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Risk recognition by prey is of paramount importance within the evolutionary arms race between predator and prey. Prey species are able to detect direct predator cues like odors and adjust their behavior appropriately. The question arises whether an indirect predation cue, such as the odor of scared individuals, can be detected by conspecifics and subsequently affects recipient behavior. Parents may also transfer their experience with predators to their offspring. In two experiments, we assessed how direct and indirect predation cues affect bank vole (Myodes glareolus) foraging behavior, reproduction, and pup fitness. Weasel (Mustela nivalis) odor served as the direct cue, whereas the odor of weasel‐scared conspecifics, alarm pheromones, was used as an indirect cue and both of those were compared to a control odor, clean wood shavings. Alarm pheromones attracted female voles, measured as time in proximity to the treatment and foraging. Both predator odor and alarm pheromones enhanced reproduction compared to the control odor. Females treated with alarm pheromone had significantly higher pregnancy rates, and pups from predator‐treated mothers were significantly heavier at birth. Our study provides two novel ideas. First, the impact of a predator can be socially transmitted. Second, predation risk likely triggers terminal investment in reproduction.
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Secondhand horror: effects of direct and indirect predator cues on
behavior and reproduction of the bank vole
Department of Biological and Environmental Science, Konnevesi Research Station, University of Jyv
a, P.O. Box 35, 40014 Jyv
Department of Biomedical Sciences, University of Veterinary Medicine, Veterin
arplatz 1, Vienna, Austria
Faculty of Biological and Environmental Sciences, University of Helsinki, Viikinkaari 1, P.O. Box 65, Helsinki, Finland
Citation: Sievert, T., M. Haapakoski, R. Palme, H. Voipio, and H. Yl
onen. 2019. Secondhand horror: effects of direct and
indirect predator cues on behavior and reproduction of the bank vole. Ecosphere 10(6):e02765. 10.1002/ecs2.2765
Abstract. Risk recognition by prey is of paramount importance within the evolutionary arms race
between predator and prey. Prey species are able to detect direct predator cues like odors and adjust their
behavior appropriately. The question arises whether an indirect predation cue, such as the odor of scared
individuals, can be detected by conspecics and subsequently affects recipient behavior. Parents may also
transfer their experience with predators to their offspring. In two experiments, we assessed how direct and
indirect predation cues affect bank vole (Myodes glareolus) foraging behavior, reproduction, and pup
tness. Weasel (Mustela nivalis) odor served as the direct cue, whereas the odor of weasel-scared con-
specics, alarm pheromones, was used as an indirect cue and both of those were compared to a control
odor, clean wood shavings. Alarm pheromones attracted female voles, measured as time in proximity to
the treatment and foraging. Both predator odor and alarm pheromones enhanced reproduction compared
to the control odor. Females treated with alarm pheromone had signicantly higher pregnancy rates, and
pups from predator-treated mothers were signicantly heavier at birth. Our study provides two novel
ideas. First, the impact of a predator can be socially transmitted. Second, predation risk likely triggers ter-
minal investment in reproduction.
Key words: alarm pheromone; ecology of fear; Mustela nivalis; Myodes glareolus; odor; stress response; terminal
Received 2 April 2019; revised 30 April 2019; accepted 2 May 2019. Corresponding Editor: Robert R. Parmenter.
Copyright: ©2019 The Authors. This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
E-mail: thorbjorn.t.sievert@jyu.
Predators decrease an individuals survival
probability (Sih et al. 1985, Murdoch et al. 2003).
Predation, and the indirect effects of predator
presence, has been recognized as strong life-his-
tory determinants across different taxa (Sih 1994,
onen and Ronkainen 1994, Werner and Peacor
2003, Nelson et al. 2004, Yl
onen and Brown 2007,
Sheriff et al. 2009). Historically, ecological
research has focused on the aforementioned
direct predation effects (Paine 1966, Taylor 1984,
Krebs et al. 1995). However, in the last decades,
the focus has shifted more and more toward the
indirect effects of predation (see reviews by Lima
1998, Creel and Christianson 2008), and it has
been recognized that perceived predation risk
alone can have large tness or survival effects on
the population level as direct mortality by preda-
tors (Schmitz et al. 1997, Nelson et al. 2004,
Preisser et al. 2005, Pangle et al. 2007).
Co-evolution of predator and prey species sug-
gests prey evolved a number of sensory and
behavioral adaptations in order to recognize and
avoid predators. In many mammalian prey spe-
cies, this includes behavioral changes such as 1June 2019 Volume 10(6) Article e02765
freezing, avoidance, and heightened vigilance,
but also the ability to detect and correctly recog-
nize the odors emitted by predators, from here
on predator odor (PO), which serve as triggers
for the adaptive behaviors previously mentioned
(Kats and Dill 1998, Dielenberg and McGregor
2001, Sundell and Yl
onen 2004, Conover 2007,
Osada et al. 2014, Apfelbach et al. 2015, Sievert
and Laska 2016). Indirect effects of predation, for
example, decreased reproduction (Yl
onen and
Ronkainen 1994, Sheriff et al. 2009, 2015), as well
as the interaction of risk and competition, are
drawing increasing attention in current literature
(e.g., Apfelbach et al. 2005, Parsons et al. 2017).
In a natural environment, the odor of a preda-
tor might be abundant in the form of excrement
or markings. It is therefore not surprising that
several studies have reported rapid habituation
to predator-born odor in a natural environment
(Cox et al. 2010, Elmeros et al. 2011, Bytheway
et al. 2013). This leads to the assumption that
prey, while detecting PO, considers it as an ambi-
ent risk (Brown et al. 2015). A study by Bleicher
et al. (2018) showed that voles reaction to preda-
tor odor returns to baseline levels after being
confronted with a live predator. This indicates
that actual predator presence outweighs the
information content of an olfactory cue alone
and that there is no increase in perceived risk
toward a predator odor cue. Prey species then
need different means to convey actual threaten-
ing or acute predation risk, allowing them to
dynamically adjust their behavior to differ-
ent threat levels (Dufeld et al. 2017). This
role is most likely covered by intra-species
Intra-species communication and signaling
about increased risk, for instance, through
Schreckstoff (Frisch 1938) or alarm pheromones
(henceforth AP), are evolutionarily widespread
in many taxa (Bowers et al. 1972, Howe and
Sheikh 1975, Stowe et al. 1995, Boissy et al. 1998,
Beale et al. 2006, Guti
ıa et al. 2007). In
several social species of sh, insects, and mam-
mals, AP secretions are recognized as a signal to
protect their colony, group, or family when in
danger (Breed et al. 2004, Kiyokawa et al. 2004a,
Gomes et al. 2013). Despite some papers raising
concern about the categorization of APs, arguing
that these chemicals cannot be classied as real
pheromones (Magurran et al. 1996, Viney and
Franks 2004), the behavioral response is the
same, given the correct context (Magurran et al.
1996). While for most mammals, the chemical
structure of APs is still unknown, it has been
identied in, for example, aphids (Bowers et al.
1972, Beale et al. 2006), sea anemones (Howe
and Sheikh 1975), and several insects (Crewe and
Blum 1970, Heath and Landolt 1988, Kuwahara
et al. 1989). To fulll their sensory warning role,
APs should be volatile or hydrophilic (Kiyokawa
et al. 2005, Inagaki et al. 2009). Given the major-
ity of experiments on mammalian APs have been
done on lab animals, their chemical structure has
been described only for mice (C57BL/6J and
OMP-GFP strains; Brechb
uhl et al. 2013) and
Wistar rats (Inagaki et al. 2014). Brechb
uhl et al.
(2013) state that both mouse APs and mam-
malian predator olfactory cues share structural
similarities, specically sulfur-containing mole-
cules. In this paper, we utilize the concept of APs
similarly as in previous studies, although we
acknowledge that in most studies the chemical or
biological nature of the different odors of stress
is not yet properly determined.
High predation risk affects mating behaviors
and reproductive success (Sih 1994, Ruxton and
Lima 1997, Kokko and Ruxton 2000). There is
strong support for the notion that predation risk
negatively affects breeding success (Sih 1994).
