The word
"personality" conjures up a variety of mental images, most of which
pertain to one animal in particular: us. It goes without saying that people
have different personalities; we experience it every day. But do other animals
have personalities as well?
In animal behaviour, the term "personality" is defined as consistent differences in behaviour displayed by individuals. An example of a personality trait is how an individual responds to a threatening situation, termed boldness or shyness. Bold individuals are undaunted by threatening situations and will approach the stimulus, while shy individuals will stay away or hide.
There are heaps of different personality traits that have been studied, including boldness, exploratory behaviour and aggression, amongst many others. Individuals' "personalities" are thought to range along a proactive-reactive continuum, where proactive individuals are aggressive and bold while reactive individuals are more passive and shy (sound familiar? It's not unlike a simplified version of the extroverted/introverted behaviour displayed by people).
There's growing evidence that "personality" is present within many groups of animals. Despite this, we don't really know much about what determines an animal's place along the proactive-reactive continuum or why this variation exists.
My study species: the Asian house gecko (Hemidactylus frenatus). Image credit: Wikimedia Commons (1 & 3) and Rebecca Wheatley (2). |
During my honours project, I
investigated "personality" in male Asian house geckos (Hemidactylus
frenatus). I measured the anti-predator behaviour (a proxy for boldness) of
100 geckos by filming each gecko for one hour and then by calculating the
proportion of time it spent inside the shelter in its terraruim.
Each gecko was
measured under three different treatments:
- "empty terrarium": where nothing (aside from the shelter) was added to the terrarium, to give me a measure of each gecko’s normal amount of anti-predator behaviour
- "terrarium with novel object": where I added a novel object to the terrarium, to see what happened to their anti-predator behaviour when something new was added to the environment
- "terrarium with threatening stimulus": where I added a threatening stimulus, to see how their anti-predator behaviour changed when something scary was added to their environment
I found that different individuals
reacted to the treatments in different ways, but the overall trend looked like
this:
We can see that when a novel object was
added to the environment, the geckos' anti-predator behaviour generally decreased
when compared to their standard level of anti-predator behaviour. This might be
because they wanted to check out the new object to make sure it wasn't food or
some other valuable resource.
However, when I added a threatening stimulus,
their anti-predator behaviour jumped back up again to around the same as its
standard level. So it seems that the threatening stimulus effectively cancelled
out the novel object effect.
How do we know if these behaviours
constitute as "personality"? Well, I found that while different
individuals displayed consistent anti-predator behaviour within treatments,
they also responded to the treatments in different ways. Some displayed more
anti-predator behaviour when the environment was altered (were
"shyer"), while others displayed less (were "bolder").
Therefore, from our definition, we can see that their anti-predator behaviour
is a personality trait: they display consistent differences in behaviour that
are context-specific.
Checking on my gecko housing set-up. Image credit: Amanda Niehaus. |
But why do individuals
have different personalities? Previous research has found that a few things can
be associated with an animals' boldness or shyness. A large body mass is often associated
with a bold personality, which is probably because heavier individuals are
usually larger and more likely to win in a fight (so they have a good reason to
be bold). Similarly, individuals with a hard bite force, a strong claw pinch or
any other performance trait which would give them an advantage in a contest are
usually bolder as well.
The possession of traits that might make it easier
for them to escape from a predator in a pinch, like fast running speed, have
also been associated with boldness. In addition, resting (or
"standard" for reptiles) metabolic rate has been linked to animal
personality; it's thought that bolder, more aggressive individuals need a
higher metabolic rate to keep up with their energetic demands.
I investigated how some of these traits
interact to effect boldness in my geckos. I measured each gecko's mass,
standard metabolic rate, maximum running speed and maximum bite force and
analysed their interactive effects on anti-predator behaviour. Contrary to what
I expected (and to what the literature would lead us to predict), I found that
none of these traits affected anti-predator behaviour. This could be due to a
few different things: one possibility is that boldness and shyness in Asian
house geckos has a hormonal basis. It could also be that
"personality" in geckos develops based on experiences rather than any
specific physiological or performance trait. To discover the answer to this
question, further research into the interactive effects of such traits on personality
needs to be done.
Anyway, why does it all matter – why
does "personality" even exist? The fact is there are costs and
benefits to being both proactive and reactive. Proactive
individuals are bolder and more aggressive, so they are usually better at
holding territories and getting laid – but they're also a lot more conspicuous
to predators, so they tend to "live hard, die young". Reactive
individuals, on the other hand, might not have the best real estate or as many
mates at any given time, but their shy behaviour means they usually live
longer. So, if we imagine an ecosystem where predation is low, it's better to
be proactive and reap the benefits without the risk of being eaten. But if the
ecosystem changes (for example, a bunch of predators move into the
neighbourhood) and all the proactive guys die off – who is left? This is the
most popular theory as to why different personalities exist; so that if
conditions change quickly, some individuals survive and the population
continues.
Although extremely interesting, these
personality experiments were only one small aspect of my honours project, which
aimed to answer questions about fighting ability (resource-holding potential)
and fighting strategies. More on that later!
- by Rebecca Wheatley
- by Rebecca Wheatley
Bit of a teaser for the rest of my project. Image credit: Amanda Niehaus. |
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