Age Determination and Growth in the Male South African Fur Seal Arctocephalus pusillus pusillus (Pinnipedia: Otariidae) Based upon Skull Material

2008·16.7 MB·English

by  C L Stewardson

Age Determination and Growth in the Male South African Fur
Seal Arctocephaluspusilluspusillus (Pinnipedia: Otariidae)
Based upon Skull Material
C. L. Stewardson^ T. Prvan^, M. A. Meyer^ and R. J. Ritchie*"*
'Botany and Zoology, AustralianNational University, Canberra, ACT,Australia
(PresentAddress, Fisheries and Marine Sciences Program Bureau ofRural Sciences, The Department of
Agriculture, Fisheries and Forestry, CANBERRAACT 2601 Australia)
NSW
^Department ofStatistics, Macquarie University, 2109;
;
^Marine and Coastal management (MCM), Rogge Bay, Cape Town, SouthAfrica; ''School ofBiological
NSW
Sciences, The University ofSydney, 2006
*CorrespondingAuthor ([email protected])
Stewardson, C.L., Prvan, T., Meyer, M.A. and Ritchie, R.J. (2008). Age determination and growth in the
male South African Fur Seal Arctocephalus pusillus pusillus (Pinnipedia: Otariidae) based upon skull
material. Proceedings oftheLinnean SocietyofNewSouth Wales 129, 207-252.
Skull remains are the most commonly found material ofmarine mammals and the most likely to be
keptinnaturalhistorycollections. Morphology,relativesizeandgrowthofthe skull in 83 SouthAfricanfur
seals,Arctocephaluspusilluspusillus, from the coast ofsouthernAfrica are described. The SouthAfrican
or Cape fur seal is very closely related to the Australian fur seal {Arctocephaluspusillus doriferus). Age
structure of populations is important in understanding the conservation status of an animal population
and the impacts ofhuman activity upon the survival ofviable wild populations ofanimal species. Skull
measurements {n = 32 variables) were examined in relation to standardbody length (SBL - defined as the
lengthfromthenosetothetailinastraightlinewiththeanimal onitsback),condylobasallength(CBL)and
chronologicalage(y)usinglinearregression.Animalsrangedfrom 1 monthsto> 12y(12*y).Twentyfour
animals were ofknown-age, while 39 were aged from counts ofincremental lines observed in the dentine
oftooth sections. Morphological observations were generallyconsistentwith earlierstudies. Condylobasal
length was highly, positively correlated with SBL and age. Overall, skull variables grew at a slower rate
thanSBL,apartfromheightofmandibleatmeatus andangularisto coronoideus,whichexpressedisometry
relativeto SBL. Condylobasal lengthcontinuedto increaseuntil atleast 12 y, withno obvious growthspurt
between 8-10 y, when social maturity (full reproductive capacity) is attained. Mean CBL was 19.4% of
SBL in yearlings; 15.5% in subadults, and 13.7% in adults. Apart from the dentition, all variables ofthe
facial skeleton followed a somatic growth trajectory. Most variables expressed positive allometry relative
to CBL, with greatest growth occurring inthe vertical partofthe mandible. Mastoidbreadth, andgnathion
to middle ofoccipital crest, expressed a strong secondary growth spurt at 10 y. Breadth ofbrain case, and
basion to bend ofpterygoid, followed a neural growth trajectory, scaling with negative slope relative to
CBL. Sutures ofthebraincase (i.e., basioccipito-basisphenoid, occipito-parietal, interparietal and coronal)
closedbefore those ofthe facial skeleton. Condylobasal length was foundto be a 'rough indicator' ofSBL
and age group (adult, subadult), but not ofabsolute age. Suture age was not a good indicator ofabsolute
ageoragegroup.Acomparisonisfinallymadebetweenskulldataonthe SouthAfricanfurseal {A.pusillus
pusillus) with available data on theAustralian fur seal {A.pusillus doriferus).
Manuscriptreceived 12November2007, acceptedforpublication 6 February2008.
