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Afr. J. Ecol. 1999, Volume 37, pages 375–385
Seed dispersal of Acacia erioloba by African bush elephants
in Hwange National Park, Zimbabwe
JOSEPH P. DUDLEY
Hwange National Park (Main Camp), Private Bag DT 5776, Dete, Zimbabwe
Abstract
Approximately two-thirds (64%) of all dry season samples of elephant dung analysed
during a 3-year study in the Main Camp subregion of Hwange National Park,
Zimbabwe, contained seed and/or pod materials from Acacia erioloba. Most seeds
were recovered intact and actively germinating seeds were not uncommon. Very little
pod mass relative to seed mass was recovered in most instances, with pod fragments
recorded from only 56% of all exhaustively sampled elephant dung piles containing
A. erioloba fruit materials. Nonetheless, large pod fragments and even entire intact
pods were recovered occasionally from elephant dung. Seeds and pods of A. erioloba
may comprise 12% or more of total wet-weight dung mass; individual dung piles
were found which contained > 5000 A. erioloba seeds. Birds and smaller mammals
search out and consume A. erioloba seeds present within elephant dung piles. The
findings of this study indicate that potential digestibility of A. erioloba seeds for
bush elephants (Loxodonta africana africana) may be much higher than expected
from previous studies. In controlled feeding trials with captive bush elephants (age
11–15 years old) maintained on predominantly free-range dry season diets, the
estimated efficiency of digestion for A. erioloba seeds consumed in pods was 81% to
96%, with a gut-transit time of between 24.5 and 36.0 h. On the basis of throughput
times determined in experimental feeding trials, potential elephant-dispersal distances
of 20–50 km are predicted for A. erioloba in the Kalahari Sands landscapes of
southern central Africa.
Key words: Acacia erioloba, bush elephant, digestibility, seed-dispersal
Re
´sume
´
Pre`s des deux-tiers (64%) de tous les crottins d’e´le´phants re´colte´s en saison se`che et
analyse´s dans le cours d’une e´tude de trois ans dans la sous-re´gion du Camp Principal
du Parc National de Hwange, au Zimbabwe, contenaient des graines ou des parties
de gousses d’Acacia erioloba. La plupart des graines se retrouvaient intactes et celles
qui germaient activement n’e´taient pas rares. La plupart du temps, on retrouvait
peu de masse de gousses par rapport a` la masse des graines puisqu’on n’a rapporte´
la pre´sence de fragments de gousses que dans 56% du total des crottins analyse´sen
profondeur qui contenaient de la matie`re provenant d’A. erioloba. On a cependant
retrouve´ occasionnellement de grands fragments, voire des gousses entie`res dans les
Present address: Department of Biology and Wildlife, Institute of Arctic Biology, University of Alaska-
Fairbanks 99775-7000, U.S.A. E-mail: ftjpd@uaf.edu
1999 East African Wild Life Society.
376 Joseph P. Dudley
crottins d’e
´le
´phants. Les semences et les gousses d’A.erioloba peuvent composer 12%
et me
ˆme plus du poids de la matie
`re se
`che totale des crottins; on a retrouve
´des
crottins qui contenaient plus de 5000 graines d’A. erioloba. Des oiseaux et des petits
mammife
`res fouillent et mangent les graines d’A. erioloba qui sont dans les crottins
d’e
´le
´phants. Les re
´sultats de cette e
´tude montrent que la digestibilite
´des graines
d’A. erioloba peut e
ˆtre beaucoup plus e
´leve
´e chez les e
´le
´phants de brousse (Loxodonta
africana africana) que ne le laissaient pre
´voir de pre
´ce
´dentes recherches. Lors d’essais
de nourrissages contro
ˆle
´s d’e
´le
´phants captifs (a
ˆge
´sde11a
`15 ans) maintenus en
grande partie au re
´gime libre de saison se
`che, on a estime
´l’efficacite
´de la digestion
des graines d’A. erioloba mange
´es par gousses entre 81% et 96%, le transit digestif
durant de 24.5 a
`36.0 h. Sur base du temps de transit de
´termine
´lors des essais de
nourrissage expe
´rimental, on pre
´voit que les distances de dispersion potentielles d’A.
erioloba par les e
´le
´phants dans les zones a
`Sables du Kalahari, au sud de l’Afrique
centrale, vont de 20 a
`50 km.
