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Early development of locomotor behavior in vervet monkeys.

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American Journal of Primatology 17:ll-25 (1989)
Early Development of Locomotor Behavior
in Vervet Monkeys
JOEL A. VILENSKY AND EVA GANKIEWICZ
Department ofdnatomy, Indiana University School of Medicine, Fort Wayne, Indiana
The locomotor development of three vervet infants across approximately
the first 2 months of life is described. Fairly normal-looking walking
movements (as compared to adults) were seen in all the animals by
approximately 1 month of age and galloping was observed by 2 months.
Early locomotor footfall patterns were often aberrant and bounding-type
gaits were sometimes exhibited. Most of the symmetrical gaits observed
were classifiable as lateral sequence. Across the 2-month period the
animals showed decreased three- and four-foot support and improvements
in joint angular displacement patterns. From their earliest locomotor
movements the infants showed significant linear relationship between
both cycle duration and swing and stance durations of the limbs. We
suggest that locomotor control mechanisms are probably fairly mature at
birth but that weight support and postural control problems explain the
initial locomotor difficulties exhibited by these infants.
Key words: primates, locomotor development, gaits, footfall patterns
INTRODUCTION
Primates are not able to locomote a t birth. But at what age do various taxa
achieve the ability to walk, run, and gallop? Surprisingly, there are few data
providing such information for most nonhuman primate species. For vervets
(Cercopithecus aethiops) Hurov [19821 provides data for three individuals; one at
age 15 days, one a t ages 56-75 days, and one at ages 81-105 days. However, since
Chalmers [19721 states that vervets achieve “steady” locomotion by a mean age of
36 days, Hurov’s description is primarily based upon animals that had already
achieved fairly mature locomotor skills.
For the present study we investigated the locomotor development of three
infant vervets from their birth date (or next day) until they were approximately 2
months of age. These animals were filmed weekly and, later, every 2 weeks, as
they locomoted or attempted to locomote on a platform.
METHODS
The three infant monkeys used in this study were all female. Animal No. 1 was
filmed on days 1,8,15,22,29,43,and 58 of life (day 0 = day of birth); animal No.
Received for publication July 8, 1988; revision accepted September 22, 1988.
Address reprint requests to Dr. Joel A. Vilensky, Fort Wayne Center for Medical Education, 2101
Coliseum Blvd., East, Ft. Wayne, IN 46805.
0 1989 Alan R. Liss, Inc.
12 I Vilensky and Gankiewicz
TABLE I. Number of Strides Available for Each Animal for Each Age (Days)
No. 1
Age
8
15
22
29
43
58
No. 2
No. 3
Strides
Age
Strides
Age
Strides
3
6
28
42
10
13
21
28
2
2
2
4
6
40
56
14
5
9
64
7
9
a
2 on days 0, 7,13,21,28,40, and 56 of life; and, animal No. 3 on days 0 , 7 , 1 3 , 21,
28,42, and 64 of life. For these filming sessions the infants were taken from their
mothers and placed on a Plexiglas-enclosed elevated surface (1.17 x 0.43 m). Their
locomotor attempts and behavior were then filmed by using a 16-mm camera set to
run at approximately 64 framesls for animal Nos. 1 and 2 and 100 framesls for
animal No. 3. Additionally, each animal was filmed while air-stepping, i.e.,
walkinglike movements exhibited by infants while suspended [cf. Vilensky et al.,
19891. Each filming session lasted about 15 minutes. Exact filming speeds for each
trial were determinable from timing markers placed on the film a t 0.01-s intervals.
Following the completion of each filming episode the animals were returned to
their respective mothers. Animal No. 1 was housed with her mother in a singleanimal cage. The other two infants were housed with their respective mothers and
other animals in large group cages.
The films, once processed, were initially viewed (using an analytical projector)
with the goal of making qualitative evaluations as to how “normal” (i.e., adultlike)
the locomotor movements appeared and of making basic gait determinations.
These qualitative evaluations were based upon our extensive observations of adult
vervet locomotion [Vilensky et al., 19881. All of the available film was used in this
analysis. The filmed locomotor sequences were then divided into actual “strides.”
