The life expectancy of the wild pig Sus scrofa L. in Ruhuna National Park, Sri Lanka

10 Pages·1998·5.9 MB·

by  Kenneth R Ashby

,
THE LIFE EXPECTANCY OF THE WILD PIG SUS SCROFA
L.
IN RUHXJNA NATIONAL PARK, SRI LANKA1
Kenneth R. Ashby2 and Charles Santiapillai3
(Withjive text-figures and oneplate)
Key words: Age determination, cementum, life expectancy, mortality, life table,
Sus scrofa herbivores
,
A pick-up collection ofskulls in RuhunaNational Park, made to determine age at death of
ruminant herbivores through study of layering in the cementum oftheir molar teeth, also
contained skullswith teeth of29 adultwild pig (Susscrofa). Layering present in thedentine
ofthe tusks cannot be used to determine age. But layering in the cementum ofthe molars,
although generally lessclearlymarked than inwaterbuffalo (Bubalusbubalis) haspermitted
y
thedetermination ofage atdeath in 23 specimens. Thepattern ofdark layers resemblesthat
in Bubalus with strongly and weakly marked lines alternating, corresponding with the
major and minor dry seasons. It differs from that in sambar (Cervus unicolor) and chital
(Axis axis) where a dark layer forms only in the main dry season each year. The mean life
expectancy of young adults is 6.0 years and the maximum 12 years, with no evidence of
distinction between the sexes, a situation very similar to that in the water buffalo. But
juvenile mortality due to predation is dramatically greater in wild pig, compensated for by
a correspondingly higher fecundity than in water buffalo. Sambar and chital both have
longer life expectancies as adults than water buffalo or wild pig.
Introduction Fig. 1). It is very active as anopportunistic feeder
onthe more succulent elements in the vegetation,
The wild pig Sus scrofa L. is an adaptable particularly underground storage organs, and on
omnivore distributed generally in the broad- carrion. Corpses are dismembered within a few
leaved forest and steppe regions of the hours ofdeath.
Palaearctic, extending through southern and Inspite of the richness of its fauna of
southeast Asia to Java, Bali, Flores and the mammals, as ofother land vertebrates, there has
Solomon Islands (Honacki, Kinman and Koeppl been little study ofthe ecology ofthe mammals
1982), and introduced as domestic or feral stock in southern and southeastern Asia. Schaller
widely elsewhere. In Sri Lanka, asthe sub-species (1967) undertook a pioneering study of the
Sus scrofa cristatus, it is aprominentmammalian ecological relations of the large herbivores and
component ofthe ecosystem, particularly in the predators in Kanha National Park, Madhya
dry zone of the country (Santiapillai and Pradesh, as representative ofthe drier forests of
Chambers 1980, de Silva et al. 1995). This peninsularIndia, including an outline analysis of
habitat is seen in a pristine form in Wilpattu and the age structure of the large herbivores, but it
Ruhuna (Yala) National Parks (for location see could onlyconcern indices ofage as his estimates
were based on tooth-wear alone. Eisenberg and
‘Accepted October, 996
! Lockhart (1972) conducted an outline survey of
departmentofBiological Sciences, University ofDurham,
Durham DH 3LE, England, UK the ecology of large mammals in Wilpattu
1
department of Zoology, University of Peradeniya, National Park in Sri Lanka. This has been
Peradeniya, Sri Lanka followed by more specific studies on the
)
34 JOURNAL, BOMBAYNATURAL HIST. SOCIETY, Vol. 95 (1998)
Fig. 1 . The study area in Ruhuna National Park (Block I, the southwestern section), and the position of
Wilpattu (#1) and Ruhuna (#2) National Parks in Sri Lanka.
population densities and annual breeding cycles high, it is possible to find skeletons ofthe large
of the large herbivores, and of the leopard mammals which have died in that area without a
Panthera pardus as the main large predator, in disproportionate expenditure oflabour. Searching
Block I, the southwestern part of the coastal is aided by the ground being firm and dry
section ofRuhuna National Park (Santiapillai et throughoutthemaindry season fromJulyto early
al 1981, Santiapillai et al. 1982, Santiapillai et October. From 1981 through 1985, annual
al 1984) including one on the pig Sus scrofa searches were made to find skulls, which resulted
(Santiapillai and Chambers 1980). However, in a collection of over 300 specimens. It was
there was no attempt to determine the age found, initially inwaterbuffalo (Bubalus bubalis
structure ofthe species concerned. that the well marked annual pattern of wet and
As much ofthe coastal section ofthe Park dry seasons was matched by corresponding
is short-grass prairie andpopulation densities are layering in the cementumofthemolarteeth with,
LIFE EXPECTANCYOF THE WILD PIG SUS SCROFA L. IN RUHUNA NATIONAL PARK 35
as in the case of the herbivores of the African consistent relationship was observed between
savannah (Grimsdell, 1973), dark lines in the numbers oflayers and age as indicated by tooth-
cementumcorresponding withperiods ofdrought. wear, itwas concluded that layering in the dentine
With increasing experience it was found that the was not a good measure of age. Attention was
number of layers could also be determined in therefore focussed, as in the ruminants studied,
sambar deer (Cervus unicolor) and chital {Axis on layering in the cementum ofthe molar teeth.