This is manifested as delayed breeding in bank
voles and gray-sided voles (Myodes rufocanus;
Mappes and Yl
onen 1997, Fuelling and Halle
2004), hindering copulations in bank voles
(Myodes glareolus; Ronkainen and Yl
onen 1994),
elevating stress levels in snowshoe hares (Lepus
americanus; Sheriff et al. 2009), or decreasing
weights of breeding individuals or their off-
spring in snowshoe hares and bank voles (Sheriff
et al. 2009, Trebatick
a et al. 2012). However, the
mechanisms and adaptive value of delayed or
suppressed breeding under risk are not clear and
continue to be debated (Ruxton and Lima 1997,
Kokko and Ruxton 2000). Several publications
have already explored the effects of increased
risk of predation in parentsenvironment on off-
spring behavior and tness, nding altered
learning behavior in three-spine sticklebacks
(Gasterosteus aculeatus; Roche et al. 2012, Feng
et al. 2015), altered stress reaction in C57BL/6
mice and Long Evans rats (St-Cyr and McGowan
2015, St-Cyr et al. 2017), or changed foraging 2June 2019 Volume 10(6) Article e02765
strategies in Sprague Dawley rats (Chaby et al.
An alternative explanation suggests that par-
ents will maximize reproductive efforts at all
costs in risky conditions (henceforth terminal
investment). In this scenario, individuals breed-
ing in a risky environment will enhance, or speed
up, reproduction in order to maximize tness by
producing a number of strong offspring despite
the high costs for the parentsor mother s sur-
vival. If offspring survive and reach a fertile age,
this then compensates for parental disappear-
ance from the reproductive pool (Kokko and
Ranta 1996, Kokko and Ruxton 2000). This strat-
egy of bet-hedging or terminal investment has
been shown in experimental studies in passerine
birds breeding under increased predation risk
onen et al. 2009) as well as in crickets
(Adamo and McKee 2017). Additionally, it has
been shown as a reaction to infections in ants
and sparrows (Bonneaud et al. 2004, Giehr et al.
The relationship between weasels and voles
has been intensively studied as the weasel is a
specialist predator of rodents and is the major
cause of mortality in boreal voles, especially dur-
ing a populations decline (Korpim
aki et al. 1991,
Norrdahl and Korpim
aki 1995, 2000). As an
adaptation against the dramatic predation pres-
sure by weasels, voles are able to detect the odor
of mustelids as an antipredator measure and
change their behavior accordingly. Bank voles
decrease their movement and foraging when
exposed to weasel odor (Yl
onen 1989, Sundell
and Yl
onen 2004, Bleicher et al. 2018). They shift
their activity times and spatial use to avoid wea-
sels (Je
zdrzejewska and Je
zdrzejewski 1990, Je
jewski and Je
zdrzejewska 1990, Sundell et al.
2008) and use more arboreal escape under preda-
tion risk (Je
zdrzejewska and Je
zdrzejewski 1990,
ainen et al. 2014). In the study by
ainen et al. (2014), weasels rarely followed
bank voles into a tree, if the bank vole climbed
one, showing the efciency of bank voles
antipredator responses.
Here, we studied in two experiments how
increased predation risk, either direct risk in the
form of least weasels (Mustela nivalis nivalis)
odor, or indirect risk in the form of odor emitted
by weasel-scared conspecics, inuenced behav-
ior and reproductive investment in bank voles.
The effect was assessed in both behavioral trials
and a breeding experiment with cue exposure of
parents and monitoring subsequent offspring
performance. Social cues, such as pheromones,
have previously been shown to be sufcient to
trigger cross-generational changes (Koyama
et al. 2015). In order to differentiate between the
effects of PO, AP, and mere social odor, we also
used non-stressed conspecic bedding as a sec-
ond control in addition to clean wood shavings.
In the behavioral experiment, we predicted
1. Voles would feel safer in control and social
odor treatments and spend more time in
boxes containing those treatments. This
would lead to increased foraging in those
treatments and foraging to be lowest in PO
treatment and second lowest in AP treat-
ment. This would be in accordance with
previous studies (Osada et al. 2014,
alez et al. 2017).
In the tness experiment, we predicted, based
on the existing body of research, that predation
cues have a detrimental effect on reproduction.
Specically, we predicted that:
2. The direct predation cue, PO, would
decrease the breeding success of parent
voles (measured as number of breeding
females and litter size) more than AP.
3. Both predation cues, PO and AP, will
decrease the number of breeding females
and cause the production of smaller litters
(Kokko and Ruxton 2000, Fuelling and Halle
4. Both PO and AP treatments will cause pups
to be smaller (Sheriff et al. 2009, 2015).
5. There would be no effect of social odor or
control odor on condition, breeding of par-
ent voles, or size of offspring.
Study species
Bank voles are common rodents in boreal for-
est areas. Vole populations cycle in Scandinavia
and specialist predators have a large role in caus-
ing this cyclicity (Hanski et al. 2001). Regular
high predation pressure in the wild maintains 3June 2019 Volume 10(6) Article e02765
bank vole antipredator behavior at a high level.
The breeding season of the bank vole in central
Finland usually begins at the end of April and
lasts until September. During the breeding sea-
son, breeding female bank voles are strictly terri-
torial and male territories overlap with several
female territories (Bujalska 1973). The gestation
period is about 20 d, after which 36 pups are
born. These pups mature after 30 d.
The least weasel is a specialist predator of
small mammals and lives in the same habitat as
its prey. Due to its small size, the weasel is able
to hunt in tunnels and burrows of voles during
both summer and winter, leaving only a few safe
places for the voles (Norrdahl and Korpim
1995, 2000). Weasels are adapted to the harsh
winter conditions by a coat change in late
autumn. The weasel, like all small mustelids,
uses strong odors in its intraspecic communica-
tion, giving the prey a means to evaluate the cur-
rent predation risk.
The studies were conducted in the laboratory
at Konnevesi Research Station in Central Finland
(62°370N, 26°200E). In the laboratory, the voles
are kept in light and climate-controlled
husbandry rooms with a 12-L:12-D daily cycle.
The animals were kept individually in
42 926 915 cm transparent cages with wire
mesh lids with ad libitum water and food supply.
Each cage had wood shavings and hay as bed-
ding. Males and females were kept in the same
room. Study animals were the F1 generation of
individuals housed in the lab during the winter
months. The average initial weight of the voles
was 16.3 g 2.8 g (mean SD). All animals
were individually marked with ear tags (#1005-
1L1, National Band & Tag Company, Newport,
Kentucky, USA).
Weasels for the odor treatment were housed
individually in 60 9160 960 cm cages in an
outdoor shelter. Each cage had a nest box and
wood shavings and hay as bedding. During the
experiment, weasels were fed dead bank voles.
Odor cues
For this experiment, the following odor cues
were used:
Predator odor (PO): 1 mL of odor solution.
The PO was obtained by collecting 6 dL of wea-
sel bedding (wood shavings soiled with urine
and fecal matter) and mixing it with 6 dL of
diethyl phthalate (CAS 84-66-2), a solvent for a
broad variety of chemical substances and often
used for fragrances (Api 2001). The mix was left
overnight in a refrigerator, and the liquid phase
was extracted after 24 h (2 h). The odor solu-
tion was renewed every 7 d and stored in a
stable temperature of +4°C(0.2°C) in a refriger-
ator in-between application. The use of extracted
olfactory cues allowed for even exposure to all
animals and reduced the stress to our captive
weasels. Alarm pheromone (AP): 1 dL of vole
beddings from individuals directly exposed to a
predator. To obtain AP, two male voles were
individually exposed to a weasel for 1 min every
other day. Each individual was placed in a wire
mesh cage, which was then put directly into the
weasel cage. The animal was immediately
returned to its cage afterward. When the treat-
ments were applied, all the bedding of both ani-
mals was thoroughly mixed together. If the voles
were scared on the same day the treatments were
applied, the bedding was collected at the earliest
1 h after the animal returned to its cage. Social
odor (SO): 1 dL of vole beddings collected from
two male voles that were not handled before col-
lection nor exposed to weasel. The bedding of
both animals was carefully mixed before applica-
tion. The control (C) odor consisted of clean vole
bedding, that is, fresh wood shavings changed
between each trial. The odor cues were renewed
for each trial.
