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Eight-year study of social and ecological correlates of mortality among immature baboons of Mikumi National Park Tanzania.

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American Journal of Primatology 16:199-212 (1988)
Eight-Year Study of Social and Ecological Correlates of
Mortality Among Immature Baboons of
Mikumi National Park, Tanzania
'Department of Psychology, University of California, Riverside, 'Department of Psychology,
University of Washington, Seattle, 'Department of Applied Biology, Cambridge University,
Cambridge, England
The Cox Proportional Hazards Regression Model was used to examine
ecological and sociodemographic correlates of mortality among 164 immature yellow baboons born over a n 8-year period at Mikumi National Park,
Tanzania. Ecological correlates were derived from seasonal rainfall. Mortality was lowest for immatures born during the late rainy season, when it
was likely that nourishment for pregnant and lactating females was greatest. High mortality was associated with above-average rainfall early in the
immature's first complete rainy season. This association may result from
one or more of the following: exposure, increased ranging, or accelerated
vegetation growth, each of which is thought to increase hazards for weanlings. When births for the 8 years of the study were pooled, a birth peak
occurred during the early dry season. The larger number of births during a
birth peak should intensify competition among mothers and among immatures. Mortality was greatest for immature females born during the season
immediately following the birth peak, while lowest for immature females
born during the season immediately preceding the peak. Immature female
mortality was greatest when the troop size a t birth was large, and was most
severe for females born to low ranking mothers. The sociodemographic
results are consistent with Wasser's hypothesis that cooperative attacks of
female baboons upon other troop females suppress the others' reproduction
and the vitality of their infants, thereby improving the relative competitive
position of the attackers' own young.
Key words: Papio, sociodemographiccorrelates, rainbow patterns, social competition
The effect of ecological and sociodemographic pressures on survival is a central
concern of behavioral ecology [Dittus, 1979; Dunbar, 1980; Malik et al., 1984; Altmann et al., 1985; Hrdy, 19861. This paper is a n investigation of such pressures on
the mortality of immature yellow baboons (Pupio cynocephalus) living in Mikumi
National Park, Tanzania. Mikumi baboons have been studied almost continuously
since 1975 in a habitat described by Norton et al. [1987]. These long-term observaReceived for publication February 9,1988; revision accepted July 22, 1988.
Address reprint requests to R.J. Rhine, Department of Psychology, University of California, Riverside,
CA 92521.
0 1988 Alan R. Liss, Ine.
200 I Rhine et al.
tions yielded a large sample of individually identified immatures needed for reliable
demographic analyses.
Among the many possible ecological influences upon mortality, rainfall in East
Africa is a prime candidate for study because of its seasonal effect upon important
aspects of the animals’ environment. Rainfall might influence the health of pregnant
females, lactating mothers, or immatures either through sickness or through food
production, depending upon the amount and timing of rainfall vis a vis immature
development [e.g., Dunbar, 1980; Altmann et al., 19851. Also, the amount and timing
of rainfall could create hazardous conditions by their impact upon ranging patterns
and upon the growth of cover in which immatures could become lost or predators
could hide. For example, Rhine et al. [1980, p. 4021 described the plight of a n infant,
Magego, the day after her mother was killed by a predator: “The troop was out of
sight over 500 m away, and Magego was wandering aimlessly by herself in heavy
The possible impact of social conditions upon the survival of immatures is
suggested by the work of Wasser, who found that competition among female baboons
at Mikumi was tied to important reproductive events [Wasser, 1983; Wasser &
Starling, 1986, 19881. In particular, cooperative female-female aggression (attack
coalitions) increased with estrous and pregnancy synchrony. Receipt of attack was
in part dependent upon the female’s reproductive state and in part upon the time a
female gave birth relative to births of other young in her offspring’s cohort: The first
few females to give birth received the highest rates of attack. The effect of such
stressful female-female aggression upon pregnancy or mothering may reduce the
number and viability of immature competitors the attackers’ own offspring will face
during its development.
