craneo sylvilagus

8/21/2019 craneo sylvilagus

1/10

The University of Notre ame

Growth of the Skull in the Cottontail (Sylvilagus floridanus) and Its Application to Age-DeterminationAuthor(s): Donald F. Hoffmeister and Earl G. ZimmermanSource: American Midland Naturalist, Vol. 78, No. 1 (Jul., 1967), pp. 198-206Published by: The University of Notre DameStable URL: http://www.jstor.org/stable/2423380.

Accessed: 05/03/2014 11:50

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at.http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of

content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms

of scholarship. For more information about JSTOR, please contact [email protected].

.

The University of Notre Dameis collaborating with JSTOR to digitize, preserve and extend access toAmerican

Midland Naturalist.

http://www.jstor.org

This content downloaded from 132.248.181.49 on Wed, 5 Mar 2014 11:50:10 AMAll use subject to JSTOR Terms and Conditions
http://www.jstor.org/action/showPublisher?publisherCode=notredamehttp://www.jstor.org/stable/2423380?origin=JSTOR-pdfhttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/stable/2423380?origin=JSTOR-pdfhttp://www.jstor.org/action/showPublisher?publisherCode=notredame


2/10

GrowthftheSkull nthe CottontailSylvilaguslori-danus) nd tsApplicationoAge-DeterminationDONALD F. HOFFMEISTER nd EARLG. ZIMMERMANMuseum ofNatural History,University f Illinois, Urbana 61801ABSTRACT: Skulls of 125 cottontails,Sylvilagus floridanus,of agesdeterminedfromlens-weight,were available fromIroquois and Cham-paign counties, east-central llinois. An attemptwas made to findfea-turesof the skull that mightbe useful in placing these rabbits into age-groups. Later, the lens-weightwas correlated with thesegroups. In very

young animals, less than 21 days old, the teeth are not fullyerupted;group 1 animals, 21-92 days lens-age, have no fusionanywhere long theexoccipital-supraoccipital uture; group 2 animals, 93-105 days lens-age,have the sutureclearlyvisible,but there s some fusionalong it, especial-ly near one end; in group 3 animals, 106-170 days lens-age, the sutureis fused along its entire lengthbut not obliterated,although it may beseen in some cases onlywitha lens or microscope; group 4 animals,morethan 170 days lens-age,have the sutureno longerdiscernible. Growth ofthe skull and some of its cranial elements from age 5 days (approxi-mate) to and beyond 260 days are graphed. Using the age-groupsandthe graphs, the age of the skull can be ascertained with fair accuracy.Ninety percent of adult size is reached by 125 days of age, 96.5% by170 days. The most rapid growthoccurs during the first 0 days. Al-thoughthere is no actual decrease in size in any part of the skull mea-sured,there s a relativedecrease in zygomaticbreadth,breadth of brain-case, postorbitalbreadth,and parietal breadth.Specimens of rabbits and hares can be aged with fair accuracyby the weight of the lens and with much less accuracy by the lengthof the tarsus. For many of the specimens used by taxonomists,systematists,wildlifemanagers, and ecologists,lenses or tarsal lengthmay not be available. It would be advantageous to have otherfeatures, especially in the skull, for aging rabbits. To ascertain ifother characters might be employed, we obtained a large series ofskulls of cottontails,Sylvilagus floridanus,from Iroquois and Cham-paign counties, Illinois. Nearly all had the lenses saved. The Iroquois

Co. specimens, obtained from hunters, were taken between mid-November and mid-January, 1960-1 , fronm the following localities:Beaverville, Donovan, and Iroquois; all are within a 6-mi. radius.The Champaign Co. cottontails (18 in number) were trapped inJune-July, 1965, on the University of Illinois farms on the southernedge of Champaign-Urbana. Specimens from the two counties areonly 60 miles apart. The observations reported here are basedentirely on wild-living animals. The results should be checked againstcaptive cottontails in which the precise ages are known. However,sometimes animals in captivity have parts of the skull and patternsof the dentition affected differently than in the wild, and the resultsmight not be readily applicable to work in taxonomy and systematics.The collected animals were salvaged by workers of the IllinoisNatural History Survey who measured the hind foot, weighed some198

http://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsphttp://www.jstor.org/page/info/about/policies/terms.jsp