Strides were defined as the interval between a hind-foot touchdown to the
subsequent touchdown of the same foot. It is important to note that not all of the
data were definable as strides because of the erratic nature of some of the
movements. Furthermore, in many cases, the strides filmed were abnormal in that
the footfall order or timings were not those typically observed in a more mature
animal. Table I shows the total number of strides available for each animal a t each
age. It is clear from the table that there were only a limited number of strides
available for animal No. 3.
All of the defined strides were initially classified as symmetrical or nonsymmetrical based upon the footfalls of the hind limbs. Specifically, if a hind-foot
touchdown occurred between 40 and 60%of the stride defined by the contralateral
hind-foot’s touchdown, we considered it to be a symmetrical stride [cf. Hildebrand,
19661. Furthermore, the symmetrical strides were classified as being either
diagonal sequence (DS) or lateral sequence (LS). In a DS gait, hind-limb touchdown is followed by the touchdown of the contralateral forelimb (e.g., left hind
limb, right forelimb). In a LS gait, hind-limb touchdown is followed by that of the
ipsilateral forelimb (e.g., left hind limb, left forelimb). The DS and LS strides were
then subdivided into single foot (SF),lateral couplets (LC), and diagonal couplets
(DC) types depending on the precise timings of the footfalls. A SF gait occurs if the
footfalls of all the limbs are evenly spaced in time. A LC gait occurs when the
footfalls on the same side of the body are coupled in time, and a DC gait occurs if
Locomotor Development in Vervets / 13
there is coupling between the diagonally opposite fore- and hind limbs [Hildebrand, 1966, 19801. Finally, if diagonal limbs move approximately synchronously
the gait is classified as a trot.
The nonsymmetrical strides were divided into typical asymmetrical gaits such
as gallops and bounds, in which the touchdowns of the fore- and hind-limb pairs are
each coupled in time (e.g., left hind-limb touchdown followed by right hind-limb
touchdown), and strides are coupled with abnormal footfall patterns. These were
strides in which alternate hind- and forelimb footfalls occurred, but the hind-limb
touchdowns were not within the 40-60% phase interval of each other. For these we
only determined the limb movement sequence (i.e., DS or LS) while we classified
the galloping strides as transverse or rotary and the bounds as complete bounds or
half-bounds. A transverse gallop is characterized by both the fore- and hind limbs
having similar leading limbs (e.g., the left hind limb striking the ground before the
right, and the left forelimb striking the ground before the right) while in a rotary
gallop the leading limbs are different. In a complete bound, the hind limbs contact
the ground in unison as do the forelimbs. A half-bound is characterized by only the
hind limbs contacting the ground in unison [Hildebrand, 19771.
In order t o ascertain if the number or organization of limbs supporting the
animals during locomotion changed as they aged, the types of limb support the
animals used (e.g., unipodal, diagonal, lateral, etc.) were determined for each
stride. These data were then averaged for each animal for each date.
The timings of the stance and swing durations of the limbs were evaluated for
each animal. That is, within the strides, each complete locomotor cycle for a
particular limb was divided into its two component parts (stance and swing). We
then examined how each component changed relative t o differing durations of the
entire cycle both within a date and across time.
Finally, hind-limb joint angular displacement patterns were analyzed. This
was accomplished by digitizing the estimated locations of the metatarsophalangeal, ankle, knee, and hip joints and by using a computer to calculate the
corresponding angles. (The hip angle was measured to the horizontal.) This
procedure was not uniformly done across animals and dates. Depending on the
quality and types of strides exhibited by each animal on each date as well as
whether the movements were roughly perpendicular to the axis of the camera lens,
we chose to digitize between zero and three strides per animal per date. Each of the
raw joint displacement patterns from these strides was smoothed by using a digital
filtering procedure [Winter, 19791 with cutoff values of either 5 or 6 .
RESULTS
Gaits
Animal No. 1. Figure 1 illustrates some of the gaits used by this animal
during the study period. On day 1of life this animal generally toppled over when
placed on the platform. She did, however, exhibit one bounding-type stride in
which some forward movement was achieved. At 8 days, animal No. 1 showed a
clear ability to support her weight for brief periods during locomotor movements
(Fig. 1). She exhibited bounding strides and nonsymmetrical gaits which were all
LS. At 15 days of age, animal No. 1 exhibited some normal-looking LS strides as
well as bounds and half-bounds. Three of the LS strides were symmetrical. Of
these, two were classified as SF gaits and one as DC.