Introduction
The camelthorn Acacia erioloba Meyer (=A. giraffae Burch.) is a characteristic and
dominant tree species in arid and semiarid Kalahari Sands landscapes of southern
central Africa. The English vernacular name is biologically inappropriate, and derived
from the Dutch-Afrikaans vernacular for ‘giraffe-thorn’ (‘kameeldoring’: Coates
Palgrave, 1983). The growth habit of A. erioloba varies from that of a bushy shrub
(2 m height) to a tall tree (up to 16 m height), with mature specimens typically
occurring in the tree form. Large specimens have bole circumferences [3.0 m,
supporting a spreading crown of up to 20–30 m diameter (this study). Repeated
browsing by elephant and giraffe establishes a foliage browse-line at a height of
5–7 m above ground level, and maintains the classic umbrella-shaped crown exhibited
by most large specimens in Hwange National Park, Zimababwe. Leaves are shed
only at the time of replacement, and the shade which these trees provide may be a
critical, and perhaps even limiting, environmental resource for elephants in southern
African desert regions (Coates Palgrave, 1983). The outer bark of large trees is
typically dark grey to blackish-brown in colour and deeply furrowed (Barnes, Fagg
& Milton, 1997); in elephant habitats, large trees often exhibit flattened reddish or
yellowish bark patches on the trunk and principal branches due to surface abrasion
and scarification caused by elephants jolting trees to shake down pods (this study).
The dark red-brown heartwood is extremely hard, dense and resistant to termites
and other wood-boring insects. The smaller branches are highly spinescent and
invested with paired, basally fused, robust and exceedingly sharp conical thorns
ranging between 0.5 and 6.0 cm in length. Leaves are complex, with 2–5 pinnae and
8–18 pairs of microphyllate leaflets per pinnae. The flowers are globular (1–2 cm
diameter), pendant and bright yellow in colour (Coates Palgrave, 1983; Barnes et al.,
1997)
The indehiscent pods are flattened, broad, lanceolate (5–13 cm length×2–5 cm
width), thick (0.5–1.0 cm) and straight or crescent-shaped. Pods have a silver-grey
velvety surface covering a robust, woody bi-layered pod capsule. Seeds are brown,
thick and lenticular or elliptic (8–14×7–10 mm) with a characteristic circular aureole
on the seed coat (Coates Palgrave, 1983; Barnes et al., 1997). The seeds are embedded
within a dense white chalky matrix which completely fills the pod capsule interior.
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
Seed dispersal of Acacia erioloba by elephant 377
Fig. 1. Seed content of Acacia erioloba pods from Hwange National Park, Zimbabwe (n=500; mean=
18.3; mode=18; SD=4.61).
Ripe pods typically contain approximately 9–24 mature seeds, with an average of
18 seeds per pod (Fig. 1). Trees flower during July–September with fruits maturing
within the period December–March (Coates Palgrave, 1983). Under ideal conditions,
large mature trees in Hwange National Park, Zimbabwe, may sustain an effective
fruiting season of 10 months duration (January–October: this study), and pod
production from a single large tree may exceed 500 kg/y (Barnes et al., 1997).
The foliage, shoots and pods of A. erioloba are consumed by elephant (L. a.
africana Blumenbach), giraffe(Giraffa camelopardalis Linn.), kudu (Tragelaphus
strepsiceros Pallas), impala (Aepyceros melampus Lichenstein),steenbok (Rham-
phicerus campestris Thunberg) and other browsing ungulates (Wilson, 1975; Smithers,
1983; this study).The foliage contains up to 17% protein with 35% digestibility, and
the pods (including seeds) contain up to 16% protein and 48% total digestible organic
matter (Barnes et al., 1997). Nonetheless, the foliage and pods of A. erioloba may
contain cyanogenic glycosides (prussic acid) in sufficient quantity to be toxic to
domestic livestock under certain conditions (Coates Palgrave, 1983). The prussic
acid content of foliage and pods varies seasonally, with highest concentrations present
in foliage under high ambient moisture conditions and lowest concentrations evident
in dry ripe pods (Barnes et al., 1997). Unlike most other tree species of Kalahari
Sands habitats, leaf replacement of A. erioloba is modulated by photoperiod rather
than ambient moisture conditions, with leaf replacement and flowering occurring
synchronously during the middle dry season period (August) and new shoot growth
commencing as flowering ends (September). The new leaf and shoot growth from
A. erioloba provides elephants and ungulates with a high-protein forage resource
during the late dry season period when elephants and other large herbivores are
subsisting on diets with a negative protein-nutrient-energy balance (Williamson,
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
378 Joseph P. Dudley
1975a; Childes & Walker, 1987). Studies in Namibia by Coe & Coe (1987) and
Hoffman et al. (1989) indicate that the ingestion of pods by elephants may increase
A. erioloba seed germination and seedling establishment rates (see also Barnes et al.,
1997).
