A wild howler monkey is filmed raiding a chicken coop for eggs (Video courtesy of Carina Barboza Muhle)
Wild howler monkeys have been caught on camera raiding chicken coops and eating bird's eggs.
The behaviour has surprised researchers as howler monkeys are thought to be exclusively vegetarian.
The egg-eating behaviour has evolved among black and gold howlers living in forest in Rio Grande do Sul in Brazil.
Despite it being one of the best studied monkeys in the New World, this is the first time the species has been recorded eating animal matter.
Details of the new behaviour are published in the International Journal of Primatology by Dr Julio Bicca-Marques and Ms Carina Muhle of the Primatology Research Group at the PUC/RS Bioscience Faculty in Porto Alegre, Brazil and colleagues.
Alouatta is the most studied New World primate genus in the wild and there is not a single observation of intentional ingestion of animal matter in the literature
Primatologist Dr Bicca-Marques
"Howlers are believed to be strictly vegetarian. Their diet is based on leaves and fruits, although other vegetable matter, such as petioles, pulvini, buds, flowers, stems, twigs and bark can also be eaten," says Dr Bicca-Marques.
Howler monkeys are also known to ingest occasional arthropods such as beetles, but the monkeys do this inadvertently as they gather plant material.
However, in 2005 Ms Helissandra Prates, a student of Dr Bicca-Marques, observed two juvenile black and gold howler monkeys (Alouatta caraya) licking the interior of a dove's nest at Estancia Casa Branca, a 2 ha orchard forest in Alegrete.
The nest contained a broken egg shell.
She later found evidence of egg remains in howler monkey dung collected from the orchard floor.
Then in 2007, Ms Muhle, another of Dr Bicca-Marques's students, witnessed howler monkeys eating birds' eggs at a separate woodland site at Beco Xavier.
Over an eight month period, she recorded 19 separate instances of egg predation by a group of five individuals.
A black and gold howler monkey licks the inside of a pilfered egg
"This study was responsible for the uncontestable confirmation that black and gold howlers monkeys can act as nest predators and eat birds eggs," says Dr Bicca-Marques.
All these later cases involved chicken eggs, which the monkeys stole from a chicken coop. While all five monkeys were seen to enter and investigate the coop, most of the eggs were actually eaten by a single subadult male.
"We were very surprised because Alouatta is the most studied New World primate genus in the wild and there is not a single observation of intentional ingestion of animal matter in the literature."
The researchers suspect the monkeys are varying their diet in response to environmental conditions, particularly where a high density of monkeys lives among forests or orchards of low diversity.
"Eggs should be a surrogate supplementary source of protein when few plant species compose the available menu," says Dr Bicca-Marques.
Evolution of egg-eating?
Dr Bicca-Marques and his colleagues may have actually witnessed the emergence of this egg-eating behaviour over many years.
Dr Bicca-Marques and his wife Claudia Calegaro-Marques first studied howler monkeys at Estancia Casa Branca in the summer of 1989, as part of their own Master's degree dissertations.
Then they recorded 15 to 17 individuals, some of which showed an unusual interest in bird's eggs.
Though the monkeys were never seen to actually take eggs, they inspected the nests of birds on three occasions, even removing an egg and examining it, before replacing it back into the nest.