Large terrestrial mammals are more vulnerable to acoustic impact of drones
than to visual impact
November 30, 2022
Large terrestrial mammals are vulnerable to the acoustic sounds of drones,
technological systems which are increasingly used to study the wildlife in
open habitats such as the savanna and marshes. This is one of the
conclusions revealed in a new study published in the journal Drones, which
has been led by the experts José Domingo Rodríguez-Teijeiro, from the
Faculty of Biology and the Biodiversity Research Institute of the
University of Barcelona (IRBio); Margarita Mulero-Pázmány, from the
University of Malaga, and Serge A. Wich, from the Liverpool John Moores
University (United Kingdom).
Several studies state that drones that are used for scientific and
recreational purposes can become a new source of disturbance for many
animal species. However, there are still few studies identifying the actual
factors associated with these devices that can negatively affect the
Drones and wildlife: opportunity or thread?
The use of unmanned aerial systems (UAVs or drones) is becoming
increasingly widespread in wildlife monitoring and conservation studies.
Obtaining scientific data with a high spatial and temporal resolution, low
operational costs, and simple logistics —without compromising the physical
safety of researchers— would explain the widespread scientific use of this
technology, especially in the study of large mammals in open or
The first author of the new study is Geison Pires Mesquita, from the
Baguaçu Institute for Biodiversity Research (IBPBio, Brazil), an
organisation committed to research, environmental education and
biodiversity conservation. The study analyses the reaction of 18 species of
large mammals to noise emitted by a drone in the large ex situ areas of the
São Paulo Zoo (Brazil).
The 18 species studied belong to 14 families, namely: addax (Addax
nasomaculatus); cattle (Bos taurus); waterbuck (Kobus ellipsiprymnus);
dromedary (Camelus dromedarius); maned wolf (Chrysocyon brachyurus); red
deer (Cervus elaphus); sambar (Rusa unicolor); Asian elephant (Elephas
maximus); imperial zebra (Equus grevyi); jaguar (Panthera onca); Bengal
tiger (Panthera tigris tigris); giraffe (Giraffa camelopardalis);
hippopotamus (Hippopotamus amphibius); giant anteater (Myrmecophaga
tridactyla); white rhinoceros (Ceratotherium simum simum); warthog
(Phacochoerus africanus); tapir (Tapirus terrestris) and the spectacled
bear (Tremarctos ornatus).
Birds and mammals, the most studied using drones
Birds and mammals are the two groups of animals most studied with drones
and the most affected by the recreational use of these devices.
“Especially, large mammals are the most studied with drones because of
their size, as they are easier to identify using aerial images”, says
Geison Pires Mesquida, postdoctoral researcher, who included this study in
his doctoral thesis defended in February 2022. “In addition to size —he
adds— the type of habitat of the species is another determining factor for
using drones in wildlife studies”.
The drone survey of wildlife was adapted to the Brazilian National Civil
Aviation Agency (ANAC) regulations, which limit drone flights to a maximum
of 120 metres. In addition, all flights were VLOS (Visual Line-Of-Sight)
flights, i.e. they were required to be within the pilot’s line of sight.
All flights were conducted at times when there were no visits to the zoo in
order to avoid any disturbance due to external factors. Audiograms were
also available in the scientific literature for 12 of the 18 species
analysed —of the same or similar species— allowing for a more specific
analysis of the influence of the frequency and intensity of drone-generated
The flights started at a maximum altitude of 120 metres. Once the drone was
over the individuals, it began to descend until the animal showed an
atypical behaviour. “A limit of 10 metres above the animals was established
if the animal showed no behavioural changes, but in no case did the drone
descend to that height because the animals showed behavioural changes at a
higher altitude”, says Pires Mezquita.
The Asian elephant, sensitive to low-frequency sounds
In general, species with higher biomass —elephants, rhinos, giraffes,
zebras and the waterbucks— showed a change in behaviour with drones at
higher altitudes (and therefore lower decibels). As this group of animals
is the most studied on land using drones —especially in open habitats such
as the African savannah— terrestrial mammalian megafauna would be more
likely to suffer from the effects of drone noises.
The results reveal that the low-frequency sound pressure level particularly
affected the behaviour of the Asian elephant, but not that of the other
species studied, which were more sensitive to noise at medium and high
“These results explain why the elephant is one of the few mammal species
capable of hearing low-frequency sounds (below 0.25 kHz), or infrasound
(frequencies below 0.0125 kHz). Both the size of the tympanic membrane and
the size of the ossicular chain and the spaces in the middle ear are
compatible with sensitivity to low frequencies”, says José Domingo
Rodríguez-Teijeiro, professor emeritus in the UB’s Department of
Evolutionary Biology, Ecology and Environmental Sciences.
“Low-frequency sounds —the expert continues— propagate more easily due to
the physical characteristics of their sound waves than high-frequency
sounds. It is believed that elephants can communicate more than 10
kilometres away by emitting and receiving these infrasounds”.
Each type of animal exhibits a specific behaviour of caution, irritation or
escape. In addition, animals in ex situ environments —such as zoos— may
exhibit even more specific behaviours. For this reason, the study involved
the participation of Luan Henrique Morais, the zoo’s head of mammal
management. This expert has known each of the animals for years and
informed the team if he noticed that any animal was affected by the noise
of the drone.
In the case of the Asian elephant, head-shaking movements were observed in
the presence of the drone. In the felines, grunting and sudden body
movements; in the spectacled bear, sudden leg and head movements. In the
case of deer and warthogs, attempts to escape from their location are
examples of behaviours that showed adverse reactions in response to the
It is noteworthy that “most of the species we studied did not show any
behavioural reactions to the presence of the drone at an altitude of 100
metres or higher, which is the altitude at which it usually flies over the
ground to carry out wildlife censuses. This confirms that the responsible
use of these systems is a low-impact tool for the study of mammals”, says
lecturer Margarita Mulero-Pázmány (UMA).
Visual Versus Acoustic Impact
Although this experiment does not allow us to fully discriminate between
the effects generated by the impact of the acoustic or visual stimulus of
the drone on the fauna, it was possible to indirectly deduce that the first
effect caused by the drone on the species is acoustic. This conclusion was
reached through the analysis of visual acuity —measured in cycles per
degree (c/g)— which determines the ability to detect, discriminate and
recognise objects against a background.
“All the species studied have a visual acuity of less than 50% of that of
the human species (60 c/g). We can therefore deduce that the first impact
caused by the drone on the species was acoustic, if we take into account
the reduced visual capacity of the mammals analysed, the difficult
detection of the drone used by the human eye at 50 metres, and the fact
that the heights at which changes in behaviour occurred were on average
higher than 50 metres”, says the researcher.
“According to the available information —the researcher continues—, this is
the first time this factor has been analysed. Understanding that drone
noise has an impact on some mammal species earlier than visual noise can
help to improve current drone studies on these species and minimise the
negative effects of recreational use in areas where these species are
In wildlife studies, the sound profile of the drone model should also be
considered, it is a factor that has so far not been considered if its
negative impact is to be minimised. “Although there are many drone models
on the market, there are still few commercial models being used to study
wildlife. Trying to understand how much noise these models generate is a
necessary step to make the use of drones in wildlife studies more
effective”, concludes José Domingo Rodríguez-Teijeiro.