Competition is expected to increase with the number of immature competitors
in the infant’s birth cohort. In this study, advantage is taken of troop-size differences
and a n &year seasonal birth peak to examine the possible association of birth-cohort
size with mortality. An original study troop of 106-130 individuals split into three
parts in late 1978 and early 1979. Recent troop sizes averaged 65, 65, and 20. The
average birth-cohort size was considerably larger pre-split than post-split. Similarly,
average birth-cohort size was largest in the season of a birth peak. Mikumi baboons
gave birth throughout the year and did not have a yearly birth season such as that
described for Japanese macaques [Lancaster & Lee, 19651; however, when all births
were pooled over 8 years, more births occurred in one season than in others. Across
those 8 years, the time of birth can be related to the birth peak, and the season of
a n immature’s birth in relation to the birth peak can be used in a n examination of
the possible association between survival and the immature’s time of birth within
its cohort.
Although social and ecological conditions are expected to influence immature
mortality, their effects are not necessarily separate or additive. Given the complexities of baboon social and ecological life, a network of interactions is more likely.
These interactions should cast light upon how and why patterns of reproduction,
social stress, and immature survivorship may be related in complex social mammals.
Mortality patterns were examined from birth through the fourth year of life for
164 immatures born during the 8 years from 1975 to 1982. Four years was chosen as
the cut-off because almost no mortality occurred between age four and adulthood. In
some cases, separate analyses were also conducted for age cut-offs of 1 or 2 years to
determine if ecological and social pressures affecting mortality of younger individu-
Mortality of Immature Baboons / 201
z 2
Fig. 1. Mean rainfall in inches (+SEM) for November, June, and the six rainy-season months from
December to May for the eight-year period from 1975 to 1982. Mean rainfall was negligible during dryseason months not shown [Norton et al., 19871.
als differ from those affecting older ones. Two years was chosen as a cut-off because
all individuals were fully weaned by this time [Rhine et al., 19851. Mortality was
examined for 0-1-year-olds because 53% of immature deaths occurred during that
age period.
The data are from part of the long-term records kept on Mikumi baboons since
1975. From that year, immatures of known date of birth and all adult animals were
individually identified, as were many of the older immatures. From November 1976,
all troop members were individually identified. They were well habituated for closeup observation even in conditions of heavy cover [Rhine et al., 1979;Fig. 11.
The censusing method used to obtain demographic information was described
by Rhine et al. [1984]. Briefly, in the morning, at and near the sleeping site, an
observer walked through the troop finding and identifying animals. Animals not
found in the morning were sought throughout the day with the help of other
observers, including those obtaining censuses in the process of recording progressions of moving troops [e.g., Rhine & Westlund, 19811. A date of birth was usually
established to within plus or minus 3 days, and the possible error was always less
than a month.
Immatures that disappeared before the end of their fourth year of life were
presumed to be dead. From the time when all animals were individually identified
and well habituated, no unknown female or immature male immigrated into any of
the Mikumi study troops. All other troops encountered were checked whenever
possible to determine if any study animals had transferred into them. Study-troop
females and males of 4 years or younger have never been observed in other troops.
In contrast, numerous subadult and adult males from the study troops were observed
in others.
The age at death was calculated as the number of days from the first observation
of the animal after its birth until the last day the animal was observed in the troop.
In some cases, there was a day or more gap in observation days before the animal
202 I Rhine et al.
was first observed, or immediately after the animal was last observed. Under these
circumstances, the number of missed observation days in each gap was divided in
half and added onto the total number of days the animal was known to be alive in
the troop.
Maternal dominance, which was quite stable among Amboseli yellow baboons
[Hausfater et al., 19821, was estimated from the outcomes of one-on-one supplant
data collected in 1975 and 1976 [Rasmussen, 19801,1977 [Klein, 19831, and 1979 and
1982 by Wasser. As occurred also in the Amboseli population [Samuels et al., 19871,
the dominance ranks of some adult females changed over time. In such cases, the
maternal rank associated with each offspring was that determined from data that
were collected closest in time to the offspring’s birth. Dominance ranks were assigned by quartile within troops to allow comparisons of the effects of dominance
rank in the pre-split and post-split conditions, irrespective of troop sizes.
Cox Model
The Cox Proportional Hazards Regression Model [Cox, 1972; Cox & Oakes,
19841 was used for survival analyses. This time-dependent, log-linear model is
employed in epidemiological and medical research to predict the effect of a condition
on a subject’s time to death, taking into account unique aspects of the subject such
as age, sex, and socioeconomic status. The model is also suitable for survivorship
analyses of natural populations where the conditions are ecologic and demographic.
