post - traumatic stress disorder ( ptsd ) represents arguably the most salient example of how fear and stress shape the mind .
because controlled prospective studies can not be conducted on humans , it is necessary to use an  animal model  to help elucidate the etiology of ptsd and explore the associated neurological changes ( cohen et al . , 2010 ) .
a suitable animal model should utilize stressors that emulate as closely as possible the relevant stressors in humans ; the behavioral , physiological , and neurological responses elicited in the animal must reflect clinical symptomatology ; and pharmacological agents known to affect symptoms in human patients should correct , with equal efficacy , comparable symptoms in the animal ( rosen and schulkin , 1998 ; roseboom et al . , 2007 ; stam , 2007 ; armario et al .
, 2008 ; masini et al . , 2009 ; mitra et al . , 2009 ; cohen et al . ,
many researchers have adopted utilizing exposure to a predator ( e.g. , showing a rat a cat ; adamec and shallow , 1993 ) , or predator odor , as a stressor , in exploring the animal model of ptsd ( cohen et al .
predator exposure was initially seized upon for practical reasons as this permits the researcher to utilize a ( 1 ) psychological stressor , that is ( 2 ) life - threatening , but ( 3 ) does not involve pain ; all consistent with the etiology of ptsd in humans ( adamec and shallow , 1993 ; roseboom et al .
takahashi et al . , 2008 ; staples et al . , 2009 ; cohen et al . , 2010 ; mackenzie et al
. , 2010 ) . of greatest importance with respect to understanding ptsd , the hallmark of which is the long - lasting or  transformational
 change in the patient in response to a trauma ( yehuda and bierer , 2009 ) , predator exposure has been demonstrated to have long - lasting effects on : anxiety - like behaviors , glucocorticoid levels , dendritic morphology , gene expression , and the release of the neuropeptide corticotrophin - releasing hormone ( crh ) in the amygdala ( the region of the brain most frequently linked to fear ) , as well as many other phenomena associated with ptsd ( adamec and shallow , 1993 ; schulkin et al . , 2005 ;
, 2008 ; campeau et al . , 2008 ; rosen et al . , 2008 ;
predator exposure early in life has also been shown to increase vulnerability to developing subsequent long - term behavioral disruptions when exposed to a predator in adulthood ( cohen et al . , 2006 ) , consistent with the growing evidence that individual variation in susceptibility to ptsd is influenced by early - life experiences ( yehuda and bierer , 2009 ) .
researchers studying the  predator model of ptsd  have increasingly begun to suggest that predator exposure offers an additional advantage in attempting to understand ptsd , because long - lasting predator - induced fear and stress is ethologically and ecologically relevant , and represents a valid experience applicable to animals in their natural environment ( roseboom et al . , 2007 ;
cantor , 2009 ; staples et al . , 2009 ; cohen et al . , 2010 )
independently , wildlife ecologists have begun to arrive at a similar conclusion , following a line of inquiry that began in the 1990s ( creel and christianson , 2008 ) .
traditionally , the view of both wildlife ecologists and comparative endocrinologists has been that predator - induced fear and stress is necessarily acute and transitory : the prey detects a predator ; freezes , flees , or fights ; survives or does not ; the event is over ; the animal returns to going about its business ; homeostasis is restored ( schulkin , 2003 ; sheriff et al . , 2009 ) .
according to this traditional view , lasting effects are necessarily maladaptive and pathological : since the evolutionary  function  of predator - induced fear and stress is to ensure immediate survival , any further or lasting effect on fitness ( i.e. , darwinian fitness ) , such as an effect on subsequent reproduction , must be maladaptive ; and since the  function  of the stress axis is to maintain homeostasis , chronic stress must be pathological ( lupien et al . , 2009 ; rodrigues et al .
