Movement studies
Animal movement is a very large and
interesting topic. In the past, most scientists were interested in describing
movement of an animal let’s say from point A to point B. This is now known as
dispersal (translocation) of an animal. Dispersal usually answers two main
questions in animal ecology which are WHERE and WHEN.
When I started working with
marine animals I got very interested in describing the “movements” of white
sharks in Mossel Bay. We put a simple acoustic transmitter on a white shark and
followed it for several days (our longest track has been 107 hours
continuously). This method allows us to look at dispersal and activity space.
The following graphs have been recently published by our team in Environmental Biology
of Fishes (2013: 96, pp.881-894), and show the activity spaces of several
sharks within Mossel Bay.
As you can see Seal Island,
Hartenbos, Kleinbrak and Grootbrak are focal areas in Mossel Bay.
Can you find
out possible reasons for that?
For answers you can look through some of our
publications within the following links:
http://awsassets.wwf.org.za/downloads/23_finding_a_balance__white_shark_conservation_and_recreational_safety_in_the_inshore_wa.pdf (paper 4 and 6)
A unique problem with active
manual tracking of marine animals from a boat is that we work on a 2
dimensional field while the animal moves within a 3 dimensional environment. We
therefore decided to incorporate transmitters with depth sensors into our tagging
protocol. This allows, for instance, calculations of speed (as rate of movement:
distance travelled divided by the time used to move that distance) in not just
two, but three dimensions, which are more realistic estimations of the true speed
of an animal. We were able to cluster different white shark behaviours in
different areas of Mossel Bay, based on obviously position, but also swimming depth,
swimming linearity and swimming speed. We differentiated behavioural types such
as “resting” (swimming very slow at the bottom, sometimes almost drifting with
current) from hunting for fish (“fishing”: swimming up and down -from the
surface to the bottom in a yo-yo pattern-, back and forth –in a very localised
area-, quite fast), from hunting for sea lions (swimming at a certain
favourable depth, back and forth, quite fast), from travelling (swimming in a
more sporadic yo-yo pattern, at medium speed) etc... This helps in answering
the HOW part of the animal movement study.
Why do you think travelling for a
white shark can not be too fast? 3 hints: metabolism, thermoregulation and
energy expenditure.
In the same way that this novel
branch of research has evolved, we soon realised that knowing the WHEN, WHERE
and HOW, was not enough for us. We struggled to further develop our understanding
of these movement processes with just simple description, so we started looking
around at possible other avenues to get a better understanding of animal
movement. We realised that a big effort was put into trying to characterise and
model movement patterns. Animals can move for several reasons such as searching
for food (hunger), searching for possible mates (passing own genes to the next
generation), avoiding being preyed upon (fear), avoiding being harassed
(stress), searching for better conditions
in terms of habitat or in terms of space (i.e. overpopulation). Models
such as random walks and Levy flights are attempts to look at underlying
parameters which could help in understanding the biggest questions in animal
movement studies which is WHY an animal moves. For instance if you were an albatross
you may search randomly an area for food (random walk model) and if you don’t
find anything you may want to move 'quite a bit' and start searching for food in
another area (Levy flight model).
Recently a new branch of animal
movement studies has gone even further. As usual in science when you answer a
question at least two more questions open in front of you: that is not a
negative consequence but rather an exciting opportunity for us to grow as
scientists. If we understand the WHY next we might want to know WHO. Do all specimens
behave in the same pattern? Are some individuals more prone to certain kind of
behaviours? Individuals may respond differently to different stimuli because of
different threshold for those stimuli. So individual may have intrinsic traits
which govern dispersal: usually age, gender, reproductive status, social status
(in social animals), and morphology; but what about personality. I can already
hear the comments from several ‘old style’ scientists: “Animals do not have
personalities! They are not humans”. Actually I have had this thought for a
long time based on what I can see while working with white sharks on the field:
some white sharks are more aggressive (“playful”) whereas others are more
skittish (“shy”). Even further, the same white sharks can change behaviour in
two consecutive days (what I call ‘moods’). Without going into too much detail,
while mood changes can be the consequence of changes in the environment that
supports that animal. The so called ‘personality’ types are something well
known in scientific literature. Cote et
al. (2010) studying mosquitofish assessed that some individuals, identified
as more asocial (bold) than the population norm, tended to disperse greater
distances, but that was not all: mosquitofish from populations characterised as
more bold overall, also dispersed more often regardless of their individual
personality type. Reale et al. (2007)
assessed individual behaviour over five fixed axes of personality: boldness,
exploration, activity, aggressiveness and sociability.
Can you guess which type of
personality in mosquitofish would be the one more efficient in finding food in
environments with scarce food supply?
Now why I am talking about all
this? What if we were able to distinguish individuals by using, for instance,
their interest in using wide areas (explorers) vs. individuals paying more
attention to the environment they move within (sitters)? You
will find out with the next blog about movement studies.
Enrico Gennari
