In the current day, combined research has helped majority
of researchers conclude that individual personalities within animals exist. Personality
is defined as differences between individuals in terms of their behaviour that
are consistently present over a vast period of time or change in context (Zidar
et al., 2017). Animals can present with various personalities such as
aggressiveness, boldness, exploration, while some can display the opposite.
Some researchers believe personalities may be related to gender, age, body size
etc., with personalities being identified as affecting an animal’s overall life.
There is a large body of research that claims that certain traits can be
inherited so are reproductively advantageous. Survival rate, hunting ability
and reproduction success can be impacted by an individual’s personality type,
creating a link between personality and adaptive significance, giving way to
evidence as to why personalities would be favoured by natural selection. Shy-bold,
aggressive-sociable and exploration-avoidance are terms that are used in both
the human and non-human world when describing the extremes of personalities and
will be terms used within this essay. Furthermore, there has been evidence to
show links between boldness, aggressiveness and exploration in a personality
type, if an individual is one of these they are typically the other two as well
(Sih & Watters, 2005) with the opposite traits being connected for a
different personality type. This essay will look at the evidence for and
against the adaptive significance of boldness, aggression, exploration and
their opposing traits within non-human animals. This essay will conclude that
there is evidence for the advantages and disadvantages of all of these traits
and a mixture of personalities is key to survival and reproduction success for

Boldness is a personality trait that
has been found within a high number of species such as mammals (Réale, Martin, Coltman, Poissant & Festa-Bianchet,
2009), fish (Ariyomo
& Watt, 2011) and birds (Cole & Quinn, 2014). Boldness can have a
strong impact on various aspects of an individual’s life and can positively influence
reproduction rates. Réale et al found within 105
bighorn sheep rams (Ovis canadensis) that
boldness was a positive and helpful trait for the males. They survived longer,
had longer horns, attained higher dominance ranking and were extremely reproductively
successful. This may be because bolder rams typically had longer horns and
therefore were more willing to fight other rams in order to win a female. Réale et al
found that rams who presented as shy were more likely to back down to a bolder
male and let them breed with the available female. This study therefore
presents a link between boldness and adaptive significance. Another study by
Ariyomo and Watt found that zebrafish (Danio
rerio) present with 3 differing personalities surrounding boldness; bold,
middling and shy. They paired males from each personality subset with females
and found that egg laying was similar amongst the group as ovulation occurs due
to the presence of male gonadal pheromones. However, the number of successfully
fertilised eggs was much higher within the bold group, providing an advantage
to both the bolder males and the females paired with them. This study therefore
provides support for the existing theory that bolder males have higher
reproduction success than shy males. A final study was by Cole and Quinn. They
found that in female great tits (Parus
major), bold birds prioritise reproduction over survival and shy birds vice
versa. In this study, they looked at adults with chicks and found when a mother
was presented with a novel object in her nest, shy individuals typically
delayed or even abandoned their parental responsibilities in fear of the object
in order to increase personal survival. Whereas bold females did the opposite,
risking their survival to prioritise their chicks and reproduction success. All
three of these studies provide evidence for the adaptive significance of the
personality trait boldness, typically bolder individuals are in a position
where they can increase their reproductive success.

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 However, due to
the evidence for the positives of being a bolder individual, there is an
argument as to why there is a need for the shy trait at all and why bolder
personalities have not dominated certain species if they constantly win
reproduction battles. One explanation for this is looking at boldness in a
risk-taking context. Cole and Quinn’s study highlighted that shy female great tits
typically prioritised their own survival over their reproduction. The need to
survive is a need that typically drives a shy, low risk-taking personality. For
example, shy personalities tend to submit to dominant and bolder individuals in
fights. One view is that this leaves them with missed opportunities to
reproduce, however another is that it increases their chance of survival and
therefore extending their opportunity to reproduce, positively affecting their
overall fitness. Smith and Blumstein (2008) conducted a meta-analysis of
published studies that compared the fitness consequence of personality types.
They found a general trend that bolder individuals have greater reproductive
success than their shy counterparts, yet overall, have a much shorter life
span. Therefore, shy individuals may have a lower successful reproduction rates
early on, their overall fitness balances out due to their longer life spans.
Smith and Blumstein concluded that when looking at a rehabilitation side of
captive animals, encouraging the shy trait within an animal fit for release
should increase the overall survival of the species, but that there will
probably be a negative effect on the reproduction rates. With both survival of
an individual and survival of the species via reproduction being key to
adaptive significance, both sides of these arguments can be used to explain why
shyness and boldness are traits that are still present in different species.