, the many lasting effects of predator exposure documented by researchers exploring the predator model of ptsd must be an artifact .
the most parsimonious explanation being  given this perspective  that such lasting effects stem from the unnatural conditions of captivity , i.e. , it is not the fact of predator exposure but the fact the predator is inescapable that must explain these effects , since the animal can not flee from the predator as it naturally would ( creel et al . , 2009 ; jngren et al . , 2010 ) .
we propose that the traditional view in wildlife ecology and comparative endocrinology , that the effects of predators on free - living animals are necessarily transitory , is no longer tenable , since the results from a growing number of experimental and observational field studies show that predator - induced fear and stress has long - lasting effects on animals in the wild ( creel and christianson , 2008 ; hawlena and schmitz , 2010 ) , comparable to those documented by investigators addressing the predator model of ptsd . for animals in the wild that are in peril every moment of every day of being torn limb from limb by any number of predators , responses resembling ptsd in humans may result from necessary trade - offs to stay alive , that are fully adaptive , because dead animals do not reproduce .
we suggest that for both , researchers studying the predator model of ptsd , and ecologists , conducting collaborative studies on predator - induced fear and stress on animals in the wild would be of enormous benefit . for investigators addressing the predator model of ptsd
, the extremity of the stressors faced by animals in the wild , in a real world context , would appear to much better emulate the circumstances leading to ptsd in humans .
for ecologists , building upon the progress that has been made in understanding ptsd in the lab provides the most expedient means of addressing the mechanisms underlying predator - induced fear and stress effects in the field .
we briefly review approaches taken to studying ptsd in the lab that appear translatable to the field ; and then describe recent field studies on songbirds and snowshoe hares showing that , predator - induced fear and stress affects reproduction in animals in the wild , and the physiological responses involved appear comparable to those documented in response to predator exposure in the lab .
behavioral responses to predator exposure in the lab include avoidance , reduced activity and increased vigilance ( blanchard and blanchard , 1989 ; stam , 2007 ; armario et al . , 2008 ; takahashi et al . , 2008 ) , and similar responses to predator exposure have been exhaustively documented in the field since at least darwin 's time ( caro , 2005 ) .
predator exposure in the lab results in changes in plasma glucocorticoid levels ( blanchard et al . , 1998 ;
2009 ) and the same has been shown in both birds and mammals in the field ( hawlena and schmitz , 2010 ) .
measuring glucocorticoid metabolites in feces provides a new , non - invasive means of assessing glucocorticoid responses to predator exposure that is particularly useful in field studies ( sheriff et al . , 2009 , 2010 ) .
studying the neurological effects of predator - induced fear and stress in animals in the wild will likely rely primarily on destructive sampling .
though effects on live animals could be studied using pharmacological methods or neuroimaging , there are practical difficulties translating these approaches to the field .
the suitability of using predator exposure in exploring the animal model of ptsd has been validated , in part , by the numerous studies showing that pharmacological agents known to affect symptoms of ptsd in human patients also correct comparable symptoms in animals exposed to predators ( cohen et al . , 2006 , 2010 ;
. some of these pharmacological agents can be administered in food ( e.g. , antalarmin ; zoumakis et al . , 2006 ; armario et al . ,
2008 ) , which is of practical advantage for use with free - living animals since it is then not necessary to capture the subject to administer the drug .
the principal constraint on using pharmacological agents on animals in the wild is almost certain to be the cost of the drugs , since the intrinsically greater error variation associated with studying any phenomenon in the field necessitates a larger sample size than that required in the lab .
a number of recent neuroimaging studies using magnetic resonance imaging ( mri ) have evaluated the neurological effects of exposure to predator odor in lab rats ( e.g. , chen et al .
mri has also been used to assess neuroactivity in response to other stimuli in mice and songbirds ( van der linden et al . , 2007 ) .
neuroimaging holds enormous promise as a technique for studying effects on animals in the wild because , being non - destructive , subjects could be returned to the field to determine if differences in brain activity predicted their subsequent behavior and reproduction .
however , though mri is non - destructive it is necessarily invasive and may be very injurious depending upon the method used ( e.g. , the manganese used in manganese - enhanced mri is potentially toxic ; silva et al . , 2004 ) .
at a minimum , neuroimaging requires restraining the subject 's head in a scanner for a protracted period . to measure effects in conscious animals requires acclimation to being restrained in this manner , which takes several days in laboratory animals ( king et al . , 2005 ) , and may be unachievable in many wild - caught animals . even if anesthetized during the procedure , the trauma of capturing a wild animal and transporting it to wherever the scanner is might render the results uninterpretable ( van der linden et al . , 2007 ) . nonetheless , we strongly recommend that using neuroimaging to study effects on animals in the wild should at least be attempted . because animals in the wild are generally challenging to capture , and limited in number , it is critical to maximize the information extracted from every animal euthanized .
moreover , because free - living animals must be captured , the conditions of capture will vary , meaning the rate at which tissue can be obtained will vary , and the circumstances will often be less than ideal .
