed
immune function and increased blood pressure It has
there-fore been argued that non-GC-mediated mechanisms of
reproductive suppression would be evolutionarily favored
in species for which reproductive suppression is a part of the
normal social organization (Creel, Creel & Monfort, 1996)
This argument is supported by field data, because there is an
increasing array of evidence that it is more common for
elevated GC levels to be a cost of dominance than of
subordination in free-living populations (Creel, 2001)
How-ever, if subordinates or normally solitary species cannot
escape the presence of dominant individuals, stress responses
might drive reproductive suppression This situation might
arise, for example, in a high-density rodent population In
such situations, GC-driven mechanisms are a plausible, and
even expected, mediator of reproductive suppression
The wolverine Gulo gulo is a large mustelid that inhabits
the northern boreal and arctic zones of North America and
Eurasia (Pasitschniak-Arts & Larivie`re, 1995) It is a solitary
species with reflex ovulation and a delayed implantation
period of c 5–6 months (Mead et al., 1993; Banci, 1994) In
the wild, wolverine males spatially overlap both other males
and several females, whereas territories of reproductive
females only overlap territories with males and recent
off-spring (Powell, 1979; Hornocker & Hash, 1981; Magoun,
1985; Banci & Harestad, 1990) Social groupings other than
mating pairs and mother and infants are very rarely observed
To investigate whether behavioral and physiological
traits that normally are associated with group living might
be latent even in a solitary species, we explored the level of
sociality and investigated causes and mechanisms of
repro-ductive failure in female wolverines that experienced a
highly aggregated social environment in captivity, with
surplus food and no predation We predicted that social
interactions would generate a social hierarchy, and that
reproduction would be heavily skewed toward specific,
top-ranked individuals We further predicted that the lack of
possibility for subordinates to escape dominant individuals
would cause an elevated stress response in subordinate
individuals, and that this would result in subordinate
reproductive failure We pursued the following specific
questions: (1) How much time do females engage in social
interactions? (2) Is there individual variance in female
reproductive success? (3) Is any such variance related to
social rank of the females? (4) If reproduction fails, at what
stage of the reproductive process does it occur? (5) If reproduction fails, is reproductive failure related to an endocrine stress response (i.e elevated GC levels)?
Methods
Animal housing and monitoring of reproduction
We conducted the study at a private facility in Washington State, USA The animals at the facility were mainly or-phaned individuals that were housed in captivity for huma-nitarian reasons The study was conducted under a license from the State of Washington, USA Animals were kept in three outdoor enclosures, each c 1500 m2 in size The enclosures consisted of mature conifer and hardwood forest enriched with water bodies and logs for climbing The animals were fed commercial mink food mixed with offal provided by local elk and deer hunters The animals had
ad lib access to food and water, and were occasionally provided with elk and deer heads, hoofs and legs to play with and chew upon
During the mating seasons (May–August) of 1995, 1997 and 2001, study groups of 12–15 females and five to six males were housed in one of the 1500 m2enclosures During the mating season of 1996, two groups were enclosed (again
in two of the 1500 m2 enclosures), each with five to six females and three to four males Animals were normally housed under these conditions and not specifically grouped together for the study Animals that required special atten-tion for medical reasons or that were believed to be pregnant (see below) were housed in individual pens, 3 6 m in size These pens were situated in semi-open buildings (i.e with a solid roof but only wire mesh walls) immediately adjacent to each of the enclosures, so that separated individuals were within sight, smell and sound of both each other and the remaining group, as well as of environmental stimuli such as photoperiod These pens had concrete floor covered with sawdust and were cleaned daily Each pen had a small sleeping box of c 1 m3, padded with straw
All but two of the females included in the study were born wild in Canada These two were born at the facilities, one by
a female not included in the study and one by an included study female (Table 1) All wild-born females were caught at different locations and can thus be assumed to have been unrelated During each mating season, females were ob-served daily for signs of mating activities Except for beha-vioral data on one female (see Behabeha-vioral data), we only included data on females that had been observed to copu-late, as several of the other females were of unknown age and could have been sexually immature Females observed to have copulated were separated during the fall (from Septem-ber to DecemSeptem-ber) and housed in individual pens as described above but with a slightly larger nest box, which was subdivided into one interior and one exterior compartment Each nest box had an infrared camera that was fitted to the interior compartment and linked to a monitor in an adjacent
