Malnutrition, hunger and thirst in wild animals

This text is part of a series examining the conditions of animals living in the wild. For more texts examining the ways animals in the wild suffer and die, see our main page on the situation of animals in the wild. For information on how we can help animals, see our page on providing for the basic needs of animals in the wild.

The most common cause of starvation is simply being born. Most species of animal reproduce in very high numbers. Arthropods and fishes, for example, can lay from thousands to millions of eggs during their lifetime. If most of these animals survived, then animal populations would grow rapidly and exponentially. This is not what happens, however – animal populations tend to stay relatively stable across generations. 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.

Sometimes the effects of hunger and malnutrition are reduced because malnourished females do not get pregnant, so fewer animals are born who would only starve to death. However, this does not eliminate the effects that hunger has on individuals in these populations. Animals normally reproduce and bring to life huge numbers of new sentient beings, many more than would keep the number of animals in the population stable. The amount of food available for these newborn animals is a key factor in determining how many of them survive. Hence, food shortage is a continual source of suffering for wild animals, particularly in the winter and early spring when food is scarcer.

Other causes of starvation and malnutrition in animals living in the wild

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 drop. 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.1 Sometimes, malnourished mothers 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 instance, 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.

Emperor penguins are another example. After a months-long journey on foot over the Antarctic ice, female penguins lay an egg and leave it in the care of the father. Having lost a third of her body weight, the female sets off on a two month search for food, leaving her mate to keep the egg warm. By the time she returns and he leaves on his own trip for food, the male has not eaten for four months and has probably lost half his body weight.2 These perilous conditions endanger the young as well as the parents, because penguin chicks will starve if they don’t receive enough food from their parents. When they are fledglings, exhaustion caused by malnutrition can deplete the energy they need to forage effectively on their own, and this can lead to starvation. During a bad year, one colony of 40,000 penguins lost all but two chicks.3

Ecological disruptions and natural disasters can devastate large percentages of populations in a short period, destroying or contaminating food supplies, soil, and water for many years, leading to starvation and malnutrition. Animals also face intermittent and seasonal periods of starvation as their habitats undergo changes. For example, deers don’t hibernate or migrate, and routinely starve in large numbers every winter due to scarcity of shelter and food.4 In some areas, more than half a population of sea turtles can die during the winter when they become stunned by the cold and too disoriented to eat.5

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.6 Migration and dormancy are common adaptive responses, but they have their own dangers. Dormant animals are still vulnerable to starvation as well as disease and stress from heat or cold. Migration takes a great deal of energy, and its success often depends on how favorable the weather and food conditions were in the spring and summer prior to migration.

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 physical stress from hunger and malnutrition, leading to death. Other insects resort to cannibalism when food is scarce.7

Throughout the animal kingdom, lack of sources of energy 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.

Even in the presence of abundant food, disease and injury can prevent animals from accessing the resources they need, causing them to starve. For example, abalones can die of starvation due to withering abalone syndrome. The disease is caused by bacteria that consume the digestive tract lining of infected animals. This can destroy the digestive enzymes, preventing the abalone from being able to digest food. To survive, the abalone consumes their own body mass. This causes a loss of muscle, resulting in a “withered” appearance. Infected animals will starve to death or be eaten by predators in their weakened state.8 Birds can starve if their beaks are injured badly enough that they can’t eat.

In some cases, the trouble is as simple as having bad teeth: aging elephants eventually become unable to chew as their teeth are gradually worn down by their tough diets, and squirrels who fail to find sufficiently hard food to file down their teeth find themselves with incisors so long and sharp that they cannot get them around new food items. In either event, the result is starvation and death for the affected animal.

Starvation is a common cause of death for animals who survive to old age. At some point, animals’ bodies simply wear out and they are no longer able to forage. Some insects invest little energy into maintenance after reaching maturity. Crucial body parts simply run down until an animal is unable to eat or cannot move. Wings and mouth parts can start to fall apart, muscles atrophy, joints wear out, and digestive systems can lose the ability to repair themselves.9 If aging animals don’t starve on their own, conspecifics might attack them or drive them away from the safety and food security provided by a group. Aging social insects like ants and bees may leave their groups voluntarily, be intentionally starved, or be chased out of their groups when they are no longer able to contribute.10

Food scarcity is worsened by the simultaneous occurrence of hunger and predation. How are hunger and predation related? First, prey animals naturally try to avoid predators as much as possible. They try to find food in places where the risks that predators 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 to predators. This leads to a rise in the number of deaths due to predation. So, predation and malnutrition combine to cause suffering and death within animal populations. The relationship between food availability and predation has been studied in detail for animals of many species.11

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.12 Second, as with malnutrition, some animals threatened by predators 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 take many risks to satisfy their need for water.13 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.14

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.15

The combination of thirst and starvation accelerates the process of dehydration that culminates in death. Many animals who live in arid conditions continue to eat as a survival strategy because there are some fluids in food. This allows animals to remain alive for longer.16 Without the availability of water directly or indirectly through food, many animals do not survive harsh climates.

