Mastoid Process
in
Sex Determination
DIANE L. FRANCE
Department of Anthropology, ColorJdo Slaıe University, fort CollL"ls, Colorado, USA
C1NS1YET A YlRIi\UNDA MASTOlD ÇIKI~T]NIN KULLANIMI TOPLUMLARARASI FARKLILIKLAR
Cinsiyet ayırımı çalışmalarmda herhangi bir iskelet kemiğinde osteometrik ölçümler yapıldığı zaman, kasJann yapışma yerlerinde veya bu alanlara yakın bölgelerin ölçümlerinde dikkatli davr3nıImalıdır; bslann
kullarıun farklılıklarına bağlı olarak beliren yapısal yoğunluk ve kemik yapılarmdaki anatomik farklıltkıann
cinsiyet ayırımı için uygulanan işlemlerde yanlışlıklara neden olabileceği unul\llmamalıdır. Kranyum
yardlJllıyla cinsiyet ayırımında en fazla kullanılan oluşumlardan biri olan processus mas/oideus, erkekleıd.: daha bLiyük ve daha engebeli oluşuyla özellik gösterir. J'rocessus mastoideus'a yapı~an kasıardan biri olan m. sternocfeidomaslOideus, öteki boyun kaslnnyla birlikte, kafa/vücut hareketlerinin uyumlıı obıasını sağlar.
Bu ar~ştınnada, Güney ve Haıı Afrika ile Güneydoğu Asya toplumlarının özelliklerini taşıyan iskelet
koııeksiyonlan kullanıldı. Bu to[llumJarda, processus nuısıoideus yardunıyla cLnsiyeı ayınını kriterlenninin ve dinıürfizm özelliklerinin karşılaştırılması yapJdı. AyTIca, processus mastoiJeus'dan yararlanarak yapillnasi
düşünülen cinsiyet ayırunı çalışmalarında, m. sıemocleidomasloideus'un kullanılma niceliğine ve uiteliğine
bağıl olarak, bu anatomik oluşumda görülen cinsiyet dimorfizmi üzerindeki etkileri tüm bulgularıyla ayrıntılı obrak göstcrilmi~tir.
Suıııınary
When osıeemeıde meaSUfcments aıe used in sex determinalion on an)' bone of tl1C skeleton, caution must be. ııseel when measming those local.ions on or ver)' ncar arC3S of musde insertion. for thcse areas nce more
!ikely ıo renecl differences in mııscle use, and may re su lt in misidentification of sex. This is demonstral.ed
wıth largely circumstanLİal evıdence of the dfects of intense use of the stemodeidoinastOld muscle on ıhe
sexual dimorphisın of the mastoid process.
Kc} words : Sex determlflllıion - Mas/oid process -S/ernocleıdornasıoid muscle
150 D.L FRAK'CE
INTRODUCTION
Osteometric analysis is increasingly used in sex determination as eorroborative evidencc, by individuals who are less experienced in sex determination by anthroposeopie means, or İn situations in which onlyaportion of the skeleton is available for study. Three premises when identifying sex is that male bones will be larger than those of females, that this difference can be measured, and that those measurements will be widely useful within a given population. Those measurements also tend to favor the most straight[orward methodology so that it will be wiclely applicable to those with lcss experience in recognizing bony landmarks.
In a previous study by the author (1, 2), of the sex dif[erences in the humerus, discriminant function scores and regression equations were developed [or 5 populations: American. Blacks, American Whites, American Indians (pucblo and Arikara) and Sudanese Nubians. Wilhin each of the five populations, as was expected, males were larger than females though, as also expected, the regression equations were not accurate across populational lines, even between Arikara and Pueblo Indians (when grouped together, Pueblo Indian males and females were generally classified as females, as they are generally smaller individuals). In all groups, the best sex discriminators in the humerus were measurements at or near the proximal epiphysis, an area of the humerus al most entirely devoted to muscle insertion. It was also discovered by other authors that the most impressive measurements for sex discrimination in the femur (3, 4) and tibia (5) occurred at or very near areas of muscle insertion, and much lcss so at areas of muscle origin.