measures that respond to an acute trauma or perturbation , such as the trauma of capture , will be largely unsuitable . several new approaches to measuring neurological effects , developed in the lab , nonetheless appear amenable to use on animals in the wild , even given these constraints .
immunohistochemistry has been used to map the expression of genes in response to predator exposure in various brain regions that appear central to the phenomenon of fear ( such as the medial amygdala ) .
whereas a number of lab studies have mapped the expression of the immediate - early gene c - fos , in response to predator exposure ( dielenberg et al . , 2001 ; roseboom et al . , 2007 ; campeau et al . , 2008 ) , c - fos is rapidly expressed ( within < 1  h ; armario et al . , 2008 ) and
rapidly down - regulated ( staples et al . , 2009 ) , which is problematic for use in the field .
two recent studies ( staples et al . , 2009 ; mackenzie et al . , 2010 ) have mapped the expression of fosb and its protein products fosb/fosb , as an alternative to mapping c - fos .
fosb can persist in the brain for weeks after chronic stimulus exposure ( mcclung et al . , 2004 ) , and staples et al .
( 2009 ) reported that fosb/fosb expression remained elevated 7 days after repeated predator exposure , making this a much more suitable marker for use in field studies .
global gene expression has been assessed in response to predator exposure using cdna microarrays ( gene chips ) in rats and chickens .
( 2007 ) euthanized rats 3  h after predator exposure , and found increased crh - binding protein gene expression in the amygdala , consistent with previous studies ( schulkin et al . , 2005 ) .
( 2010 ) euthanized chickens 2 week after predator exposure and identified 13 significantly differentially expressed genes in the midbrain .
roseboom et al.s ( 2007 ) findings confirm that cdna microarrays can be used to identify the expression of genes expected to be upregulated in response to fear , and jngren et al.s ( 2010 ) study shows that this approach can be used to detect long - lasting effects , even in non - mammalian subjects .
quantifying dendritic morphology appears ideally suited for indentifying individual variation in susceptibility to predator - induced fear and stress in field studies , and may be useful in evaluating predator - induced changes in neural architecture .
( 2009 ) evaluated behavioral differences in subjects 2 weeks after predator exposure and found differences in the architecture of the neurons in the basolateral amygdala .
total dendritic length , dendritic extent , and total branch points were all greater in individuals that continued to demonstrate anxiety - like behaviors as compared to those that no longer showed anxiety - like symptoms .
though the design of mitra et al.s ( 2009 ) study did not allow them to determine whether these differences in dendritic morphology were pre - existing or induced by predator exposure , mitra and sapolsky ( 2008 ) reported changes in dendritic morphology in response to a single day of stress , suggesting that predator - induced fear could indeed induce such changes in neural architecture .
yehuda and bierer ( 2009 ) recently reviewed the potential role of epigenetic changes in the etiology of individual differences in susceptibility to ptsd .
epigenetic modifications involve long- lasting , often environmentally induced , changes in gene expression and function , that can be inter - generationally transmissible ( i.e. , heritable ) , though the dna sequence itself remains unchanged ( champagne and curley , 2009 ; yehuda and bierer , 2009 ) .
several lines of evidence point to epigenetic changes as potentially being involved in predisposing individuals to ptsd , including the association of ptsd risk with maternal ptsd , the relevance of childhood adversity to the development of ptsd , and recent evidence of a relationship between childhood abuse , dna methylation ( in gene promoters , an epigenetic marker of gene silencing ) and suicide ( mcgowan et al . , 2008 , 2009 ; yehuda and bierer , 2009 ) . as noted above , cohen et al .
( 2006 ) reported that early - life predator exposure increased vulnerability to behavioral disruptions in response to exposure in adulthood , though there have been no studies looking specifically at predator - induced epigenetic changes . in the aforementioned suicide study
, subjects had been dead an average of 24  h before sampling , suggesting that changes in dna methylation ought to be detectable in the brains of animals in the wild collected under less than ideal field conditions , as recently corroborated by pilsner et al .