Elephants appear to feed principally and preferentially on the bark, shoots and
foliage of smaller and shrubbier immature A. erioloba trees; severe bark-stripping
of mature A. erioloba was found in this study to be uncommon relative to that
experienced by other some other acacias (A. leuderitzii, A. nigrescens, A. tortilis)in
Hwange National Park (this study). Elephant bark-stripping damage was recorded
from seven of 50 large A. erioloba trees sampled within a 100-km
2
area located just
west of the Main Camp tourist entrance; bark damage of [20% of bole circumference
was found on only three trees; the maximum observed extent of debarking damage
was 35% of bore circumference for a single specimen. Although elephant bark-
stripping has been identified as the cause of widespread mortality among A. erioloba
in past decades (Williamson, 1975a), girdling or ring-barking by elephants does not
appear to be a particularly serious threat within the focal tourism areas of Hwange
National Park at this time. Most of the elephant-caused damage or mortality to
mature A. erioloba trees observed in the course of this study was evidently attributable
to the shaking of trees by elephants harvesting pods, which sometimes results in the
breaking-offof principal branches and occasionally the felling of an entire tree.
Study site
Hwange National Park is located on the north-eastern fringes of the Kalahari Sands
region of central southern Africa at 19°S26°E, and lies within the Zambezian
biogeographic region of southern central Africa. The park covers an total area of
14 600 km
2
, about two-thirds of which is covered by Kalahari Sands terrains and
semiarid Kalahari Sand deciduous woodland and scrubs, interspersed with a few
patchily distributed areas of edaphic grasslands and savanna habitats. The northern
and extreme southern sections of the park are dominated by mopane (Colo-
phospermum mopane) woodland and scrub habitats. Topography is generally flat to
slightly rolling, depending on the prominence of fossil aeolian dunes; altitude ranges
from 900–1100 m a.s.l. Regional climate is semiarid and subtropical with sporadic
severe frosts occurring during the winter dry-season months. Mean monthly maximum
temperatures range between 24°C and 33°C; mean monthly minima range from 4°C
to 18°C, with a record low of – 14°C. Frosts resulting in widespread damage to woody
vegetation have occurred historically at approximately 5-year intervals (Childes &
Walker, 1987). Rainfall is highly variable and patchily distributed over space and
time, but tends to be concentrated within a single rainy season during the summer
months (November–April). Annual precipitation within the focal study area averages
some 650 mm/year (observed range 335–1160 mm), decreasing along an east to west
gradient within the park to about 550 mm at the Botswana border. Aerial survey
estimates indicated a dry season population of approximately 25 000 elephants for
Hwange National Park at the time of this study (Price Waterhouse, 1996).
The focal study area for this research was located in the eastern central portion
of Hwange National Park, within the area circumscribed by a 25 km radius with its
origin at the Main Camp headquarters site. The area is dominated by Baikiaea
woodland and Combretum–Acacia shrubland habitats interspersed with patches of
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
photo courtesy of
DOUGLAS GROVES
Living with Elephants
Seed dispersal of Acacia erioloba by elephant 379
A. erioloba woodland and savanna (Childes & Walker, 1987). The north-eastern
fringe of the focal study area is transitional in species composition between trees
characteristic of Kalahari Sands woodland types (Baikiaea plurijuga, A. erioloba,
Ricinodendron rautenenii) and those typical of miombo (Brachystegia boehmii, Bra-
chystegia spiciformes) and mopane (Colophospermum mopane) woodland associations
(Rogers, 1993). The northernmost sampling site, the Mambange River seeps, was
located just outside the park boundary in an area of non-Kalahari Sands terrains
and habitat.