Trang 3building At the time of expected parturition, nest boxes
were inspected daily through the remote sensing system to
determine day of parturition We followed the reproductive
outcome of 20 breeding attempts where copulation was
confirmed in eight females during four breeding seasons,
1995/1996, 1996/1997 and 1997/1998 and 2001/2002 (Table 1)
Throughout the paper, we have used the term ‘breeding
attempt’ to describe the entire time period from mating season
(in which copulations had been observed) through delayed
implantation, gestation and lactation
Behavioral data
During the 1996 mating season we collected behavioral data
on five females, of which two were housed in one enclosure
and three in another During 1997, we collected data on four
of these females In this mating season all were housed in
the same enclosure During 1996, we collected data from
18 May to 25 August, and during 1997 from 10 May to
20 July This provided a total of 135 h of behavioral
observa-tions Two females observed in 1996 produced kits the
subsequent spring, and one produced kits also after the
mating season 1997 None of the other three females
pro-duced kits, although two were observed to copulate The
female not observed to copulate was regarded as an
unsuc-cessful reproducing female in the analyses because at the time
she was at least 3 years old, and hence should have been
sexually mature (Persson, 2003) During 1996, both of the
successfully reproducing females were housed in the same
enclosure Each animal was observed for 1 h three times a
week, using scan sampling at 30 s intervals Each 1 h scan
sampling session was carried out either between 9:00 and
12:00 h or between 18:00 and 21:00 h We classed behaviors
into five categories: (1) inactive (any inactive behavior such as
sleeping or resting), (2) moving (any movement, such as
walking, running or pacing), (3) social interactions (any clear
social interaction between two or more individuals),
(4) grooming (grooming behavior), (5) other, including
dig-ging, carrying objects and swimming (any other behavior)
These definitions were based on pilot observations of the
study animals In addition to scan sampling, we recorded all occurrences of social behavior and scent marking during focal hours For social interactions, we recorded the gender, and if possible the identity, of the initiator of the interaction Scent marking was divided into four categories: defecation, urina-tion, pelage rubbing and abdominal rubbing Pelage rubbing
is a distinct behavior in which the animal vigorously rubs one
of its sides on an object or on the ground Abdominal rubbing
is an equally distinct behavior in which the animal drags or rocks its abdomen against a surface or object
Fecal collection and steroid extraction
We collected fecal samples from 15 breeding attempts by eight females for endocrine analyses (Table 1) The number
of analyzed fecal samples per breeding attempt ranged from
6 to 54 (mean = 21,SD= 11) The data were collected during the 1995/1996, 1996/1997 and 2001/2002 breeding seasons During the 2001/2002 breeding season, we only collected fecal samples from January to May During periods when females were communally housed, we collected feces oppor-tunistically whenever a female was observed to defecate In addition, we hand-fed females with food items labeled with colored plastic pellets and other identifying markers to facilitate identification of feces During periods when fe-males were housed in individual pens, feces were collected as part of the daily maintenance of the facilities Each scat was frozen immediately after collection and stored in 20 1C until further analyses
We derived endocrine data non-invasively from enzyme-linked immunosorbent assays (EIA) and radioimmuno assays (RIA) on the fecal material We validated the assays according to standard criteria, described below for each assay (Cekan, 1975) A total of 580 feces was used for these analyses
We extracted steroids using previously published meth-ods (Creel et al., 2002) Each sample was homogenized, and
a subsample was dried in a vacuum evaporator The dried sample was pulverized, and a subsample of c 0.2 g was boiled for 20 min in 10 mL of absolute ethanol After
Table 1 Female wolverines Gulo gulo included in the study, their origin and year of birth, number of breeding attempts followed (with copulation
confirmed), number of litters produced, number of mating seasons with behavioral data and number of breeding attempts with endocrine data
Number of breeding attempts
Number of litters produced
Number of seasons with behavioral data
Number of breeding attempts with endocrine data
a Gave birth to three litters, but killed her single kit in one of these cases.
b
Daughter to female not included in study.
c Daughter to female #9500.