Experimental designbehavioral assays
For the rst experiment, we used 50 bank voles
(28 males, 22 females). We applied two behav-
ioral measures to study the response to olfactory
cues in the voles: The rst was a test measuring
the individuals perceived risk using optimal
patch use (Brown 1988, Lima and Dill 1990) and
the other investigating spatial avoidance or pref-
Brown (1988) framed the harvest rate an ani-
mal makes at a given patch as a balance of the
energetic gains and costs attributed to foraging
effort, predation, and missed opportunity costs.
The density of food remaining in a patch after
the forager stops foraging is called a giving-up
density (GUD; Brown 1999) and reects the point
where the energy remaining in the patch is equal
to or outweighed by the combined costs to the
forager. The GUD, as a method, has been 4June 2019 Volume 10(6) Article e02765
adapted to test a large variety of elements affect-
ing the strategic decisions animals take (Bedoya-
Perez et al. 2013) and has been widely applied as
a measure for habitat use (Yl
onen et al. 2002,
Orrock et al. 2004, Bleicher 2017, Bleicher et al.
Each individual was placed in a 190 9190 cm
cross-shaped system (Appendix S1: Fig. S1) for
three hours. At the center of the cross is a release
cage (20 920 cm). Going outward in the four
horizontal directions, the odor chamber is con-
nected via an opaque tube (10 cm long, 4 cm
diameter) to an antechamber (30 920 cm) with
a metal grid lid. This prevented the odors enter-
ing and mixing in the central area of the maze
and minimized the chances of an odor contami-
nation. From there, going outward, an opaque
tube (5 cm long, 4 cm diameter) led to a closed
and opaque odor chamber (40 925 cm). This
tube was considered as part of the odor chamber
for the analysis. Each odor chamber contained
one of four odor treatments together with a box
acting as a foraging patch (henceforth patch).
The odor cues were attached to the lid of the
chamber to avoid contaminations by the vole
and renewed for each trial. PO was applied to l-
ter paper (article no. 120002, grade 1001; Munk-
tell Filter AB, Falun, Sweden). The spatial
orientation of the odors was randomly changed
for each trial to avoid a spatial bias. Two mazes
were used simultaneously, both were located in a
dimly lit room 2 m apart. The ventilated experi-
mental room was 7.5 97.5 m with the height of
4 m, allowing a large overhead space to dilute
escaping odors from the systems. The experi-
ment was performed during day time. Two trials
were run simultaneously for a total of four to six
trials per day. After each trial, every segment of
the maze was cleaned with denatured ethanol
(70%) and dried, to avoid odor contamination
between trials.
The design of the patches was a lidless box
(19 919 96 cm) containing 8 dL of sand into
which 20 husked sunower seeds were mixed.
Each animal was allowed to forage in the system
for three hours (henceforth trial). The optimal
trial length was determined beforehand with
pilot trials. After each trial, the sand was sieved
and the remaining untouched seeds were
counted to obtain the GUD. To avoid cross-con-
tamination of olfactory treatments, the sand was
left to air out for 3 d between trials. Bytheway
et al. (2013) showed that even though predator
odor still elicited increased investigative behav-
ior after 24 h, it no longer elicited a change in for-
aging behavior. Based on this, it seems
reasonable to assume that if the voles were still
able to detect the odor after 72 h, the information
conveyed drastically changed. To encourage for-
aging in the novel systems, the animals were
starved for three hours prior to each trial.
Each trial was recorded using a GoPro4 for
later analysis. During the video analysis, the fol-
lowing parameters were measured for each of
the four arms: choice of the rst odor box
entered, time spent in the connection tubes, and
time spent in the odor box. The rst hour of each
trial was analyzed separately from the whole
duration to account for a possible habituation
Experimental designtrans-generational effects
The 240 bank voles (120 males, 120 females)
were divided equally into four treatment groups
for the second experiment. Prior to grouping the
animals, every individual was weighed and the
dominance of the male individuals was assessed
following the urine marking of males as
described by Horne and Yl
onen (1996) and
Klemme et al. (2006). The males were placed in
the urine marking arena for 4 h and had access
to a small amount of food and water. The urine
markings were analyzed twice by two observers
independently and the average score was
recorded. Each individual received a dominance
score from 1 (no marking, a subordinate male) to
6 (markings all over the arena, a dominant male).
During the group assignment, we made sure
that all treatment groups consisted of an equal
number of males and females, the weight distri-
bution for each sex was similar and that the dom-
inance distribution for each treatment was
similar. Within these constraints, the animals
were assigned randomly into four different hus-
bandry rooms.
The voles were kept in the rooms for seven
days to acclimate to their new husbandry rooms.
The treatments consisted of the following four
odor cues (measurements per cage). Predator
odor (PO): 1 mL of odor solution on lter paper
(article no. 120002, grade 1001; Munktell Filter
AB, Falun, Sweden), Alarm pheromone (AP): 5June 2019 Volume 10(6) Article e02765
1 dL of male vole beddings from scared individ-
uals, social odor (SO): 1 dL of male vole bed-
dings and control (C): 1 dL of dry, aired wood
shavings. Each treatment was directly applied
through the lid of the cage, without handling the
animal or the cage itself. Treatments were
applied three times per week for a total of seven
weeks. The treatments were collected and pre-
pared identically to what was outlined before.
All animals were moved to clean cages after the
mating phase. This is the standard procedure in
our laboratory. It allows the pregnant female to
build a nest in a cage free of the odor of a male
conspecic. Furthermore, it reduces the need to
disturb the female to clean its cages during preg-
After the rst week, the animals within the
treatment were randomly paired for mating,
avoiding pairing of rst-degree siblings. For pair-
ing, the animals were housed in a joined cage for
seven days. From 18 d on after the beginning of
the pairing, female cages were checked for pups
twice per day. When litters were found each pup
was weighed one day after birth and the size of
the litter was recorded. The treatments were
stopped as soon as all pregnant individuals had
given birth. All individuals were weighed again
and the dominance of the males was reassessed.
The females were weighed again 5 d after giving
birth. The experiment and all measurements
ended at this point.
At the end of the habituation, prior to the odor
treatment, fecal samples were collected from all
voles for stress analysis. The voles were put indi-
vidually in smaller cages without bedding for a
maximum of three hours, after which all fecal
pellets not contaminated with urine were col-
lected into Eppendorf tubes then stored at
20°C. This procedure was repeated for all indi-
viduals, including nursing females, after the
treatment was stopped. Corticosterone metabo-
lites in the samples were analyzed following the
method outlined by Sipari et al. (2017) at the
University of Veterinary Medicine in Vienna.
Statistical analyses
All statistical analyses were performed in R (R
Core Team 2018). Plots were generated with
ggplot2 (Wickham 2009) and ggsignif (Ahlmann-
Eltze 2017). To analyze the directional choice of
voles as they entered the behavioral assays, a
multinomial log-linear regression (MLM), pack-
age nnet (Venables and Ripley 2002), was run.
This was combined with a Wald z-test to deter-
mine P-values, package AER (Kleiber and Zeileis
2008). In order to analyze not only the distribution
of litter sizes between treatments but also the dif-
ferences in successful pregnancies, and GUDs,
zero augmented generalized linear models, from
the package pscl (Zeileis et al. 2008) were used.
The time spent in each compartment, the differ-
ences in weight, the weight of the pups, and the
difference in stress metabolites were analyzed
with a linear model (LM) or linear mixed model
(LMM) for repeated measurements, packages
lme4 (Bates et al. 2014) and lmerTest (Kuznetsova
et al. 2017). Other measurements were analyzed
with linear or generalized (mixed) models,
depending on the measurement in question. Data
points with missing observation were excluded
from the data set, resulting in an effective sample
size for the statistical tests of 93 breeding pairs for
the breeding success part of the experiment.