It utilizes data on all subjects by censoring data from individuals who lived beyond
a specified cut-off time, such as survival to 4 years.
In the Cox analysis, a regression coefficient reflects the degree to which a
covariate increases the probability of mortality before a given time. Specifically, the
9-1 I
Fig. 2. Frequency of births by month of birth: December to February (early wet season), March to May
(late wet season), June to August (early dry season), and September to November (late dry season).
Mortality of Immature Baboons / 203
9-1 I
Fig, 3. Percent mortality before two or four years of age by months of birth December to February (early
wet season), March to May (late wet season), June to August (early dry season), and September to
November (late dry season).
regression coefficient is the log difference in the hazard function per unit change in
the covariate. The regression coefficient divided by its standard error approximates
a z of the normal curve from which the statistical significance of the covariate can
be determined. Throughout this paper, statistical significance is at the .05 level or
Logistic or log-linear regression differs from standard regression by determining
the change in probability of an event, rather than the amount of variance accounted
for. This probability is reflected by taking e to the exponent of the fi coefficient,
expressed as exp@).This expression is interpreted as the multiplicative change in
the risk of death caused by one unit increase in a given covariate. In other words,
one unit change in covariate i multiplies the probability of death before time x by e
to the power of pi. For example, for the number of females in the individual’s birth
cohort, an exp(fi)of 1.4 means that an increase of one female in the birth cohort
multiplies the individual’s risk of death by 1.4.
The covariates entered into one or more of the Cox analyses described below are
as follows: rainfall, season of birth, sex of the infant, number of males and females
in the infant’s birth cohort, maternal dominance rank, and overall troop size at birth
(pre-split-larger; post-split-smaller).
Ecological Conditions by Season
Rainfall. Mikumi has a single rainy season that usually begins in late
November and extends into early June. Details of rainfall for 20 years are provided
by Norton et al. [1987, Figs. 4 and 51. Figure 1 shows the mean monthly rainfall in
204 I Rhine et al.
inches (+SEM) for November through June from 1975 to 1982, the period of the
present study; negligible rainfall occurred in the dry-season months (not shown).
Based on these data, which are consistent with the 20-year averages, the year was
divided into four 3-month seasons as follows: early wet (December-February), late
wet (March-May), early dry (June-August), and late dry (September-November).
The ecological year was taken to begin with the early wet season. An infant’s birthcohort size was defined as the number of infants born in its ecological year of birth
Birth peak. When the birth data were pooled across all eight years of this
study, a birth peak occurred during the early dry season (Fig. 2). The overall pattern
of birth frequencies across the four seasons differs significantly from chance
expectancy (x2 = 7.82, df = 3, P < .05), and the largest contribution to this
difference is from the peak frequency, which is the only one of the four seasonal
frequencies differing significantly from chance (z = 2.71, P < .01).
Mortality by season of birth. It might be expected that immature mortality
would be lowest for infants born during the birth peak. In fact, mortality was lowest
for individuals born in the late wet season, the season just prior to the birth peak,
and was highest for those born in the late dry season, the season immediately
following the birth peak (Fig. 3). The difference in immature mortality between
these two seasons is significant in the Cox analysis for mortality before 1year (z =
2.52, P < .025, exp(0) = 5.0), 2 years (z = 2.47, P < .025, exp(0) = 3.551, and 4
years of age (z = 2.23, P < .05, exp(0) = 2.23). Thus, in comparison with immatures
born in the late wet season, those born in the late dry season have a 5.0-fold
increase in their probability of mortality before age 1, a 3.55-fold increase before
age 2, and a 2.23-fold increase before age 4. Mortality rates of immatures born in
the early wet and dry seasons were intermediate and not significantly different
from any other season.
Food availability. Survivorship is the inverse of the mortality pattern by birth
season shown in Figure 3. Survivorship to 4 years corresponds in Figure 4 to two
broad measures of food usage-the number of different foods eaten and the number
of parts eaten (flower, fruit, pod, etc.) from 185 food species [Norton et al., 19871.
The number of foods and parts eaten were highest during the late wet season, the
season of birth for which immature survival was also highest. Survival was lowest
among immatures born in the late dry season when the number of foods and parts
eaten were lowest. Survival to 2 years also closely parallels the feeding curves.