KEYWORDS: dl\ovas\xy,Arctocephaluspusillusdoriferus,Arctocephaluspusilluspusillus,Australian fur
seal, Otariidae, skull growth, skull morphology, SouthAfrican furseal.
AGE AND GROWTH IN SOUTH AFRICAN FUR SEAL SKULLS
INTRODUCTION and mandibular growth based on animals aged from
tooth stmcture, or on animals of known-age (i.e.,
Skull remains are the most commonly found animals tagged orbranded as pups), is only available
material of marine mammals and the most likely for a small number of species including Callorhinus
to be kept in natural history collections. It would ursinus, northern furseal (Scheffer andWilke, 1953);
be useful to be able to gain as much information
Zalophus californianus, Califomia sea lion (Orr et
as possible about sex, age, probable size, breeding
status and even in many cases positive identification al., 1970); andEumetopiasjubatus, northem (Steller)
of such material in terms of modem taxonomy and sea lion (Fiscus, 1961; Winship, Trites and Calkins
nomenclature (Brunner 2003). The South African 2001). Sometimes very few skull variables have
or Cape fur seal {Arctocephalus pusilhis pusiUus) been recorded. Currently, there is limited information
occurs on the Namibian and SouthAfrican coasts and on cranial growth according to age (y) in southem
nearby offshore islands (Schaffer, 1958; King, 1983; hemisphere fur seals: the main problem being small
Wameke and Shaughnessy, 1985) but does not occur
samplesizes(King 1969;King 1983;Bmnner 1998ab;
on Subantarctic Islands betweenAfrica andAustralia.
The SouthAfrican fur seal andtheAustralian fur seal Bmnner et al, 2002; Brunner et al., 2004; Daneri et
{Arctocephalus pusiUus doriferus) are now regarded al., 2005), particularly the small numbers of tagged
as closely related varieties of the same species individuals ofknown age that are available.
{Arctocephalus pusiUus) (King 1972; King 1983; In the mammalian skull, there are two growth
Wynen et al., 2001; Brunner et al., 2002; Brunner models, neural and somatic, each with two types of
2003). Historically, the Australian fur seal was found growth,monophasicandbiphasic(ToddandSchweiter,
on the southern Australian coast from Kangaroo
1933; Scott, 1951; Moore, 1981; Sirianni and
Island (South Australia) to Seal Rocks (mid coast
NSW) with its distribution centred on Bass Strait Swindler, 1985). In neural growth, skull components
and Tasmania (King 1969). The identity of the fur associated with the nervous system (i.e., braincase,
seals that originally inhabited Macquarie Island until orbital and otic capsules) grow rapidly during
wiped out in the early 19* century is uncertain. Today prenatal and early postnatal life, completing most of
breedingcolonies aremore orlessrestrictedto islands their growth well before the rest ofthe body (Moore,
ofthe Bass Strait region and Tasmania (Kirkwood et 1981). In somatic growth, all other skull components
aelt.,al.1,99220;02A;mAoumlodu,ldanedt Wala.,m2e0k0e3),.2002; Shaughnessy (i.e., facial skeleton) follow a moreprotracted growth
course (Moore, 1981). After the initial growth spurt
It is usefiil to as fully as possible investigate
morphometric measurements of seal skulls to experienced during early development, growth may
correlate with age and maturity and breeding status. bereasonably constant(monophasic growth), orthere
Earlier cranial growth studies in pinnipeds were may be a secondary growth spurt in older animals
based on unreliable age determination techniques, (biphasic growth) when they reach sexual maturity.
including: (i) the extent ofclosure ofcranial sutures; Bmnner (1998a) and Bmnner et al. (2004) drew the
(ii) body length, colour of vibrissae, pelage and overall conclusion that growth pattems in fiar seal
general appearance; (iii) ovarian structure; and (iv)
skulls were similar to that found in other camivores
baculum development (e.g. Doutt, 1942; Rand,
1949a, b, 1950, 1956; King 1969; King 1983; such as canids (dogs) (Wayne, 1986; Morey, 1990;
Bmnner 1998ab; Bmnner et al., 2002; Bmnner et Evans, 1993) and other marine mammals (Bryden,
al., 2004; Daneri et al., 2005). A common feature 1972).