( 2010 ) in a study that examined dna methylation in the brains of polar bears shot by aboriginal hunters in eastern greenland .
evolutionarily , the  function  of staying alive is to reproduce , i.e. , to transmit genes to the next generation . for ecologists
ecological factors such as food and parasites , with obvious long - lasting effects ( malnutrition and disease ) , have always been considered to be those most likely to affect reproduction , because reproduction ( giving birth and rearing young ) is a slow process .
traditionally , predators have not been thought to affect reproduction because predator - induced fear and stress has been considered to be far too acute and transitory .
behavioral ( e.g. , kotler , 1992 ) and physiological ( e.g. , boonstra et al . ,
1998 ) studies began , in the 1990s , to suggest that predator - induced fear and stress could have lasting effects on animals in the wild , but because of the logistical challenges involved the critical experiments necessary to demonstrate effects on reproduction have only very recently been conducted .
free - living animals can , and do , simply flee or avoid , a predator in a cage , predator models , or predator odor stations ( e.g. , stankowich and blumstein , 2005 ) . because sound travels , and thus occupies space , field studies often use playbacks of recorded predator calls and sounds to investigate effects of predator exposure . moreover , for organisms that rely more on sound and sight than smell , such as birds and humans , auditory stimuli are generally more meaningful than olfactory ones ( jarvis , 2004 ) , and acoustic cues may frequently be more alarming than visual ones ( cohen et al . , 2010 ) .
only one study to date on a bird or a mammal has , to our knowledge , exposed free - living prey to increased predator cues in the field , and demonstrated a resulting effect on the number of offspring produced per year .
( submitted ) used an array of speakers spaced over several hectares to expose nesting female song sparrows to playbacks of either predator calls and sounds , or non - threatening calls and sounds .
females exposed to elevated predation threat produced almost 40% fewer offspring than controls ( 3.8    0.4 vs. 6.0    0.4 , mean   se ) , over the 4-month breeding season , because they laid fewer eggs , fewer of their eggs hatched , and more of their chicks starved to death .
these effects on reproduction were most likely mediated in part by predator - threat - induced changes in glucocorticoid levels , because work on the same study populations has demonstrated lasting effects on glucocorticoid levels associated with variation in predator abundance ( clinchy et al . , 2004 , 2011 ) , and the probability of suffering nest predation ( travers et al . , 2010 ) .
sheriff et al . ( 2009 ) recently reported correlative results suggesting that predator exposure affects glucocorticoid levels and reproduction in free - living snowshoe hares , consistent with the results from zanette et al.s ( submitted ) experiment . to corroborate their findings , sheriff et al .
( 2009 ) presented a live predator ( a trained dog ) to pregnant hares housed in 4  m   4  m outdoor pens , and demonstrated that predator - exposed females had dramatically elevated fecal glucocorticoid metabolite ( fcm ) levels , and were significantly less likely to give birth to live young . in a subsequent study on snowshoe hares , sheriff et al .
( 2010 ) showed that predator exposure may have very long - lasting effects on animals in the wild , extending from one generation to the next .
sheriff et al . ( 2010 ) reported that at a population level , predator exposure , mean maternal fcm levels , and mean juvenile fcm levels , were all correlated , suggesting that predator - induced glucocorticoid changes in mother hares affect their offspring 's glucocorticoid levels . to corroborate these findings , sheriff et al .
( 2010 ) measured fcm levels in pregnant hares , housed in 4  m   4  m outdoor pens , and demonstrated that each mother 's fcm level was highly correlated with her offspring 's glucocorticoid responses to a hormonal challenge , when the latter was 28 days old .
thus , in animals in the wild , maternal or early - life exposure to predators may increase responsiveness to predators later in life , consistent with cohen et al.s ( 2006 ) lab results demonstrating that early - life predator exposure increases vulnerability to behavioral disruptions when exposed to a predator in adulthood .
life - long maternal effects on the glucocorticoid responsiveness of their offspring , resembling the results shown by sheriff et al .
