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Comparing laterality in Canis familiaris and
Homo sapiens: Differences between the
left paw and right hand
Annette Gagliano
University of Toronto. Corresponding author: Annette Gagliano, [email protected]
Abstract
Studies indicate that humans are not the only species with asymmetric characteristics, in that they have two distinct hemispheres
of the brain controlling motor laterality. Non-human species such as dogs, monkeys, cats, and fish exhibit laterality. This review
compares laterality between the domestic dog (Canis familiaris) and humans. Previous studies have indicated that, like humans, dogs
possess asymmetric characteristics. Yet, dogs do not display a preference for a particular paw as humans do. The existing research
raises the question of why human beings favour motor laterality on a dominant side (the right-hand side), while other species such as
dogs tend to exhibit equal dominance on both sides. The “Right Shift Theory” developed by psychologist Dr. Marian Annett from the
University of Leicester presents one possible explanation for the human right-side dominance phenomenon. Understanding laterality
in the domestic dog is important, as it may have important applications such as in the effectiveness of training guide dogs.
Introduction
100
A1
Percent (%)
Percent (%)
Human beings are the only species known to possess a dominant side; approximately 90% of the human population is righthanded [1].
100The “Right Shift Theory” proposed by Marian Annett,
a psychologist from the University of Leicester, speculates that this
75
dominance may be due to the existence of a gene, which she has
named the Right Shift (RS) gene. According to Annett’s theory, the
RS gene is expressed in the left hemisphere of the brain in most
75
right-handed individuals [2]. All other animals, including dogs
50
and primates, do not display such dominance because they do
not have the RS gene [3]. However, research suggests that dogs,
like humans, possess asymmetric characteristics as they have two
50
distinct hemispheres of the brain that control motor movements.
25
Studies assessing various attributes of the brain’s hemispheres,
such as speech control and visual recognition, have yielded a rich
body of literature
concerning motor lateralization. Motor lateral25
0
ization is the preference for using one side of the body more often
Hand Prefe
than the other when performing a task. For over 150 years, it has
Left
been known that the right hemisphere of the brain in most humans
Paw Prefe
Right
specializes in 0speech, while the left hemisphere is related to spatial
Dominance
relations and emotions [4]. The association between speech and
Hand Preference in Humans
Left
the left hemisphere of the brain was determined in 1864 when Figure
1. Comparing
Laterality in the Human and Dog Populations
Paw Preference
in Dogs
Right
French neurologist Paul Broca
discovered
speech
impairments
in
Dominance
Figure 1: Comparing Laterality in the Human Dog Research indicates
patients with a lesion in an area of the left hemisphere [5].
that laterality differs significantly amongst humans and dogs. Excluding
In
the past
decades,
studies have
expanded
from
cases of ambivalence, 90% of humans have a right-hand bias while there
Figure
1. several
Comparing
Laterality
in the
Human
andnonDog
Populations
human primates and rodents to domesticated animals, such as the is an equal split between left and right-paw preference in dogs [1, 6, 7].
domestic dog [3]. Previous studies (Tan, 1987; Wells, 2003) suggest
Journal of Undergraduate Life Sciences • Volume 6 • Issue 1 • Spring 2012
Comparing laterality in Canis Familiaris and Homo sapiens: Differences between the left paw and right paw
that laterality in the domestic dog, characterized by means of paw
preference, differs from the human right-side bias. Unlike humans,
dogs do not significantly favour one paw over the other. Instead,
they have an equal probability of being left or right-pawed [6, 7].
Therefore, results from existing studies are an indication that a dog
can favour one paw over the other paw for motor movements.
The Right Shift Theory
Studies (Tan, 1987; Wells, 2003) have revealed that there is an
equal probability for a dog to be left or right-pawed which suggests
that dogs possess asymmetrical characteristics similar to humans.
One theory to explain the differences in laterality between
humans and dogs is Marian Annett’s “Right Shift Theory”. Annett
proposes that the RS gene is polymorphic, and that the majority
of human beings have two copies of the dominant allele (rs+) [1].
According to Annett’s theory, the RS gene likely evolved because it
played a significant role in cerebral lateralization in language in the
left hemisphere of the brain, which controls muscles on the right side
of the body [1]. Annett proposes that the rs+ allele is mainly expressed in right-handed individuals and controls the shift to the right
side of the body in the left hemisphere of the brain. According to the
theory, the majority of left-handed individuals do not possess two
copies of the rs+ allele. Therefore, individuals who lack the dominant gene can be classified as either left-handed or right-handed. As
a result, humans with different alleles of the RS gene would have an
equal probability of being either right or left-handed [7].
