The male deer strutting their stuff in the Phoenix Park are a noisy lot when the mating season comes along. They use a sequence of guttural sounds to announce to other males their location in the pecking order and more importantly to impress females who may choose them as mates.
A team at University College Dublin has been watching the fallow bucks throwing shapes for over a decade in an ongoing research effort to understand the behaviour of the park's 700-strong deer population. The group's world standing in this research has enabled it to publish a sequence of papers about its work in leading journals.
Interest in the deer population, aka Dama dama, living under the noses of Dublin's citizenry was heightened in the mid-1980s when people thought they were dying out, explained Dr Alan McElligott of UCD. He has been involved for many years in the research effort by the mammal research group within UCD's department of zoology.
The population included brown, black and spotted deer, but people seemed particularly attached to the spotted animals, he explained. Having seen the Disney film Bambi is the closest most urban dwellers get to deer and because Bambi was spotted, people may have expected that the park's animals should also be spotted.
As the numbers of spotted fallow deer declined, people started to complain and the politicians heard them. There were even Dail questions about it at the time. The Office of Public Works began looking into it and contacted UCD's Dr Tom Hayden. "People were querying what was happening to the spotted deer. The OPW contacted Tom to find out what was going on," Dr McElligott said.
Thus began an unbroken annual pilgrimage from UCD to the park to study the deer and monitor their behaviour. Early work involved watching which males were mating and trying to understand "dominance interactions between the males" which determined their ranking in the herd, Dr McElligott said.
The spotted deer question was quickly solved. "It wasn't that the spotted deer were dying out. The most successful bucks were black and more black deer were being born."
The work developed and the team began a detailed analysis of the "fighting tactics" used by the males to establish dominance rank. "Males that were closely aligned in the dominance rank were more likely to fight," Dr McElligott said. Fighting was also age related, with youngsters never getting into serious rows with older animals because of a genuine risk of death. "It is better for them to wait four or five years until they are able to compete."
The most recent work relates to the connection between the sounds or "vocalisation" used by the bucks and their mating success. A paper in last May's Animal Behaviour described Dr McElligott's and Dr Hayden's work on the "long-term investment" made by bucks in these vocalisations during the annual rut and a paper to be published in the same journal in December will look at the "short-term" aspects of vocalisation.
The sounds made by a buck during the rut are called "groans" by zoologists but they are difficult to describe. "It is like a loud belch," Dr McElligott suggested, short, sharp and repeated many times. "They do an awful lot of it." The researchers had to tape them because of the rapidity and number of the sounds. "Some of the males were vocalising over 3,000 times per hour."
This represented a huge energy expenditure and so must be important in terms of rank and mating potential, Dr McElligott said. "Only the high-quality males can do this. If this is important in the mating decisions of the female, then you have to do it." The top bucks begin vocalising around the third week in September, long before the rut begins in mid-October and before lesser animals down the dominance rank begin to make themselves heard.
"The long-term vocalisation is probably aimed at females," Dr McElligott said. "It is in a female's interest to mate with the high-quality males. We think that the initiation of vocalisation is the male advertising they are good quality males and are trying to transmit this evidence to the females."
The males don't eat during the rut and will lose between 20 and 25 per cent of body weight, so sustaining the vocalisation must be significant to mating success. "The theory is that the male controls everything that the females do, but that is not what happens in Phoenix Park." The female will walk away if the buck doesn't convince her. "It is really up to the males to impress the females if they want to have any chance at all."
While producing these noises over a long period in the run-up to the mating season is directed at the females, males are involved in short-term vocalisations most likely aimed at other males. These are thought to be a way to dictate their position in the buck pecking order. As a top animal's energy wanes and he loses weight and stamina at the end of the rut, he is likely to encounter more challenges from other males who will seek to hop past him in the rank.
There is much more to be learned from the herd, Dr McElligott said. There is a "mate guarding" dimension to vocalisation and the issue of scent has yet to be explored.