At 22 days of age, animal No. 1 exhibited normal-looking LS and bounding
strides. One of the LS strides was classifiable as a SF gait.
At 29 days of age, the same three types of basic gaits seen earlier were observed
14 I Vilensky and Gankiewicz
8 days
22 days
29 days
58 days
Fig. 1. Tracings from film of animal No. 1 locomoting at the noted ages. Gaits illustrated are: 8 days, LS, SF;
15 days, LS, SF; 22 days, LS, SF; 29 days, LS, SF; and 58 days, transverse gallop.
(LS, half-bounds, and bounds). Only one LS stride was symmetrical; it was of the
SF type. Note in Figure 1the tail posture during this week and also the elevated
position of the hindquarters reflecting increased joint extension (cf. below).
At 43 days of age, LS and bounding-type gaits were again observed. None of the
LS sequences were sufficiently symmetrical to be defined. At 58 days, LS and
galloping gaits were observed as well as two occasions of DS limb movements. The
five symmetrical LS gaits were all of the DC type. The galloping sequences were
transverse.
Animal No. 2. Figure 2 illustrates some of the gaits used by this animal
during the study period. On her day of birth this animal was able to sit but
immediately toppled over when she tried to move. At 7 days of age, the animal was
able to move across the platform but, with her ventral surface contacting the
platform for much of the “stride” (Fig. 2). At 13 days of age, this animal still had
some difficulty supporting her weight (Fig. 2) and many of her strides were
aberrant in appearance. Of two symmetrical strides, one was a DS, DC type and
one was a trot.
At 21 days of age, bounding-type gaits as well as fairly normal-looking
symmetrical gaits were observed. Within the latter, both LS and DS sequences
were observed with one classifiable as DS, DC. Note the elevated body position
compared to the earlier dates (Fig. 2). At 28 days of age, four symmetrical LS gaits
were recorded; three were DC and one was a SF. Additionally, a half-bound
sequence was observed.
At 40 days, this animal only exhibited highly coordinated trotting-type gaits.
On day 56, animal No. 2 exhibited trots and both rotary and transverse galloping
sequences.
Animal No. 3. This animal was uncooperative during the initial filmings and
did not exhibit any usable locomotor movements until 28 days of age. At this age
Locomotor Development in Vervets / 15
7 days
13 days
21 days
.-
28 days
56 days
Fig. 2. Tracings from film of animal No. 2 locomoting at the noted ages. Gaits illustrated are 7 days, not
classifiable; 13 days, DS, DC; 21 days, DS, DC; 28 days, LS,DC; and 56 days, rotary gallop.
she used normal-looking LS walking gaits. Two strides were symmetrical and
classifiable as LC. At 42 days, this animal also showed LS gaits; one sequence was
classifiable as DC and the other as a trot. At 64 days, both transverse and
rotary-type galloping sequences were observed. One symmetrical sequence was
classified LS, DC.
Support Patterns
Table I1 presents the limb support patterns as a percent for each of the animals
for each date. For animal No. 1,three-legged combinations (triplets) supplied the
greatest percentage of body support during all weeks, providing nearly 50%a t 8
days of age. The only clear trends evident over time for this animal are an increase
in lateral support across all but the last date and a decrease in quadrupedal and
three-legged support during the later dates compared to the earlier dates.
For animal No. 2, three-legged combinations supplied the bulk of support
during the early dates but were replaced by diagonal support at the final two ages.
Thus, there is evidence of an overall trend to increase diagonal support over time
with a corresponding trend toward decreased three-legged support. Table I1 also
indicates a general decrease in four-foot support.
Animal No. 3 showed very similar results to animal No. 2 with a decrease in
triplet and quadrupedal support and an increase in diagonal support. This animal
also had notable (30%)unipodal support at 64 days of age.