That these areas should be more impressive in sex determination is not surprising, and is easily explained by observation of muscle aetion at origin and insertion. A general observation in gross anatomy, when describing the action of a muscle, is that
the insertion of a musele is pulled toward the origin. That is, the point of origin
remains fairly stationary while the insertion moves. The muscle tendon at the insertion point must move through a greater angular ran ge of motion than does the origin and must therefore be asmaller point on the bone (Figure 1). If the tendon fibers covered a large area, some fibers at the extreme edges of the insertion would streteh farther during muscle contraction, and might be strained or broken. The origin may be attaehed over a greater area of bone as it must travel through a very limited range of motion, and the muscle fibers in general remain more parallel to the bone. The same force is acting on each end of the muscle, but since the insertion is smailer, more foree per unit area is
placed on the insertion than on the origin, both at the muscle tendon and then, of
course, at the bone. Sinee bone reacts, by the piezo-electric effect, to the amount of this force being applied by the muscle, the area of insertion will react more for a given pull of muscle than will the origin. This effect results in the addition of more bone and the inerease in density of bone at the areas of insertion than at the areas of origin for a gi ven
Figure ı. Muscle action at origin and insertion.
muscle. One would expeet, then, that the areas of muscle insertion, and the nearby joint
surfaces, will show more aetivity-or oecupation-related differenees, and be less reliable
in indieating general sex-related differenees. The mastoid process is an area of the
eranium long reeognized to show signifieant sex differences within populations: the
male mastoid process is generally larger and more rough than the female. This area is
devoted to the insertion of the sternocleidomastoid muscle, which is a very strong,
band-shaped muscle originating from the mcdial third of the clavicle and the front of the
manubrium and inserting on and very near the mastoid process. It acts to "ben d the head
and neck toward the shoulder and rotate them toward the opposite side. When both
muscles act, the neck isflexed toward the thorax and the chin is raised ... " (6, p. 413).
Various muscles of the anterior neck also aid in the flexion of the neck, and thcse
muscles would act in concert with the sternocleidomaslOid to, for instance, balance a
weight on the cranium (Figure 2) or to counteract a force acting to pull the cranium
posteriorly (as with a tump line). That the sternocleidomastoid muscle is used in
counteracting forces of this kind may be demonstrated by exerting force against the
forehead with one hand while noting the tension created in the sternocleidomastoid
muscle with the other hand.
Because the mastoid process is the area for insertion of the sternocleidom<ıstoid
muscle, this area should reflect use-related changes as described above. This are<ı of the
cranium should show less sexual dimorphism in those groups in which females are the
primary burden carriers, and in which the weİght of those burdens İs at lcast in part
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BUSH-MAN DOGON ANDA-MAN
A','E. OF
'i7 VAR. [ ]
MASiOlD
BREADTH
Figure 3. Percentage of sexual di.morphi;;m ([emale mean/rnalc mcan) ın [our populatton>, bascd on
Howe!/s (7) measuremcnts.
dimorphism than the Bushman (Zulu = 92.3%, Bushman - 87.1%). If the differences in sexual dimorphism are caleulated for each of the 57 me.asurements independentiy, and that differcnce averaged, the Zulus and Bushman differ by 2.4%. Thcrc İs İn other words, relatively little difference bctween the groups in average sexual dimcrphism, there is almost 15% lcss sexual dimorphism in the Zulu poputaLion in mastoid process \\-icith than İn the Bushman population. Stated another way, the males anel females of the Zulu popularion are m ııch more al ike in masroid process width than woııld be expccted from the general dcgree of sexual dimorphism, and a rcsearcher is that much less likely ro correctly identify sex in the Zulu crania based on this fcaıure. There is no significant differcnce bctween the general dcgrcc of sexual dimcrphism in these two populations and the degrce of sexual dimOl1)hism in the mastoid process lcngth: only in the widıh.
In comparing the Dogon and Andanıan Islanda crarıia to the Zulu, acldiıional
interesIİng patterns emerge. Again, using the average sexual dimorphism index for all
51 measurements, the Zulu are morc sexually dimorphic than eıther the Dogon or AndameJl Islanders. As can be seen, hmvever, L"ıcy are much lcss dimorphic in mJstoıel process breael th.
154 DL FRA\'CT
CONC ONS
According to several published historical and ethnographic accounts (8, 9, 10), both women and men of all ages in the Bushman society carried heavy loads, usually tied to the waist or supported on the shoulders, but there was little sexual division of labor in
carrying In the society (11-1 the were knmvn
the sok carrying and in faet,
(ll) stated aman walks completcly unencumhered, "earrYlrıg mııhing heavın
his own dignity."
One would expeet less sexual dimorphism in the areas of musele insertion on the Zulu erania, then, and this seems to be eonfirmed by the results of this study. The
Dogon women holh carried burdem their heads, with litık
division 7), while Andaman Islamkcs earried such
burdens tump lines brcgmatic furrows lıave developee! the frontal
the skull (18-20). In the AndalIlan Island populalion, females are said to carry burclerıs III
this way more of ten than men, and that tump line s were used to such an extent that sexing was almost impossible in the cranium due to extreme muscle use(20).