Rainfall and mortality patterns. Cumulative rainfall during the ecological year
of the infant’s birth was not significantly related to immature mortality, neither
was rainfall in the immature’s third ecological year (the 13th through the 24th
months after the end of the ecological year of birth). On the other hand, high
cumulative rainfall during the immature’s second ecological year, the time when
weaning was well along or completed (Rhine et al., 1985), was positively associated
with mortality before one (z = 3.08, P < .0005, exp(0) = 1.05), two (z = 2.93, P <
.005, exp(P) = 1.09, and 4 (z = 3.86, P < .0005, exp(0) = 1.05) years of age.
In a n attempt to determine more precisely the source of these statistically
significant findings, the Cox model was applied to rainfall by month during the
first full rainy season after the immature’s birth. Except for a brief period in the
latter part of November, this is the rainy season of the second ecological year. Only
the amount of rainfall in December, the first full month of the rainy season, was
significantly associated with immature mortality (z = 2.33, P < .025, exp(0) = 1.2
for all of 1, 2, and 4 years of life). The association between rainfall and mortality is
highly significant if December rainfall is entered into the model alone (z = 3.29, P
< .0005, exp(0) = 1.24 for mortality before 1 year; z = 3.51, P < .0005, exp(/3) =
Mortality of Immature Baboons / 205
TABLE I. Regression Coefficients (R.C.), z's of the Normal Curve, and exp(P) Generated
From the Cox Proportional Hazards Regression Model for the Effect of December Rainfall
on Mortality as a Function of Immatures' Season of Birth
Mortalitv before
1 Yr
Late wet season
Early dry season
Latedry season
2 Yr
exp(0) R.C.
4 Yr
exp(0) R.C.
* P < .05.
**P < .01.
***P < ,001.
1.22 for 2 years; and z = 3.98, P < .0001, exp(/3) = 1.21 for 4 years). In addition, as
shown in Table I, December rainfall was significantly associated with mortality of
immatures born during all seasons except the late wet season, and the greatest
effect was on immatures born during the late dry season.
Social Effects and Sex Differences
The Cox model was used to explore associations between the immature's probability of mortality and its sex, the number of males and females in its birth cohort,
and whether the immature was born before the troop split, when the troop was
relatively large, or after the split, when the resulting three troops were smaller.
Although none of these variables yielded significant main effects, some key interactions were statistically significant.
Interaction terms were calculated by multiplying the independent variables
together for continuous variables. When one of the independent variables was
discontinuous or categorical (e.g., sex of immature) interaction terms were calculated
by creating a new variable for each category of the discontinuous variable (e.g., the
interaction terms for sex and birth-cohort size become birth-cohort size on males and
birth-cohort size on females). Each of these new variables was set equal to the value
of the continuous independent variable for those cases which contain that category;
otherwise its value was set to zero. For example, for male immatures, birth-cohort
size on males was set equal to birth-cohort size, and birth-cohort size on females was
set equal to zero. The opposite would hold for female immatures. These variables
were then entered into the regression together. Applying this method via the Cox
model, the effects of birth-cohort size on the mortality of each sex were examined.
The mortality of a n immature male was unaffected by the number of males or
females in its birth cohort; in contrast, female mortality before the fourth birthday
significantly increased as the number of females in the birth-cohort increased (z =
2.29, P < .025, exp(/3) = 1.32), and significantly decreased as the number of males in
the cohort increased (z = 3.29, P < .001, exp(/3) = .72). Only the latter association
was significant for mortality before 2 years of age (z = 2.18, P < .05, exp(/3) = 233).
When the same comparisons were made using the Cox model after dividing the data
into pre- and post-split conditions, the above effects of birth-cohort size on female
mortality remained significant only for animals born in the pre-split condition, when
troop size was larger. These results for mortality before 4 years are shown in Table
11. For mortality before 2 years, female mortality again decreased as the number of
males in the cohort increased (z = 2.36, P < .025 exp(/3) = 2301, but the effect of
female-cohort size on female mortality was no longer significant.