( 2010 ) , have been well - studied in relation to stress effects on maternal care in laboratory rodents , and have been shown to be associated with dna methylation of genes affecting glucocorticoid receptor function in the hippocampus ( weaver et al . , 2004 ; kappeler and meaney , 2010 ) . in an example of the kind of collaboration between neuroscientists and ecologists we are herein hoping to encourage , mcgowan and boonstra are currently examining the brains of juvenile snowshoe hares , collected in the field , whose mothers were subject to naturally varying levels of predator exposure , to test if maternal predator exposure affects dna methylation in their offspring 's hippocampus in a manner similar to the way in which childhood abuse evidently affects dna methylation in humans , as shown in the aforementioned suicide study ( mcgowan et al . ,
calisi and bentley ( 2009 ) recently proposed that studying neurobiology and behavior in semi - natural settings may provide a means to merge lab and field approaches .
our focus here is on the lasting effects of predator - induced fear and stress on neurobiology and ecology . as noted above , the principal challenge in studying such lasting ecological effects concerns space , and this applies equally to studying such effects in a semi - natural setting 
the subject must have the same amount of space available as it would if it were free - living , to flee or avoid a predator , otherwise any effects seen could be attributed to the unnatural conditions of captivity ( creel et al . ,
sheriff et al.s ( 2009 , 2010 ) exposure of caged hares to a predator , for example , can not be considered definitive , for this reason ( clinchy et al . , 2011 ) . moreover , since , as noted above , the ecological  currency  that matters is reproduction , the subject must be able to reproduce as it naturally would .
very large ( e.g. , several hundred square meter ) outdoor enclosures may fulfill these requirements when studying very small animals ( e.g. , mice or songbirds ) , whereas housing an animal in a somewhat larger cage than usual in an animal care building ( e.g. , blanchard and blanchard , 1989 ; choi and kim , 2010 ) does not meet these criteria .
the scope for future collaborations between neuroscientists and ecologists will almost certainly involve studying many more species than just sparrows and hares .
as further field experiments on the effects of predator exposure on reproduction are conducted , we have no doubt such effects will be found to be common .
effects on components of reproductive success have already been documented in experiments on several other species .
( 2006 ) reported effects of predator call playbacks on the number of eggs laid by siberian jays , and fontaine and martin ( 2006 ) found that where predators were removed songbirds laid heavier eggs .
( 2000 ) similarly showed that where predators were removed the proportion of arctic ground squirrel females weaning young was increased .
lasting behavioral and physiological effects pointing to likely effects on reproduction have been shown in an even larger number of species ( creel and christianson , 2008 ; hawlena and schmitz , 2010 ) .
what effect such predator - induced fear and stress has in shaping the minds of free - living birds and mammals is a question that is almost completely unexplored , presenting a wide - open field of study replete with opportunities for new discoveries .
numerous laboratory experiments have shown that predator - induced fear and stress has lasting neurological effects , and wildlife ecologists have begun demonstrating that predator - induced fear and stress has lasting effects on reproduction in free - living animals in the field .
we propose that the next two critical questions to answer are : ( 1 ) whether predator - induced fear and stress has lasting neurological effects on free - living animals , and if so ; ( 2 ) which of the effects seen in the lab appear most frequently in wild animals in the field . the insights neuroscientists have to offer ecologists in exploring the effects of predator - induced fear and stress on the minds of wild animals in the field include , where to begin , and what to measure .
the insights ecologists , in turn , have to offer researchers studying the predator model of ptsd include , establishing which effects seen in the lab are observed in the greatest number of species and circumstances , and which are most biologically meaningful as gaged by their association with effects on reproduction .
we suggest that if , as the predator model assumes , ptsd in humans has evolutionary precursors , then it is virtually certain that collaborations between neuroscientists and ecologists will greatly enhance our understanding of the etiology of ptsd and the associated neurological changes .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 that the fear and stress of life - threatening experiences can leave an indelible trace on the brain is most clearly exemplified by post - traumatic stress disorder ( ptsd ) . 
 many researchers studying the animal model of ptsd have adopted utilizing exposure to a predator as a life - threatening psychological stressor , to emulate the experience in humans , and the resulting body of literature has demonstrated numerous long - lasting neurological effects paralleling those in ptsd patients . even though much more extreme , predator - induced fear and stress in animals in the wild was , until the 1990s , not thought to have any lasting effects , whereas recent experiments have demonstrated that the effects on free - living animals are sufficiently long - lasting to even affect reproduction , though the lasting neurological effects remain unexplored . 
 we suggest neuroscientists and ecologists both have much to gain from collaborating in studying the neurological effects of predator - induced fear and stress in animals in the wild . 
 we outline the approaches taken in the lab that appear most readily translatable to the field , and detail the advantages that studying animals in the wild can offer researchers investigating the  predator model of ptsd .  
Introduction
Approaches Translatable to the Field
Field Studies Showing Long-Lasting Effects of Predator Exposure
Conclusion
Conflict of Interest Statement