Trang 4centrifuging at 1500 r.p.m (504 g) for 15 min and decanting
the supernatant (containing extracted steroids), the residual
pellet was weighed as a measure of indigestible matter The
recovered supernatant was evaporated under air, rinsed in
1.5–2.5 mL of absolute ethanol, sonicated, vortexed and
re-dried The re-dried samples were reconstituted in 1.0 mL of
absolute methanol, sonicated for 45 s and then vortexed for
1 min to free particles adhering to the tube walls We then
transferred the extracts to 2 mL cryovials with silicone
O-rings for long-term storage The methanol extracts were
stored at 80 1C until analyses
Fecal progesterone assay
We measured fecal progesterone using an EIA kit from
R&D Systems Inc (Minneapolis, MN, USA), with low
reported cross-reactivity to other steroids (3.5% for
17-OH-progesterone, 0.77% for corticosterone and o0.10% for
the other steroids tested) Serial dilutions of standards and
fecal extracts gave parallel antibody binding for 7 points
from 16-fold to 16 000-fold dilution The recovery of known
amounts of progesterone to fecal extracts (50 mL at
1.95–2000 pg mL 1) was 88 18% (y = 60+1.04x,
SEb= 0.02, r2= 0.999) Fecal extracts were assayed in
200-fold dilutions following the protocol provided with the
assay kit Intra-assay variation was 8.4%, and inter-assay
variation was 11.3 and 5.7% for pooled high and low
controls, respectively Sensitivity was 0.195 pg (per
determi-nation, or 3.9 pg mL 1of extract)
Fecal estrogen assay
We measured fecal estrogens using a commercially available
double antibody125I RIA kit for total estrogens from ICN
Pharmaceuticals (Costa Mesa, CA, USA), previously
vali-dated for fecal assays with other carnivores (e.g Creel et al.,
1997) This antibody does not distinguish between
estradiol-17aand estrone, but shows little cross-reactivity with other
steroids (9.0% for estriol, 7.0% for estradiol-17b, 2.5% for
equillin ando0.10% for the other steroids tested) Serial
dilutions of standards and fecal extracts gave parallel
anti-body bindings for 6 points from a two-fold to a 64-fold
dilution All unknowns were run within this range of
dilution The recovery of known amounts of estrogens to
fecal extracts (50 mL at 5–200 pg mL 1) was 88.7 8.54%
(y = 0.90+0.85b,SEb= 0.03, r2= 0.993) Aliquots of 10 mL
of sample extract (equivalent to a 10-fold dilution) were
evaporated under air and reconstituted with dilutent buffer
Aliquots of 0.5 mL of the reconstituted samples were
assayed following the instructions supplied with the assay
kit Intra-assay variation was 3.5% and inter-assay
varia-tion was 9.1% Sensitivity was 2.5 pg tube 1
Fecal corticosterone assay
We measured fecal GCs using a commercially available
double antibody125I RIA kit for corticosterone from ICN
Pharmaceuticals (Costa Mesa, CA, USA), previously
vali-dated for fecal assays with other carnivores (e.g Creel et al.,
2002) The antibody shows little cross-reactivity with other steroids (0.34% for desoxycorticosterone, 0.10% for testos-terone and less than 0.10% for the other steroids tested) Serial dilutions of standards and fecal extracts gave parallel antibody bindings for 8 points from a fivefold to a 640-fold dilution All unknowns were run within this range The recovery of known amounts of corticosterone added to fecal extracts (50 mL at 12.5–250 pg mL 1
) was 67 15% (y = 34.6+0.88x, SE b= 0.05, r2= 0.979) Fecal extracts were assayed in 100-fold dilutions, following the supplied protocol for the kit, with the exception of halving volumes Intra-assay variation was 5.3% and inter-assay variation was 7.7 and 4.3% for pooled high and low controls, respectively Sensitivity was 2.5 pg tube 1
We evaluated the biological validity of the corticosterone assay by measuring the GC response to injections of 25 IU Cortrosyns
(Amphastar, Rancho Cucamanga, CA, USA) injected intramuscularly into the quadriceps muscle We injected four wolverines (three females and one male) and collected all feces from 48 h before the injection until 72 h after the injection There was a significant increase in fecal corticosterone between 12 and 72 h post-injection (pre-injection, meanSE: 8.82 1.03 mg g 1dry feces; post-injec-tion, meanSE: 12.65 2.26 mg g 1
dry feces; t3= 3.14, P= 0.05, calculated on individual means), although the timing of the corticosterone peak varied among individuals Although the GC response was significant, we believe that the measured response to the ACTH challenge was probably muted by elevated baselines in this trial, because all animals had to be shifted to new, isolated housing just before the experiment for logistical reasons
Statistics
To account for non-independence of different behavioral frequencies during the scan sampling, we log-ratio trans-formed the frequencies of each behavior (i.e number of scan events with the behavior), and used this transformed vari-able in an ANOVA to analyze behavioral time budget data (Aitchison, 1986) For social and scent marking data (col-lected by all-occurrences sampling), basic behavioral rates were expressed as observations of each behavior per obser-vation hour We square root transformed this variable to approximate normality and used this transformed value as a response variable in an ANOVA (Crawley, 2002) In ana-lyses of behavioral data, we used breeding attempt as the smallest sample unit