3/10

1967 HOFFMEISTER ET AL.: GROWTH OF SKULL 199of the animals, sexed them, and saved the heads. Of the 186 headssaved, 125 had the lenses preserved. We are indebted to the IllinoisNatural History Survey, especially to Glen Sanderson, William Ed-wards, James Bailey, and Rexford Lord for their cooperation. TheNatural History Survey not only salvaged these skulls but alsotreated and weighed the lenses forus. The method employed is thatoutlined by Lord (1959). The skulls were cleaned by dermestidbeetles so that no parts of the skull were damaged other than thatresulting from gunshot or through some handling by the collector,and all are preserved in the Museum of Natural History.

The work was supported in part by a grant from the NationalScience Foundation, GB 1432.The skulls studied intensively were those for which lenses hadbeen saved. These skulls were arranged by us in what appeared to bean age-sequence without any knowledge of the ages as determinedfrom the lenses. Characters other than size were sought, such aspresence or absence (closure) of sutures, profiles of the skull, in-flation of the bullae, porosity of the bone, and wear on the teeth.Fourteen cranial measurements for each specimen, taken for us byMr. John Lynch after the fashion described by Orr (1940:3-4) andHoffmeisterand Lee (1963:502) were also used to segregate thespecimens into "ages." Cranial measurements, especially greatestlength, basilar length, and zygomatic breadth, proved useful inseparating only the younger animals. All of the morphologicalfeatures gave no help in putting animals more than 3 weeks oldinto age-categories except for the closure of the exoccipital-supraoc-cipital suture. This one feature, depending upon the degree offusion as described by Hoffmeisterand Lee (1963), did segregatethe animals into four groups.The use of the lens-weightfor cottontails from our study areafor determining age to the month for their firstyear of life wasadvocated by Lord (1963:33). More recently, Rongstad (1966)working with cottontails in southern Wisconsin maintains that lens-weight is sufficientlyccurate only to separate animals less than oneyear from those older than one year. Additional studies on thegrowth, variation, and significanceof the lens in the cottontail seemnecessary. We have converted lens-weight to age in days for ourskulls but are cognizant, especially in view o,fRongstad's (1966)and Dudzinski and Mykytowycz's (1961) reports, that such preciseage may not have significance. Age down to days did permit us toplace the skulls in a sequence for visual and measurable analysis.Furthermore, n our analysis of age-categories (1 to 4), we are awarethat precise age-limits in days cannot be assessed although we havesuggested them. The orderly increase in size in age-intervals formost measurements, as shown in Table 1, would indicate that theaging sequence must be fairly eliable.Cottontails less than 38 days of age are not available from thearea studied. However, there are three unaged animals judged to



4/10

200 THE AMERICAN MIDLAND NATURALIST 78(1)be between 5 and 15 days. From southwesternOhio, we have anothersix animnals f about the same age and one animal possibly as oldas 21 days. Animals less than 21 days do not have the cheek teethfully erupted and can be distinguished in this manner from olderanimals. 1cm-

2 3 4Fig. 1.-Posterior view of the skulls of Sylvilagus floridanusshowing thechanges in the exoccipital-supraoccipital uturefrom ge-category1 to 4. Draw-ings prepared by Alice Boatright.20 40 60 80 100 120 140 160 180 200 220 240 260 280 Ad.

10698940 ~~~~~~~~~9086f t ~~~~~~82787478 _ t < 4 4 _ ] _ - _ _ 370

66~ ~ ~~ ~~~~~~~~~~~~~~~674638

20 400 80 00 20 140 60 180 200 220 240 260 280 Ad.