For each analysis, the most complex model
included an interaction between Treatment and
Sex. Other factors, such as litter size and weight,
were added to the most complex model if appro-
priate, but never in interaction with other factors.
To achieve the best model t, rst the interaction
was removed, then other factors, only leaving
Treatment for the simplest model. The individual
animal was always included as a random factor
in analyses with repeated measurements. Each
treatment was compared to the C (control) treat-
ment. For each analysis, the most tting distribu-
tion and model were chosen based on AICc,
package MuMIn (Barton 2018). A model was
considered the best if the difference in AICc from
the next model was greater than 2.5. In the cases
where there was no clear best model, all models
within a DAICc of 2.5 were weighed based on
their differences to the best tting model and
weighed averages of the parameter estimates
were reported. The tables with all tted models
for each statistical test can be found in
Appendix S1: Tables S1S14.
Foraging behavior and giving-up densities
The rst choice of animals did not show a sig-
nicant preference for or avoidance of entering 6June 2019 Volume 10(6) Article e02765
any specic odor compartment. However, we
found a tendency (MLM, P=0.056, df =3,
n=46) that voles were 2.25 times more likely to
enter the SO compartment rst. Otherwise, it
seems likely that the sex of the individual had a
negligible role in the decision to enter either odor
compartment of the maze, as it was not included
in the best model.
In contrast to the rst choice for odors, we
found that over the full experimental duration of
three hours there was a signicant interaction
between the sex of the vole and the time spent in
the AP tube (LMM, P=0.031, df =10, n=50,
Fig. 1). When both sexes were analyzed together,
female voles spent on average 2.5 min (147.5 s)
longer in the AP tubes compared to the males.
When the two sexes were analyzed separately,
male voles did not show a preference or avoid-
ance for the tubes (LMM, P>0.05, df =6,
n=28), but females spent two minutes (121.4 s)
longer in the tubes connecting the AP compart-
ment (LMM, P=0.027, df =6, n=22) compared
to the tubes leading to C compartment (close to
four minutes, 225.2 s). For the time spent in the
odor compartment, there are no signicant dif-
ferences for the whole trial (LMM, P>0.05,
df =10, n=50).
The analysis of the GUD showed that about
1.1 seeds more (weighted average) were
harvested from the AP compartment compared
to control independent of the animals sex
(GLMM, Poisson, P=0.019, df =5, 6, n=50,
Fig. 2).
Effect on parents and offspring
Weight change in parental generation.On aver-
age, female voles gained 0.92 g in weight during
the experiment. However, the weight gain of
females was solely dependent on the number of
pups born, as for every additional pup the
females gained 1.2 g of weight (LM, P<0.001,
n=93, df =3) and was not affected by the treat-
ments (Appendix S1: Fig. S2).
The two best models show a signicant (LM,
P=0.023, n=120, df =5, 6, n=93) weight
increase for the males in the SO treatment. The
weighted average of those models indicates that
the male voles in the SO treatment gained about
1.49 g more weight than males in the control
group (Fig. 3). Change in male dominance may
also play a role in the weight change since it was
included in the second best model. Our treat-
ments, however, did not affect male dominance
(LM, P>0.05, df =5, 6, n=93).
Breeding success and offspring weight.During
the experiment, a total of 74 litters were born
from 120 breeding pairs. The analysis of the litter
Fig. 1. Time spent in the connective tube by sex and treatment. Females reacted signicantly different from
males to AP (P<0.05). Asterisks indicate a signicant difference from C at P<0.05. 7June 2019 Volume 10(6) Article e02765
rate per treatment showed a clear best model.
Signicantly, more female voles gave birth under
the AP treatment compared to those in the con-
trol treatment (Hurdle GLM, Poisson & Binomial,
P=0.0095, df =8, 10, n=93). About 36.8% of
the females in the control treatment successfully
gave birth, whereas about 84.5% of those under
AP treatment gave birth (weighted averages,
Fig. 4).
Seventy-four litters resulted in a total of 290
pups across all treatments. There were no signi-
cant differences in litter sizes between the treat-
ment groups (Hurdle GLM, Poisson & Binomial,
P>0.05, df =8, 10, n=93). However, pups
from the PO group weighed about 2.81 g and
were signicantly heavier one day after delivery
than the control pups which weighed about
2.44 g (LMM, P=0.026, df =6, 7, n=262 pups,
64 mothers, Fig. 5). This was independent of
litter size.
Fecal corticosterone metabolite levels.From the
240 experimental animals, we non-invasively col-
lected pre- and post-exposure fecal samples from
230 individuals. The difference of stress metabo-
lites per 50 mg of fecal matter between the two
measurements was assessed with a LM. No clear
Fig. 2. Giving-up density by treatment. Asterisk
indicates a signicant difference from control at
Fig. 4. Female average pregnancy rates for each
treatment weighted by AICc. Whiskers show the 95%
condence intervals. Asterisks symbolize a signicant
difference from the control group at P>0.01.
Fig. 5. Weighted (by AICc) model averages of the
individual pup weight at day 1. Whiskers show 95%
condence intervals. Asterisk indicates a signicant
difference at P<0.05.
Fig. 3. Boxplots of the weight change of male voles
separated by treatment. Asterisks symbolize a signi-
cant difference from the control group at P>0.05. 8June 2019 Volume 10(6) Article e02765
best model was found; however, both models
within the AICc range show a signicant differ-
ence between the C and SO treatments (LM,
P=0.037, n=225). In the control treatment
stress metabolites rose by 82.7 ng/50 mg but in
the SO treatment the metabolites only rose by
32.5 ng/50 mg (weighted averages by AICc,
Fig. 6).
Our study brings new insights into the com-
plex system of predatorprey interactions. We
propose a novel way how prey can determine
predator presence and how prey change their
behavior in response to olfactory cues. First, we
show that the voles were able to distinguish
between an ambient or conservative level risk,
that is, PO (Dufeld et al. 2017) and a cue of an
acute or reliable risk signaling life-threatening
imminent possibility of predator attack, that is,
AP. Second, we also show how these different
perceived threat levels differently affected repro-
ductive investment and success. In the AP treat-
ment, most of the females were breeding, and in
the PO treatment, voles were investing in larger
pup size.
Compared to the information that a direct
predator odor presents a more long-lasting habi-
tat level risk, the acute risk information in the
form of odor of a recently scared conspecic vole,
seemed to outweigh in importance the weasel
odor. Actually, the conspecic carried cues may
include both predation risk levels, as the preda-
tor must have been close enough to scare a prey
individual, who carries then the immediate
threat cue to other conspecics. Non-olfactory
conspecic cues, for example, vocalizations, have
already been known to convey information
about predator presence (Blanchard et al. 1991,
Barati and McDonald 2017, Forti et al. 2017), the
perceived risk, or even the identity of the preda-
tor (Manser et al. 2002, Ouattara et al. 2009, Bar-
ati and McDonald 2017, Collier et al. 2017).
Thus, in the total assessment of predation risk
both cues may well be complimentary: Predator
odor increases vigilance from the base level and
a scared conspecic vole having survived a close
encounter with a predator may signal more accu-
rately and more rapidly for a group of con-
specics that the real danger is acute and near.
We found females treated with both PO and
AP showed a positive response in their reproduc-
tive states compared with control females. This
manifested itself in two major ways: (1) AP-trea-
ted females had a higher successful insemination
rate, and (2) PO-treated parents had heavier
pups shortly after birth. This further indicates
that both predator presence cues and alarm cues
from conspecics can work at the same time,
both to increase vigilance but also to trigger
enhancement of reproduction in the form of ter-
minal investment. Contradictory to our expecta-
tions derived from previous experiments (Yl
and Ronkainen 1994, Fuelling and Halle 2004,
Haapakoski et al. 2012), volesbreeding effort
increased under elevated predation risk. How-
ever, our results are in accordance with Haa-
pakoski et al. (2018) where female bank voles
had larger litters in the AP-treated eld enclo-
sures compared to social odor treated females.