Materials and methods
Elephant dung was sampled during the dry season months of three consecutive years
(1995–97) to determine the presence and relative abundance of woody plant seeds
consumed by elephants. The seed content of elephant dung was determined through
analyses of data collected from plot samples, road transect samples, and opportunistic
ad libitum site samples. Standard data measures recorded included: the number of
dung boli per defaecation, dung bolus size (diameter, thickness), condition (intact/
fragmented, recent/old), gross contents (presence/absence of grass, woody, leaf, seed
and/or fruit materials) and net seed and/or fruit contents. Vertebrate or invertebrate
macrofauna discovered within or under dung piles were noted, and observational
data collected on the utilization of elephant dung by birds and mammals. When
field identification of seed or leaves was uncertain, specimens were collected for
subsequent verification. Voucher specimens of intact leaves, fruits and seeds recovered
from elephant dung were deposited with the Hwange Main Camp-Research Unit
herbarium.
To ascertain the exact seed content of dung samples, entire dung piles were
dissected by hand, and usually in situ. Dung piles which were moved to other
locations before dissection were evaluated and measured before loading en masse
within heavy-duty plastic bags for transport. The presence or absence of A. erioloba
seeds in elephant dung censused on road transects and plots was evaluated by
preliminary inspection of the dung pile, followed by the complete dissection of a
single randomly selected bolus in instances where no seeds were immediately evident
upon superficial inspection. Acacia erioloba seeds still embedded within intact pods
or pod fragments were not included in seed count totals.
The average seed content of A. erioloba pods, used for estimating pod consumption
rates and seed digestibility quotients from dung samples, was determined from a
sample of 500 ripe, intact wind-fallen pods collected from a grove of 26 mature trees
in the tourist housing area of Hwange Main Camp (Fig. 1). Experimental feeding
trials using A. erioloba pods were conducted with captive elephants at the Wild
Horizon Safaris Elephant Camp, located approximately 17 km south-west of Victoria
Falls, Zimbabwe. These elephants were all taken into captivity as juveniles during
culling operations in Zimbabwe between 1982 and 1986 (R. Hensman, pers. comm.).
Ages of these elephants at the time of this study ranged from approximately
11–15 years and included individuals of both sexes. At the time of the experimental
trials, elephants at the Camp were maintained on largely free-range diets with very
limited amounts of non-indigenous forage supplements provided (micromix pelleted
horse feed). The experimental site is located outside the natural range of A. erioloba
and no seeds closely resembling those of A. erioloba were found during preliminary
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
380 Joseph P. Dudley
evaluations of dung seed contents prior to initiation of the feeding trials. All A.
erioloba seeds recovered in dung would therefore have been derived from pods
provided by the investigator. A gut-transit time (‘throughput’) feeding trial was
conducted with four elephants in 1996, and a digestive efficiency trail was conducted
using eight elephants (the original group+four additional elephants) in 1997.
In the experimental feeding trials, A. erioloba pods were fed to one elephant at a
time in order to monitor individual consumption precisely. In digestive efficiency
trials, the elephants were fed pods after having been secured for the night, unchained,
within separate box stalls at 18.00 hours. Pods were offered to elephants in successive
batches of ten pods each, in order to maximize accountability and minimize wastage;
when the last pod of one batch was consumed, more pods were offered. Elephants
were fed as many pods as they would eat. Uneaten pods were collected immediately
and discarded. Approximately 1 h was required to complete the feeding trial.
No A. erioloba seeds were located during inspections of dung deposited prior to
the return of the elephants to their stalls on the following evening.
Elephants were taken out of their stalls at 07.00 hours on the second morning
following the feeding, at which time the author and two assistants dissected all
accumulated elephant dung within each pen. All seeds were collected and set aside
in labelled plastic bags for identification and counting. The most recent defaecations
noted from most stalls contained no A. erioloba seeds, and no A. erioloba seeds were
found in dung deposited by elephants during the morning exercise routine after
release from their stalls.
Results
Acacia erioloba seeds were the most abundant, in terms of both seed number and
seed mass, plant seed/fruit recorded from the dung of elephants in Hwange National
Park, Zimbabwe. The next most frequently recovered seeds were those of a wild
melon (Acanthosicyos naudinianus: Wilson, 1975). Other woody plant seeds recorded
from elephant dung piles in relatively high abundances (n> 50/defaecation) included
Ricinodendron rautenenii,Schlerocarya birrea,Grewia flavescens,Zizyphus abyssinica
and Zizyphus mucronata.