Temporal Characteristics
Figure 3A presents the hind-limb stance durations plotted against cycle
duration for all animals across all weeks. The appropriate regression lines are
depicted for each animal. The overall r value is .95 with a standard error of .04. The
r values for the individual monkeys range from .92 to .99 with standard errors
between .02 and .05. Clearly, the slopes are similar for all the animals. When the
hind-limb stance/cycle duration data are examined over time for animals Nos. 1
and 2 (there were insufficient data for animal No. 3) no consistent trends in slope,
intercepts, or correlation coefficients are apparent.
16 I Vilensky and Gankiewicz
TABLE 11. Limb Support Patterns (%) for Each Animal at Each Age*
Quad.
Triplet
Diag.
Lh/Rh
Lf/Rf
Lat.
Unipodal
None
l(8)
l(15)
l(22)
l(29)
l(43)
l(58)
9.3
12.1
9.9
1
3.3
2.5
48
39.9
46.4
39
29.3
33.8
30.7
24.4
17.2
10.5
13.7
44
6.7
13.6
9.1
24
21
0.8
0
1.4
4.7
6.5
14.3
14.3
5.3
8
12.4
19
16.7
4.8
0
0.6
0.4
0
1.7
0
No. 2 (13)
No. 2 (21)
No. 2 (28)
No. 2 (40)
No. 2 (56)
29.6
13.2
8.6
18
10
47.6
58.8
35.7
23.4
23.5
22
20
31.3
58.6
40.8
0.4
8
9.7
0
17
0
0
7.1
0
2.8
0.4
0
6.6
0
5.5
0
0
0.6
0
0
No. 3 (28)
No. 3 (42)
No. 3 (64)
16
0
0
82
41
9
0
23
28
0
0
20
Animal & age (days)
No.
No.
No.
No.
No.
No.
2
26
13
0
10
30
0
0
0
*Quad. = quadrupedal support; triplet = support by any three-legged combination; Diag. = support by either
pair of diagonal limbs; LMRh, Lf/Rf= support by either the left and right hind limbs or the left and right
forelimbs; Lat.= support by either pair of ipsilateral limbs; Unipodal= support by any single limb; None=
flight phase.
Figure 3B depicts the hind-limb swing durations plotted against cycle duration
for the three animals across all ages. For all the animals across all dates the r
value equals .68 with a standard error of .04. The regression equation for animal
No. 3 has a very high overall r value (.96) compared to the other two (.66 and .63),
but that may simply reflect the relatively small number of strides available for
this animal (cf. Table I). The regression lines for animals Nos. 1and 2 are similar
while that for animal No. 3 is divergent. There are no consistent changes in slopes,
intercepts, or correlation coefficients across the dates for animals Nos. 2 and 3.
Figure 4A depicts plots of forelimb stance duration vs. cycle duration for
animals Nos. 1 and 2 (insufficient data were available for animal No. 3).
Remarkably, the composite slope, intercept, standard error, and r values for these
data are identical to the hind-limb stance composite data (.77, - .05, .04, .95,
respectively). The values for the individual animals (across all dates) are similar to
the composite values. As with the hind-limb stance data there are no consistent
changes in coefficients across time.
Figure 4B depicts the forelimb swing durations for animals Nos. 1 and 2
plotted against cycle duration. As would be expected based on the stance data, the
composite slope, intercept, standard error, and r values for forelimb swing are
virtually identical to those for the hind limb. Interestingly, animal No. 1 has a
notably higher r value across all the weeks (.75) than animal No. 2 (.48).
There are
no consistent changes in any of the forelimb swing regression values across time
for either animal.
Joint Angular Displacement
Figure 5 depicts the hip displacement patterns for the individual monkeys for
each date. On each plot the shaded area represents the range of values observed for
an adult vervet over a wide assortment of speeds. Animal No. 1, beginning a t days
8 and 15 (Fig. 5A), shows somewhat normal-looking hip displacement patterns
with distinct periods of flexion and extension which are generally within the adult
Locomotor Development in Vervets / 17
I::
0.30
I
I
I
0.00
0.20
0.40
0.60
0.80
1 .oo
0.30
0.20
0.1 0
4
0.00
0.00
I
I
I
I
0.20
0.40
0.60
0.80
I
I
1.oo
CYCLE DURATION ( S )
Fig. 3. Plots of hind-limb cycle duration vs. hind-limb stance duration (A) and hind-limb swing durations (B)
for all the animals across all ages with regression lines for each animal. Circles and solid line= animal No. 1;
squares and dashed line= animal No. 2; triangles and dashed-dotted line = animal No. 3.
range. During subsequent weeks this animal's hip displacement curves are
generally normal in appearance and stay within the adult range (Fig. 5B,C).
