A new study has shown that visitors who feed the deer in Dublin's Phoenix Park could be driving the artificial selection of "harassment behaviour" in some species.
The park is home to more than 600 fallow deer, which have been an attraction since it was first established in 1662.
The study by researchers at UCD found that fawns from mothers who consistently begged for food were significantly heavier than those whose mothers rarely approached visitors.
The research, published in the Journal of Animal Ecology, associates this begging behavioural trait with those animals with bolder personality types, which could potentially cause some animals becoming more aggressive in order to obtain food.
The study showed that the entire deer population in the park fell into three categories: consistent beggars, occasional beggars, and rare beggars, with about 24% of the population consistently begging for food.
Those deer that begged more received the largest amount of human food - including bread, crisps, carrots, apples, and biscuits, leading them to have a drastically different diet from those classed as rare and occasional beggars.
Feeding the deer at Phoenix Park is prohibited by the Office of Public Works but the Covid-19 pandemic saw an increase in the numbers visiting the park and interacting with the deer.
Laura Griffin, a researcher at the UCD Laboratory of Wildlife Ecology and Behaviour, said that deer who are fed by visitors to the Phoenix Park could have issues giving birth.
Speaking on RTÉ's Morning Ireland, she said: "In terms of this weight gain that we're seeing there are concerns for the animals in the future. The deer in the park give birth naturally. They go off on their own into covered areas and they give birth
"If there is a point where the fawns are getting larger because more and more people are feeding them, you might see issues with natural births in the future. The mother and the fawn could die as a result."
She added: "The offspring could certainly learn it from their mothers. There’s also some evidence that there’s a genetic element to it.
"If your [fawn's] mother is one of these bolder individuals there's a possibility that you’ll inherit that boldness and that will mean that you engage in these behaviours as well."
Elsewhere, the Department of Agriculture, Food and the Marine is urging members of the public not to feed horses and donkeys without the owner's permission.
A spokesperson warned: "While it may be well-intentioned, the feeding of bread, food scraps, fruit or vegetables to horses or donkeys may cause severe illness, choking or death.
"What may seem like a treat could be potentially fatal for the animals.
"Additionally, the horses and donkeys may be put at risk of developing laminitis (a painful swelling of the feet) or suffering colic (a condition which can be serious and even fatal)."