Approximately two-thirds (64%) of all elephant dung analysed from road transects,
survey plots, and ad libitum site samples contained seed and/or pod materials from
A. erioloba (Table 1). Little pod mass relative to seed mass was recovered in most
instances, and pod fragments were recovered from only 56% of all exhaustively
sampled elephant dung piles with A. erioloba materials present. Nonetheless, large
pod fragments and even entire intact pods have been recovered from the dung of
elephants on occasion, usually from dung piles known or assumed on the basis of
size to have been deposited by mature bulls. The dung of fully mature bulls
(approximately 40 years and older) is distinguishable on the basis of size alone, since
only mature bulls are sufficiently large to produce the largest diameter class of dung
boli: 19–23 cm).
The average seed content of A. erioloba pods (n=500) was found to be 18.3 (SD=
4.61), with a mode of 18 seeds per pod. On the basis of these data, the estimated
efficiency of digestion (or predation) for A. erioloba seeds consumed by seven
captive African elephants (ages 11–15) in a controlled experimental feeding trial was
calculated at between 86–99%; the minimum gut-transit time was determined at
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
Seed dispersal of Acacia erioloba by elephant 381
Table 1. Frequency of Acacia erioloba seeds in dry season samples of elephant dung from
Hwange National Park, Zimbabwe
Total Number with %
number of A. erioloba containing
Date(s) dung piles seeds and/or pods A. erioloba
Survey 10.7.95 34 29 85
Plots 18.7.95 27 17 63
19.7.95 34 29 85
20.7.95 28 11 39
21.7.95 7 5 71
19.8.95 31 19 58
N total 161 110 68
Road 5–7.6.1996 187 122 66
Transects 9–11.7.1996 45 25 56
3.9.96 8 0 0
N total 240 147 61
Total 1996 69 47 68
Seed count 1997 38 19 50
Samples N total 107 66 62
Combined totals 507 322 64
24.5 h, with an estimated maximum gut-transit time at approximately 36.0 h (Table 2).
Acacia erioloba seeds/pods constituted up to 12% or more of the total wet dung
mass. An average of 267 A. erioloba seeds per defaecation, with a standard deviation
of 952 seeds, was determined for 99 exhaustively sampled dung piles. One dung pile
sampled in 1997 was found to contain 5690 undigested A. erioloba seeds. Acacia
erioloba pod/seed matter constituted approximately 11.5% (7.6 kg) of the wet-weight
dung mass from two consecutive defaecations (47 kg and 18 kg; 65 kg total) by a
very large bull estimated at between 50 and 55 years of age. After hand cleaning
under field conditions, the wet-weight mass of the loose seed component (n=4763)
was 2.78 kg, and that for the undifferentiated seed/pod matter was 4.82 kg.
The average number of isolated seeds recovered from dung piles containing seed
or pod materials (n=407) was equivalent to the total seed content of approximately
22 seed pods (Fig. 1), with an extrapolated air-dry pod mass of 210 g (Table 3). An
effective digestive efficiency of 50% would increase this estimate to 44 pods and an
estimated ingested pod mass of 430 g; at 90% digestive efficiency the average number
of ingested pods per feeding bout would be approximately 203 pods, with a estimated
ingested seed/pod mass of 1.95 kg.