axis), where the main problem in counting The technique of preparation, which had to be
resulted from irregularity in layering, and finally unsophisticated, was based on the method which
in the pig {Sus scrofa where the main problem had proved successful with water buffalo and
),
has been the frequency of a combination of a deer. First lower molars were bisected vertically
densely opaque white background coloration of and transversely to give vertical sections of the
the cementum with weakness ofdefinition ofthe cementumand dentinebetweenroots. The surface
incremental lines. Despite the problems of the cementum and dentine was polished
encountered, 85% to 95% of the molar teeth successively with a coarse and a fine grade of
yielded counts ofthe number oflayers in the four carborundum powder and then examined under
species involved. Preliminary accounts of the a 12 x handlens and a strong unidirectional light,
findings were published by Ashby and normally sunlight. It was found helpful to
Santiapillai (1986), Ashby and Santiapillai examine the specimens with incident light from
(1991). Final publication was delayed in the various angles. Initially the number oflayers was
expectation that further collection of material counted in specimens where layering was clear.
would permit calculation based on larger sample It was then extended to those where it was less
sizes, which was particularly desirable inthe case easy to discern. In all specimens, several
ofpig and chital. However, added to the difficulty independent assessments were made.
of organizing collection of material in the field This process eventually left a residium where no
in the years since 1985, the epidemic of swine count of layers was possible. In addition the
fever in 1989 (De Silva et al., 1995) massively depthofboth dentine and cementuminthe section
disturbed the age structure ofthe pig population. were measured with a micrometer. Where
Thereforewe decidedto publish the dataresulting available, the two lower first molars were used
from the skulls collected from 1981 through 1985 and average values for parameters calculated.
in the belief that the life table of the pig which Where this was not the case, attention concern-
has been constructed and the statistical data on ing layering was turned to upper first molars
resulting wear, though lacking in detail, are and if necessary, to more posterior molars.
reliable in their broad characteristics. But the depth of dentine and cementum were
used in calculating regressions only from first
Material and Methods molars.
Estimate was made of tooth-wear in all
Material obtained inthe annual searches of specimens, based on Schaller’s (1967) nine point
coastal dunes, grass prairies, margins of water scale which had been devised with deer
holes and open bush was supplemented by particularly in mind. While the wear pattern in
specimens found by game guards in the course pigs is not identical to that in ruminants, given
of patrols. In the case of the pig, sections were the differences in tooth and jaw structure and
made initially of both canines and molar teeth functioning, it is believed that a parallelism
and both dentine and cementum examined. between given wear-classes in ruminants andpigs
A
Layering was evident in the dentine ofboth types was achieved. key wear-class in the series is
of teeth in some individuals, but given that no numberIII whichoccurs atthe endofthe subadult
36 JOURNAL, BOMBAYNATURAL HIST SOCIETY, Vol. 95 (1998)
Fig. 2. Relation in wild pig (Sus scrofa ofwear ofdentition to age as estimated from layering in
)
dental cementum.
phase whenthe lastmolar is completing eruption. Giventhatthe tusks ofmales are largerthan
Analysis of data was simplified in the present those offemales, it was possible tojudge the sex
instance by the fact that all the specimens found ofspecimens where canine teeth were present or
were already fully grown; where the whole of at least the front end of the jaw was intact and
the lowerjaw was available, its length provided therefore their sockets remained in cases where
confirmation ofthis fact. To make the estimation the canine had fallen out.
oftooth-wearmore sensitive, each grade ofwear
was sub-divided into three, giving besides the Results and Conclusions
typical expression of the class of wear (W), a
W+ and a W- grading, with the + sign indicating There were 29 specimens with teeth
one third progression towards the next older present, andsatisfactory sections offirstorsecond
grade, and W- one third ofa grade more youthful molars were obtained in 27 of these. Estimates
than the standard, giving effectively a 27 point ofage from layering were obtained in 23, and of
scale. The most youthful specimen was judged depth of cement and of dentine in Ml in 20
to be in grade III. specimens. For individuals where the two
Bombay nat. Hist. Soc. 95 Plate
J. 1
K.R. Ashby and C. Santiapillai: Wild Pig (Sus scrofa L.)
