Geese, sacred to Juno, were kept on the Capitoline Hill in ancient Rome. In 390BC, Gallic warriors tried to creep into the city under the cloak of darkness. Sleeping dogs failed to notice the danger but the geese did. They began cackling loudly, alerting the garrison and saving the ‘eternal’ city from disaster. The Romans never forgot their debt to the geese: one was carried in commemorative procession each year.
The ‘canary in the coal mine’ helped deal with another threat. Dangerous gases can be released when coal seams are opened. Carbon monoxide, invisible and odourless, is particularly lethal; it combines with the haemoglobin in the blood rendering it incapable of absorbing oxygen. Victims become drowsy comatose and die. Small birds need much more oxygen than we do, so canaries were taken down mineshafts. If noxious gases built up, the canary would die, alerting miners to the danger.
Humans are not the only creatures to employ other species as security guards; many animals eavesdrop on the vocalisations of their neighbours. Zoologists Roan Plotz and Wayne Linklater have investigated one, seemingly unlikely, relationship; that between African black rhinos and red-billed oxpeckers.
There are two rhino species in Africa. The white rhino, a grazer, feeds out in the open; it’s the one everybody sees on safari.
‘White’ comes from Africaans meaning ‘wide- lipped’. The other species, no darker in colour, is called ‘black’ by default. Being a browser, the black remains under cover, one reason why it is difficult to observe. Another reason is more ominous; the species has been driven to the brink of extinction by hunters and poachers. Numbers have declined by 99% since the 1950s.
Virtually blind, black rhinos detect approaching humans by smell, not very effectively when the threat comes from downwind. Rhinos need the likes of geese or canaries to look out for them.
According to local African tradition, they already have a minder. What we call the ‘oxpecker’, is known as ‘askari wa kifaru’, ‘the rhino’s guard’, in Swaheli. A distant relative of starlings, the oxpecker eats ticks and creepy-crawlies living on the skins of large mammals. It prefers some animal hosts to others; cattle are targeted, but not camels.
The black rhino seems to be the ‘tickbird’ animal of choice.
Both pecker and rhino should benefit from this ‘cleaner species’ arrangement. However, a study published in 2011 suggested that the birds were actually parasitic. They drink blood from wounds on the rhino, preventing them from healing. But is the oxpecker also a ‘sentinel species’?
Platz and Link placed radio transmitter implants in the horns of 14 black rhinos in Hluhluwe-Umfolozi Park, South Africa. They then made 86 experimental approaches to the animals in the wild, noting the numbers of oxpeckers present. When the birds called, the rhinos faced downwind, the direction from which poachers were likely to come.
‘Oxpeckers enabled rhinos to evade detection by us in 40% to 50% of encounters’, they wrote, ‘every additional oxpecker improved detection distance by 9m’.
This was no ‘ivory-tower’ research. The authors recommend that oxpeckers be reintroduced to areas where they are no longer present, to help protect critically endangered black rhinos from poachers.
Roan Plotz & Wayne Linklater. Oxpeckers help rhinos evade humans. Current Biology. 2020.