We cannot rule out that AP produced by our
voles exposed to predator might communicate
Fig. 6. Difference in fecal stress metabolites pre- and
post-treatment per 50 mg fecal matter. Asterisk indi-
cates a signicant difference at P<0.05. 9June 2019 Volume 10(6) Article e02765
more than just alarm. We do not know yet what
kind of physiological processes are involved in
the odor production of a scared individual; that
is, the odor may be a combination of being
scared but also relief due to being able to escape
predation. This issue needs further studying. In
fact, at least two studies show that male mice
exposed to competitors or predators are more
attractive to females. In the rst one, chronic
exposure of cat odor enhanced aggression, uri-
nary attractiveness, and sex pheromones in mice
(Zhang et al. 2008). In the second one, chronic
co-housing with rats increased the competitive-
ness of male mice and their urines were more
attractive to females (Liu et al. 2017). Liu et al.
(2017) also found that the levels of major urinary
proteins (MUP) and some volatile pheromones
were increased in the co-species-housed mouse
urine, along with their serum testosterone levels.
It is known that MUP functions as a pheromone
and stimulates sexual attraction (Roberts et al.
2010) and estrus in female mice (Marchlewska-
Koj et al. 2000). We have an ongoing bioassay
study for clarifying and analyzing the body odor
compounds of AP voles compared to non-dis-
turbed voles. After this information, we hope to
nd more answers to the role of MUP and AP on
the vole reproduction.
The behavioral experiment suggested that the
voles were more likely to inspect the maze arm
containing the AP cue than the arm containing
the control cue. In Haapakoski et al. (2018), vole
females also preferred the AP odor compared to
SO while males preferred SO over AP odor. This
is also partly reected in our result that female
voles spent signicantly more time in the maze
arm leading to the AP compartment compared to
males and signicantly more time compared to
the control. We attribute this to exploring the
arms to gain information from the signal (Baro-
cas et al. 2016, Parsons et al. 2017). In contrast,
AP cue enhanced foraging compared to C, caus-
ing lower GUDs in the experimental patches con-
taining the AP, which is suggestive of lower
vigilance (Embar et al. 2011). Further, this could
be a result of a heightened energetic need (Arenz
and Leger 2000) and the rst indicator of termi-
nal investment.
Increasing reproductive investment despite
severe negative changes in the breeding environ-
ment seems maladaptive at rst glance.
However, Dufeld et al. (2017) propose a new
dynamic model for adaptive reproductive strate-
gies. At low-to-medium perceived risk levels,
reproduction is affected negatively, as parents
invest in own survival. Above a certain threshold
a coping mechanism, that is, terminal invest-
ment, would be triggered to compensate for the
loss in an individuals own reproductive value. A
similar idea is described with the insurance
hypothesis, where individuals increase their
reproductive investment in anticipation of an
unfavorable environment (Promislow and Har-
vey 1990, Forbes 1991, Houston et al. 2012). The
increased number of offspring or increase in fer-
tility is designed to counteract expected low sur-
vival chances of offspring. While this has been
mainly shown in birds (Anderson 1990a, Forbes
1990), there is also evidence in humans (Ander-
son 1990b, Strassmann and Gillespie 2002). As
the majority of studies about terminal investment
in mammals focuses on the aspect of senescence
(e.g., Ericsson et al. 2001, Hoffman et al. 2010,
Weladji et al. 2010), it is difcult to assess the
benets of this strategy on an evolutionary scale
considering high predation pressure. We invite
others to investigate this phenomenon further
and to incorporate it into existing evolutionary
For our study species, the bank vole, Eccard
et al. (2011) describe a similar pattern for a dra-
matic increase in breeding effort after a critical
threshold vole density was surpassed. Breeding
bank vole females require a breeding territory
and if the breeding habitat is occupied, the sur-
plus females cannot breed. In the Eccard et al.
(2011) study, the number of females was gradu-
ally increased from normal to four times the sus-
tainable number of territory owners. As the
density of females became far too high and no
opportunities for an individuals own breeding
territory existed anymore, all females started to
breed regardless of costs (Eccard et al. 2011).
This was explained by incomplete control of a
social behavior (Reeve et al. 1998). Similarly,
onen et al. (2002) found in their study with
Australian house mouse during a plague, that
despite an extremely high predation risk, mice
were taking high risks in exploiting food sources
in open habitats with a diverse guild of predators
including mammals, birds, and snakes. As the
number of competitors becomes intolerably high 10 June 2019 Volume 10(6) Article e02765
and food becomes scarce, risk-taking is the only
solution. The authors described the desperate
behavior of the mice as Stalingrad effect, after
the behavior of desperate soldiers during the
siege of Stalingrad in World War 2. While those
two studies do not investigate the effect of preda-
tor odor exposure, they investigate the results of
extreme stress situations due to crowding and
social cues or crowding and direct predation risk.
It is conceivable that the underlying mechanism
is similar to what we found in our study in giv-
ing up a conservative strategy for risky strategies
under high risk.
Levels of stress hormone metabolites rose sig-
nicantly less in the SO, compared to C, while no
change was observed for the two predation cues.
This was unexpected as the SO, signaling compe-
tition environment, was affecting male weights
as discussed below. It also contradicts our
hypothesis as we expected to see a strong
increase in those metabolites under both preda-
tor cues. However, as the treatment period lasted
a total of seven weeks, it is possible that the treat-
ments elicited an initial spike in corticosterone
metabolites, but then adapted to the new per-
ceived stress level, causing only mildly elevated
levels due to new environment and handling.
With this in mind, it was interesting to see that
the SO treatment, the odor of an unstressed con-
specic male, reduced the increase in stress hor-
mone metabolites. This could be caused by social
buffering, the decreased impact of stress by a
social interaction. It has been shown before in
rodents that the presence of an unfamiliar
conspecic is sufcient to cause social buffering
(Terranova et al. 1999, Kiyokawa et al. 2004b,
Klein et al. 2015, Kiyokawa and Hennessy 2018).
A potential explanation is that voles are poten-
tially able to adapt to a prolonged period of
stress, while the benets of social interactions do
not vanish with prolonged exposure.
The weight of the voles varied signicantly
throughout the experiment. The changes we
observed suggest that there is a difference in
how the sexes respond to different cues. In
females, the strongest effect was offspring care
the larger the litter, the greater the weight
increase. However, we also found an effect of
conspecic cues. Only the odor cue of conspecic
rival males caused an increase of weight for the
males. The SO treatment possibly simulated a
high-density environment with increasing intra-
sexual competition in males. So it might be favor-
able for the males to invest in growth in order to
outcompete competitors, as shown in Kalahari
meerkats (Suricata suricatta; Huchard et al. 2016).
It is therefore interesting that the alarm phero-
mone treatment, which is the odor of a stressed/
anxious male vole, failed to elicit a similar
response. Thus, it is possible that the addition of
the external risk pheromone cancels out the effect
of the social cue, as alarm pheromones are only
excreted in extreme situations.
Our study contributes to the picture of how
mammals with highly developed olfactory
senses can interpret the information carried in
olfactory signals correctly and are able to differ-
entiate between ambient level predation risk and
socially validated risk. We collected evidence
that voles are able to gather information about
acute risk levels from conspecic odor cues,
which in turn triggered a higher successful
insemination rate and heavier offspring. This is
in accordance with terminal investment ideas. If
a female takes the risk of investing in producing
offspring (Yl
onen and Ronkainen 1994 vs. Kokko
and Ranta 1996), then it pays to put all efforts
into breeding (M
onen et al. 2009). As preda-
tor odor and alarm pheromones yield such
different results, we propose that both odors
provide the voles with different information, that
is, the predator odor simulates a low-to-medium
threat environment, but the alarm pheromone
clearly represents a high and immediate threat.
