This is a time of quiet anticipation for the zoo’s southernmost residents. Zookeepers have brought Petre here from the Taronga Zoo to mate with Felix, an import from the Cairns Wildlife Safari.

The mating process is delicate. Zookeepers are monitoring the hippos’ behaviour to gauge the right moment for the two to be introduced; too early and Felix could feel the full force of Petre’s teeth. The process could take a year or two. Meanwhile, the pygmy hippos wait in separate enclosures in the zoo’s royal park home.  Patience is everything.

For 150 years, the zoo has been a haven for endangered species. But the animals may now be facing a threat to their sanctuary. The plans for the first stage of the state government’s proposed 18 kilometre cross-city toll road include a six-kilometre tunnel running less than 300 metres away from the zoo’s southern corner. 

The Australian Institute of Landscape Architects recently concluded that the tunnel would “substantially damage” Royal Park. Its map of the proposed construction shows the hippos and gorillas would be less than 100 metres from the “severe impact” zone of the development that will touch the zoo’s south-east boundary (shaded yellow, below), as well as widened roadway (red).

David Nichols, an urban planner from the University of Melbourne, says he is frustrated that the Linking Melbourne authority, which is responsible for planning the tunnel, has not considered the impact of the noise and vibrations of construction on the zoo’s inhabitants. “The people who propose to build the East-West Link have not given one moment’s thought to the animals in the zoo.” Just because the roadway will not be seen, he says, does not mean its presence will not be felt.

Ten minutes away from Dr Nichol’s office, the zoo rainforest feels like another world. The zoo’s troop of western lowland gorillas sit with their backs to visitors, arms folded. This is a trying time for the troop. Their alpha male, Rigo, died of a sudden heart attack last month. In the wild, a troop would be expected to disband after the alpha male’s death.

Gorillas —whose DNA is 98 per cent identical to that of humans — are extremely sensitive to change. Researchers at the San Diego Zoo found that increasing surrounding noise can make the primates more aggressive towards visitors. Evidence of their increased stress can be detected physically, too: as noise increases, so does the level of adrenalin in gorillas’ faeces.

“Gorillas are rather sensitive, and I can well imagine that they would be disturbed by increased noise like this” — Professor Colin Groves

Australian National University’s Professor Colin Groves, the author of Primate Taxonomy, describes the proximity of the East-West Link to the gorilla enclosure as disturbing. “Gorillas are rather sensitive, and I can well imagine that they would be disturbed by increased noise like this.”

Beneath the rainforest canopy at the end of the path, Petre and Felix remain oblivious to the city’s infrastructure battles. Despite their separation, hippos, like elephants and whales, can communicate via infrasound – sound waves transmitted at frequencies too low for humans to hear. These waves can travel up to five kilometres through solids, water and air.

Like most animals, Petre and Felix are accustomed to the everyday cacophony of infrasound that can also be created by weather and geological shifts, as well as humans using heavy machinery. But this familiarity does not ensure they will not be affected by the East-West works.

Michelle Nussey, a zoo presenter, recalls when three new elephants arrived from Thailand in 2006, they were terrified of the trams that pass nearby. The resident female, Mek Kapah, would place herself near the trio to calm the newcomers.

But Pygmy hippos are solitary creatures, so there will be no one to calm them in late 2014, when tunnel construction is planned to begin. Or in 2019, when the tunnel traffic begins to pulse.  

In small doses (below 90 decibels), infrasound vibrations are considered harmless. But repeated exposure to higher levels can cause effects similar to being drunk, according to a 2008 Polish study by the Motor Transport Institute. Prolonged exposure may cause permanent damage; infrasound above 170 decibels can kill an animal. A Korean study found a truck travelling 100km/hr through a tunnel produced 100 decibels. 

A spokeswoman for the Authority said the project’s Comprehensive Impact Statement – to be released in November – would include a specialist vibration report assessing likely effects on the zoo. Contractors would be required to develop a management plan taking into account the impacts of any vibration from tunnelling.

Melbourne Zoo director Kevin Tanner last week told The Age the zoo had raised with the Linking Melbourne Authority the issue of the potential effects on animals. Local residents have also started an online petition at Change.org, calling on Zoos Victoria chief executive, Jenny Gray, ”to insist on rigorous investigation into the impacts of the east-west link”.

Petre, meanwhile, passes the time in her Parkville quarters, listening perhaps to infrasound messages from the elusive Felix – or to the distant rumble from the test drilling that has now begun.

► This story was co-published with The Sunday Age.