Vertical section of cementum ofMl ofwild pig with dentine above. Position ofa main Dark
X
Layer is marked by and of an intermediate Dark Layer by an arrow. In this individual, faint
intermediate lines were formed several times per year in its last years

LIFE EXPECTANCYOF THE WILD PIG SUS SCROFA L. INRUHUNA NATIONAL PARK 31
Fig. 3. Relation in wild pig ofdepth ofcementum in bisected Ml to age as estimated from layering
in cementum.
parameters concerned were determined, the expected to form in Ml at about the age of one
relation ofage as indicated by layering to wear- and a half years. It was therefore assumed that
class is given in Fig. 2, its relation to depth of the age estimated from layering in Ml was the
cementum in Fig. 3 and to depth of dentine in number ofsuch layers plus one. Where both Ml
Fig. 4. In the absence of data from research, it and M2 were sectioned there was generally one
has been assumed that the eruption of Ml is more such layer in the cementum ofMl than in
completed towards the age of one year and that that ofM2, therefore estimates of age based on
cementum starts to form soon after eruption M2 alone were calculated as the number ofsuch
finishes. Since most births occur in March and layers +2.
April (Santiapillai andChambers 1980) (although In the specimens where layering is clearly
striped young are seen at other seasons), the shown (see Plate 1), there are faint dark layers in
first dark band in the cementum corresponding the cementum alternating with the main dark
to the latter part ofthe main dry season, can be layers. This characteristic was apparent in most
38 JOURNAL, BOMBAY NATURAL HIST. SOCIETY. Vol. 95 (1998)
—
o
cf
D -
9
X - ?
Fig. 4. Relation in wild pig ofdepth ofdentine in bisected Ml to age as estimated from layering
in cementum.
ofthe teeth in specimens ofwater buffalo, but in or trees which, being deep-rooted, will be little
&
only a few specimens in deer species, and then affected by the short dry season. From Figs. 3
only in old age. It was concluded that the faint 4, it will be seen that the rate of deposition of
dark layers correspond to the subsidiary dry cement and ofwear ofdentine in upper Ml was
season which occurs around February, and that not obviously different from that in lower Ml
it was much more prominent in buffalo than in and the data from upper Ml teeth have therefore
deer because the former is a species grazing out been included when calculating regressions.
in the open where the quality offorage is quickly There was, likewise, no evidence ofthe relation
affected by drought, whereas deer, more ofthe various parameters differing in males and
particularly sambar, aremainlybrowsers and feed females. These conclusions applied also to water
on vegetation not much affected in quality by a buffalo and sambar deer, and with the exception
shortperiod ofdry weather. The similarity ofpig of a delayed start in cementum formation, to
to buffalo in the pattern of layering may be chital.
attributed to its feeding mainly in open areas and FromFig. 2, itis seenthatthere was astrong
at ground level and not on the shoots of shrubs correlation between the estimates ofwear and of
LIFE EXPECTANCYOF THE WILD PIG SUS SCROFA L. IN RUHUNA NATIONAL PARK 39
age as estimated from layering, the regression fact once eruption was completed, age as
equation for the range of age classes which indicatedbylayering in Ml teeth ofwaterbuffalo
occurred in the sample being: and the two species of deer was proportional to
the logarithm ofthe depth ofthe dentine.
Age = Wear Group x 1.53 - 1.96 Since there was no evidence ofany marked
=
r 0.82 difference in the life expectancy of the sexes, it
is possible to construct an approximate life table
This compares with a value over the same for the pig based on the fairly small size of the
=
range ofwear-classes ofr 0.76 inwaterbuffalo sample ofskulls obtained, and to compare it with
(N = 93), 0.83 (N = 77) in sambar deer, and 0.78 those for the ruminants where samples were
(N =48) in chital forthis relation. There was also larger. The resulting curves deduced for the pig
a good overall correlation between depth of (N = 23) and that for water buffalo (N = 126) are
cement of Ml and number of annual layers as given in Fig. 5. Since the youngest specimen of
shown by Fig. 3. The regression equation with pig was at least 1 .5 years old at death the present
depth ofcement (in mm) is: study gives no direct evidence ofmortality prior
to that age. However, its approximate value can
Age = Cement depth x 0.13 + 1.93 be deducedfromobservational studies, given that
=
r 0.79 the young pigs are easily seen and counted and
not cryptic like the young of sambar deer. It is
This compares with a correlation certain that neonatal and juvenile mortality is
coefficient for the relationship of 0.92 in water heavy and that predation by leopard (Panthera
buffalo, 0.89 in sambar deer and 0.74 in chital. pardus) is an important cause. Eisenberg and
There is a broader scatter ofvalues in older pigs Lockhart (1972) working in Wilpattu in a similar
than in water buffalo and sambar, where the habitat, reported that 50% of the young pigs
relationship of these parameters remains close disappeared within one month of birth.
