Aging
in Non-Human Primates
With
increased age comes decline in one’s motor, sensory, and mental faculties. We
have spent a great deal of time in class discussing these different aspects of
aging and how their subsequent effects impact the lives of the elderly. I was
curious, however, to find out if other organisms, as well, experienced similar
signs of aging – especially other primates. From what I have learned I can see that non-human primates
experience very similar signs of aging. As a result, they serve as excellent
models of age-related functional decline.
Picture taken from http://www.primates.com/baboons/index.html
Balance
A key component in walking or maintaining posture is balance
(Tideiksaar 2002). The loss of one’s ability to steadily balance oneself can
have tremendous repercussions on an individual – both physically in the sense
of increased risk of falling and mentally in terms of an individual’s feeling
of well being and independence. Balance is maintained as a result of the
coordination of both sensory and perceptual systems (Tideiksaar 2002). These
systems include the proprioceptive, visual,
vestibular, and auditory systems (Tideiksaar 2002). The loss of one system, but
especially the loss of two or more, decreases one’s ability to balance oneself
and subsequently increases the risk of falling (Tideiksaar 2002). In a healthy
young adult, a sudden loss of balance is detected by the sensory system (Tideiksaar 2002). Upon detection, signals are relayed to
muscles throughout the body, which then spring into action in attempt to
correct this loss of balance and avoid a fall (Tideiksaar 2002
).
The ability to maintain balance in humans has been shown to
decrease with age (Tideiksaar 2002). Such a decrease has, as well, been
observed in primates. This was shown by a 2003 study by Florence Nemoz-Bertholet and Gabienne Aujard that assessed balance performance, and as well
physical activity, as a function of age in gray mouse lemurs. Gray mouse lemurs
are nocturnal primates and are arboreal – meaning that they spend most of their
time in the trees, jumping from one to another (Nemoz-Betholet
and Aujard 2003). The lifespan of these primates in
captivity is approximately 8-10 years (Nemoz-Betholet
and Aujard 2003). As Nemoz-Bertholet
and Aujard (2003) describe, “After they have reached
5 years of age, mouse lemurs exhibit typical morphological and physiological
modifications related to aging” (pg. 408).
In Male Gray Mouse Lemurs
These Include: ·
Fur whitening ·
Reduced aggression ·
Decreased territory marking ·
Decreased sexual hormone levels ·
Decreased reproduction (Nemoz-Betholet
and Aujard 2003)
Picture from
http://www.americazoo.com/goto/index/mammals/86.htm
A Rotarod
test was used to determine each gray mouse lemur’s ability to balance itself (Nemoz-Betholet and Aujard 2003).
This test consisted of a rotating, accelerating rod on which the animals were
placed (Nemoz-Betholet and Aujard
2003). Acceleration would increase with time and balance was assessed by the
amount of time each animal was able to remain on the apparatus without falling
off (Nemoz-Betholet and Aujard
2003). As Nemoz-Bertholet and Aujard
(2003) describe, “balance performance at the Rotarod
test showed a clear decrease with age” (pg. 407).
As shown in the figure to the left, gray mouse lemurs of 1-2
years of age were able to maintain balance on the rotating, accelerating
rod for the longest period of time. The latency to fall mean time for this
group was 154.7 + 13.5 seconds. A dramatic decrease in this time was noted in the oldest group (>6 years), having a latency to
fall mean time of 31.2 + 5.4 seconds. (Nemoz-Betholet and Aujard 2003)
Figure taken from Nemoz-Betholet
and Aujard 2003
Physical activity as a function of age was also investigated in
this study. It was observed that the type of physical activity that older and
younger gray mouse lemurs engaged in was significantly different (Nemoz-Betholet and Aujard 2003).
While younger individuals frequently chose the more difficult paths, although
quicker, in their cages that necessitated riskier actions such as jumping and
climbing, older gray mouse lemurs (>5) preferred to take easier paths that
merely involved walking or running (Nemoz-Betholet
and Aujard 2003). Although these chosen paths were
less risky and consumed less energy, they were longer (Nemoz-Betholet
and Aujard 2003). However, this was a compromise that
the aged gray mouse lemurs seemed perfectly fine with accepting.
Figure
taken from Nemoz-Betholet and Aujard 2003
Hearing
The auditory system is a complex system that has endowed humans,
and many other animals, with the capacity to hear. The detection of sound is
really the detection of pressure waves produced by vibrating air molecules (Purves (ed.) 2001). Humans can hear sounds having a
frequency of 20 Hz to 20kHz (Purves
(ed.) 2001). The human ear is partitioned into three different sections: the
inner, middle, and external ear (Purves (ed.) 2001).