TS, MH, HV, and HY designed the study. TS, HV,
and RP collected data. TS, MH, RP, and HY were
involved in writing the manuscript. The study has
been funded by an Academy of Finland grand
awarded to HY. We would like to thank Joni Uusi-
talo and Olga Yl
onen for helping in the lab. Sonny
Bleicher, Emily Bureld-Steel, and Liam Murphy
provided comments at different stages of the writing
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2765/full 16 June 2019 Volume 10(6) Article e02765
... This assumes that the odors are carried into the nest in the fur of the mother and the combination of the odor and potential changes in maternal care and/or increased stress hormone levels in the milk will trigger aversive behavior. Third, we predicted that offspring would forage less in the presence of PO and CAC, regardless of their mother's treatment (Brown, 1988;Sievert et al., 2019). Fourth, we predicted increased latencies to investigate foraging options, reduced time spent in foraging chambers, and fewer foraging chamber visits in chambers with PO or CAC compared to the control chamber (Apfelbach et al., 2015;Sievert and Laska, 2016;Parsons et al., 2018;Sievert et al., 2019). ...
... Third, we predicted that offspring would forage less in the presence of PO and CAC, regardless of their mother's treatment (Brown, 1988;Sievert et al., 2019). Fourth, we predicted increased latencies to investigate foraging options, reduced time spent in foraging chambers, and fewer foraging chamber visits in chambers with PO or CAC compared to the control chamber (Apfelbach et al., 2015;Sievert and Laska, 2016;Parsons et al., 2018;Sievert et al., 2019). Prediction three and four assume that PO and CAC carries information about an increased risk and therefore these compartments will be largely avoided and not used for foraging (Brown, 1988). ...
... For example, exposure might communicate more about the unfamiliar conspecific male (Eccard et al., 2017) than about the predator that male encountered. While this possibility cannot be ruled out in the current study, previous work in bank voles revealed different reactions to the CAC produced by predator exposed male voles and the "normal" odors produced by unexposed male voles Sievert et al., 2019). Furthermore, we have recently analyzed possible molecular candidates for alarm substances in the bank vole (Sievert, 2020) showing that differences in chemical odor composition are only caused by the treatments (i.e., no handling, handling, and weasel exposure) and show no significant differences based on the sex of the individual. ...
Full-text available
Prey animals can assess the risks predators present in different ways. For example, direct cues produced by predators can be used, but also signals produced by prey conspecifics that have engaged in non-lethal predator-prey interactions. These non-lethal interactions can thereby affect the physiology, behavior, and survival of prey individuals, and may affect offspring performance through maternal effects. We investigated how timing of exposure to predation-related cues during early development affects offspring behavior after weaning. Females in the laboratory were exposed during pregnancy or lactation to one of three odor treatments: (1) predator odor (PO) originating from their most common predator, the least weasel, (2) odor produced by predator-exposed conspecifics, which we call conspecific alarm cue (CAC), or (3) control odor (C). We monitored postnatal pup growth, and we quantified foraging and exploratory behaviors of 4-week-old pups following exposure of their mothers to each of the three odour treatments. Exposure to odors associated with predation risk during development affected the offspring behavior, but the timing of exposure, i.e., pre-vs. postnatally, had only a weak effect. The two non-control odors led to different behavioral changes: an attraction to CAC and an avoidance of PO. Additionally, pup growth was affected by an interaction between litter size and maternal treatment, again regardless of timing. Pups from the CAC maternal treatment grew faster in larger litters; pups from the PO maternal treatment tended to grow faster in smaller litters. Thus, in rodents, offspring growth and behavior are seemingly influenced differently by the type of predation risk perceived by their mothers.
... After perceiving increased predation risk, multiple mechanisms and adaptations by prey animals are possible, from simple immediate behavioural responses to long-term physiological or even intergenerational adaptations (Abrams 2000). Anti-predatory behaviours employed in prey range from simple avoidance of high-risk areas (Ferrero et al. 2011;Clinchy et al. 2013;Pérez-Gómez et al. 2015) and freezing to decrease detectability (Wallace and Rosen 2000;Sundell and Ylönen 2004), over changes in vigilance and foraging (Brown 1999;Ylönen and Brown 2007;Embar et al. 2011), to drastic changes in the reproductive behaviours (Ylönen and Ronkainen 1994;Sih 1994;Mappes and Ylönen 1997;Mönkkönen et al. 2009;Haapakoski et al. 2012Haapakoski et al. , 2018Sievert et al. 2019). ...
... Several of the aforementioned species live in social groups, so the secretion of AP serves to warn the group, family or colony. Previous behavioural studies have already shown alarm pheromone effects on reproductive behaviour in bank voles, specifically differences in the number of offspring , the amount of parturitions (Sievert et al. 2019), and several transgenerational effects (Sievert et al. 2020). While the effects of an alarm pheromone exposure have been studied, the actual nature remains unclear. ...
... In our field experiment, no clear difference in foraging effort was observed in the AP GUD was observed on the first day, which is in line with our previous results (Sievert et al. 2019). However, a clear increase in foraging effort in AP patches after just one day, we suggest two factor for explaining this result. ...
Full-text available
Chemical communication plays an important role in mammalian life history decisions. Animals send and receive information based on body odour secretions. Odour cues provide important social information on identity, kinship, sex, group membership or genetic quality. Recent findings show, that rodents alarm their conspecifics with danger-dependent body odours after encountering a predator. In this study, we aim to identify the chemistry of alarm pheromones (AP) in the bank vole, a common boreal rodent. Furthermore, the vole foraging efficiency under perceived fear was measured in a set of field experiments in large outdoor enclosures. During the analysis of bank vole odour by gas chromatography–mass spectrometry, we identified that 1-octanol, 2-octanone, and one unknown compound as the most likely candidates to function as alarm signals. These compounds were independent of the vole’s sex. In a field experiment, voles were foraging less, i.e. they were more afraid in the AP odour foraging trays during the first day, as the odour was fresh, than in the second day. This verified the short lasting effect of volatile APs. Our results clarified the chemistry of alarming body odour compounds in mammals, and enhanced our understanding of the ecological role of AP and chemical communication in mammals.
... Conversely, Haapakoski et al. (2018) demonstrated that secondary exposure of wild bank voles (Myodes glareolus) to bedding used by individuals who had been directly exposed to predator odors increased the former's subsequent litter size. Sievert et al. (2019) also found that in bank voles, exposure to predator odors and conspecific alarm pheromones led to higher pregnancy rates and increased offspring birth weights. Defensive responses in rodent species may also be sensitive to other environmental cues, besides predator olfactory cues, in the immediate vicinity. ...
Full-text available
In rodents, defensive behaviors increase the chances of survival during a predator encounter. Observable rodent defensive behaviors have been shown to be influenced by the presence of predator odors and nearby environmental cues such as cover, odors from conspecifics and food availability. Our experiment tested whether a predator scent cue influenced refuge preference in meadow voles within a laboratory setting. We placed voles in an experimental apparatus with bedding soaked in mink scent versus olive oil as a control across from four tubes that either contained (a) a dark plastic covering, (b) opposite-sex conspecific odor, (c) a food pellet, or (d) an empty, unscented space. A three-way interaction of tube contents, subject sex, and the presence of mink or olive oil on the preference of meadow voles to spend time in each area of the experimental apparatus and their latency to enter each area of the apparatus revealed sex differences in the environmental preference of meadow voles facing the risk of predation. The environmental preference of female, but not male, meadow voles was altered by the presence of mink urine or olive oil. A similar trend was found in the latency of males and females to enter each area of the experimental apparatus. These differences suggest that each sex utilizes different methods to increase their fitness when experiencing a predation risk. The observed sex differences may be explained by the natural history of voles owing to the differences in territorial range and the dynamics of evasion of terrestrial predators.
... This might raise the question whether the method to collect AP is suitable. However, studies using a similar method Sievert et al. 2019) clearly showed significant changes in voles when presented with the AP cue. ...