The osteometric results are interesting in this regard: as would be expected, the
degree sexual dimorphisın the Andaman Island populaLion closely
that of while is interrnccl iaL,~. This data that wh ı
is a differenee in the degrec sexual between groups, the
differenee in the mastoid process is mueh greater than would be expected solely genetically. If something is deereasing the sexual differenee in this area of the Zulu crania, the logical explanation would include the biomechanieal differenees of the
stcrno-cleidomastoicl muscle bCl\-veen the sexes. action of sternocleidomasl.oiCı
muscle counteracıiııg forces attempting throw the off balance,
since inİLial forees great1y İncreas(:d when mass it is also
cal to assuıne that these kİl1ds of forees are inereasing the piezo-electrie effeel in ıhis
area of musc1e insertion, and will deerease the Hkelihood of eorrect assessmcnt of sex.
DISC N
In this hypoth(~sis, very difficnH find a collcetion ol' skeletal
in which this kind of oeeupational use is known for eaeh individual. We only know the eultural norms from eaeh group, and those reports are from historieal aceounts from travelers and missionaries (some of whom had little respeet for the groups they were
invading, had rather views of the amount done by eadı
It is reeogn the results report are on eireumstan
evidence, evidence cııough to a good argumenı. for the
novices should theoretica1ly be able to take measurements and plug in the formulae for correct sex assessmenl. This can stiU be done but with two very important caveats:
O)
the measurements are, as has been shown in many other bodies of research (2 ı
-
25 and others), strictly population specific - they should be used only on the populations from which the measurements were derived, and (2) the lifestyles of those target populations should be studied so that the researcher is aware of the effects of occupation and lifestyle on the areas of the skeleton to be used for sex determination.REFERENCES
France, D. L. (1983) Sexual Dimorphism in the Human Humerus, Ph.D. Dissertaıion, University of Colorado, Boulder.
2 France, D. L. (1988) Am. J. Phys. Anthropol., 76, 515-526.
3 Dwight, (1905) Am. 1. Anat., 4, 19-3 ı.
4 Stewart, T. D. (1979) Essentials of Forensic Anthropology, Especially as Developed in the United
States, Charles C. Thomas, Springfield, IL.
5 Işean, M. Y., Miller-Shaivitz, P. (1984) 1. Forensic. Sci., 29, 1087-1093.
6 Schaeffer, J. P. (ed.) (1947) Morris' Human Anatomy: A Complete Systematic Treatise, The Blackstone Co., Philadelphia.
7 Howells, W. W. (1973) Cranial Variation in Man, Papers of the Peabody Museum of Archaeology and Ethnology, 67, Harvard University, Cambridge.
8 Marshall, L. (1959) Africa, 29, 335-364.
9 Thomas, E. M. (1959) The Harmless People, Alfred A. Knopf, New York.
10 Lee. R. B. (1966) Subsistence Ecology of !Kung Bushmen, Ph.D. Dissertation, University of Califomia, Berkeley.
11 Shooter, J. (1957) The Kafirs of Natal and the Zulu Country, E. Stanford, London.
12 Krige, E. J. (1965) The Social System of the Zulus, Shuter & Shooter, Pietennaritzburg.
13 Bryant, A. T. (1929) Olden Times in Aululand and Natal Containing Earlier Political History of the Eastern-Nguni Clans, Longmans Green, London.
14 Bryant, A. T. (1949) The Zulu People as They Were Before the White Man Came, Shuter & Shooter, Pietennaritzburg.
15 Kohler, M. (1933) Marriage Customs in Southem Natal (Wannelo, N. J. V., ed.), Union of South Africa, Department of Native Affairs, Ethnologieal Publ., 4, Pretoria, GOV'[ Printer.
16 Griaule, M. (1965) Conversations wi/h Ogo/emmeli : An Introduction to Dogon Religious Ideas,
Oxford University Press, London.
17 Palau Marti, M. (1957) Les Dogon, Presses Universitaires de France, Paris.
18 Man, E. ıı. (1932) On the Aboriginallnhabitants of the Andaman Islands, The Royal Anthropological Institute of Great Britain and Ireland, London.
19 Cipriani, L. (1961) An/hropos, 56,481-500.
20 Cipriani, L. (1966) The Andaman Islanders, (Cox, D.T., ed.) Frederick A. Praeger, New York. 21 Birkby, W. H. (1966) Am. 1. Phys. Anthropol., 24, 21-28.
M. (1981) 1. I/um. Eı o!., J (EISS) Hum. Bio/., 27, 12-21
24 Kccn, J. A. (1950) Am. J. Phys. Anlhropol., 8, 65-79. 25 Kajanoja, P. (1966) Am. 1. Phys. Anlhropol., 24, 29-34.
Rcprints request to
Diane L Francc Department of AnthropoJogy, ("olarada State l'niversity, Colorado 80523, USA