We used mixed linear models to test for differences in fecal steroid levels between successful and unsuccessful breeding attempts during different phases of the breeding cycle One female that was observed to kill her newborn kit was regarded as a successful breeder in these analyses, because she was not physiologically suppressed according
to the definition described in the introduction We divided each breeding attempt into four phases: mating, diapause (recall that wolverines show delayed implantation), gesta-tion and lactagesta-tion The mating season was defined as May, June and July, because mating was recorded during this
Trang 5entire period Diapause was defined from the beginning of
August until 50 days before parturition or expected
parturi-tion (i.e after mating and before gestaparturi-tion) Gestaparturi-tion was
defined as the time from 50 days before and until the time of
parturition or expected parturition, and lactation was
de-fined from parturition or expected parturition until the end
of April The expected time of parturition for unsuccessful
breeding attempts was calculated as the average parturition
date for each specific breeding season or as the average date
for all breeding seasons if a season was without any
success-ful breeding attempts To avoid temporal pseudoreplication,
we used days until parturition or expected parturition as a
random covariate for successful and unsuccessful breeders,
respectively Further, to avoid pseudoreplication related to
multiple breeding attempts by the same female, we nested
each breeding attempt within each female and added this as
a random factor in the models We included scat water
content and per cent indigestible matter (indexed as weight
of centrifuged pellet divided by amount of extracted dried
sample) as fixed methodological covariates The models
were fitted using restricted maximum likelihood (REML)
We evaluated main and interaction effects by a conditional
F-test, which is appropriate when fitting models using
REML (Pinheiro & Bates, 2000) For interaction effects,
we used t-tests on breeding attempt means to test for
differences between successful and unsuccessful breeders
within each time period Analyses of progesterone and
estrogen concentrations were conducted on log-transformed
endocrine data to meet the assumption of homoscedasticity
To approximate normality, we arcsine transformed scat
water content (expressed as per cent water of sample before
being dried) and used the transformed variable in the models
(Zar, 1996)
Values are presented as meanSE, unless otherwise
noted All test statistical tests were two-tailed Statistical
analyses were performed with the software R, version 1.8.1
for Linux (http://www.r-project.com)
Results
Time budget of females
The wolverines spent most of their time inactive (1996:
33.6 6.2% of scans, n = 5 individual females; 1997:
54.7 3.9%, n = 4) or just moving (1996: 21.3 4.6%;
1997: 27.0 4.9%; Fig 1) They were involved in social
interactions 10–20% of total time observed (1996:
19.1 3.5%; 1997: 9.9 3.1%) and 20–36% of active time
(1996: 36.6 7.2%; 1997: 21.2 5.6% of the scans when
animals where active) Social interactions mostly consisted
of chasing and various forms of playing, and were generally
accompanied by various types of vocalizations Aggression
was overall rare among the observed females
Because no clear dominant or submissive behaviors were
observed, we could not utilize the data on social behavior to
derive relative rank between the observed individuals
Dif-ferences in behavior for females that bred successfully or
failed to reproduce are discussed below
Distribution of reproductive success among females
Overall, 20 breeding attempts by eight females were mon-itored Of these eight females, three produced offspring during the study (Table 1) Seven out of 10 breeding attempts resulted in offspring for these three females (one out of three, three out of three and three out of four breeding attempts, respectively) However, one female killed her single kit 5 h after birth, limiting the total number of successful attempts to six out of the total 20
Behavioral correlates of reproductive failure
We observed no open aggression involving any of the three females that bred successfully However, successful females showed a significantly higher rate of scent marking than unsuccessful females (F1,9= 11.9, Po0.001), particularly abdominal and pelage rubbing and urination (Fig 2) Reproductive success did not seem to affect the time budget
of the females (F5,18= 0.52, P = 0.75; Fig 1) nor the amount
of social interactions either with other females (F1,6= 0.10,
P= 0.75) or with males (F1,6= 2.86, P = 0.14) Further, reproductive success did not affect whether females initiated social interactions, either with females (F1,6= 0.25,
P= 0.63) or with males (F1,6= 0.41, P = 0.54)
Endocrine correlates of successful reproduction
The total gestation time (measured from copulation to parturition) was 252 7 days (mean SDof four breeding
70 60 50 40 30 20 10 0
1996
70 60 50 40 30 20 10 0
1997
Inactive Moving Social Groom Other
Figure 1 Behavior of female wolverines during the 1996 (n = 5) and the
1997 (n = 4) breeding seasons The number of successful breeders was two in 1996 and one in 1997 There was a significant difference between the behavioral categories recorded (F5,18= 3.43, P = 0.02), but reproductive success did not seem to affect the time budget of the females (F 5,18 = 0.52, P = 0.75) The figure represents means and standard errors of per cent of 30 s scans.