of most of these studies is the limited number of Current populations of South African fur seals
specimens available and the unknown age of most numbermorethan2 millionindividuals althoughthey
ofthe material. Therefore, observed growth pattems reacheda lowlevelofabout 100,000 early inthe 20th
could not be quantified with any real meaning (King,
century. Their larger populations, occasional culling,
1972); generally a division into mature and immature
animals was all that was possible (King 1969; King drowning in fishing nets and shooting of "problem"
1983). Fortunately a reasonably precise method of animals, have made more specimens available for
age determination of untagged South African fur study than their Australian relatives {A. pusiUus
seals has been established from incremental growth doriferus). Furthermore, another consequence of
lines inthepulp ofteeth (Scheffer, 1950; Laws, 1953; the much smaller populations ofAustralian fur seals
McCann, 1993; Oosthuizen, 1997) but this involves (about35,000-60,000: Kirkwoodetal., 1992; 67,000
ndeegsottmicatbilveeosnammpulisneguomfmmaatteerriiaall.wDheinctihtimoinghwtasnotalbseo - 200,000, Shaughnessy et al., 2002; Amould and
used for aging 69 male and 163 femaleAustralian fur Wameke 2002;Amouldetal., 2003) andmore limited
seals collected from an island in Bass Strait in 1970- accessibility is that very little cranial morphometric
1972, but unfortunately the skulls were not measured data are available onAustralian fur seals (King 1969;
as part ofthe study (Amould and Wameke 2002). Bmnner 1998ab), particularly of definitively known
Within the Otariidae, information on cranial ages based on tagged individuals (Bmnner 1998ab,
208 Proc. Linn. Soc. N.S.W., 129, 2008
.
C.L. STEWARDSON, T. PRVAN, M.A. MEYERAND R.J. RITCHIE
Amould and Wameke 2002). 1997. The date of collection, method of collection
Here we examine the skulls of 83 male South and approximate location of specimens are listed in
MCM
African fur seals, Arctocephahis pusilhis piisillus, Appendix 1. seal specimens are accessioned as
from southern Africa. Specific objectives were to: AP followed by a number inAppendix 1
(i) describe the general morphology of the skull;
(ii) quantify growth of skull measurements {n = 32 East Coast and West CoastAnimals
variables) relative to standard body length (« = 74 Additional skulls from Sinclair Island (West
animals), condylobasal length {n = 83 animals) and coast of southern Africa, 27° 40'S, 15° 3rE) were
chronological age {n=63 animals); and(ii) determine measured (condylobasal length only) to determine if
if condylobasal length and suture closure are useful Eastern Cape seals (« =28 males) were ofsimilarsize
indicators of age and/or standard body length. This to those inhabiting west coast waters (« = 12 males).
is a very large data set compared to recent studies on PEM animals were adults 7- > 12 y. West coast
the Antarctic fur seal {Arctocephahis gazella) and animals were adults of undocumented-age. West
Southern fur seal {A. australis) (Daneri et al., 2005), coast animals were collected by Dr R. W. Rand in
the Australian fiir seal {A. pusillus doriferus) (King the 1940s and housed in the SouthAfrican Museum,
1969; Brunner 1998ab) and theNew Zealand fur seal Cape Town. These skulls were divided into sub-adult
{A. australis forsteri: King 1969; Brunner 1998ab) or adult classes based upon size and suture criteria
and the recent review of cranial ontogeny of otariid (see below).
seals by Brunner et al. (2004).
A limited comparison is also made between the Preparation and measurement ofskulls
available data on the SouthAfrican fur seal from the Skulls were defleshed and macerated in water
present study with published material from King for 2-3 months. Water was changed regularly. Skulls
(1969)andfromBrunner(1998ab,2000)onAustralian werethengentlywashedinmilddetergent(orbrushed
furseals. Modemmultivariatemorphometric analyses with water), and air dried at room temperature. A
of skull parameters complete data sets rather than small number of skulls were defleshed and gently
just means and variances of variables need to be boiled. Dry specimens were measured (« = 32 linear
mm
available. measurements) to the nearest 0.5 using a vernier
calliper (Table 1, Figure 1).
