Sonny,

 Fun question, and one I don't actually have an answer for.  However, it reminded me of this paper in Nature: McAllister and Roitberg 1987 - Adaptive suicidal behaviour in pea aphids.

Hope it provides some new thoughts!

Jeremy

“Just because you’re paranoid doesn’t mean they’re not after you” (Heller). I don't know whether "paranoia" is a properly defined or definable behavioural phenomenon for the species of animal under your investigation. However, even in humans the condition does not normally just pop up out of nothing. So I would recommend you to try finding an explanation for your observations rather than to try "justifying" the outcome from the evolution of the research experiment.

I will have to check out that paper Jeremy, thanks for that. 

Roland, 

I define paranoia as a game theoretic response to a predator that is not there, or at low risk of being there. in essence it is the opposite of naivete when describing behavioural attributes of prey species.

Sonny 

I find the torpor link intriguing - are you saying this species uses torpor? There are papers that propose endotherms may limit torpor or nocturnal hypothermia under perceived predation risk. I'm thinking specifically of:

Laurila, M. & Hohtola, E. (2005) The effect of ambient temperature and simulated predation risk on fasting-induced nocturnal hypothermia of pigeons in outdoor conditions. Journal of Thermal Biology 30, 392-399.

I would like to know how you can link body size and torpor to paranoia directly. They could result from other environmental pressure !

I don't know this subject much, but I would suggest reading optimal foraging theory under predator pressure.

Sonny - Sorry, my response was not 100% serious. Still, reading your question I would say that the behaviour may just look paranoid within the context of your observations (resp. game theoretical context). If you observe orangutans in a forest in Sumatra you may find that they rarely touch ground in fear of being hunted down by tigers. You may find the same behaviour present for many generations even after tigers have actually gone extinct in the forest, whereby it is the tigers' extinction which makes the behaviour look "paranoid" for future game theoretic researchers (it may then not make sense for orangutans to always hang out in the trees when much orangutan food is actually available on the ground). In orangutans the behaviour may be "culturally" transmitted and hence may be relaxed after a relatively short time, but it could also be genetically encoded through natural selection, and hence persist for a long time. If the species you refer to is more primitive than orangutans the species' behaviour is likely genetically encoded, hence "paranoia" is "justified" via evolution. So, I guess "yes" would be the general answer to your relatively general question, if I understood it correctly...

I noted striking intraspecific behavioral variation among individual rodents that I worked with over the years.  Small mammal trappers all encounter "trap-happy" and "trap-shy" individuals.  Variation in risk aversiveness may be due to past individual experience and indicate genetically-based behavioral plasticity.  It would be important to show that the paranoia you describe is widespread and not limited to individuals that have had a similar environmental history.

Couldn't torpor and smaller body size be a result of paranoia, rather than an adaptation to cope with paranoia? E.g., a reduced foraging time would decrease body size, rather than a smaller body size demands less foraging time.

An interesting question, yet I wonder to what degree the concept of 'paranoia' has been loaded by the DSM and concepts of human psychiatric illness? Could your observations be accounted for by more usual concepts of fear, anxiety and 'flight' rather than a term that is suggestive of pathology?

A range of seemingly paranoid behaviours (e.g. neophobia) can be selected for and can certainly confer fitness depending on the context - the classic example being the selection of bait aversion in rats. Alterations to the hypothalamic–pituitary–adrenal axis (HPA) can be selected for rapidly in many species of course and this is related to flight distances.  From an anthropomorphic perspective we might see animals with extreme flight response as 'paranoid', but this might only be related to the adaptation to stressors in the environment. 

Alternatively, captive animals may well have a psychological condition akin to paranoia - a disease state.

So, perhaps the context of the observation is important?

Thanks Clive for interesting take on the matter. The context in which I am interested in studying the phenomenon is one related to invasion ecology. The scenario in which my species (Cheatodipus penicilltus) was exposed to risk was in a semi-natural arena where we controlled predation risk from two viper species and an owl. One viper was "known" to the pocket mice from the environment in which they co-evolved. However we also released into a section vipers that are convergent evolutionarily to the "know vipers" however come from another continent all together and hunt using different strategies. What was interesting was the abandonment of foraging where-ever there was cues of any snake activity. My take on the matter, being the smallest competitor in it's community of desert granivors, and lacking physical tools to avoid predation effectively it evolved "paranoid" response to cues coupled with ability of prolonged torpor. In essence, given no better choice, accentuating the fear response to cues is likely one of drivers that allows it to co-exist with all the "stronger" competitors.    

In my opinion, paranoia is an indicator that the particular species you found might be subject to strong predation and paranoia might be a phenotypically plastic response to predation.

See also: https://www.researchgate.net/publication/235416497_Phenotypically_plastic_neophobia_a_response_to_variable_predation_risk

Article Phenotypically plastic neophobia: A response to variable pre...

according to me... paranoia will be good strategy in case of strong predation and with fast and generalist grazing animals.