Variations in the Brains
of Monogamous and Promiscuous Species
Evolutionary neurologists have sought
to determine the significance of overall brain size, as well as the
size of different regions of the brain. It has been noted that an
overall increase in size of the brain correlates positively (as the
occurrence of one increases, the likelihood of the other increases as
well) to a higher level of social bondedness, the complexity of their
habitat, and their behavior flexibility. In other words, they were
able to experience a higher level of social complexity in their
lives, could maintain living in a more complex environment and were
able to adapt their behaviors accordingly. For species such as birds,
bats, ungulates, and carnivores, the brain size is shown to be
correlated to their pair-bonding, while anthropoid primates were a
different case. The size of the group tended to influence the overall
brain size, while their pair-bonded monogamy did not. Due to these
observations, the social brain hypothesis came to be.
The social hypothesis posits that
species who come from a more complex social structure have developed
larger brains overall. The reason for the development would be due to
the increasing cognitive demands implemented from the social group.
Animals would have to be able to monitor their relationships with
others and learn how to respond appropriately to different
individuals in the group. For example, wolves act differently when
socializing with average members of their pack, than when interacting
with the alpha male in the group. The hypothesis has been proven in
studies with haplorhine primates and two families of the Carnivora.
However, it has not been proven in but not in lemurs, bats and
multiple families of Carnivora. In these cases, other variables come
into play. Factors such as the complexity of their foraging, their
flexible activity patterns, and even the size of their testis are
shown to be better predictors of relative brain size. However, direct
comparison across the studies is difficult due to evolutionary
influences. It is possible that evolutionary changes in whole cortex
or overall brain size may reflect selective expansion. As in, they
may be bigger in that species to serve as a different evolutionary
advantage. It has been shown in numerous studies that the size of a
local area in the brain is positively correlated with a specialized
behavior.
In the study performed, the
limbic-associated cortical area was examined to see if there was a
correlation between size and whether or not the species was
monogamous or promiscuous. Within the genera Microtus and Peromyscus,
prairie voles and California mice are two highly social, monogamous
species that experience pair-bonding. The species exhibit
affiliation, copulation, nests sharing and biparental care with their
mate. Meadow voles (Microtus pennsylvanicus) and white-footed mice
(Peromyscus leucopus), on the other hand, are promiscuous species
that do not form pair-bonds. For these two species, it is the female
who rears the kids.
Studies in the past have shown that
social structure plays a role in determining the size of various
regions within the brain. However, the goal of the current study was
not to explain variations in brain or cortex size, but rather to
determine whether two specific areas of cortex that have been linked
to particular mating systems exhibits shape or structure (morphology)
in monogamous and promiscuous species, and to see whether convergent
patterns (two unrelated species becoming more similar over time) of
cortical evolution are observed across species of voles and
Peromyscus mice that have independently evolved similar mating
systems. The mPFC, is a cortical region that has receptors for the
neuropeptides oxytocin and vasopressin, and the neuromodulator
dopamine – all of which are important for the forming of social
bonds. In promiscuous species, the mPFC region is larger than in the
brains of their monogamous counterparts. The findings show support
for the convergent evolution of mPFC size. However, despite the
patterns with the mPFC when it came to mating systems, there was
nothing to signify differences in the mPFC size had any influence on
the mating behaviors of the male prairie voles. Additionally, in
the present study, there were no observed differences in the size of
the RS that proved to be significant.
An explanation for the size
differences in the mPFC involves the function of the area. The mPFC
is important for ‘complex tasks’ that require memory for a
sequence of events in time, the association of place and information
regarding an object, as well as behavioral flexibility such as when
animals are required to select new rules based on the social cues.
These abilities would be more important for species who demonstrate a
promiscuous mating style. To be more specific, males with multiple
partners need to remember where their mates are located and at what
time they ovulate in order to maximize their offspring output. To
support this idea, scientists used data gathered from other studies
that show how the daily range a promiscuous meadow vole changes over
time. Their ranges have a tendency to overlap the ranges of females
and other males, with the latter occurring most often when a nearby
female is ovulating. A reproductive advantage is seen by those who
are better able to remember where to go and when to go there. Another
study has offered further proof to the idea by showing that
promiscuous white-footed mice were more likely to be found in the
areas of female mice that were getting close to ovulation, than when
the female was in the early stages of pregnancy.
In the case of the promiscuous
species, no long-term associations are found. Neither the same
females nor males have been recaptured together. In fact,
white-footed mice females only seem to tolerate having males around
during their mating period, but not afterward. In both species,
females raise their offspring alone.
The gender-bias seen by the size of
mPFC could also be attributed to a female's need to keep track of
whom they've mated with. By coupling with multiple males, the females
can decrease the likelihood of infanticide occurring, increase
genetic diversity and even prevent the occurrence of inbreeding.
On the other hand, monogamous prairie
voles and California mice do not require as large of an mPFC region
due to their lifestyle. Males and females have a tendency to mate
primarily with their chosen partner, a fact that has been proven in
studies where the same male and female have been recaptured together.
Paternity tests have also been used to indicate the same-partner
bias. Unlike their promiscuous counterparts, prairie voles and
California mice both play a role in raising their offspring. The
ranges of the monogamous species are also smaller and more exclusive
than those of the promiscuous species.
Another explanation for the size
difference of the mPFC between monogamous and promiscuous species
could involve how the region is utilized. For instance, while
promiscuous voles do have larger ranges than their monogamous
counterparts, however this difference is primarily due to the fact
that the promiscuous voles have a much larger range than the females.
However, since females have a large mPFC region and smaller ranges
than males, this does not support this hypothesis. Additionally,
white-footed mice have smaller ranges than their monogamous
counterparts, which do not agree with the findings. Therefore, the
size of the mPFC region is not relative to the size of the ranges
(spatial ability). Instead it relates more to the extended social
networkings that require using spatial knowledge with object-place
and information regarding timing.
So why is all of this important?
Understanding what influences the size
of brain regions can help scientists better understand the functions
of the regions. The regions studied in this experiment already have
information available to researchers about their functions, but that
is not always the case with the brain. While there is a lot we do
know and understand in regards to the brain, there is a lot still
needed to be determined. Additionally, the social hypothesis could
further benefit future studies into animals that have a tendency to
avoid contact with humans. Suppose scientists discover a new species
on, let’s say, Galapagos Island; a species that has managed to
avoid being noticed by scientists for years - an animal such as the
chupacabra. If a chupacabra is happened to be found, scientists can
then use their findings from the study to determine what potential
mating habits the species haves, as well as what type of social
interactions they may experience with others in their species -
knowledge which can be beneficial to future studies. The more we know
about how the size of the brain influences a species, the better able
we are to predict behavior.
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