Animal No. 2's hip displacement curves are grossly abnormal a t 13 days (Fig.
5A). Even at 21 days, one of the depicted curves (Fig. 5B) shows very little
flexion-extension oscillation while the other is more normal in appearance. At 28
days, one of the curves shows more flexion than is normal while the other is
somewhat typical. During the final two filmings, the hip curves for this animal
appear fairly typical (Fig. 5C).
A hip displacement curve is only presented for animal No. 3 at 42 days and is
mature looking.
Adult monkeys show a quadriphasic knee displacement pattern (Fig. 6). For
18 I Vilensky and Gankiewicz
0.80 i
0.70 -0.60
--
0.50 --
0.40-0.30-0.20
--
0.1 0
0.10--
0.00 t
0.00
I
0.20
I
0.60
0.40
0.80
1 10
0.30
. ...
0.20
..
s
.
0.1 0
0.00
0 0
I
I
I
0.20
0.40
,
I
0.60
0.80
1.oo
CYCLE DURATION (S)
Fig. 4. Plots of forelimb cycle duration vs. forelimb stance duration (A) and forelimb swing duration (B) for
animals Nos. 1 and 2 across all ages with regression lines for each animal. Symbols as in Figure 3.
animal No. 1 a t 8 and 15 days, the knee displacement curves (Fig. 6A) are not
consistent with each other and generally are not adultlike in appearance. Similarly, at 22 and 29 days, animal No. 1’sknee displacement patterns extend beyond
the adult range and do not show much initial flexion (Fig. 6B). The patterns for
this animal during the final two ages are more normal-looking in appearance
although they still tend to show increased flexion (Fig. 6C).
The curves for animal No. 2 at 13 days of age show grossly abnormal knee
displacement patterns characterized by little oscillation and extreme flexion (Fig.
6A). At 21 and 28 days of life, the knee curves for animal No. 2 (Fig. 6B) are less
abnormal but exhibit little initial flexure and show periods of flexion with values
outside the normal range. During the final two filmings, the knee displacement
Locomotor Development in Vewets / 19
130
120J A
110
100
90
80
70
60
50
40
30
20
10
-20
-30
-50
-40
110
100
n
90
w
80
70
cc
2z
60
50
40
30
20
10
-20
-30
-40t
-50
0
I
I
I
,
25
50
75
100
PERCENT OF CYCLE
Fig. 5. A Plot of some hip displacement curves for animal No. 1 at 8 days (open circles) and 15 days (filled
circles), and animal No. 2 at 13 days (filled squares). B Similar plot for animal No. 1 at 22 days (open circles)
and 29 days (closed circles) and animal No. 2 at 21 days (open squares) and 28 days (filled squares). C: Similar
plot for animal No. 1at 43 days (open circles) and 58 days (closed circles),animal No. 2 at 40 days (open squares)
and 56 days (closed squares), and animal No. 3 at 42 days (open triangles). The shaded area in each plot
represents the range of values exhibited by an adult animal.
20 I Vilensky and Gankiewicz
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
40-30--
Fig. 6. Plot of some knee displacement curves for the animals. A-C: Symbols and shaded area a s in Figure 5.
Locomotor Development in Vervets I 21
patterns for animal No. 2 tend to show more normal initial flexion periods, but the
secondary periods show greater than normal maximum flexion values.
The knee displacement curve for animal No. 3 a t 42 days shows a normallooking pattern except for greater maximum flexion.
The ankle joint’s displacement pattern in adult animals is similar to that of the
knee (Fig. 7). Animal No. 1 a t 8 days of age shows an ankle displacement pattern
which is highly abnormal and characterized by greater than normal flexion values
during much of the cycle. At 15 days of age, the displayed patterns for this animal
vary, with one being almost normal in appearance and range and the other two
showing periods of abnormal flexion and atypical patterns. At 21 days of age, the
curves for animal No. 1 reveal two fairly normal-looking patterns (although
exhibiting a high degree of flexion) while the pattern displayed for day 29 is
atypical (Fig. 7B). Two of the ankle displacement patterns shown for animal No. 1
on days 40 and 56 (Fig. 7C) are fairly typical while one (day 56) is not.