Variables used largely correspond to those
MATERIALS AND METHODS
reported in earlier otariid studies (Sivertsen, 1954;
Orr et al., 1970; Repenning et al., 1971; Kerley
Collection ofspecimens and Robinson, 1987; Daneri et al., 2005; Brunner
South African fur seals were collected along et al., 2004; Brunner et al., 2004) and in particular
the Eastern Cape coast of South Africa between the studies of Brunner (1998ab) focusing on the
Plettenberg Bay (34° 03'S, 23° 24'E) and East Australian fur seal {A. pusillus doriferus). Care was
London (33° 03'S, 27° 54'E), from August 1978 taken to measure standard parameters, measured
to December 1995, and accessioned at the Port in the same way as described in previous studies.
Elizabeth Museum (PEM). The circumstances under Variables were grouped by region in an attempt to
which most specimens were obtained are listed in reflect a fijnctional cranial analysis and to assess
PEM
Appendix 1. Apart from specimens collected before overall skull size (Hartwig, 1993) (Table 1). All
May 1992 {n = 16), all specimens were collected by measurements(andmeasurementstakenformSinclair
the firstauthor. One animal (PEM2238)was collected Island skulls) were recorded by the first author. The
MCM
NE ofthe study area, at Durban. From this collection, majority of measurements were recorded by
skulls from 59 males were selected for examination the third author (M.A. Meyer).
(Appendix
1).
Thesamplewassupplementedwithmeasurements Suture index
from 24 skulls collected by staff from Marine and Eleven cranial sutures (Figure 2) from 48 skulls
Coastal Management (MCM), Cape Town, South wereexaminedandassignedavalue 1-4, accordingto
Africa. These skulls were fi-om males that had been the degree ofclosure (1 = suture fully open; 2 = open;
tagged as pups, and were therefore ofknown-age (1- 3 = suture halfclosed; 4 = suture closed), according
12 y). They were collected from the west coast, south suture index (SI), ranging from 11 (all fully open) to
westcoast, southcoastandthe EasternCapeProvince 44 (all sutures closed). These values were added to
of South Africa, between February 1984 and July give a total suture index (SI), ranging from 11 (all
fully open) to 44 (all sutures closed). The suture
Proc. Linn. Soc. N.S.W., 129, 2008 209
AGE AND GROWTH IN SOUTH AFRICAN FUR SEAL SKULLS
Table 1: Linearskull measurements (n=32) taken from male SouthAfrican furseals in this study. Meas-
urements illustrated in Figure 1. Note that L26 and L27 were difficult to measure accurately. Param-
eters are broadly associated with the following functions; A- articulation, BC - braincase, F- feeding,
RA^- respiration/vocalisation.
