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It's not just humans that seek out drugs to relieve a stomach ache, get rid of a headache or treat skin conditions - animals do it too.

The theory of self-medicating animals, known as zoopharmacognosy, has been spotted in macaws in Brazil, elephants in Kenya and even dogs and cats in the UK and US.

There is even a four-step process to help researchers determine whether an animal is self-medicating, or simply hungry.

Despite the fact eating grass makes them sick, domestic and wild dogs around the world actively seek out the plant. It is believed to be a way for the animals to relieve a stomach ache and get rid of anything in their gut that is causing discomfort

According to Joel Shurkin, writing in the journal PNAS: 'The science of animal self-medication is derived from the roots zoo ('animal'), pharma ('drug'), and gnosy ('knowing').

'It's not clear how much knowing or learning is involved, but many animals seem to have evolved an innate ability to detect the therapeutic constituents in plants.

'Although the evidence is entirely circumstantial, the examples are plentiful. The practice is spreading across the animal kingdom in sometimes surprising ways.'

For example, despite the fact eating grass makes them sick, domestic and wild dogs and cats around the world actively seek out the plant.

It is believed to be a way for the animals to relieve a stomach ache and get rid of anything in their gut that is causing discomfort.

'Dogs do not have the means to digest grass, as they lack the enzymes needed to break down the fibres,' explained Vancouver-based vet Dr. Michael Goldberg.

'Thus, there is little nutritional value in it for them. One reason for eating grass may be due to a feeling of nausea. 

'It is possible that dogs learn this is a temporary solution for stomach irritation.'

He continued that tests on dogs that have exhibited these behaviours have later revealed inflammatory conditions in their stomachs, as well as signs of gastric reflux or bowel disease. 

Elsewhere, chimpanzees have been spotted swallowing the leaves of up to 35 Aspilia plants whole, in a behaviour thought to treat parasites.

The leaves contain a chemical known as thiarubrine-A, which kills certain intestinal parasites, while the roughness of certain plants can act like 'sandpaper' to remove the parasites.

In a similar way to dogs, chimps have also been known to eat certain bushes to make themselves sick.

Red and green macaws have been known to eat kaolin clay to help with digestion problems. This is said to be a 'detoxifying' strategy.

The diet of the Amazonian macaw, for example, is made up primarily of seeds and this can lead to them eating toxic berries and other items that resembles seeds.

Red and green macaws (left) have been known to eat kaolin (right) to help with digestion. The diet of the Amazonian macaw, for example, is made up of seeds and this can lead to them eating toxic foods. Clay on riverbeds is said to detoxify the birds by binding to toxic compounds and removing them from their bodies

Heavily pregnant elephants have been known to eat the plant boraginaceae, which is not part of their regular diet. The same plant is used by women in African regions to induce labour, and it is believed elephants eat the plants to have the same effect

Conservation zoologist Charles Munn believes that many macaws will lick clay on riverbeds to obtain minerals and to counter the bitter tannin and alkaloid compounds found in many of these seeds.

This strategy is said to help detoxify the birds by binding to the compounds to remove them from the birds' bodies.

Clay is also known to absorb bacteria and alleviate diarrhoea in tapirs, forest elephants and mountain gorillas.

In 1996, biologist Michael Huffman devised a four-step process of establishing whether the plants and minerals being eaten by the animals are a form of self-medication.

It the plant isn't part of the animal's regular diet, if it gives little or no nutritional value, if it's eaten at certain times of the year when parasites are rife, and if other animals in the group aren't seen eating it at the same time, Mr Huffman claims it is being used as a medicine, and not just food.

Earlier this year, researcher Holly Dublin spent a year observing a group of elephants in Kenya.

Elsewhere, chimpanzees (left) have been spotted swallowing the leaves of up to 35 Aspilia plants (right) whole, in a behaviour thought to treat parasites. The leaves contain a chemical called thiarubrine-A, which kills intestinal parasites, while the roughness of certain plants acts like 'sandpaper' to remove the parasites

Other animals, including birds such as the ruffled goose (pictured), apply remedies topically. For example, they grasp ants in their bills and wipe them across their feathers. In particular, ants that spray formic acid are used to kill off feather lice and protect birds from infection

She spotted a heavily pregnant elephant ate the plant boraginaceae, which was not part of her regular diet.

The elephant ate the entire plant, before reverting back to her normal feeding routine, and four days after the unusual pattern, she gave birth.

The same plant is used by women in the region to induce labour, and it is believed the elephant was eating the plant to have the same effect.

Pregnant lemurs have been spotted eating tamarind and bark to boost milk production. They also feed the bark to their offspring to help kill parasites

Meanwhile, pregnant lemurs have been spotted eating tamarind and bark to boost milk production and increase the chances of a successful birth.

They also feed the bark to their offspring to help kill parasites.  

In October last year, photographer Paolo Seimandi, 34, captured a herd of mountain goats scaling a brick wall in the Gran Paradiso National Park in Northern Italy.

It is thought the goats were grazing, by licking the stones for their salts and minerals.

For most of the year, males and females occupy different habitats - females tend to stick to steep terrain and males use lowland meadows.

But when snow appears, both sexes, which are strictly herbivorous, move to steep rocky slopes to avoid the snow.

Known as mineral licks, this behaviour is similar to that seen among macaws, and the goats use it as a supplement when food supplies are low, or they are unwell.

Other animals, including birds such as the ruffled goose, apply remedies topically.

More than 200 species of birds, for example, grasp ants in their bills and wipe them across their feathers. This is known as anting.  

In particular, ants that spray formic acid can kill off feather lice and protect the birds from infection.

Female capuchins have been known to rub sugary syrups on wounds, while North American brown bears treat insect bites using a paste of Osha roots and saliva.

Osha contains compounds that repel insects, too.

Mr Shurkin continued: 'The scientists researching zoopharmacognosy are convinced that humans can learn from the animals, particularly in finding new medications.

'Much of folk medicine, particularly in the undeveloped world, likely came from medicine men watching animals self-medicate, and in the case of the plant used by the bonobos, what they saw works.' 

Goats have been spotted scaling walls in Italy (pictured) and licking the stones for their salts and minerals. Known as mineral licks, this behaviour is similar to that seen among macaws, and the goats use it as a supplement when food supplies are low, or when they are unwell