206 I Rhine et al.
TABLE 11. Regression Coefficients (R.C.), z's of the Normal Curve, and exp(0) Generated
From the Cox Proportional Hazards Regression Model for the Effects of Male and Female
Cohort Size on Sex-Specific Mortality Before 4 Years of Age, Pre- and Post-Split
Female immatures
No. females in cohort
No. males in cohort
Male immatures
No. males i n cohort
No. females i n cohort
- .07
- .04
- .05
"The regression coefficients for sex and pre- versus post-split as main effects are not significant.
*P < .05.
**P < .001.
Ecological and Social Factors Combined
The above association of social variables with mortality of immature females
appears to be stronger for mortality before 4 years than before 2 years, This trend
became more apparent when ecological and social variables were combined into a
single Cox analysis of mortality before 4 years. In this combined 4-year analysis,
December rainfall was no longer significantly associated with immature mortality;
however, it was significantly associated with mortality before 2 years (z = 2.40, p <
.025, exp(0) = 1.24). In contrast, the birth-cohort size variables previously associated
with immature mortality (Table 11)remained statistically significant for mortality
to 4 years (Table 110,but were not significant for mortality to 2 years. These results
suggest that the effects of rainfall have their greatest impact on immature mortality
up to the end of weaning, whereas social factors have their greatest impact after
TABLE 111. Regression Coefficients (R.C.), z's of the
Normal Curve, and exp@)Generated From the Cox
Proportional Hazards Regression Model for Female
Immatures Re-Split, Showing the Effect of Maternal
Rank by Season of Birth and of Birth-Cohort Size
Upon Mortality Before 4 Years of Age
Maternal rankb
Early wet season
Late wet season
Early dry season
Late dry season
No. females in cohort
No. males in cohort
- .24
- .64
3.46"* *
3.94 * * *
"The birth-season variables for immature females post-split and for
immature males pre- and post-split were included in the combined
regression analysis (they were not significant).
"To account for different troop sizes, highest rank is membership in
the first quartile of an animal's own troop and lowest rank is
membership in the fourth.
* P < .05.
+ a p < ,01,
< ,001.
Mortality of Immature Baboons / 207
The association of maternal rank with immature mortality before 4 years was
also examined in the combined Cox model. Maternal rank was significantly associated with immature mortality, not as a main effect, but as an interaction with the
pre-split season of birth. As shown in Table 111, maternal rank was associated with
the mortality of immature females born in the early and late dry seasons. Females
born to low-ranking mothers during the season of the birth peak (early dry), and
especially those born in the following season (late dry), had a significantly higher
probability of mortality than did females born at these times to high-ranking
Ecological Effects
Rainfall and hazards. High mortality was associated with high December rain
during the immature’s second ecological year. Brief descriptions will be given of
three speculative hypotheses that, individually or in combination, might account
for this association of rainfall with mortality. Adequate empirical support of these
hypotheses is not available.
The first of these hypotheses is that high rainfall early in the rainy season
produces scattered rainpools in areas that lack a permanent water supply, making
possible increased ranging, which in turn increases the probability of young baboons
becoming lost. To test this hypothesis, ranging data by month over several years
are needed to determine if mortality and ranging early in the ecological year are
greater in years of heavy December rain than in years of light December rain.
Immature baboons were seldom known to be sick before they died. With the
exception of injured animals, the large majority of mortalities were recorded for
individuals present and healthy-appearing one day and gone the next. These
observations are consistent with the first hypothesis, but not with the second.
The second hypothesis is that high rainfall early in the second ecological year
increases the health hazard to immatures through direct exposure [e.g., Dunbar,
19801. Confirming data would show that sickness and mortality are greatest during
second ecological years having heavy rain early in the rainy season. Obtaining
reliable data on the health of free-ranging immatures without seriously disturbing
natural troop behavior is a considerable challenge. If exposure were an important
cause of immature mortality, high rainfall during months of the rainy season other
than December might also be expected to show some association with immature
mortality, which was not the case.
The third speculative hypothesis is that high rainfall early in the rainy season
accelerates early grass and herbaceous growth, increasing the probability of
weanlings becoming lost or being taken by predators, and possibly making food
harder to find. To test this hypothesis it is necessary to show that grass growth is
accelerated during years of heavy December rain and that increases in the number
of lost or preyed-upon immatures accompany such accelerated growth. This is a
formidable task. Since data and analyses confirming or disconfirming the three
hypotheses are not yet adequately available, and since the three hypotheses are
not mutually exclusive, future analyses may show that conditions specified by all
three in some degree contribute t o immature mortality.