Code Variable Region ofSkull Function
Dorsal
Dl Condylobasal length (posteriorpointonthe occipital - -
condyles to the most anteriorpoint onthe premaxilla)
D2 Gnathionto middle ofoccipital crest - RA^
RN
D3 Gnathiontoposteriorendofnasals splanchnocranium
Greatest width ofanteriornares (distance betweenthe
D4 nasal R/y
anteriormargins ofthe nares)
Greatest length ofnasals (distancebetweenthe anterior
D5 nasal RJV
andposteriormargins ofnasals)
^^^^
D6 Breadth atpreorbital processes -
D7 Least interorbital constriction frontal -
D8 Greatestbreadth at supraorbital processes frontal F
D9 Breadth ofbrain case (atthe coronal suture, anteriorto neurocranium BC
the zygomatic arches)
Palatal
Palatal notch to incisors (posteriormargin offirst
PIO palate RA^,F
incisor alveolus to palatal notch, excluding cleft)
Length ofupperpostcanine row (anteriormargin of
Pll postcanine one alveolus tothe mostposteriormargin of palate (dentition) F
postcanine six alveolus)
P12 Greatestbicanine breadth palate RAA,F
P13 Gnathion to posteriorend ofmaxilla (palatal) palate RA',F
P14 Breadth ofzygomatic root ofmaxilla zygomatic arch F
P15 Breadth ofpalate atpostcanine 1 (excluding the palate F
alveoli)
Breadth ofpalate atpostcanine 3 (excludingthe
P16 palate F
alveoli)
P17 ^^^ Breadth ofpalate atpostcanine 5 (excludingthe palate F
alveoli)
P18 Gnathion to posteriorborderofpostglenoidprocess - RA^,F
Bizygomatic breadth (maximum distancebetween the
P19 zygomatic arch F
lateral surfaces ofthe zygomatic arches)
P20 Basionto zygomaticroot(anterior) - -
Calvarialbreadth (greatesttransversewidth across of A
P21 basicranium
the skullbase, anteriortothe mastoid)
P22 Mastoidbreadth (width across theprocesses) basicranium A,F
Basiontobend ofpterygoid (anterior ofbasion to
P23 basicranium BC,A,F
anteriorofpterygoid)
210 Proc. Linn. Soc. N.S.W., 129, 2008
7
C.L. STEWARDSON, T. PRVAN, M.A. MEYERAND R.J. RITCHIE
Table 1 (continued)
Code Variable Region ofskull Function
Lateral
Gnathionto anteriorofforamen infraorbital (= lateral
L24 splanchnocranium RA^
face length)
L25 Gnathionto posteriorborderofpreorbital process splanchnocranium RA'
L26 Heightofskull atbase ofmastoid (excluding crest) -
L27 Heightofsagittal crest neurocranium BC,F
Mandibular
Length ofmandible (posteriormargin ofcondyle to
M28 mandible F
anteriormargin ofthefirst incisor alveolus)
Length ofmandibulartoothrow (anteriormargin ofthe
M29 first incisoralveolus to themostposteriormargin of mandible (dentition) F
postcaninefive alveolus)
Length oflowerpostcanine row (anteriormargin of
M30 post- canine one alveolus to the mostposteriormargin mandible (dentition) F
ofpost- caninefive alveolus)
M31 Height ofmandible atmeatus (dorsal margin of mandible A,F
coronoidprocess to the base ofthe angularis)
M32 Angularis to coronoideus (dorsal margin ofcoronoid mandible F
process to thetop ofthe angularis)
numbering system and the method ofjudging degree age animals and canine aged animals) was 63. All
ofclosure and calculation ofthe suture index follows MCM skulls (n = 24) were of known-age based on
Moore (1981). The suture index has been frequently tagging. Ofthe 59 PEM animals in the study: (i) 28
and successfiilly used as a criterion for separating were aged from counts ofincremental lines observed
immature and mature skulls of mammals (Moore inthedentineofuppercaninesasdescribedinSchaffer
1981), including seals (eg. Rand 1949; King 1969; (1950), i.e., range 1-10 y; (ii) 11 were identified as
Orr et al., 1970; Bryden, 1972; King 1972; Brunner adults > 12 y(i.e., pulp cavity of the upper canine
1998ab; Brunner et al., 2004; Daneri et al., 2005). closed); and (iii) 20 could not be aged because of
missing or decayed teeth. In South African fiar seals,
Age determination animals > 13 y cannotbe agedfrom counts ofgrowth
The age of animals was estimated from counts layer groups in the dentine ofupper canines because
of growth layer groups (GLGs) observed in the the pulp cavity closes at that age which terminates
dentine ofthin tooth sections (Scheffer, 1950). Upper tooth growth, hence the age group '>12 y or 12* y)'.
canines were sectioned longitudinallyusing acircular Dentition has also been used to age Australian fur
diamondsaw. Sectionsweregrounddownto280-320 seals (Amould and Wameke 2002) who claim that it
^im, dehydrated, embedded in resin and viewedusing is possibleto age some maleAustralian fur sealsto 1
a stereomicroscope in polarised light (Oosthuizen, years old based on upper canine dentition.
