Stress, Behavior, and Sodium Appetite

Larry L. Berger, Ph.D.
University of Illinois 

Introduction: 

Animals face stress, whether in natural or production settings.  It’s normal.   Wild animals’ stress comes in seeking food – or seeking to avoid becoming some predator’s food!  Then there’s heat, cold, antagonistic social interactions, etc., all of which cause stress.  In this article, the term “stress” as applied to farm animals is a potential damaging stimulus that evokes a largely adaptive response (Ewbank, 1992).  Such responses can range from the relatively innocuous licking behavior to the extreme of cannibalism.

Efforts to minimize stress are not only good for the animals, but they can also benefit the producer’s bottom line.   Most sources of stress will increase the animal’s maintenance requirement.  For example, heat or cold stress can increase the calories used to maintain a normal body temperature by 30-80%.  This in turn reduces production and increases feed costs.  Disease stress not only causes discomfort to animals resulting in decreased feed intake, but also stimulates the immune system leading to increased maintenance requirements.   This combination of reduced nutrient intake and increased maintenance requirement can devastate production efficiency. 

Modern production agriculture exposes animals to environments with different sources of stress than those to which they would experience in the wild.  Although animal and livestock producers try to minimize the stress these animal experience, some animals do experience increased stress that is reflected in their endocrine profile.  Recent research suggests that the changes in hormonal profile may cause an increased appetite for sodium.  This increased appetite for sodium may encourage stereotypies behavior, behavior of an unvarying, repetitive nature with no direct purpose (Mason, 1993).  It is the purpose of this article to review our current understanding of the relationship between stress, behavior, and the appetite for sodium. 

Stress and Behavior: 

Stress encourages stereotypies behavior in laboratory and farm animals.  Sodium-deficient lab rats exhibit stereotyped fixed action patterns that are ingestive in nature (Berridge et al., 1984).  Sodium-deficient cattle frequently display excessive licking behavior (Sly and Bell, 1979).  Cattle that are tethered in a restricted area or raised individually as calves in isolated stalls exhibit similar licking behaviors. 

In the past few years scientist have learned a great deal about how hormonal changes resulting from stress can affect brain chemistry and behavioral changes.  Animals respond to stress by releasing adrenocortiotropic hormone (ACTH) from the anterior pituitary gland.  The ACTH then causes the adrenal cortex to release aldosterone and corticosterone.  Aldosterone is the main hormone that controls sodium balance by changing the kidney’s reabsorption of sodium and thus the amount excreted in the urine.

Corticosterone increases blood glucose and carbohydrate metabolism to supply energy.  These hormones also act directly on the brain through the activation of the neuropeptide angiotensin II.  Angiotensin II is a powerful stimulus for thirst and sodium appetite (Fitzsimons, 1998).      When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in sodium intake. However, the appetite for salt is more persistent and may be affected by previous experience.  Some researchers believe that the angiotensin II may influence neuronal organization in the brain that can cause long-term changes in sodium appetite (Fitzsimons, 1998).   Stress has been shown to increase the salt appetite in rats, mice, rabbits and sheep.   

Cattle: 

Phillips et al., (1999) conducted an experiment to determine whether salt intake influenced the behavior of cattle in stressful environments.  In this experiment,   36 Estonian Red dairy cows were allocated to three treatments, 0, 200, or 400 grams of salt   added to a standard winter ration, daily. The basal diet was grass silage and ground barley.  The final diets contained 1.0, 6.0 and 11.0 g sodium/ kg dry matter for the control, low and high sodium diets, respectively.  The salt supplements were mixed with the barley and no feed refusals observed.  Cows were individually housed and milked twice daily in their tied stalls.  Each cow was observed for a total of 18 5-minute periods and the amount of time doing various behaviors recorded.  Stereotypies behavior recorded included: mouthing the feed trough bars or tethering chain, rubbing against feed trough bars or tethering chains, pawing the ground or self-grooming.  None of the individual sterotypies behaviors was significantly affected by sodium level, but collectively there was a reduction in total time spent in stereotypic behavior at the high sodium level.  The fact that stress increases the sodium appetite of other herbivores suggests that the reduction of stereotypies measured in this experiment may be a consequence of the physiological relationship between stress and sodium status. 

In a second experiment (Phillips et al., 1999), 16 British Friesian female calves were selected at birth and allocated to pairs of similar weight.  Within each weight, calves were assigned to no additional salt or 13.5 grams of salt/kg of concentrate fed.  Adding the salt to the concentrate increased the sodium concentration from 4 to 9 g sodium/kg concentrate.   Calves were housed in individual pens and weighed weekly for 6 weeks.  Behavior was recorded for 12 hours after the calves received their concentrates on day 1 of each week.   Adding sodium to the concentrates increased feed intakes, water intakes, and live weight.  Calves with supplementary sodium spent less time grooming themselves, licking the pen, licking the buckets and ear sucking.  The sterotypies behaviors was more pronounced in calves than in the cows in the previous experiment.  The sodium intake of the control treatments was greater than the requirement given by the British Ministry of Agriculture.  In that sense they were not sodium deficient diets.  However, the stress experienced may have increased the desire for sodium that resulted in behavior patterns associated with stress.  Increasing the sodium level was helpful in controlling abnormal behaviors. 

