Most animals who are ever born die of thirst or starvation shortly after being born or hatched. Those who survive may have to search for food and water at great risk to their lives and wellbeing. We know that, at least for some animals, thirst can be a terrifying and painful experience. Anxiety is also a major harm to animals in the wild, and for some it appears that living in a constant state of anxiety is an adaptive response that increases their chances for survival. In this video, you ‘ll see the most common conditions that contribute to hunger, thirst, and psychological stress.
Also available as a chapter of our companion ebook to the video course Introduction to wild animal suffering: A guide to the issues
Another important factor that can seriously affect the lives of animals is the lack of food. Many animals suffer through long periods of hunger and malnutrition, yet they survive. Others starve to death, often shortly after birth. The most common cause of starvation in the wild is simply being born in an environment where there is not enough food for all. Unfortunately, this is the situation of most animals who are ever born. Most species of animals reproduce in very high numbers. Many different species of arthropods and fishes, for example, can lay from thousands to millions of eggs during their lifetime. This means that populations would grow out of control if most of the offspring survived. In order for a population to remain stable, on average only one offspring per parent can survive to adulthood. The rest will die. Some eggs don’t hatch, some animals are killed by predators, siblings, or even parents shortly after birth, but one of the most common forms of death is by starvation just after being born or hatched. For those who do survive, there are multiple challenges and dangers that can easily lead to malnutrition, starvation, and thirst.
Parents are at greater risk of starvation just before and after mating, when their energy levels and fat stores can drop by 50% or more. Babies are also more vulnerable, even in species that have few children and care for their young. Young mammals prematurely separated from their mothers rarely find the food they need to survive. When food is scarce, a mother may starve herself in an effort to nourish her children. Alternatively, she may reject her children, refusing to feed them or let them suckle. Sometimes, malnourished mammals are unable to produce milk. In these circumstances, babies either starve in the nest or den, or are abandoned, as is often seen among squirrels. Non-mammals can be at even greater risk of starvation during mating and parenthood, as their fat reserves drop and their access to food is severely restricted. Salmon, for example, endure an exhausting journey upriver to their breeding grounds, swimming against the current and leaping up waterfalls. Throughout this period, they do not eat. Some survive to make the journey again in subsequent years, but many do not, expending the last of their energy to reproduce, and dying shortly thereafter.
Animals also face intermittent and seasonal periods of starvation. For example, different deer species don’t hibernate or migrate, and they routinely starve in large numbers every winter due to scarcity of shelter and food. In some areas, more than half a population of sea turtles can die during the winter when they become stunned by the cold and are too disoriented to eat or move around.
Under food stress, mammals, birds, and fishes first shed accumulated stores of fat and then begin consuming muscle mass as an emergency source of energy, which can be debilitating and eventually becomes fatal as organs atrophy. Migration and dormancy are common adaptive responses to lack of food, but they have their own dangers. Migration takes a great deal of energy, and its success often depends on how favorable the weather and food conditions were in the previous spring and summer. Dormant animals are still vulnerable to starvation, as well as disease and stress from heat or cold.
Invertebrates employ similar strategies to cope with starvation periods, and many invertebrates, including insects, have evolved to survive for months or even years without food. Others migrate, but their ability to take off and to fly can be reduced by hunger stress and malnutrition, leading to death.
Throughout the animal kingdom, lack of energy sources is common. During times of food scarcity, the animals who starve first are those with lower fat stores, such as juveniles, animals who have lost energy due to breeding, animals too weak to migrate, and those with lower social status who have less access to food. Food scarcity is worsened by the simultaneous occurrence of hunger and predation. How are these two things related? First, to avoid being killed, animals try to find food in places where the risks that other animals pose to them are lower. For example, they will look for food in wooded areas where they can hide instead of in open plains where predators can more easily see them. When there is not enough food in the areas where they hide, they face hunger and malnutrition. When malnutrition becomes critical, they start leaving safer areas, increasing their vulnerability.