Full-text available
Predator-prey interactions are a major evolutionary driver, affecting not only the direct mortality of prey species, but also their behaviours and reproduction. Prey species behavioural adaptations aim to mitigate the effects of predation and to maximise survival and individual fitness. These adaptations include the ability to signal a threat to conspecifics, e.g. via alarm calls or alarm secretions, or to detect predator presence via odours. In this thesis, I studied the effects of predator odours and conspecific alarm secretions on behaviour and reproduction bank voles (Myodes glareolus), a small mammal species inhabiting boreal forests. My work focused on three major points in comparing the direct predator cue and indirect conspecific cue: first, how the reproductive behaviour is affected by the predator odour or alarm pheromone, second, whether there are transgenerational effects and how they are exhibited in offspring, and third, what the chemical nature of these alarm secretions is. I conducted four experiments, which included both trials in semi-natural enclosures and under controlled laboratory conditions. I found evidence that exposure to conspecific alarm secretions causes a shift in voles’ reproductive behaviour, switching towards terminal investment. This became apparent with an increase in parturitions and an increased growth rate in larger litters, which did not occur when exposed to predator odour. I also found evidence of transgenerational effects, which affect aspects of the offspring’s exploratory and foraging behaviour. Additionally, I discovered that these behavioural effects are context-dependent and do not occur in every environment. Lastly, I identified a group of chemicals from voles’ alarm secretion, which are likely to be responsible for the observed effects. The results of my thesis fill a knowledge gap concerning chemical communication in mammals, and help to further understand the implications of predator presence on prey behaviour and reproduction.
... When the chance of survival for offspring is already low, it would benefit Brandt's voles to produce more female offspring. Sievert et al. (2019) found similar results, wherein both predator odors and alarm pheromones enhanced reproduction compared to that observed with exposure to a control odor. The impact of a predator can be socially transmitted, as predation risk can exert population-level effects through alarm pheromones, which are pheromones released by predatorstressed conspecifics (Haapakoski et al. 2018). ...
Full-text available
Animals may use different reproductive strategies depending on environmental conditions. This study investigated the effects of maternal stress induced by exposure to predator odors on reproductive output and adult offspring quality in pregnant Brandt’s voles (Lasiopodomys brandtii). We exposed pregnant Brandt’s voles to cat urine, rabbit urine, or distilled water for 18 days (1 h/day). Our results indicated that pregnant voles in the cat odor-exposed group tended to produce more offspring and the number of viable female offspring was larger. However, we did not observe any differences in the sex ratios of vole offspring among the three treatment groups. Compared with the control (distilled water) group, female offspring of voles in the cat odor-exposed group had lower body weights, lengths, and smaller body weights in relation to body length, whereas they had larger ovaries in relation to body weight. Furthermore, the female offspring of the voles exposed to the cat odor had higher concentrations of serum estradiol and higher levels of gonadotropin-releasing hormone mRNA in the hypothalamus compared with the female offspring of the voles in the control group. However, we observed no differences among the male offspring in the three treatment groups. Our findings suggest that pregnant Brandt’s voles suffering from chronic exposure to predator odors will produce greater numbers of low-weight female offspring that probably possess higher breeding potential to improve the fitness via regulation of the hypothalamic-pituitary-gonadal axis.
... This might raise the question whether the method to collect AP is suitable. However, studies using a similar method (Haapakoski et al. 2018;Sievert et al. 2019) clearly showed significant changes in voles when presented with the AP cue. ...
Full-text available
In the predator–prey arms race, survival-enhancing adaptive behaviors are essential. Prey can perceive predator presence directly from visual, auditory, or chemical cues. Non-lethal encounters with a predator may trigger prey to produce special body odors, alarm pheromones, informing conspecifics about predation risks. Recent studies suggest that parental exposure to predation risk during reproduction affects offspring behavior cross-generationally. We compared behaviors of bank vole (Myodes glareolus) pups produced by parents exposed to one of three treatments: predator scent from the least weasel (Mustela nivalis nivalis); scent from weasel-exposed voles, i.e., alarm pheromones; or a control treatment without added scents. Parents were treated in semi-natural field enclosures, but pups were born in the lab and assayed in an open-field arena. Before each behavioral test, one of the three scent treatments was spread throughout the test arena. The tests followed a full factorial design (3 parental treatments × 3 area treatments). Regardless of the parents’ treatment, pups exposed to predator odor in the arena moved more. Additionally, pups spend more time in the center of the arena when presented with predator odor or alarm pheromone compared with the control. Pups from predator odor–exposed parents avoided the center of the arena under control conditions, but they spent more time in the center when either predator odor or alarm pheromone was present. Our experiment shows that cross-generational effects are context-sensitive, depending on the perceived risk. Future studies should examine cross-generational behavioral effects in ecologically meaningful environments instead of only neutral ones. Significance statement We exposed bank voles to odors signaling predation risk to assess the effects parental predation exposure on the behavior of their offspring. Besides predator odor, we also assessed the role of a conspecific alarm cue as a novel way of spreading the predation risk information. Pup behaviors were assessed in the open-field arena, a standard way of assessing animal behavior in a wide range of contexts. We found that also alarm pheromone increased the time pups spend in the center of the arena similarly to predator odor. While previous studies suggested that offspring would be more fearful, our results indicate that the cross-generational effects are very context-dependent; i.e., they differ significantly depending on which scent cue is presented in the open-field arena. This shows the need for better tools or measurements to translate laboratory results into ecologically meaningful frameworks.
... The partly contradictory studies may provide a synthesis in the form of variability of breeding strategies in animals: whether to invest in own survival and decrease breeding activity, suppressing or delaying reproduction (Ylönen 1994;Ylönen & Ronkainen 1994), along with other activities like moving and foraging. Alternatively, the second option is to invest in intensive reproduction, even with the risk of being the last one, with the hope that at least one pup will survive over the period of high risk (Duffield et al. 2017;Haapakoski et al. 2018;Sievert et al. 2019). The latter theoretical strategy, bet-hedging or terminal investment, has been documented in numerous taxa depending on either intrinsic factors like individuals' age or extrinsic threat factors for survival, like predation or parasitism (see tables 1 and 2 in the review by Duffield et al. 2017). ...
Full-text available
Climate change, habitat loss and fragmentation are major threats for populations and challenge for individual behavior, interactions, and survival. Predator‐prey interactions are modified by climate processes. In the northern latitudes strong seasonality is changing and the main predicted feature is shortening and instability of winter. Vole populations in the boreal Fennoscandia exhibit multiannual cycles. High amplitude peak numbers of voles and dramatic population lows alternate in 3–5 years cycles shortening from North to South. One key factor, or driver, promoting the population crash and causing extreme extended lows, is suggested to be predation by the least weasel. We review the arms race between prey voles and weasels along the multiannual density fluctuation, affected by the climate change, and especially the change in duration and stability of snow cover. Snow provides for ground‐dwelling small mammals thermoregulation, shelter for nest sites, and hide from most predators. Predicted increase in instability of winter forms a major challenge for species with coat color change between brown summer camouflage and white winter coat. One of these is the least weasel, Mustela nivalis nivalis. Increased vulnerability of wrong‐colored weasels to predation affect vole populations and may have dramatic effects on vole dynamics. It may have cascading effects to other small rodent – predator interactions and even to plant – animal interactions and forest dynamics.
Technical Report
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Updated tables, figures and references of Palme, 2019, and the respective supplements (Date: 2nd Jan 2023)
Scent marks are an important means of transmitting information between rodents, and they can be produced from several body sources. Previous studies have shown that scents from multiple sources can convey the same information; female meadow voles, for example, have three scent sources that communicate sex. However, possessing three separate sources that convey the same information is likely costly due to the metabolic energy required to produce these signals and the increased chance that eavesdropping individuals may intercept information present in these signals. In this study, we investigated if these scent sources could communicate other information, in addition to scent donor sex, by determining if male meadow voles could distinguish scent marks taken from different sources of a single female scent donor. This was accomplished with a habituation-test method, that allowed us to compare how male meadow voles differently investigate scent from a familiar and novel source of a female scent donor. Male meadow voles could distinguish between faeces and urine scent marks of a female, but could only distinguish mouth from urine and faeces scent marks when first familiarized with mouth scent marks. Our findings suggest that mouth, urine, and faeces scent marks of female meadow voles produce both redundant and distinct information. The overlap in information between scent marks produced from separate sources may be needed to provide social context, which allows receiving individuals to accurately weigh the tradeoffs associated with responding to an olfactory cue. While this overlap in information remains costly, this cost may be minimized by the different fade-out times of scent marks from distinct body sources, which may limit the amount of time information in a scent mark is available to a time period where this information is socially relevant.