Trang 6attempts where the date of mating was known) Parturition
occurred on 25, 26, 28 February and 3 March These
breeding attempts came from three females Successful
breeding attempts (seven breeding attempts by three
fe-males) were characterized by baseline progesterone levels
during mating (5.78 0.77 mg g 1
dry feces) and diapause (6.01 0.77 mg g 1 dry feces), elevated progesterone levels
during the gestation period (14.90 3.80 mg g 1
dry feces;
here defined as the period between implantation and
parturition) and baseline levels throughout lactation
(6.59 53.00 mg g 1
dry feces) (Fig 3a) Estrogen levels were elevated during the mating season (32.59 3.93 ng g 1dry
feces) and lactation (31.09 3.41 ng g 1
dry feces), but low during diapause (22.81 1.39 ng g 1dry feces) and
gesta-tion (22.60 3.41 ng g 1
dry feces) (Fig 3b) Progesterone started to increase 50 days before parturition, indicating
implantation (Fig 4a) There was no detectable change in
estrogen levels during implantation and gestation (Fig 4b)
Endocrine correlates of reproductive failure
Progesterone profiles during the nine unsuccessful breeding
attempts closely resembled profiles from the six successful
ones, with one exception (Figs 3a, b and 4c–f) The excep-tional case did not show any endocrine signals of either implantation or gestation (Fig 4e) Excluding this breeding attempt, there were no significant differences between suc-cessful and unsucsuc-cessful breeding attempts in either fecal progesterone (F1,7= 0.04, P = 0.85) or fecal estrogens (F1,7= 0.82, P = 0.39), nor significant interaction effects between breeding period and breeding outcome (proges-terone: F3,283= 1.14, P = 0.33; estrogens: F3,347= 1.77, P= 0.15) Thus, with one exception, sex-steroid profiles did not distinguish successful breeding attempts from failed However, there was a significant interaction between breed-ing outcome and breedbreed-ing period for fecal corticosterone (F2,252= 3.37, P = 0.036; Fig 5) Corticosterone levels were significantly higher during the mating season for success-ful breeders than for unsuccesssuccess-ful breeders (t8= 2.74, P= 0.025), while there were no statistically significant differences during either diapause or gestation (diapause:
t9= 0.77, P = 0.46; gestation: t8= 0.64, P = 0.54)
Discussion
Female behavior and social flexibility
The amount of social interactions among the studied wol-verines approached the levels found in group-living carni-vores (e.g Biben, 1983; Macdonald, 1996) Although our behavioral data were limited to only a few individuals, it still highlights that wolverines possess behavioral traits that permit them to participate in frequent social interactions This indicates that sociality in this solitary carnivore is probably more flexible than previously believed
Traditionally, solitary social systems have been regarded
as the original social system in carnivores, from which more complex forms of sociality have evolved (e.g Packer, 1986; Gittleman, 1989) An alternative interpretation might be that social flexibility was the original state, from which the full range of patterns found across the Carnivora today has radiated This could have happened, for instance, by ecolo-gical constraints in aggregating or in living solitary Such an approach offers an evolutionary interpretation that may explain how a species that normally does not form social groups manages dense social aggregations such as those
2
1.5
1
0.5
0
TOTAL AR+PR DEF UR
Figure 2 Frequency of scent marking of different types (AR,
abdom-inal rubbing; PR, pelage rubbing; DEF, defecation; UR, urination) for
successful (n = 2) and unsuccessful female breeders (n = 3),
ex-pressed as scent marks per hour of observation The difference in
total scent marking rates between successful and unsuccessful
females is statistically significant (F1,9= 11.9, P o0.001) The figure
represents means and standard errors.