Thirst is another major contributor to high mortality rates in wild animals. There are two fundamental ways the lack of water causes wild animals to suffer and often to die painfully. First, during times of drought, there are not enough resources available for a large population of animals, so many of them die of thirst. Second, as with malnutrition, some animals show a reluctance to seek water because of the risk posed by predators. They hide in safe places where there is little or no water. Eventually, thirst forces animals to takemany risks to satisfy their need for water. When they finally leave their hiding places, they are so debilitated that they become easy prey at watering holes or in open fields. Others stay in their hiding places until they are so dehydrated that they cannot move. Thus, they are unable to reach water and they die of thirst.1
Extreme thirst is a frightening experience. It produces a sense of exhaustion caused by reduced blood volume, and the body attempts to compensate for the lack of water by raising the respiratory and heart rates. Next comes dizziness and collapse, and ultimately death.2
Diseases can also lead to dehydration. For example, frogs can be infected by the chytrid fungus, which thickens their skin so much that they can’t absorb water and essential nutrients. Since frogs primarily hydrate themselves through their skin, this is usually deadly if left untreated. A treatment exists and the infection is simple to cure, but currently there is no way to treat large populations of frogs in the wild. The disease can be further complicated by other factors such as heat stress. Heat stress can worsen the condition of a dehydrated frog, even at temperatures that do not harm them when they are hydrated.
At times, authorities respond to droughts or lack of food in ways that harm the animals who are suffering. Sometimes measures are approved to deliberately starve animals. This happens, for example, to urban pigeons.
In addition to facing physical harms like the ones we have seen above, animals can also suffer psychologically due to the situation they are in. While the effects of stress in domesticated animals have been well documented,3 there have been fewer studies on wild animals, and the severity and number of stressors that afflict wild animals have likely been underestimated by scientific research, except for the effects of captivity on wild animals.
Wild animals have to face adverse circumstances on a daily basis that can be stressful: physical trauma, disease, food shortages, conflicts with others of their species or group, dislocation due to severe weather conditions or natural disasters. They can also be frightened by loud or unfamiliar sounds. In mammals, birds, and arthropods, there is evidence of animals showing PTSD-like symptoms in response to stressful events, of mood and anxiety disorders, and of negative moods spreading within social groups. Here we will cover stress related to external threats and various aspects of social living.
Stress caused by the risk of being attacked by other animals seems to arise in two major ways. The first is directly from the conflict itself, in which animals must face the stress of fleeing or fighting. The confrontation may be so intense that the prey animal dies of stress.4 Second, stress can arise in prey animals who are forced to balance their need for food against the risk of being killed, and decide whether to decrease foraging or to risk exposure to predators.5 Often, animals decrease the likelihood of being caught by choosing to eat less. In those conditions, additional stress responses are likely to be triggered by starvation and dehydration.
This can be made worse by certain human interventions that are carried out for ecological purposes, such as the reintroduction of predators into an area to preserve a threatened species of plant. One way this is done is when wolves are reintroduced in an attempt to prevent large herbivores (for example, an elk or a deer) from eating certain foods. Not only do the wolves eat these animals, but their presence causes the grazing animals to hide in places where they are less visible and where they eat less plentiful, less nutritious plants. The dynamics that result from this are called the “ecology of fear.”
Living in social groups involves costs for animals,primarily due to social conflict and competition. Many species of animals that are social and subsocial (such as crickets and lobsters) have dominance hierarchies, as do solitary animals like octopuses, who compete over territory. Although a lot of fighting for position is ritualized, some involves actual violence or ongoing harassment. The social status that each animal has in the hierarchy dramatically influences her level of wellbeing, particularly when it comes to stress-related diseases. It has been well documented that social subordination constitutes a stressor in different social species among primates, rodents, and some types of aquatic animals. In low-ranking animals of these social species, depressive responses and a decrease in reproductive opportunities are often observed.6
Other subordinate animals may face frequent threats and intimidation in order to remain in their group. Common causes of intimidation include food access and sexual competition—often together because of the extra energy demands of reproduction. Dominant males might attack or threaten other males who try to mate, and kill the children of other fathers, causing grief to the mothers, who are then coerced into mating. We give more detail about this in the following section about conflicts between animals.
