Interpretation of Behavioral Responses to Stress

1. Interpretation of BehavioralResponses to Stress

2. In fact the neurophysiological circuits and the neurochemical systems control the behaviour. Both behavioural and physiological responses to stress appear to be controlled, at least in part, by the same central neuroendocrine systems. Hence the correlations that can sometimes be found between animals’ behavioural and physiological responses to stress are not surprising. Most of the research has focused on corticotropin-releasing hormone (CRH) because of the obviously central role it plays in regulating hypothalamic–pituitary–adrenal (HPA) activity, and because of the demonstrations of its involvement in human anxiety and depressive disorders .

3. Research has clarified the neurocircuitry underlying the effects of CRH on the HPA axis. However, less is known of the motivational mechanisms by which CRH secretion triggers behavioural stress responses, or the relationship with the animal’s emotional reactions. One instance of the interaction between behavioural and physiological responses to stress that has gained considerable attention in the research on human medicine but has been relatively overlooked in the area of applied animal ethology is the relationship between stress, sleep and growth hormone (GH) secretion.

4. CRH, sleep and GH There is now evidence that both CRH and growth hormone-releasing hormone (GHRH), one of the main stimulants of GH secretion, have an influence on sleep. In fact there seems to be a reciprocal interaction between GHRH and CRH in controlling sleep. Numerous studies show that CRH administration can reduce REM (rapid eye movement sleep), SWS (slow wave sleep) and GH secretion, while GHRH administration increases sleep (especially SWS) and GH secretion and reduces HPA activity.

5. GHRH Higher GH SWS Sleep Higher GH Decreased Stress Lower CRH

6. Sleep disturbances Stress Lower SWS Sleep REM Sleep Decreased GH Decreased GH

7. The effect of CRH on sleep suggests that stress on an animal is likely to result in sleep disturbances, which in turn may be apparent in reduced GH secretion. Indeed, the relationship between stress, sleep and GH secretion has been demonstrated, at least in humans. It has been reported that SWS in depressed patients is reduced along with a reduction in the secretion of GH during the first period of sleep. However, sleep also appears to influence the secretion of both CRH and GHRH. Studies on rats and humans have shown that GH tends to be released during sleep. There is evidence of a link in some species between the various phases of sleep and GH secretion. The results are partly contradictory, but in humans GH secretion appears to occur during SWS.

8. Sleep is also related to endocrine systems involved in the responses to stress such as the sympathetic nervous system or the HPA axis. In humans, cortisol secretion appears to be suppressed by sleep onset and is high towards the end of sleep. However, the different phases of sleep may also be related differently to HPA activity. It has been reported that, in people, urinary free cortisol and adrenaline concentrations are positively correlated with the amount of REM sleep. Sleep is often increased during periods of infectious disease, and there is evidence that enhanced sleep may help some animals, e.g. rabbits, to recuperate from infection. Intense stress can lead to increased REM sleep in rats when the stress is terminated and the reciprocal link between sleep and HPA activity suggests that sleep may play a similar role in helping animals to adapt to or recuperate from stress. It is also clear that disturbances to sleep could have wide-ranging physiological consequences.

9. Vocalizations and stress It is often suggested that communicative behaviours especially vocals may provide some measure of the internal or subjective state of an animal, and so may be useful as measures of stress. There have been many attempts to use signalling or communicative behaviour to assess animals’ responses to various stressors, and usually these involve examining vocalizations. For example, vocalization has been used to assess piglets’ responses to castration, cattle’s responses to branding or preslaughter handling, and sheep’s responses to castration and tail docking. The assumption behind these studies is that there is variability in the vocalization given by the animals, in terms of either the likelihood of vocalization occurring, or the rate, amplitude or some aspect of the acoustic structure of the calls, and that some aspect of this variability provides reliable information about the inner state of the animal.

10. 1. vocalization does contain some information about the level of risk to the animals and the degree of stress they are under. These authors examined the vocalizations shown by young pigs when they are separated from their mother. They examined what information was transmitted to the receiver of the calls by examining how sows responded to playback of the calls. Sows responded to the calls by orienting towards and approaching the loudspeaker, and they responded more strongly to calls that were recorded from needy piglets (i.e. smallest, slowest growing piglets that had missed a milk ejection and were in a cold environment).

11. These experiments show that the vocalizations of piglets separated from their mothers do contain reliable information about the level of need of the piglets, and that the sows use this information in judging how to respond to the calls. However, research with a number of species shows that calls contain other types of information besides the level of risk.

12. 2. research with primates suggests that alarm calls may not just provide information about the degree of risk, but also about the type of risk. Vervet monkeys have a number of predators, most notably leopards, snakes and aerial predators such as eagles. The predator avoidance behaviour that vervets show is specific to the particular type of predator. For example, vervets react to the presence of types of predators. Playback experiments demonstrated that these eagles by looking up in the sky or running under bushes, to the presence of snakes by standing bipedally and looking at the ground, and to leopards by running up trees.

13. Vervets also give acoustically different alarm calls when faced with different calls contain information about the type of predator rather than just the degree of risk: vervets show the appropriate predator avoidance behaviour when they hear a particular type of call. These findings indicate that vocal signals may not so much contain information about the internal state of the sender, but information about objects in the environment, i.e. semantic information. The importance of this finding for the present discussion is that the calls that animals give when faced with a stressful challenge may not so much indicate the degree of stress that an animal is under (or the degree of risk of predation) but rather the type of risk.

14. 3. Vocalizations can show a high degree of variability, and some of this variability is used to identify individuals rather than to communicate the emotional state of an individual. This individual recognition appears particularly important for the contact calls that many mammals give when separated from a group or from particular individuals within the group. 4. Information about the absence or presence of an audience, age or the sex of the animal that should be signaled. The alarm calls that cockerels gave in response to the sight of an aerial predator and found that the rate of calls given was significantly higher when the cockerel could see a hen, although the breed or degree of familiarity of the hen did not seem to make a difference.