20
15
10
5
0
Mating Diapause Gestation Lactation Mating Diapause Gestation Lactation
40 35 30 25 20 15
Successful Unsuccessful
Figure 3 Concentrations of (a) fecal
progester-one (mg g 1 dry feces) and (b) fecal estrogens (ng g 1dry feces) during mating, embryonic diapause, true gestation and lactation in suc-cessful (n = 7) and unsucsuc-cessful (n = 9) breed-ing attempts There are no statistically significant differences between successful and unsuccessful breeding attempts (proges-terone: F 1,7 = 0.04, P = 0.85; estrogens:
F 1,7 = 0.82, P = 0.39) The figure represents means and standard error of breeding attempt means.
Trang 7found in this study, and shows behavioral patterns similar to
those found in traditionally group-living species
Female reproductive failure and social rank
Of the eight females that were observed to copulate, only
three gave birth The reproductive success of these females,
compared with the absence of successful reproduction in the
other females, highlights a fairly substantial variation in
reproductive success among individual females This
varia-tion indicates either that reproductive failure was related to
social rank or that the successful females better adapted to
non-social factors in the captive environment that could
have impaired reproductive function Although we could
not use our data on social behavior to derive rank relation-ships between individuals, there are three lines of evidence that support the first case, that is social suppression First, scent marking rates, which have been shown to increase with social rank in many carnivores (Ralls, 1971; Ough, 1982; Gorman & Trowbridge, 1989; Clutton-Brock et al., 1998b), were substantially higher in the successful breeders than in the unsuccessful ones Second, the endocrine profiles of the unsuccessful breeding attempts imply that the endocrine functions in all but one of these females were intact and not impaired by environmental effects Third, a potential stress response to the captive environment itself should have resulted in elevated GC levels in females that failed to breed successfully However, this was not observed In fact, the opposite was true, where GC levels were actually elevated at the time of mating for successful breeders
However, no individual female fully managed to monop-olize breeding Two separate models have been suggested
to explain such incomplete reproductive skew (i.e the extent
to which reproduction is monopolized by dominant indivi-duals): ‘optimal skew’ models and ‘limited control’ models (Clutton-Brock, 1998; Reeve, Emlen & Keller, 1998) Opti-mal skew models suggest that dominant individuals have full control over reproduction, but grant reproductive con-cessions to subordinates to keep them in the social group Limited control models, on the other hand, suggest that subordinate breeding occurs because of the lack of domi-nant control There are no a priori reasons to predict that these captive wolverine females would benefit from granting reproductive concessions, because these are normally attrib-uted to benefits in terms of increased group size or kin selection (Clutton-Brock, 1998) Hence, we suggest that the intermediate reproductive skew among these wolverine females is due to either the lack of control of a single, dominant female or a more complex, non-linear rank hierarchy that divides individuals into high- or low-ranking
40
30
20
10
40 30 20 10
40 30 20 10
75
50
25
75
50 25
75
50 25
Days from birth
Figure 4 Profiles of fecal progesterone
(mg g 1 dry feces) and fecal estrogens (ng g 1
dry feces) during the period of true gestation for pooled successful breeding attempts (a, b; n = 7), pooled unsuccessful breeding attempts (c, d; n = 8) and one unsuccessful breeding attempt (e, f) that showed no signs
of ovarian activity at the time of expected implantation Samples were pooled over
5 consecutive days, and the figure represents means and standard error of breeding attempt means for each 5-day period.
4
3.5
3
2.5
2
Successful Unsuccessful
Figure 5 Fecal corticosterone levels during mating, embryonic
dia-pause and gestation The difference between successful (n = 7) and
unsuccessful (n = 9) breeding attempts is statistically significant
dur-ing the matdur-ing season (t8= 2.74, P = 0.025), but not during either the
diapause or gestation (diapause: t 9 = 0.77, P = 0.46; gestation:
t 8 = 0.64, P = 0.54) The figure represents means and standard error
of breeding attempt means.