Rainfall and nourishment. It would be surprising if the amount and quality of
nutrition available for pregnant or newly lactating females were unrelated to
immature mortality. During the dry season at Mikumi, as at Amboseli, there may
be a “vitamin or protein deficit in baboons due to reduced availability of fresh
green vegetation and perhaps insects as well” [Hausfater, 1976, p. 561. At Mikumi
where proteinaceous insects are an important part of the diet [Rhine et al., 19861,
208 I Rhine et al.
p 200
50 3
3- 5
Fig, 4. Percent survivorship to four years plotted in relation to the mean number of plant parts and foods
eaten per month during three-month seasonal blocks. Food data are from Tables VIII and IX of Norton et
al. 119871.
insect eating is at its height during the late wet season and is lowest during the
late dry season. During the dry season, the browned range is swept by fires, and
“prior to the start of the rains a large proportion of the study animals’ range is
barren of grass and herbaceous cover and most trees are leafless [see Rhine, et al.,
1980, fig. la]” [Norton et al., 19871. Grasses and herbs are among the baboons’
most important food staples [Norton et al., 1987 Table V]. In contrast to the dry
season, the range is lush and green by the late wet season during which time, as
Mortality of Immature Baboons / 209
Figure 4 indicates, the number of plant-food species and parts available and eaten
were greatest.
MacArthur and Pianka [1966] predicted that such dietary diversity would be
associated with decreased quality and abundance of food. It is difficult to see how
their model can explain the close seasonal correspondence for Mikumi baboons
between foods eaten and mortality (Fig. 4)unless one makes the dubious argument
that food quality and abundance are inversely related to mortality. Given the
difference in the character of the vegetation from the dry to the wet season [Norton
et al., 19871, it is probable that available food increases in the late wet season, not
only in diversity, but also in abundance and quality. MacArthur and Pianka’s
predicted relationship makes good sense for animals normally with a limited diet
which can be expanded into less salubrious foods during adverse times when their
preferred foods are in short supply. But, baboons are eclectic feeders [Norton et al.,
19871. Thus, favorable feeding for baboons might occur through selective feeding
on only a limited subset of quality foods from among those available, through lesser
use of each of many available foods which together provide a quality diet, or more
likely, a combination of both [Norton et al., 19871.
If the number of food species and parts used per season is associated with the
level of nourishment received, which seems likely for Mikumi baboons, then
nutrition was highest for mothers of immatures born during the late wet season
when survival was also highest, and lowest for mothers of those born in the late
dry season when survival was lowest. In the intermediate cases, those born in the
remaining two seasons, the early dry and early wet, nutritional conditions were
probably less salutary than in the late wet season but less deleterious than in the
late dry. Feeding conditions taken together with the possible hazards of rainfall,
discussed above, suggest that rainfall patterns produce both benefits and costs,
depending upon their timing vis a vis birth and weaning.
Sociodemographic Effects
In addition to ecological effects, the mortality pattern of Figure 3 may have
resulted in part from social pressures associated with the birth peak. Such social
pressures were documented by Wasser and Starling [1986,1988],who found rates of
aggression among adult females to be highest at the onset of the birth peak.
Individuals born during the birth peak in the early dry season had the most
comparably-aged competitors also born during that time. Individuals born earlier,
during the late wet season when mortality was lowest, did not have this same
competitive disadvantage. On the contrary, they had an age advantage in competition with younger cohort mates born during the birth peak a few months later.
Individuals born in the season following the birth peak, the late dry season when
mortality was highest, seem likely to have had the greatest risk because they had
an age disadvantage in competition with cohort mates. In addition, during this
period, quality nourishment for pregnant or lactating mothers probably was least
The sociodemographicresults further suggest that competition was particularly
deleterious for immature females born to mothers of relatively low dominance
status. The disadvantage of low dominance was most evident for immatures born
during the early and late dry seasons (Table III) when birth-cohort competition
presumably was highest. Mortality was most marked in females born before the
troop split (Table III), when the large troop size is believed to have intensified social
competition. Indeed, exacerbation of competitive conditions resulting from large
group size may have contributed to the splitting of the troop. Why pre-split competition did not also adversely affect male immature mortality is a mystery remaining
to be solved Icf., Dittus, 19791.
