Every day, along the river banks of the Amazon basin, hundreds of brightly coloured parrots, parakeets, and macaws gather to collect clay from eroded cliff faces. Some fly from as far as 15 km away to reach the clay lick, before retreating to nearby trees where they slowly chew and ingest it.
The spectacular phenomenon draws tourists from around the world who come to observe these beautiful birds in their natural habitat. It has also attracted scientists, keen to understand why parrots, who in the main, stickly to a mostly vegetarian diet, gorge themselves on the mineral-rich soil.
The practice of eating clay and other earth-like substances is known as geophagia. It is widespread in the animal kingdom and was first recorded by the Greek surgeon, physician, and philosopher Galen in the second century AD. Known as the father of experimental physiology, he noted sick and injured animals would eat clay supposedly to heal themselves.
Biologists have debated for years about what it is that drives parrots to take the risk of joining a vast, squawking flock, that is particularly vulnerable to predators, to get their daily clay fix.
One theory suggested that the grit in the clay helped aid digestion, others proposed the clay may contain essential minerals and nutrients, or aid in curing sickness, while another said it may help the parrots deal with hunger pangs.
However, at the turn of the century, James Gilardi, now the executive director of the World Parrot Trust, while working on his PhD at the University of California, Davis, put forward a new theory.
As part of his research, he studied a flock of Amazon parrots that congregate at a particular bend in the Manu River in Peru. Over a thousand birds from 21 different species arrive each morning at the clay lick and Gilardi found that they had a preference not only for a specific location on the river but also for one specific band in the soil ignoring those just a few metres above and below it.
Gilardi took samples of the soil from the preferred and rejected bands so he could compare their properties and try and find out what the reason was for the parrots’ attraction.
He found that the clay by the Manu river contained very fine particles measuring from 0.05 mm to 0.2 mm in diameter, much smaller than the particles of grit and stones usually eaten by birds to grind food in their gizzards. The rejected bands of soil by contrast contained larger particles, and therefore Gilardi dismissed the theory that the parrots were eating clay to aid digestion. This makes sense; parrots have very strong bills which can break even the hardest nuts into tiny pieces.
He then analysed the mineral content of the soils, but found that the preferred bands of clay contained fewer minerals than the rejected bands, and in much lower quantities than the parrots’ usual diet of fruits, seeds, and nuts. This meant the parrots weren’t licking clay to supplement their diet for essential nutrients.
Food supplies for the parrots were plentiful and all of the parrots Gilardi studied were in good health, so he also rejected the theories that the clay was eaten because the parrots were hungry or sick.
However, upon analysing the cation-exchange capacity – a measure of the number of positively-charged ions that can be retained on soil particle surfaces – of the clay, Gilardi came up with a new theory.
Amazon parrots regularly eat seeds and unripe fruits which have a high content of alkaloids and other toxins including strychnine, quinine and tannic acids which make them bitter and even poisonous, and in some cases lethal to humans.
The clay binds to the poisons rendering them safe and provides a protective lining in the birds’ stomachs to protect them further.
Gilardi tested his hypothesis by exposing brine shrimp to extracts of seeds usually eaten by macaws. Most of the shrimp died confirming the toxicity of the seeds, but when the seeds were mixed with clay the toxicity was reduced by up to 70% and many more of the shrimp survived.
He also gave Amazon parrots an oral dose of quinidine, an alkaloid derived from the bark of the cinchona tree. When clay was provided along with the dose, the levels of quinidine in their blood was reduced by about 60%.
From an evolutionary perspective, Gilardi’s theory makes sense. For a plant to spread its seed, the ripe fruit should be attractive to animals which encourages them to eat it and disperse the seed. However, the seed itself must be either be inedible so they spit it out, or indigestible so it passes through the animal’s body in one piece. Unripe fruit though, should be unattractive as a food stuff so that animals don’t eat it before the seed is ready to germinate.
If an animal is able to somehow bypass the defence mechanisms that the plant puts in place to protect its seed, it will have an advantage over other species by having more food available to it.
In other words, there is an evolutionary arms race with plants evolving to produce more potent toxins and animals adapting to have more sophisticated ways of detoxification.
Previous research had mostly focused on detoxification by enzymes where the poisons are synthesised in the body and converted into harmless substances. Gildari’s work suggested that Amazon parrots may have evolved a further way to neutralise toxic foods.
For several years, the theory was widely accepted but there is one obvious problem with it. Parrots all over the world eat unripe fruit and toxic seeds, yet geophagy is practised in just a few very specific locations.
In 2017, Elizabeth Hobson a postdoctoral fellow at Arizona State University, and Donald Brightsmith a professor at Texas A&M University and director of the Tambopata Macaw Project analysed the data collected from a 16-year project to observe wildlife along the clay cliffs Peru’s Tambopata River.
They revisited one of the other popular theories; that the clay provides nutrients that may be missing from the birds’ diet.
Analysing the soil, they found it contained relatively high levels of sodium, about 40 times greater than the parrots’ plant foods. Sodium intake is critical for a parrot’s nerve function, muscle contraction, and body fluid regulation, and in the tropical rainforests where the birds usually live, sodium is quickly washed away from the ecosystem as the high levels of rainfall leach it out of the soil.
They also found that parrot geophagy is more often observed in areas far away from the ocean, and peaks during breeding season when parents fed their chicks.
This evidence, coupled with the fact that parrot geophagy occurs only in the Amazon would suggest that the clay licks are providing the parrots with sodium that would otherwise be missing from their diet.