Germinating A. erioloba seeds and pods were not infrequently encountered inside
dung boli during dung seed count analyses, despite the fact that dung analyses were
undertaken during the dry season period. Various species of birds and mammals
function as secondary consumers of A. erioloba seeds by eating seeds from elephant
dung piles. Species most frequently observed consuming A. erioloba seeds from
elephant dung were crowned guinea fowl (Numida melagris Linn.), yellow-billed
hornbill (Tockus flavirostris Rupp.), red-billed francolin (Francolinus adspersus Water-
house), ostrich (Struthio camelus Linn. 1758), baboon (Papio ursinus Kerr), vervet
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
382 Joseph P. Dudley
Table 2. Gut-transit times and digestibility of Acacia erioloba as determined from experimental feeding trials with captive African bush elephants
(L. a. africana)
Throughput trial
Elephant Quantity A. erioloba seeds
name consumed passed in dung
26.3.1996 27.3.1996
1500 h
Jack > 20 16.10 h 17.30 h
Jock [3 15.30 h 17.45 h
Ellie [3 15.30 h 17.50 h
Jumbo 0
Digestive efficiency trial
No. of pods
eaten No. of A. erioloba seeds recovered from dung Estimated digestibility
18.00 hours Estimated 18.00 hours 10 May – 07.00 hours 11 May∗95% CI
Elephant 9 May 1997 seed
I.D. (# pods×18) content Intact Ruptured Total Mean Low High
Jack 65 1170 10 4 14 99% 0.968 0.992
Jock 9 162 11 9 20 88% 0.673 0.925
Ellie 9 162 4 7 11 93% 0.82 0.958
Sharu 45 810 37 74 111 86% 0.637 0.917
Chikwe 20 36054997%0.9330.984
Mana 10 18012398%0.9550.989
Sapi 17 306 17 20 37 88% 0.679 0.926
Jumbo 0 n/a000
Aggregate mean 93% 80.90% 95.59%
∗No. A. erioloba seeds evident in dung prior to 18.00 hours 10 May or after 07.00 hours 11 May 1997.
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
Seed dispersal of Acacia erioloba by elephant 383
Table 3. Nutrient content of Acacia erioloba foliage, pods and seeds (from data in Williamson,
1975a; Barnes et al., 1997)
Foliage Pods with Seeds
% Digestible nutrients 57 Metabolizable energy (MJ/kg 9.4
DM)
% Crude protein 12.9 % Digestible organic matter 48
% Digestible crude protein 8.1 % Crude protein 16.5
% Carbohydrate 48.8 % Crude fibre 27.3
% Crude fibre 29.1 % Ether extract 6.3
% Ether extract 3.3 % Ash 3.8
% Ash 4.9
Pod (without seeds) Mean SD Min. Max.
mass (g) 9.56
% Crude protein 9.93 1.39 6.56 12.78
% Fibre – NDF 44.92 5.21 33.36 58.84
% Fibre – ADF 34.31 3.54 28.6 44.97
% Lignin 4.33 1.32 1.31 7.57
% Carbohydrate 10.86 2.68 4.37 16.96
% Starch 25.82 5.11 13.2 36.1
Seed Mean SD Min. Max.
Seed mass/pod (g) 3.79
% Crude protein 27.42 2 23.1 31.39
% Fibre – NDF 32.05 6.8 21.71 47.78
% Fibre – ADF 20.91 2.41 17.35 29.98
% Lignin 4.22 0.92 2.46 6.17
% Carbohydrate 5.7 1.19 3.56 9.31
% Starch 26.05 6.25 15.2 35.1
monkey (Cercopithecus pygerythrus Cuvier) and tree squirrels (Paraxeras cepapi
Smith) (McLachlan & Liversidge, 1982; Smithers, 1983).
Discussion
Elephants in the Kalahari Sands region of Hwange National Park consume large
quantities of A. erioloba pods and it seems possible that observed shifts in elephant
foraging ranges in these habitats (Williamson, 1975b) may be modulated in part by
the distribution and availability of fruiting A. erioloba trees. During the dry season
months when A. erioloba are at their peak of palatability, elephants in the Kalahari
Sands habitats of Hwange National Park may travel 20 km or more during a 24-h
period (Conybeare 1991); given an estimated minimum gut-transit time of 24 h for
ingested seeds, potential dispersal distances for elephant-ingested A. erioloba seeds
in Hwange National Park would be about 20–30 km. Potential elephant-dispersal
distances for A. erioloba in desert regions of western Namibia are predicted to be
substantially greater, on the order of 50–70 km or more (Viljoen & Bothma, 1990).
Large mature A. erioloba trees in the Main Camp area of Hwange National Park can
maintain a effective fruiting season of at least 10 months duration (January–October).
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
384 Joseph P. Dudley
Most of the earliest falling pods from large A. erioloba trees in Hwange National
Park (January and February) appeared to be sterile or heavily parasitized by insects,
and contained few, if any, fully developed or apparently viable seeds. Pods may
be dropped continuously throughout the fruiting season, but there is typically a
pronounced peak in pod-fall rates during the early dry season (April to June) (Barnes
et al., 1997). When wind-fallen pods are not available, elephants (particularly large
bulls) will jolt or shake A. erioloba trees in order to shake loose ripe pods from the
canopy. When pods are ripe, elephants may harvest most of the pod crop sys-
tematically from particular trees over a relatively brief period (several days to weeks
depending on tree size and intensity of utilization). Bull elephants were observed
on several occasions systematically feeding on pods from a single
A. erioloba tree for more than 1 h, shaking the tree and then gleaning fallen pods
offthe ground, repeating this process several times before moving away to feed
elsewhere.