Trang 8individuals rather than into a dominant and subordinates
(which has been suggested for ungulates, e.g Veiberg et al.,
2004)
Apart from the difference in scent marking rates, we
found no other behavioral differences between successful
and unsuccessful females Because there was no evidence for
strong, overt assertion of social rank among these females,
maintenance of social rank in this population is not clear
However, it could be through olfactory means or by
beha-viors not identified by our observations
Timing of reproductive failure
All females that failed to give birth, except one, exhibited
endocrine profiles similar to each other In the only female
that showed an inconsistent progesterone profile compared
with the others, the lack of a progesterone response during
the period of expected implantation and gestation agrees
with endocrine patterns related to ovulation failure in this
species (Mead et al., 1993; F Dalerum, S Creel & S B Hall,
unpubl data) For the other females, the increase in
proges-terone levels during the time of expected implantation and
gestation indicates that they ovulated, because a major part
of this progesterone response is produced by the corpus
luteum (Hodgen & Itskovitz, 1988) Hence, either obligate
pseudopregnancies occurred or we observed actual
pregnan-cies but failed to record parturition in the majority of the
females We feel that the latter case is highly unlikely with
our nest box monitoring program In primates, both
abor-tion and re-absorpabor-tion is followed by a sharp decline in
gonadotropin and sex steroid levels, typically to below
baseline (e.g Kuehl, Kang & Siler-Khodr, 1992; Fortman
et al., 1993; Steinetz, Randolph, & Mahoney, 1995) We did
not find any such endocrine evidence for gestation failure in
any of our failed breeding profiles Gestation failure has
been observed in some cases with intact progesterone
profiles (Kuehl et al., 1992) Hence, we cannot entirely rule
out the possibility that gestation failure occurred in our case
However, without any endocrine indications of gestation
failure, we believe that, except for one female that showed
strong signs of ovulation failure, reproductive failure in
most cases was related to either a failure in fertilization or a
failure in maintaining the embryo during diapause
Female reproductive failure and endocrine
stress
Our results contradicted the fact that elevated GC levels
mediated reproductive failure On the contrary, these results
are in accordance with recent studies on group-living
ani-mals that have opposed the hypothesis that elevated GC
levels in subordinates mediate reproductive failure (see the
review in Creel, 2001) Hence, not only behavioral
tenden-cies among wolverine females appeared similar to
group-living species, but also physiological It is worth noting that
these results were derived under experimental conditions,
that is an abnormally crowded social environment for an
essentially solitary species, similar to earlier studies that
were used to support the hypothesis that GC levels mediated subordinate reproductive failure (e.g Bronson & Elefther-iou, 1964; Louch & Higginbotham, 1967; Manogue, 1975) Considering the very low levels of aggressive behaviors observed, it is in fact surprising that we found any differ-ences between successful and unsuccessful breeders in the endocrine stress response at all As there does not seem to be
a strong behavioral assertment of social status among these females, it is unclear as to what causes the elevated stress response in successful breeders during the mating season
Conclusions
In this solitary species, when housed in a communal setting,
we found similarities in the level of social interactions to what is found in many obligate group-living species Repro-ductive success seemed to be related to social rank, but reproduction was not entirely monopolized by a single individual female Physiological correlates of reproductive failure appeared similar to what is found in many group-living species This indicates that both the behavioral and physiological mechanisms observed could be viewed as reaction norms to the social environment, rather than as fixed traits that have coevolved with various forms of social structures We suggest that sociality in solitary carnivores may be more flexible than commonly observed, and that a solitary social system may be maintained by ecological constraints that limit the formation of social aggregatations
Acknowledgements
This project was supported by the Mt Hood National Forest, Bureau of Land Management, Oregon Zoo, North-west Trek Wildlife Park, Center for Wildlife Conservation and grants from the private foundations of J A Ahlstrand and Roland Nilsson We are grateful to Dale Pedersen and Audrey Magoun for logistical support and constructive discussions, to Shane Iverson for support during the data collection and to Kathleen Cooper for help with early stages
of the laboratory work Anders Angerbj ¨orn, Bertil Borg and Magnus Tannerfeldt improved earlier drafts of the manu-script
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