1997). Each section was read by one individual For this study, the following age groups were
five times, without knowledge ofwhich animal was used: yearling (10 mo to 1 y 6 mo); subadult (1 y 7
being examined (repeated blind counts). Ages were months to 7 y 6 months); and adult (> 7 y 7 months)
rounded offto the nearestbirth date. The median date (Table 2). Unfortunately, no South Afiican fiir seals
of birth was assumed to be 1 December (Amould skulls were available from wild-tagged individuals
and Wameke 2002; Shaughnessy et al., 2002). The with definitive ages greater than 12 y. The estimated
median of the five readings was used too estimate longevity ofSouthAfrican fiar seal males based upon
age. Outliers were discarded as reading errors. zoo animals is c. 20 y so it appears that they have
Thetotalnumberofagedanimals(taggedknown- a similar potential lifespan to the Australian variety
Proc. Linn. Soc. N.S.W., 129, 2008 211
AGE AND GROWTH IN SOUTH AFRICAN FUR SEAL SKULLS
Figure 1: Diagram ofa SouthAfrican fur seal skull (PEM554) indicating individual measurements
taken. Measurements are defined in Table 1.
(Amould and Wameke, 2002). such as postcanines, can be aged using growth ring
Currently, examination ofcanine tooth structure counts in the pulp cavity up to the point where the
is the most precise method of age determination in pulp cavity closes up. For recent assessments ofthe
untaggedpirmipeds; however, pulp cavityring counts reliability of this method see Ambom et al., (1992),
are not without error. In principle, other seal teeth McCann (1993) and Oosthuizen (1997).
212 Proc. Linn. Soc. N.S.W., 129, 2008
C.L. STEWARDSON, T. PRVAN, M.A. MEYERAND R.J. RITCHIE
Figure 2: Diagram ofa Soutli African fur seal sliull (PEM554) showing the position ofsutures examined
in this study. 1. Occipito-parietal; 2. interparietal; 3. coronal; 4. interfrontal; 5. internasal; 6. premaxil-
lary-maxillary; 7. basioccipito-basisphenoid; 8. basisphenoid-presphenoid; 9. squamosal-parietal; 10.
squamosal-jugal; 11. maxillary.
Classification ofgrowth patterns skull measurement for subadults at 7 y). Where the
In the present study, neural and somatic growth percent increase in variable size was < 6%, growth
patterns were distinguished as follows: [(mean was classified as neural, i.e., most growth was
skull measurement for adults > 12 y - mean skull completed as subadults. Where percent increase was
measurement for subadults at 7 y) x 100%]/(mean > 6%, growth was classified as somatic, i.e., growth
continued to increase in adults. Percentage increase
Table 2: The age distribution ofCape Fur Seals for each variable is given in Table 3.
Age group Age Frequency Percentage
(y)
Yearling 1 2 3.2 Pages 214-218 comprise Table 3
Subadult 2 2 3.2
Summary statistics for dorsal (Dl-9),
3 2 3.2 palatal (PlO-23), lateral (L24-27) and
mandibular (M28-32) skull variables
4 8 12.7
according to age (y) and age group.
5 4 6.3 Data presented as mean skull variable
mm
6 4 6.3 in ± S.E., followed by coefficient of
variation in round brackets, and skull
7 12 19.0
variable expressed as a percentage of
Adult 8 7 11.1 skull length. Maximum value of each
variable (males of unknown-age), and
9 5 7.9
^H classification of growth pattern, are
10 5 7.9 also presented.
>12 12 19.0
Total 63 100
Proc. Linn. Soc. N.S.W., 129, 2008 213
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AGE AND GROWTH IN SOUTH AFRICAN FUR SEAL SKULLS
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214 Proc. Linn. Soc. N.S.W., 129, 2008
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C.L. STEWARDSON, T. PRVAN, M.A. MEYERAND R.J. RITCHIE
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Proc. Linn. Soc. N.S.W., 129, 2008 215
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AGE AND GROWTH IN SOUTH AFRICAN FUR SEAL SKULLS
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216 Proc. Linn. Soc. N.S.W., 129, 2008