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. Because frogs primarily hydrate themselves through their skin, this is usually deadly if untreated. A treatment exists and the infection is simple to cure, but there is not yet a way to treat large populations of frogs in the wild.17 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.18

At times, authorities respond to droughts or lack of food in ways that harm the animals who are already at risk. Sometimes measures are approved to deliberately starve animals. This happens, for example, in the case of urban pigeons. Another instance occurred in 2010 in Kenya, when a drought caused the deaths of 80% of the animals typically preyed upon by lions in the Amboseli National Park. Using helicopters and trucks, humans captured 7000 zebras and wildebeests from other areas and transported them to the park to “serve” as live food for starving lions. Humans living there were interested in the presence of lions in the park because of the economic benefit of tourism.19

You can learn about how we can help on our page Providing for the basic needs of animals.

Further readings

Bright, J. L. & Hervert, J. J. (2005) “Adult and fawn mortality of Sonoran pronghorn”, Wildlife Society Bulletin, 33, pp. 43-50.

Creel, S. & Christianson, D. (2009) “Wolf presence and increased willow consumption by Yellowstone elk: Implications for trophic cascades”, Ecology, 90, pp. 2454-2466.

Hansen, B. B.; Aanes, R.; Herfindal, I.; Kohler, J. & Sæther, B.-E. (2011) “Climate, icing, and wild arctic reindeer: Past relationships and future prospects”, Ecology, 92, pp. 1917-1923.

Holmes, J. C. (1995) “Population regulation: A dynamic complex of interactions”, Wildlife Research, 22, pp. 11-19.

Huitu, O.; Koivula, M.; Korpimäki, E.; Klemola, T. & Norrdahl, K. (2003) “Winter food supply limits growth of northern vale populations in the absence of predation”, Ecology, 84, pp. 2108-2118.

Indiana Wildlife Disease News (2009) “Starvation and malnutrition in wildlife”, Indiana Wildlife Disease News, 4 (1), pp. 1-3 [accessed on 22 October 2014].

Jędrzejewski, W.; Schmidt, K.; Theuerkauf, J.; Jędrzejewska, B.; Selva, N.; Zub, K. & Szymura, L. (2002) “Kill rates and predation by wolves on ungulate populations in Białowieża Primeval Forest (Poland)”, Ecology, 83, pp. 1341-1356.

Kirkwood, J. K. (1996) “Nutrition of captive and free-living wild animals”, in Kelly, N. C. & Wills, J. M. (eds.) Manual of companion animal nutrition & feeding, Ames: British Small Animal Veterinary Association, pp. 235-243.

Lochmiller, R. L. (1996) “Immunocompetence and animal population regulation”, Oikos, 76, pp. 594-602 [accessed on 18 February 2013].

McCue, M. D. (2010) “Starvation physiology: Reviewing the different strategies animals use to survive a common challenge”, Comparative Biochemistry and Physiology – A Molecular and Integrative Physiology, 156, pp. 1-18.

Messier, F. & Crête, M. (1985) “Moose-wolf dynamics and the natural regulation of moose populations”, Oecologia, 65, pp. 503-512.

Mykytowycz, R. (1961) “Social behavior of an experimental colony of wild rabbits, Oryctolagus cuniculus (L.) IV. Conclusion: Outbreak of myxomatosis, third breeding season, and starvation”, CSIRO Wildlife Research, 6, pp. 142-155.

Okoro, O. R.; Ogugua, V. E. & Joshua, P. E. (2011) “Effect of duration of starvation on lipid profile in albino rats”, Nature and Science, 9 (7), pp. 1-13 [accessed on 13 January 2013].

Punch, P. I. (2001) “A retrospective study of the success of medical and surgical treatment of wild Australian raptors”, Australian Veterinary Journal, 79, pp. 747-752.

Robbins, C. T. (1983) Wildlife feeding and nutrition, Orlando: Academic Press.

de Roos, A. M.; Galic, N. & Heesterbeek, H. (2009) “How resource competition shapes individual life history for nonplastic growth: ungulates in seasonal food environments”, Ecology, 90, pp. 945-960.

Tomasik, B. (2016) “How painful is death from starvation or dehydration?”, Essays on Reducing Suffering, Jan-Feb [accessed on 10 April 2016].

Wobeser, G. A. (2005) Essentials of disease in wild animals, New York: John Wiley and Sons.

Notes