A personality trait linked to boldness is aggression.
In certain species where reproductive resources such as mates are highly
competitive, aggression can be a useful trait to overcome competition. Aggression
can be useful in species with dominance rankings. Muller and Wrangham (2003)
found that within a group of common chimpanzees (Pan troglodytes), when a female sexually swelled, indicating mating
availability, a male’s aggression levels would increase. This has been
explained as ‘the challenge hypothesis’. This proposes that male testosterone
raises aggression levels in order to increase reproduction success and can
explain patterns in testosterone levels and aggression amongst species with
breeding seasons (Wingfield, 2017). Muller and Wrangham’s study found that
higher ranking males presented as much more aggressive than the lower ranking
males and were more likely to mate with the sexually available females. Therefore,
implying a link between aggression and reproduction success. Rogers (1987)
hypothesised that females may be attracted to aggression as these males would
be more successful in maintaining territory and better at guarding the young. Another
study that examines the link between aggression and reproduction is by Dewsbury
(1984). He found that aggression in Deer Mice (Peromyscus) was a valuable trait for males to have in order to
increase their reproduction success. Dewsbury found that male-male aggression
levels went up in the presence of oestrous females, implying this cause of
aggression being for the females’ benefit and may be related to the
determination of choosing a mating partner. Aggression has also found to be a
valuable trait in females as well as males. Rosvall (2008) found that when tree
swallows’ (Tachycineta bicolor) nest cavities were reduced, something required for female
reproduction success, the more aggressive females were much more likely to
obtain a nest cavity, consequently having higher reproduction success. Supporting
this, Cain and Ketterson (2013) looked at aggression levels in female dark-eyed
juncos (Junco hyemalis) with
nestlings. They found that during poor breeding seasons, females who displayed
higher aggression levels produced larger eggs and achieved greater nest success
than females with lower aggression levels. Aggressive females were also found
to, although spend less time with their nestlings, spend more time feeding
them. They did find however that there was no real detectable relationship
between aggression and nestling mass after hatching, this suggests that
although aggression provides a benefit during poor breeding seasons, the
fitness is cost is fairly inconsistent. Therefore, having a more aggressive
personality for certain species can be beneficial when it comes to reproduction
rates, however there are some inconsistencies for this trait in other species,
with some animals preferring non-aggressive partners.

Thus, although there is evidence for aggression being a
valuable trait, various studies have provided evidence to the contrary. Eibl-Eibesfeldt
(1961) explained that an aggressive personality is more likely to be killed by
a conspecific due to its raised likelihood of fighting. Whilst Collias (1944)
highlighted that quick to aggression animals can be killed in fights with other
species, for example the vicious ‘fight to the death’ attitude by nocturnal
reptiles such as alligators. Herbert (1968) studied mate preference in rhesus
macaques (Macaca mulatta). He found
that the females sought out the males who were less aggressive and more
sociable to the other subjects in the study for breeding purposes. Supporting
this, Bernstein (1976) found in various baboon societies, females show an
active preference for non-aggressive males. These studies contradict claims by
researchers such as Russell and Russell (1971) who claimed aggression is a key
trait that attracts females to males, yet they are not the only studies with
this view. Spritzer, Meikle and Solomon (2005) compared mate preference in
female meadow voles (Microus
pennsylvanicus) towards males with better or worse spatial ability and high
or low aggression levels. They found that females had a preference for males
with better spatial ability and low aggression levels and showed no real
interest in the higher aggressive males. This implies that aggression may not
be as valuable as other traits. Another piece of evidence that supports the
idea that a sociable personality can lead to a higher reproduction success is
from Sih and Watters’ (2005) water strider (Aquarius
remigis) study. They found that the presence of hyper-aggressive males led
to a reduction in mating rates in the social group due to their continual
harassment of the females and males attempting to mate. As a result of the
hyper-aggressive individual’s behaviour, females were virtually inactive when
these males were present whereas mating rates were much higher when they were
around the sociable and lower aggressive males. Again, this is an example of
how individuals with a sociable personality can increase their mating success.
Not only can less aggression benefit individuals when their high aggression
counterparts can be viewed as too aggressive, it can also increase their
overall survival rates. Sapolsky and Share (2004) found that within a troop of
olive baboons (Papio anubis), the
more aggressive males were the ones who were more likely to die. They found
this was due to the more aggressive males being the members of the troop to eat
from a local garbage dump near a local human inhabited lodge. These males may
have chosen to eat here due to their ability to have a stronger chance of
fighting off any other animals eating there. Unfortunately, this study found those
who chose to eat at the dump exposed themselves to tuberculosis, with the
majority of them dying from the disease. Therefore, in this study it was
advantageous to have a non-aggressive personality. Consequently, in some
situations a weak trait would be a disadvantage, however in the case of
aggression it would appear that a non-aggressive and sociable personality can
be advantageous. It can increase the chances of reproduction success and also
survivability rates.