Acacia erioloba pods may constitute an key nutritional resource for elephant
populations inhabiting the Kalahari Sands landscapes of southern central Africa.
The total digestible organic matter of A. erioloba pods has been estimated at 48%;
the crude protein content of seeds has been estimated at 27% to 33% and the crude
protein content of pods without seeds at 6–11.4% (Table 3). Elephants in Kalahari
Sands habitats are typically in a negative protein/nutrient balance during the dry
season months when ripe pods are at their peak of availability (Williamson, 1975a)
and palatability (Barnes et al., 1997); the protein and calories available from
A. erioloba seeds and pods may be an component of the dry season diet for elephants
in the Kalahari Sands habitats of southern central Africa. Experimental feeding
trials conducted during this study indicate that the potential digestibility of
A. erioloba seeds for elephants, especially for adolescent and young adult elephants,
may be much higher than the 16% ratio found by Coe & Coe (1987). The limited
data available from preliminary feeding trials with captive elephants indicate that
80% or more of the A. erioloba seeds ingested may be broken up during mastication
and subsequently digested (Table 2). An age-dependent factor is believed to be
involved in the digestive efficiency of seed consumption, as juvenile elephants may
not possess adequate shearing strength in their jaws to break up seeds sufficiently
to permit digestion, while older elephants (particularly bulls) may be less able or
less inclined to masticate pods thoroughly prior to swallowing, due to advanced
tooth wear or other factors.
Older elephants, especially mature bulls, appear to be far less efficient in utilizing
the nutritional content of seed and pod mass and thus may be correspondingly more
effective dispersal agents for seeds. The dung of very old animals is generally readily
identifiable on the basis of relatively large dung bolus size, poorly digested condition
and loose consistency of dung contents. These characteristics appear attributable in
large part to a limited degree of forage mastication preceding ingestion, a phenomenon
which in captive Asian elephants (Elephas maximus) is frequently correlated with
age-related physical debilitation (advanced tooth wear and/or malocclusion, coupled
with poor muscle tonus in the digestive tract, abdomen, and rectum (Williams, 1950).
Within the elephant population of Hwange National Park, large bulls appear to
be the most frequent and systematic consumers of A. erioloba pods. Only the prime
mature bulls (35–45 years) are massive and powerful enough to jolt the boles of
large mature trees (> 0.75 m d.b.h.) strongly enough to effectively shake loose ripe
East African Wild Life Society, Afr. J. Ecol.,37, 375–385
Seed dispersal of Acacia erioloba by elephant 385
pods from the upper canopy. Cows and juveniles are generally restricted to wind-
fallen pods from the larger trees or those which can be shaken down from the smaller
less productive trees. Passive or aggressive displacement of subordinate elephants by
dominant individuals was observed in situations where there was apparent com-
petition for access to A. erioloba pods. Nonetheless, mature bulls harvesting A.
erioloba pods occasionally tolerate the presence of satellite bulls or cows with calves,
permitting them to feed communally upon pods which the bull has shaken out of
the tree canopy. The high relative frequency and abundance of A. erioloba seeds in
elephant dung, coupled to the frequency with which elephants were observed feeding
on A. erioloba pods, provide evidence for the potential nutritional significance of A.
erioloba fruits in the dry season diet of elephants in Hwange National Park.
Acknowledgements
This research was supported under grants from the U.S. National Science Foundation
(Grant No. DEB 9200019) and the Center for Field Research-Earthwatch. The
Zimbabwe Department of National Parks and Wild Life Management provided
logistical and technical support facilities. Peter Ngwenya, Felix Banda, and Fibion
Ndiweni assisted with identification of plant specimens. Craig White, Rory Hensman
and Buck DeVries granted access to captive elephants for experimental feeding trails.
Dung plot and transect analyses and seed count data were collected with the assistance
of EarthCorps volunteer field assistants of the Earthwatch ‘Elephant Factor’ project.
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(Manuscript accepted 17 December 1998)
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