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Prey strategically respond to the risk of predation by varying their behavior while balancing the tradeoffs of food and safety. We present here an experiment that tests the way the same indirect cues of predation risk are interpreted by bank voles, Myodes glareolus, as the game changes through exposure to a caged weasel. Using optimal patch use, we asked wild-caught voles to rank the risk they perceived. We measured their response to olfactory cues in the form of weasel bedding, a sham control in the form of rabbit bedding, and an odor-free control. We repeated the interviews in a chronological order to test the change in response, i.e., the changes in the value of the information. We found that the voles did not differentiate strongly between treatments pre-exposure to the weasel. During the exposure, vole foraging activity was reduced in all treatments, but proportionally increased in the vicinity to the rabbit odor. Post-exposure, the voles focused their foraging in the control, while the value of exposure to the predator explained the majority of variation in response. Our data also suggested a sex bias in interpretation of the cues. Given how the foragers changed their interpretation of the same cues based on external information, we suggest that applying predator olfactory cues as a simulation of predation risk needs further testing. For instance, what are the possible effective compounds and how they change “fear” response over time. The major conclusion is that however effective olfactory cues may be, the presence of live predators overwhelmingly affects the information voles gained from these cues. Significance statement In ecology, “fear” is the strategic response to cues of risk an animal senses in its environment. The cues suggesting the existence of a predator in the vicinity are weighed by an individual against the probability of encounter with the predator and the perceived lethality of an encounter with the predator. The best documented such response is variation in foraging tenacity as measured by a giving-up density. In this paper, we show that an olfactory predator cue and the smell of an interspecific competitor result in different responses based on experience with a live-caged predator. This work provides a cautionary example of the risk in making assumptions regarding olfactory cues devoid of environmental context.
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Abstract Predation involves more than just predators consuming prey. Indirect effects, such as fear responses caused by predator presence, can have consequences for prey life history. Laboratory experiments have shown that some rodents can recognize fear in conspecifics via alarm pheromones. Individuals exposed to alarm pheromones can exhibit behavioural alterations that are similar to those displayed by predator-exposed individuals. Yet the ecological and evolutionary significance of alarm pheromones in wild mammals remains unclear. We investigated how alarm pheromones affect the behaviour and fitness of wild bank voles (Myodes glareolus) in outdoor enclosures. Specifically, we compared the effects of exposure of voles living in a natural environment to a second-hand fear cue, bedding material used by predator-exposed voles. Control animals were exposed to bedding used by voles with no predator experience. We found a ca. 50% increase in litter size in the group exposed to the predator cue. Furthermore, female voles were attracted to and males were repelled by trap-associated bedding that had been used by predator-exposed voles. Movement and foraging were not significantly affected by the treatment. Our results suggest that predation risk can exert population-level effects through alarm pheromones on prey individuals that did not encounter a direct predator cue.
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Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on the context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual’s residual reproductive value beyond a terminal investment trigger—the dynamic terminal investment threshold.
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Anurans emit distress calls when attacked by predators as a defensive mechanism. As distress calls may trigger antipredator behaviour even in individuals that are not under attack, we tested whether this defensive behaviour induced behavioural changes in neighbouring conspecifics. We compared the behavioural responses of two species of Neotropical hylid frogs (genus Boana) to conspecific distress calls and white noise. Individuals of both species interrupted their vocal activity and decreased call rate after hearing the distress call. Natural variation on signal intensity calibrated among the nearest neighbours did not influence the response and we did not observe negative phonotaxis after any acoustic stimulus. Despite the fact that many predators are acoustically oriented, we could not determine if such response (reduced call rate) was induced by risk assessment or by the masking effect on advertisement calls. Boana faber responded similarly to white noise and distress calls, while B. bischoffi responded more intensely to distress calls. Duration of silence after playbacks in B. faber was longer than B. bischoffi. We suggest that, if the signals are interpreted as a risk cue by neighbouring conspecifics, each species may be preyed upon by different predators, as they may have led to distinct defensive strategies and different responses to distress calls. If risk assessment information is included in distress calls, it triggers behav-ioural responses only in the nearest neighbours, as we did not observe responses on the vocal activity of the interspecific chorus. Our results add relevant data about acoustic communication and interpretations by anurans, highlighting the importance of considering cues within common and widespread signals. K E Y W O R D S alarm signal, animal communication, anuran, defensive strategy, playback experiment, predator-prey interactions
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Landscapes of Fear (LOF), the spatially explicit distribution of perceived predation risk as seen by a population, is increasingly cited in ecological literature and has become a frequently used “buzz-word”. With the increase in popularity, it became necessary to clarify the definition for the term, suggest boundaries and propose a common framework for its use. The LOF, as a progeny of the “ecology of fear” conceptual framework, defines fear as the strategic manifestation of the cost-benefit analysis of food and safety tradeoffs. In addition to direct predation risk, the LOF is affected by individuals’ energetic-state, inter- and intra-specific competition and is constrained by the evolutionary history of each species. Herein, based on current applications of the LOF conceptual framework, I suggest the future research in this framework will be directed towards: (1) finding applied management uses as a trait defining a population’s habitat-use and habitat-suitability; (2) studying multi-dimensional distribution of risk-assessment through time and space; (3) studying variability between individuals within a population; (4) measuring eco-neurological implications of risk as a feature of environmental heterogeneity and (5) expanding temporal and spatial scales of empirical studies.
How prey animals determine predation risk remains uncertain. We propose that one signal of high predation risk is repeated activation of fight-or-flight behaviour. We activated escape runs in the cricket Gryllus texensis by blowing air on the cerci. Escape runs were induced for 5 min, three times per day, three times per week for 4 weeks. Repeated fight-or-flight behaviour led to a loss in mass and decreased life span, suggesting a decline in somatic maintenance. However, there was an increase in egg laying, which we interpret as terminal reproductive investment. Stress responses remained robust. Octopamine (OA), a stress neurohormone in insects, increased in concentration in the haemolymph after running, and the magnitude of the increase was the same even after repeated activation (i.e. there was no habituation of the response). There was also no increase in basal OA haemolymph levels. In a second experiment, crickets were exposed to a mantid (predator, Tenodera sinensis), a walking stick (nonpredator, Carausius morosus), or an empty container. None of the crickets exhibited fight-or-flight behaviour. However, mantid-exposed crickets decreased egg laying. There was no decrease in life span or mass. There was no change in basal levels of OA, or in the magnitude of the OA increase after running. These results are consistent with the hypothesis that repeated fight-or-flight behaviour induces reproductive responses that would be adaptive for a shortened life span. These responses differ from those produced by predator cues alone. Even short-lived animals, such as crickets, appear to alter reproduction depending on the relative predation risk and their residual reproductive potential.
KIYOKAWA, Y. and HENNESSY, M.B. Social buffering of stress responses: A consideration of approaches, terminology, and pitfalls…NEUROSCI BIOBEHAV REV XXX-XXX, .- Over the past decades, there has been an increasing number of investigations of the impact of social variables on neural, endocrine, and immune outcomes. Among these are studies of "social buffering"-or the phenomenon by which affiliative social partners mitigate the response to stressors. Yet, as social buffering studies have become more commonplace, the variety of approaches taken, definitions employed, and divergent results obtained in different species can lead to confusion and miscommunication. The aim of the present paper, therefore, is to address terminology and approaches and to highlight potential pitfalls to the study of social buffering across nonhuman species. We review and categorize variables currently being employed in social buffering studies and provide an overview of responses measured, mediating sensory modalities and underlying mechanisms. It is our hope that the paper will be useful to those contemplating examination of social buffering in the context of their own research.