210 I Rhine et al.
Female Competition
The finding of increased mortality with increased cohort size relates to a hypothesis proposed by Wasser [1983] to explain attack coalitions of female baboons. He
suggested that attack coalitions benefit an attacker’s own offspring at a cost to
competitors. Attack coalitions against females infrequently involved males, and
were tied to the number of females simultaneously in estrus or in late pregnancy
[Wasser, 1983; Wasser & Starling, 1986, 19881, which are related to the size of the
forthcoming birth cohort. Moreover, females with new infants often received high
levels of aggression from the remaining pregnant females who were observed on two
occasions to commit infanticide [Wasser & Starling, 19881. Wasser hypothesized that
females could enhance the survivorship of their own infants by suppressing the
reproduction of others, as well as by diminishing the vitality of female infants
already present in the troop. Female infants are potential long-term competitors of
cohort females who, unlike natal males, do not usually disperse.
Complex Relationships and Long-term Studies
The analyses in this paper show that many variables can contribute to a
population’s mortality patterns, but not necessarily as main effects. For example,
the effect of a mother’s dominance rank on the mortality of her offspring was only
significant for immature females born during or just after the birth peak, which are
thought to be competitively disadvantageous times. The large number of variables
and the complexity of the analyses necessary to ascertain interactive effects illustrate the value of long-term studies in providing the large sample sizes necessary to
detect such effects in nature.
1. Reproductive success of baboons is the outcome of an intricate web of ecological, social, and demographic conditions. Interactions among the ecological and
sociodemographic variables of this study were significantly associated with immature mortality.
2. Rainfall patterns, depending upon their timing in relation to birth and
weaning, were both costly and beneficial to immatures. Immatures born during the
late wet season, when nutrition for their recently pregnant and newly lactating
mothers was probably at its height, received the benefits of rainfall. The costs arose
from increased mortality of weaning or recently-weaned young as a function of
above-average rainfall early in their first full wet season. It was speculated that
heavy rainfall early in the rainy season could adversely affect health or increase
ranging and early grass growth, which in turn could increase the weanlings’ chances
of becoming lost or preyed upon.
3. In addition t o rainfall patterns, analyses of sociodemographic variables suggest that immature mortality was a negative outcome of female-femalecompetition.
The mortality of female immatures increased as a function of troop size at birth and
of the number of females in the birth cohort. There was a greater chance of mortality
for female infants of low ranking mothers if the infants were born during or
especially just after a birth peak, when competition was thought to be high.
Research at Mikumi National Park evolved from planning started in 1971 at
the Gombe Stream Research Centre whose Director, J. Goodall, introduced us to
Mikumi and gave invaluable advice and support. The research described herein is
part of a long-term effort to which many individuals have contributed [Rhine, 19861,
including J. Rogers who assisted in the initial rainfall investigations. Over the
years, Mikumi research has been supported by grants to the senior author from the
Mortality of Immature Baboons / 211
National Institute of Mental Health, the National Science Foundation, the Harry
Frank Guggenheim Foundation, the Leakey Foundation, Biomedical Science Support grants to the University of California (Riverside), Intramural and Intercampus
Opportunity grants (Riverside campus), and the Center for Social and Behavioral
Sciences Research (Riverside campus). Grants to Wasser from the Harry Frank
Guggenheim Foundation have helped to support Mikumi operations. We thank M.
Kahn, A. Raftery, and G. Sackett of the University of Washington for their advice
in the use and interpretation of the Cox model. For the opportunity to work a t
Mikumi, we are grateful to the people and leaders of Tanzania, particularly to the
late Hon. D. Bryceson and to the leadership of the Serengeti Wildlife Research
Institute, the Tanzanian National Scientific Research Council, and the Tanzanian
National Parks. The manuscript benefitted from critical and editorial comments
from M. Costello, R. Cox, D. Rhine, G. Sackett, and L. Wasser.
In this paper, data from part of one subset of a larger, long-term data base were
made available to Samuel K. Wasser for tests of hypotheses about female competition. He is primarily responsible for the following contained herein: Cox analyses,
associated definitions, and interpretations of the effects of December rain and female
competition on immature mortality. Additional and more complete analyses will be
forthcoming from Ramon J. Rhine and Guy W. Norton.
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