Pigs: 

Abnormal behaviors may also be influenced by sodium levels in the diets of other farm animals. For example, tail biting in finishing pigs can be a real problem in high-density confinement buildings.  Tail biting begins with the occasional chewing of another pig’s tail.  Once a wound has been established, the biting becomes more frequent and intense. Docking the tail at birth has become standard practice to try to avoid this problem later. 

Diet containing less than 0.3% salt are associated with high levels of tail-biting (Gonyou and Bergeron, 1996).  Most swine nutritionist recommend 0.5% salt in the diet.  However, salt concentrations are often raised to 1% of the diet following an outbreak of tail biting. Other factors that may contribute to tail biting in pigs includes protein deficiency, amino acid imbalance, thermal stress, high ammonia levels, overcrowding, large group sizes, and poor ventilation.   

Because blood is relatively high in sodium, some researchers have proposed that tail biting was an effort to find more sodium.  Canadian researchers ( Widowski and Jankevicius, www.thepigsite.com /FeatureArticle) have tried to determine if this was the case by allowing pigs access to ropes (similar to pig tails) soaked in blood, salt water, and pure water.  The pigs were given ACTH injections to simulate stress conditions.  In this study the blood-soaked ropes were the most popular, but there was no difference in the number of pigs that preferred the salt water and pure water ropes.  This suggest that salt taste may not be the only factors that makes blood attractive to stressed pigs. 

Poultry:

Poultry illustrate a more extreme problem associated with a salt deficiency:  cannibalism.  Cannibalism occurs when birds peck at the feathers, toes, heads, and vents of other birds.  If there is bleeding and further pecking, it may result in the death of the bird.  Poultry nutritionists often recommend that the diet contain 0.15 to 0.20% sodium to minimize cannibalism.   If cannibalism does become a problem, sometimes it can be controlled by adding 5-10 grams of salt per gallon of drinking water (PBCPFA Newsletter April 1996, www.afn.org/~poultry/newsletr/ltr96apr.htm). Other factors that can contribute to cannibalism include vitamin and amino acid deficiency, feed deprivation, over-crowding, over-heating, inadequate ventilation and bright lighting.  

Humans:

The drive to consume adequate sodium can have a powerful influence on behavior. Dr. Derek Denton in his book The Hunger for Salt builds a strong argument that the incidence of cannibalism in primitive people was highly correlated with a lack of sodium in the their diet.  Cannibalism was most common in the tropical areas of the world that lacked access to salt.  The equatorial jungles and mountains are noted for their very low sodium status. Requirements for sodium were also increased in these hot environments due to its loss in sweat.  

Summary: 

Production environments that increase the stress in farm animals will also increase the appetite for sodium.  The endocrine changes in the brain as a result of stress will stimulate the appetite for salt.  Stereotypies behaviors that seem to have no meaningful purpose may be aimed at increasing the contact with and consumption of sodium sources.  Salt requirements have been determined in production settings where efforts have been made to minimize stress.  Sodium levels required to minimize undesirable animal behaviors in stressful environments may be greater than that required to meet nutrient needs in a low-stress production setting. In that sense animals have two sodium requirements, one to maximize animal production and another to modify behavior. In stressful production settings, adding extra salt to the diet may be helpful to the animals in coping with the stress and improve the producers bottom line at the same time. 

Literature Cited 

Berridge, K.C., F.W. Flynn, J. Schulkin, and H. J. Brill.  1984. Sodium depletion enhances salt palatability in rats.   Behavioral Neuroscience 98:652. 

Ewbank, R. 1992. Stress: a general overview. In Farm Animals and the Environment (ed. J.C. Phillips and D. Piggins), p. 255. CAB International, Wallingford, UK 

Derek, D. 1984. The Hunger for Salt. Springer-Vaerlag, Berlin 

Fitzsimons, J.T. 1998. Angiotensin, thirst, and sodium appetite.  Physiol Rev. 78(3):583. 

Gonyou, H., and R. Bergeron. 1996. Is abnormal behavior really abnormal? In: Proceeding of the 15^th Annual Centralia Swine Research Update. Jan. 31, 1966. 

Mason, G. 1993. Forms of stereotypic behavior. In Stereotypic Animal Behavior (ed. A.B. Lawrence and Jl Rushen) p. 7. CAB International Wallinford, UK 

Phillips, C.J.C., M.Y.I. Youssef, P.C. Chiy, and D.R. Arney. 1999. Sodium chloride supplements increase the salt appetite and reduce stereotypies in confined cattle. Animal Science 63:741,