throughout life. Santiapillai andChambers (1980) foundthat75%
In the relation between depth ofdentine of ofyoung pigs disappeared within one year. Since
Ml and age, it is seen from Fig. 4 that there are their study was based on visits made every three
large differences in the rate of attrition of the months, with the spring visit in May and the
Ml
dentine of in different individuals inspite of majority of births occurring in April, it can be
the fact that for the overall dentition the concluded as an approximation that the young
relationship ofwear to age is close. This feature averaged one month ofage when first seen. As a
was obvious on initial inspection of the working hypothesis, itcan thereforebe concluded
specimens: individual teeth sometimes wore at that mortality during the first thirteen months or
considerably different rates, andsuch differences so oflife is in the order ofseven eighths ofthose
could occur within the length of individual bom. De Silva et al. (1995) give results in line
molars. Ifindividualvariability is discounted, the with this conclusion.
data suggestthatthe rate ofattrition ofthe dentine From the data given in Fig. 5, it can be
ofMl with increasing age is approximately linear. deduced that once thejuvenile phase is past, the
This pattern differs from that seen in the life expectancy ofpig andwaterbuffalo is similar.
ruminants in this study. In these, wear was more For individuals surviving at one year ofage, the
markedly concentratedatthe Ml level ofthejaws total mean life expectancy ofthe pig is 6.0 years,
than was the case in the pig: while the depth of and of the water buffalo 5.6 years, with no
dentine remained closely correlatedwith age, the evidence of a significant difference in this
rate ofwear progressively lessened with time. In parameter between males and females. The
40 JOURNAL, BOMBAYNATURAL HIST. SOCIETY, Vol. 95 (1998)
1000 - predators take a large crop of herbivorous and
polyphagous mammals ofa size which they can
500-
profitably attack. The tiger Panthera tigris)
(
being absent from Sri Lanka, and leopard being
200 -
the largestcarnivore present anda solitary hunter,
00- adults of water buffalo and probably also of
1
sambar deer are too large to be predated. Water
i 50-
buffalo in good condition can also protect their
young against the leopard. Its main prey, apart
fromjuvenile pig, appears to include young and
subadult chital, monkeys, and such medium sized
species as theblacknapedhare Lepus nigricollis).
(
Thejuvenile pig may also be substantially atrisk
from the marsh crocodile Crocociyluspalustris)
2b (
and the estuarine crocodile (C. porosus which
),
_L I I t I i J l l i I i I at Ruhuna National Park (RNP) though not
0 2 4 6 8 10 12
Age (years) growing to the size of crocodiles found in large
rivers and other extensive water bodies,
Fig. 5. Survival of wild pig compared with that of compensates for rather small individual bulk by
water buffalo (Bubalus bubalis) their abundance in the larger water holes in the
in Ruhuna National Park.
Park. As there appears to be little in the way of
expected of length of reproductively active life fish on which they can feed, their diet is
in the females of the two species at Ruhuna is dominated by corpses of animals which die
therefore similar, although perhaps rather longer nearby (which they drag into water within a few
in the pig than in the water buffalo as the former days, ending competition for the carrion from
can give birth for the first time at 2 years ofage pigs), and the smaller of the mammals which
and the buffalo at 3 years. The observed birth come down to drink. Thus the pattern in RNP is
rate in the pig in Ruhuna National Park is in the for the largest of the herbivores, elephant and
order of4 + per adult female per year, while in water buffalo, not to be substantially affected by
the water buffalo it is one per adult female per predation at any age, while the young of the
two years. On these data and assumptions, medium sized species, chital and more
approximately 88% mortality in the pig when particularly the pig, suffer heavy predation prior
immature is balanced by a production of young to maturity and have a highjuvenile mortality in
per adult female which reaches maturity, eight consequence. Presumably the smaller mammals,
times as great as in the water buffalo. Although apart from the bats and perhaps species like the
the estimates are based on approximate data, the Indian porcupine Hystrix indica) with anti-
(
observations on juvenile mortality and on birth predator devices, will suffer heavy predation at
rate forthe two species give concordant estimates all stages of the life cycle, by the leopard and a
and strengthen the likelihood that these are variety ofsmaller predators.
substantially accurate. The larger ofthe grazing mammals inRNP
appear to be subject to a second important
Discussion constraint affecting their life tables. Most ofthe
oldest specimens ofboth water buffalo and pig,
The regime inRuhunaNational Park is one, those approaching 12 years of age, had heavily
like that in the savannahs of Africa, where worn dentition, and because of the reduction in