Subordinate females in matrilineal groups can face constant threats and deprivation, in which dominant females use aggression and intimidation to limit the access of subordinates to mating opportunities and food. The children of subordinate females might also be killed by the dominant female. She may force the subordinates to serve her, especially to further her reproductive success. This is common in meerkat gangs. The mothers whose children were killed must help care for the young of the dominant female or else be evicted fromtheir colony and face the hazards of trying to survive on their own.
Stress due to the adverse effects of maternal separation has been studied in numerous social species. Maternal separation can have a long-lasting effect on the physiology and behavior of both mother and child. After separation, common responses of the mother are reducing activity, moving with a bent-over body, and exhibiting other sickness behaviors induced by the stressful event. Amother who loses a child may carry the dead child around or refuse to leave the body for days. This has been observed in primates, birds, elephants, cetaceans, and many other animals.
Orphaned animals face fear and loneliness. Infants who are separated from their mothers show increased reactivity to stress throughout their lives and increased risk of disease. In wild animals, this has been observed in cetaceans, elephants, rodents, and primates, and other social species are likelyto experience similar effects.
In addition to the effects of maternal separation, there are many documented cases of elephants, cetaceans, dogs, birds, and other animals exhibiting grieving behavior at the loss of family members or friends. Animals like geese and ducks mate for life and grieve the loss of a partner. A mourning goose will lose weight, separate from her flock, and exhibit submissive behavior (and if she partners again, it will typically be with another goose who has also lost a partner).
2 Gregory, N. G. (2004) Physiology and behavior of animal suffering, Ames: Blackwell, p. 83.
3 See for example, Dantzer, R. & Mormède, P. (1983) “Stress in farm animals: A need for reevaluation”, Journal of Animal Science, 57, pp. 6-18; Wiepkema, P. R. & van Adrichem, P. W. M. (eds.) (1987) Biology of stress in farm animals: An integrative approach, Hinglaw: Kluwer Academic; Broom, D. M. & Johnson, K. G. (1993) Stress and animal welfare, Hinglaw: Kluwer Academic; Moberg, G. P. & Mench, J. A. (2000) The biology of animal stress: Basic principles and implications for animal welfare, New York: CABI.
4 McCauley, S.; Rowe, J. L. & Fortin, M.-J. (2011) “The deadly effects of ‘nonlethal’ predators”, Ecology, 92, pp. 2043-2048.
5 Clinchy, M.; Zanette, L.; Boonstra, R.; Wingfield, J. C. & Smith, J. N. M. (2004) “Balancing food and predator pressure induces chronic stress in songbirds”, Proceedings of the Royal Society B: Biological Sciences, 271, pp. 2473-2479.
6 Fox, H. E.; White, S. A.; Kao, M. H. & Russell, D. F. (1997) “Stress and dominance in a social fish”, The Journal of Neuroscience, 17, pp. 6463-6469; Koolhas, J. M.; Boer, S. F. de; Meerlo P.; Strubbe, J. H. & Bohus, B. (1997) “The temporal dynamics of the stress response”, Neuroscience and Biobehavioral Reviews, 21, pp. 775-782; Koolhas, J. M.; Boer, S. F. de; Rutter, A. J. de; Meerlo, P. & Sgoifo A. (1997) “Social stress in rats and mice”, Acta Physiologica Scandinavica. Supplementum, 640, pp. 69-72; Shiverly, C. A.; Laber-Laird, K. & Anton, R. F. (1997) “Behavior and physiology of social stress and depression in female cynomolgus monkeys”, Biological Psychiatry, 41, pp. 871-882; Sapolsky, R. M. (2004) “Social status and health in humans and other animals”, Annual Review of Anthropology, 33, pp. 393-418; Abbott, D. H; Keverne, E. B.; Bercovitch, F. B.; Shively, C. A.; Mendoza, S. P.; Saltzman, W.; Snowdon, C. T.; Ziegler, T. E.; Banjevic, M.; Garland, T., Jr. & Sapolsky, R. M. (2003) “Are subordinates always stressed? A comparative analysis of rank differences in cortisol levels among primates”, Hormones and Behavior, 43, pp. 67-82; Sapolsky, R. M. (2005) “The influence of social hierarchy on primate health”, Science, 308, pp. 648-652.