The ankle displacement patterns shown for animal No. 2 at 13 days are grossly
abnormal and display excessive flexion (Fig. 7A). For 21 and 28 days of age,
abnormal patterns are again evident for this animal although the flexion is not as
exaggerated (Fig. 7B). The patterns exhibited for the final two filmings are quite
variable with one of them (day 40) looking fairly normal.
For animal No. 3, the ankle displacement pattern exhibited in Figure 7C (42
days of age) is fairly normal in appearance although some flexion values exceed
the adult range.
DISCUSSION
Gait Patterns
By approximately 1 month of age all three infants exhibited relatively
normal-looking symmetrical gaits as well as gaits with nonsymmetrical footfall
patterns that were not characteristic gallops. Additionally, bounds and halfbounds were common at younger ages. One of the animals “walked as early as 8
days while another still had some difficulty supporting her weight at 13days. Most
of the symmetrical gaits recorded were LS. Toward the later dates there was a
trend toward gaits that emphasized diagonal limb coordination (DC or trots). True
galloping patterns were first observed in the animals at about 2 months of age.
Thus, vervets appear to achieve the mature complement of gaits by this age.
Hurov [19821 states that a 15-day-old vervet made locomotor attempts which
involved LS limb movements. The report also notes that an older animal (56-75
days) used both LS and DS gaits as well as an atypical gait. Hurov concludes that
vervets reach locomotor maturity by a t least 81 days. This is based on the fact that
one animal consistently used DS gaits by this age. However, since some adult
animals readily use LS as well as DS gaits [Vilensky et al., 19881this is not a valid
test for determining locomotor maturity. Nevertheless, the paucity of DS gaits
exhibited by the infants in our study does suggest that further modifications of
locomotor control mechanisms do occur after 2 months of age which result in a
higher frequency of DS gaits.
The only additional data on vervet locomotor development are provided by
Chalmers [19721. This report states that five captive vervets achieved “steady”
locomotion a t ages ranging from 31 to 49 days, with a mean of 36 days.
Detailed studies on the locomotor development of monkey species other than
vervets are not abundant except for the rhesus monkey. Unfortunately, much of
the data are inconsistent for this species. Specifically, for Macaca mulatta, studies
report walking as first appearing at 3 days [Castell & Sackett, 19731, 13 days
22 I Vilensky and Gankiewicz
1754
165
155
145
135
125
115
105
95
85
75
65
55 !
v,
w
LLI
[y.
?I
I
165
155
145
135
125
115
105
95
a5
I
I
I
I
25
50
I
55
,
175
165
155
145
135
125
115
105
95
85
q
_-
55
0
---+-____(
75
100
PERCENT OF CYCLE
Fig. 7. Plot of some ankle displacement curves for the animals. A-C: Symbols and shaded area as in Figure 5.
Locomotor Development in Vervets / 23
[Foley, 19341, 1-5 days [Tinklepaugh & Hartman, 19321, 12 days [Lashley &
Watson, 19131, 2 weeks [Hildebrand, 19671, and 3 weeks [Lawrence & Hopkins,
19761. Hines [19421 also provides detailed data on the development of gait in
rhesus monkeys, reporting that the earliest type of diagonal progression is
observed on days 1 and 2 of life. This variation in the noted ages of first walking
probably reflects differing methodologies (mother-raised vs. isolates), different
definitions of “walking,” and variability among rhesus neonates as in vervets (cf.
above; [Taylor et al., 19801). Additional data on the early development of gait in
other monkey species may be found in Rose [1977] for baboons, Negayama et al.
[1983] for Japanese macaques, Rollinson and Martin [19811 for talapoin monkeys
and mangabeys, and Chalmers [1972] for Syke’s and DeBrazza’s monkeys and for
mangabeys.
Support
The only clear trend evident from all of the support data is that three- and
four-foot support decreases across time. Clearly, this demonstrates a decreased
need for stability as the animals aged. Hurov [1982] reports that for a vervet infant
across days 56-75 of life, triplets supported the animal up to 74% of a locomotor
cycle and quadrupedal support was used for up to 22% of a cycle. Our data suggest
that vervets of that age do not normally require such stable support patterns.