Figure taken from http://mimp.mems.cmu.edu/~ordofmag/ear.htm
The function of the external ear is to maximize the collection of
sound waves and to subsequently direct them into the ear towards the tympanic
membrane, otherwise known as the eardrum (Purves
(ed.) 2001). While sound travels through the medium of air in the external and
middle ear, sound must be transmitted to the fluid-filled cochlea of the inner
ear (Purves (ed.) 2001). Such transmission goes
without significant deflection (as would normally be expected upon transmission
from a low-impedance medium to a high-impedance medium), as a result of the
increase in pressure generated by the middle ear (Purves
(ed.) 2001). The middle ear is able to accomplish this increase in pressure by,
“focusing the force impinging on the relatively large-diameter tympanic
membrane on to the much smaller-diameter oval window, the site where the bones
of the middle ear contact the inner ear” (Purves
(editor) 2001, pg. 279) and as well through the actions of the ossicles, or the three middle ear bones (Purves (ed.) 2001). Auditory stimulation is detected by
hair cells located in the cochlea of the inner ear (Purves
(ed.) 2001). Signals are then sent via the auditory nerve fibers to the CNS (Purves (ed.) 2001).
Figure taken from http://mimp.mems.cmu.edu/~ordofmag/ear.htm
Unfortunately, like all other systems, the auditory system is susceptible
to disease and diminished capacity with increasing age (Tideiksaar
2002). As a result, 37% of the elderly population suffers form
hearing-impairment (Tideiksaar 2002). Sensorineural hearing loss (presbycusis)
is one example of impairment due to aging (Tideiksaar
2002).
To Learn
More About The Ear and How It Works Go to the Following Website:
http://www.bcm.tmc.edu/oto/research/cochlea/Volta/12.html
Do other non-human primates as well
suffer from decreased auditory function with age? Hawkins et al. answered this
question in 1985 after discovering that cochlea function did indeed decrease
with age in rhesus monkeys. One of the greatest advantages of investigating the
biology of animals is their potential to serve as models for human function. Torre III and Fowler (2000) have recently continued this
inquiry into the auditory function of rhesus monkeys and its correlation with
age in hopes of attaining a model for age-related changes in humans. An
enormous benefit that comes from working with animals instead of humans is the
ability of the experimenters to control certain factors such as diet and
environmental circumstances (for example, noise level) that would be impossible
to control for in humans (Torre III and Fowler 2000).
Previous comparative studies had determined that, concerning the design of the
ear, rhesus monkeys are physiologically very similar to humans – having, for
example, a similar middle ear and cochlea. As reported by Torre
III and Fowler 2000 study by Pfingst et al. (1987),
as well, “concluded that the threshold contour of rhesus monkeys resembles the
threshold contour in humans” (pg. 132).
Pictures taken from http://www.bbc.co.uk/nature/wildfacts/factfiles/211.shtml
In this experiment auditory function was
partitioned into two main categories: cochlear function and neural function (Torre III and Fowler 2000). The question of cochlear function
was addressed by the measurement of distortion product otoacoustic
emissions (DPOAEs) and investigation into neural
function was conducted using auditory brainstem responses (ABRs)
(Torre III and Fowler 2000). As compared to younger
monkeys, older monkeys were seen to have smaller DPOAE levels at each frequency
tested (Torre III and Fowler 2000). As well, the
shape of the DP-gram (the graph of frequency vs. mean DPOAE level) displayed
increasing DPOAE levels with increasing frequency in young monkeys (Torre III and Fowler 2000). DPOAE levels for aged monkeys,
however, appeared flat across all frequencies examined (Torre
III and Fowler 2000). The differences between DPOAE levels of young and old
monkeys were shown to increase with increasing frequency (Torre
III and Fowler 2000). As Torre III and Fowler (2000)
state, “The decrease in cochlear function found in older monkeys of this study
is consistent with Bennett et al. (1983) who found that older monkeys had
reduced pure tone hearing sensitivity across the frequency range tested” (pg.
137). The results obtained in this aspect of the study, as well, were shown to
agree with similar findings in humans. Aged human participants in a study
conducted by Lonsbury-Martin et al. (1991) were
observed to have decreased DPOAEs as compared to
younger individuals and in addition expressed a relatively flat DP-gram (Torre III and Fowler 2000)..