Linked with boldness and aggression is the exploratory personality
type. Exploratory personality types are those that will explore novel
surroundings more than others, this can benefit them by exposing them to more
food sources, better breeding spots and sometimes more mating opportunities. Nettle
(2006) outlines that the trait of exploration in non-human animals is
consistent and substantially heritable, the same as in humans, and therefore
can be classed as a personality trait. In support of this, Drent, van Oers and
van Noordwijk (2003) found that 30% of exploration in chicks could be explained
via exploratory parents, implying strong advantages the trait can bring. Various
experiments using great tits such as Dingemanse and de Goede’s study (2004), have
found that higher levels of exploration have the benefit of providing an
individual with territory and resources when these are limited. Alternately,
Garamszegi, Eens and Torok (2008) found exploratory male collared flycatchers (Ficedula albicollis) are more likely to sing
in the presence of an observer than non-exploratory males, ultimately bonding
faster with a female. McCowan, Mainwaring, Prior and Griffith (2015) carried
out a study looking into personality in zebra finches (Taeniopygia guttata) and their
consequent reproduction success. They found that although there was little preference
from females to males based on personality, males who were more exploratory
made more breeding attempts and overall raised more nestlings than those who
were less exploratory. This again provides evidence for the adaptive
significance of the exploratory personality trait. The willingness to explore a
novel habitat has also been found to be beneficial for animals kept in
captivity who are later on rehabilitated, with the trait being a sign of a more
robust individual. Fraser, Gilliam, Daley, Le and Skalski (2001) found that in Trinidad
killifish (Rivulus hartii) kept in
captivity and later released back into the wild, the individuals that explored
the initial captive habitat were later more successful in the wild and
travelled a further distance after release, and had higher overall reproduction
rates and resource opportunities due to this. Linked to this, Liebl and Martin
(2012) studied the introduction of house sparrows (Passer domesticus) to Kenya in 1950 and noted that those found with
nests at the edge of the expansion areas and access to more resources rated
higher on exploratory scales. Implying this personality can create an advantage
for an animal in a novel area, with these individuals being most likely to
thrive. Furthermore, Nilsson,
Brönmark, Hansson & Chapman (2014) highlight that a personality willing to explore can help increase an
individual’s survival rate when faced with a sudden reduction in food
resources. In support of this, van Overveld and Matthysen (2010) found that
when they purposely reduced the food source of a group of great tits, it was
the fast exploring individuals who were able to adapt and shift their behaviour
quickly. By expanding their foraging radius and successfully finding alternate
food sources, these individuals were able to reduce their mortality chances
compared to the individuals more averse to exploration. Most of the literature
discussing exploratory personalities links it with survival rates rather than
higher reproduction, but stating that exploration can expose an individual to
different mating opportunities. Therefore, the adaptive significance of
exploration is mainly linked to an increased survival rate, which still
ultimately improves overall fitness of the individual.


In terms
of reproduction, individuals who are classed as being more cautious to
exploration appear to be more successful. Both, Dingemanse, Drent and Tinbergen
(2005) studied a group of great tits and categorised them based on their
personality into fast and slow explorers. They found that, whilst faster
exploring individuals were much better at obtaining high quality territory, it
was the slow exploring individuals that had higher reproduction success. They
found that the slow exploring birds made better parents, with higher nest
success and had larger fledglings compared to their fast exploring
counterparts. In support of this, Jones and Godin (2010) found that slow
exploring convict cichlids (Amatitlania
nigrofasciata) were faster than their fast exploring
conspecifics in reacting to predators and ultimately avoiding death. Another
piece of research is from Wolf, van Doorn, Leimar and Weissing (2007) who
stated that in species such as fish, birds and rodents, individuals face a
trade-off regarding reproduction in year 1 and year 2 of their lives, and that
this decision is mediated by exploration behaviour. High exploring individuals
typically put off reproduction until year 2 to see if they can better their
resources, this ultimately leads to an overall reduction in reproduction
success in high explorers. Finally, from a female’s viewpoint, exploring out of
their territory could be too costly. Larsen and Boutin (1995) found that female
North American red squirrels preparing for offspring (Tamiasciurus hudsonicus)
refused to change territory when given the option, they hypothesised this may
be because relocation is too risky for them and their offspring. Therefore,
this combined evidence implies the fitness advantages of having a low
exploratory personality in both personal survival and reproduction success. Throughout
this essay both sides of personality traits have shown to have advantages and
disadvantages, implying a need for varying personalities. This view is
supported within literature by outlining that successful groups need to be
composed of a mixture of personalities, some that bring an advantage at the
start of a life cycle and some that can bring advantages later on (Chapple,
Simmonds & Wong, 2012).


This essay has discussed 3 personality
traits; boldness, aggression and exploration, and their opposing traits. It has
shown evidence for and against their adaptive significance, in terms of
individual survival and reproduction success. Each trait has its own advantage
and disadvantage, with some displaying more positive consequences, such as
boldness. However, with all of these traits they appear to be species and
context specific. In certain species such as great tits an individual may
succeed more if they are bolder, aggressive and more exploratory, whereas in
primates a female may choose to mate with a less aggressive male who will stay
near the territory and offspring. It is clear that as each trait typically
brings a strong advantage, for a species to thrive or for large social groups
it would be advantageous to have a mixture of these traits for survival. They
can expand, defend territory and have strong leaders because of certain
individuals’ traits whilst also securing overall survival and reproduction due
to individuals at a different spot on the personality scale. Similar to the
advantages humans gain from having a mixture of personality, there is a clear
strong adaptive significance from having varied personalities in non-human