Non-humans do not possess the RS gene, which means that
their dominant side is random. For instance, a study performed
on monkeys asserts that 50% of monkeys are right-handed [8].
Another study examining the laterality of cats, mice, and rats notes
that 54% of the animals consistently use a dominant paw as much
as humans favour a dominant hand [8].
Annett asserts that asymmetrical brains are not unique to
humans, but rather, right-handedness is unique [7]. In her model,
Annett asserts that laterality in animals, such as dogs, is not biased,
but random. Hence, animals have an equal probability of favouring their left or right paw. For humans, however, the distribution
of preference is shifted to the right side of the body or the left hemisphere of the brain. This is a result of the complexity of human
speech in comparison to that of other species. Consequently, since
cognitive functions such as speech are located on the left hemisphere of the brain, there is a greater possibility for humans to be
right-handed because the left hemisphere is dominant in controlling the right side of the body.
Annett proposes that the RS gene arose because it was important for cerebral lateralization for language. As a result, right hand
dominance was an indirect consequence [9].
I.C. McManus proposes a similar model to Annett’s, but uses
a “dextral” (D) allele, analogous to Annett’s rs+ allele, to denote an
individual’s handedness [10]. Another counter-theory proposed by
Kevin Laland et al. also suggests a genetic bias for right-handedness,
but hypothesizes that the gene in question is not polymorphic and
that handedness is largely influenced by cultural and evolutionary
forces [11]. Recent studies have indicated that environmental influences may contribute in determining an individual’s handedness as
well [12]. Currently, the genetic basis, if any, involved with hand
preference, has yet to be determined.
Review Articles
Societal Implications
Humans are the only studied species that display a developed
dominance for their right-hand side. In fact, the high frequency
of right-handed humans was apparent over 500,000 years ago as
determined by the examination of scratch patterns on fossilized
teeth [13]. Although theories such as Annett’s Right Shift Theory
attempt to explain right-hand dominance in humans, the specific
mechanism for this bias remains unknown. It is possible that a
number of factors are involved [14].
Humans have displayed a keen interest in understanding
their own laterality and, more recently, that of other species
such as the domestic dog. Right-hand dominance in the human
population, which has an approximate ratio of nine to one, has
been apparent throughout history. Marian Annett’s “Right Shift
Theory” suggests a genetic basis to handedness that is passed
down through generations. However, the reasons underlying the
greater proportion of right-handed individuals remain debated.
In addition to having a neurological basis by means of linguistics, other non-genetic factors such as social, anthropological
and cultural factors, also likely influence laterality in humans
[15]. Nonetheless, additional studies and analyses should be
conducted in order to gain further insights into the right hand
preference in humans.
The traditional way to train dogs is on the left side of the
handler. Dr. McGreevy, from the Faculty of Veterinary Science
at the University of Sydney, states that this custom may be disadvantageous to some dogs due to their asymmetrical brains, as one
side of the dog is favoured over the other side [16]. Therefore,
if the dog is left brain dominant, meaning that the right plane
of the body is the principal side, vision from the right eye will
be hindered by the handler. This dog will not make an effective
guide dog since the dog’s dominant eye does not have a clear view
due to visual interference from the handler. This suggestion may
explain why 50% of dogs fail their training programs as sniffing
or guide dogs [16]. Indeed, it is possible that the majority of
dogs that fail specialized training programs, such as guide dog
training, are right-pawed dominant. If trainers and handlers were
to customize their training methods based on a dog’s laterality,
they could potentially improve training techniques. In addition,
the role of the handler’s handedness in comparison to the dog’s
paw preference is worth studying in order to optimize laterality
preferences in both species. Through various simple exercises,
handlers can obtain a good understanding of whether a dog is
left or right pawed. Paw preference can be determined in several
ways. For example, placing a blanket on top of the head of the
dog and observing which paw the dog uses to remove a blanket
or observing which paw the dog uses to retrieve a treat enclosed
in a chew toy, such as a Kong [6]. Further research to support the
McGreevy’s claims would be a worthwhile endeavour.
Conclusion
Studying motor laterality in humans and nonhumans is an
intriguing field that allows scientists to gain insights on functional
lateralization of the brain, and the associated evolutionary and
genetic roots of laterality dominances in different species. In turn,
this can lead to the development of strategies for optimal performance in every day tasks.
Journal of Undergraduate Life Sciences • Volume 6 • Issue 1 • Spring 2012
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Review Articles
Comparing laterality in Canis Familiaris and Homo sapiens: Differences between the left paw and right paw
References
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Journal of Undergraduate Life Sciences • Volume 6 • Issue 1 • Spring 2012