Concerning data obtained from ABR testing, it was
found that, in comparison to younger monkeys, older monkeys had longer peak latencies
and smaller peak amplitudes (Torre III and Fowler
2000). This finding is similar to the one found in humans. Previous studies
showed that ABR testing in older humans resulting in longer peak latencies as
well as smaller peak amplitudes in comparison to younger humans (Torre III and Fowler 2000). However, it should be noted
that the rhesus monkeys examined in this study display a larger decrease in ABRs than observed in humans (Torre
III and Fowler 2000). This experiment has also found that “few older monkeys
had measurable ABR responses at lower levels (70 and 50dB pSPL)
whereas almost all the younger monkeys had measurable ABR responses at the
lowest level (50 dB pSPL) … suggesting a decrease in
hearing sensitivity in the older monkeys” (Torre III and
Fowler 2000, pg. 137).
These results demonstrate that rhesus monkeys have
a very similar auditory structure to that possessed by humans. As a result,
rhesus monkeys serve as great models for age-related functional decline in the
human auditory system. Future research on rhesus monkeys will reveal a greater
understanding of how our own auditory system functions.
Caloric Intake
For almost a century now it has been known that
caloric restriction can produce an array of beneficial effects in numerous test
organisms (Lane et al. 2002). Caloric restriction was first reported by McCay et al. (1935) to increase both mean and maximal
lifespan in laboratory rats (Lane et al. 2002). Besides such an impact on
lifespan, caloric restriction has, as well, been observed to decrease
morbidity. As Lane et al. (2002) state, “[Caloric restriction] also delays the
onset or prevents age-related diseases, such as diabetes, cardiovascular
disease, and cancer, and maintains a host of physiological functions at more
youthful levels” (pg. 335). The study conducted by the National Institute on
Aging that began in 1987 on rhesus and squirrel monkeys, however, represents
the first attempt to examine the effects of caloric restriction on an organism
with a lifespan greater than four years (Lane et al. 2002). Since then, similar
longitudinal studies have been initiated, including studies by Lane et al. 1992
and Kemnitz et al 1993.
The study by Lane et al. found that after reducing caloric
intake by thirty percent, rhesus monkeys were observed to experience a decrease
in weight and abdominal obesity, as well as a decrease in temperature and,
temporarily, metabolic rate (Lane et al. 2002). These observations correspond
to the results obtained in previous studies on laboratory rodents (Lane et al.
2002). However, the most exciting finding was caloric restriction’s effect on
morbidity. The rhesus monkeys examined in this study displayed:
· Reduced blood glucose
levels
· Reduced insulin levels
· Improved insulin
sensitivity
· Reduced blood pressure
· Reduced triglyceride
levels
· Increased HDL2B levels
(“low levels of [HDL] are associated with increased cardiovascular risk in
humans” (pg. 335)
(Lane et al.
2002).
These effects are similar
to those experienced by previously examined laboratory rodents (Lane et al.
2002). These findings suggest that the effect of caloric restriction might be
universal, and in that case, could have significant effects, as well, on the
general well being of humans (Lane et al. 2002). Although no studies have yet
been conducted on humans, there is some evidence that caloric restriction
might, as well, affect humans (Lane et al. 2002). In their 2002 paper, Lane et
al. describe a particular natural situation that might yield some support.
Although residents of Okinawa have a decreased caloric intake as compared to
other parts of the country, “Okinawa has a greater proportion of centenarians …
and boasts a lower overall death rate as well as fewer deaths due to vascular
disease and cancer” (pg. 336).
Photo taken from http://www.monkeys-monkeys.com/rhesus.htm,
Photo credited to Rhesus Monkey Vocalization
References:
Tideiksaar, Rein. (2002).
“Sensory Impairment and Fall Risk.” Generations.
Volume 26, Number 4. Winter.
Purves, Dale (main editor). 2001. Neuroscience, Second Edition. Sinauer
Associates, Inc.
Nemoz-Bertholet,
Torre III, Peter and Cynthia G. Fowler. 2000. “Age-related Changes
in Auditory Function of Rhesus Monkeys (Macaca mulatto).” Hearing Research 142, 131-140.
Lane,
Mark et al. 2002. “Caloric Restriction and Aging in Primates: Relevance to
Humans and Possible CR Mimetics.” Microscopy Research
and Technique 59, 335-338.
Website
by Kathryn Post