Accordingly, Peters [ 19801 states that although l-week-old kittens use only threeand four-foot limb support, by 4 weeks of age gaits that maximize support on
diagonals are used. Our animals Nos. 2 and 3 also showed a trend toward
increasing diagonal support as they aged.
Temporal Parameters
In adult vervets, stance durations decrease linearly with cycle duration while
swing durations tend to remain constant for all cycle durations [cf. Vilensky et al.,
19881. For the infant vervets, both stance and swing durations were significantly
correlated with cycle duration although the latter was poorly correlated. This may
reflect some immaturity in the animals’ locomotor control apparatus; however,
since even more mature animals can show significant relationships between swing
and cycle durations [Vilensky et al., 19881our findings are not conclusive evidence
of locomotor control immaturity. Furthermore, the general similarity in regression
lines and coefficients among the animals and between the fore- and hind limbs
suggests that similar “hard-wired” locomotor control networks are operative in
these animals at or before birth. Such a conclusion has also been reached regarding
kittens as well as frogs, chickens, and rats [cf. Bradley & Smith, 19881. Accordingly, Taub [1976] suggests that the motor programs for locomotion are probably
already established within the primate CNS by the end of the second trimester of
pregnancy. Finally, because there were no consistent changes in the slopes,
intercepts, or correlation coefficients of the computed regression equations over
time, we attribute the variation we did observe to the aberrant nature of some of
the strides and low weekly sample sizes.
Angles
Animal No. 1exhibited much more normal-looking hip displacement curves at
the earlier ages than did animal No. 2. By 28 days, however, this animal was also
showing a fairly adultlike hip displacement pattern.
The knee displacement curves were grossly abnormal for animals Nos. 1and 2
during the earliest weeks. During these weeks and during the middle weeks
excessive flexion was often exhibited. By the final filmings, the patterns were
24 1 Vilensky and Gankiewicz
much more normal in appearance, although exaggerated flexion was still apparent.
The results for the ankle joint are basically similar to those for the knee except
that the patterns appear to be somewhat more abnormal at every age, and even
those from the final filming are typically not adultlike. Accordingly, human data
[Vilensky et al., 19871 and cat data [Peters, 19801 suggest that the ankle may be
the most labile hind-limb joint during locomotion, easily modifying its movement
as locomotor demands change.
In accord with our results, exaggerated flexion of the joints during early forms
of locomotion has been reported for rhesus macaques [Hines, 19421, cats [Peters,
19801, and humans [Wickstrom, 19771.
CONCLUSIONS
1. Based upon the temporal data reported here as well as a related study on
air-stepping, the basic neural mechanisms for intra- and interlimb coordination in
vervets are apparently operative at birth although gaits, support, and angular
displacement patterns are not fully normal for many weeks after birth.
2. Muscle weakness andlor immaturity in postural control mechanisms probably creates weight support problems for neonates which generally prevent the
intrinsic locomotor control circuitry from exhibiting mature-looking locomotion.
As muscle strength andlor postural systems mature, the extrinsic appearance of
locomotor movements becomes progressively more adultlike so t h a t by 6 weeks of
age there are often only minor differences in limb angular displacement patterns
compared to adults.
3. Very young infant vervets may use bounding gaits to achieve movement.
This probably represents a n attempt by the CNS to overcome muscle weakness.
Nevertheless, some animals are able to exhibit relatively normal-looking symmetrical gaits at a very young age (e.g., 8 days).
4. Subsequent to 6 weeks of age, vervets develop true galloping patterns, and
in most animals, a preference for DS symmetrical gaits.
ACKNOWLEDGMENTS
We are grateful to Ms. Penny Wilson for assistance with the animals, to Mr.
Gregory Duncan for assistance during filming, to Ms. Diana Andrews for typing
the manuscript, and to Ms. Roberta Shadle for help with the illustrations. We also
thank M.D. Rose and two anonymous reviewers for their constructive comments on
a n earlier version of this paper. Support for this study was provided by the Indiana
University School of Medicine.
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