10001 0

Fig. 2.-Changes in the length of the hind foot and size (or parts) of theskull with increasing age in Sylvilagus floridanus. Age in days and age-groupsare indicated along the abscissa; size of parts in mm along the ordinate. Thearrow indicates which group of ordinate measurements o use.



5/10

1967 HOFFMEISTER ET AL.: GROWTH OF SKULL 201The teeth are fully erupted but the exoccipital-supraoiccipitalSuture is open in skulls in group 1, and there is no, evidence offusion. nywhere along the suture. Fig. 1 shows the condition of thesuture. In group 2, this suture is clearly visible, but there is fusionat one end or somewhere along the suture. In group 3,5 the suturecan be discerned usually only with the aid of a hand lens or micro-scope. The suture is fused,but not obliterated,along its entirecength,resulting in a slightlyelevate-dridge on the occipital region of the

20 40 60 80 100 120 140 160 180 200 220 240 260 280 Ad.36323028262438 P22

36 1 1 1 lI-3 534333029 3029282700 ~~~~~~~~26

0 ~~~~~~~~~~~~~~2426 O25 40 I23I22212019181713I12I0986

20 40 .60 80 100 120 140 160 l80 200 220 2-40 2-60 280o Ad.'o 0#01 0Fig. 2.-(continued)



6/10

202 THE AMERICAN MIDLAND NATURALIST 78(1)skull. In group 4, the exoccipital-supraoccipitalsuture is no longerdiscernible, and there is no ridge on the occiput.Within a group it is not possible to arrange skulls in an age-sequence except possibly by size. After our attempt to arrange theskulls in an age-sequence, we checked the resultsagainst the ages ofthe skull as determined fromthe lens-weight.Age of the animals as based on lens-weightrathercloselycoincideswith our age-categories 1 to 4 about as follows: group 1 includesanimals fromabout 21 to 92 days of age; group 2, about 93 to 105days; group 3, about 106 to 170 days; group 4, more than 170 days.Nearly all skulls placed in the four categories fell within the properage-sequences, but two skulls in group 2 are 127 and 133 days,respectively, nd thus should have been in group 3. Three skulls ingroup 4 should have been in #3, and three skulls in group 3 shouldhave been in #4. Our age-arrangementof skulls within each categorydid not correlatewell with the lens-age of the animals, but we wereaware that our arrangement within groups was questionable basedsolely on suture-fusion, nd that lens-weight s variable also.Skulls aged by lens-weightrange from38 days to 260 days, plus12 others that are in excess of 260 days, but whose ages cannot beprecisely allocated to days. These 12 have been averaged togetherand plotted on graphs as adults (Ad). In addition, there are threevery young animals (5 to 15 days old). Animals less than 70 daysold are few. The most continuous series is the 165-day intervalbetween 80 and 245 days with 93 skulls.Cranial measurements and length of hind foot for each individualwere plotted on a separate graph. Such graphs indicated the changeswith increasing age but also show the individual variation (Fig. 2).The sex of nearly all skulls is available and we graphed the measure-ments separately for males and females. No consistent differences nthe graphs can be noted so the sexes are combined in the tables andfigures. Some cranial measurementswere so, variable -postorbitalconstriction,width of nasals, least length of palatal bridge, depth ofrostrum, ength of bullae, breadth of occipital shield-that theyarenot shown.Study of the measurements and graphs (Fig. 2 and Table 1)indicates that not all features reach adult-size at the same time.However, by 170 days (5 2/3 months), all have reached 96.5'% ormore of adult-size and as early as 125 days (4 months), all havereached 90%o or more. Body-weight and total length correspondrather closely to these percentages also (Lord, 1963: 29; Petrides,1951: 322). Hind foot, breadth of braincase, and parietal breadthattain adult-size earliest. Nasals continue to grow throughout thelife of the animals, perhaps more than othermeasurements,but theremay be a slightincrease in all cranial measurements.No actual decrease in size with increasing age occurs in anymeasurement. A relative decrease (relative to greatest length) doesoccur in zygomatic breadth, breadth of the braincase, postorbital



7/10

1967 HOFFMEISTER ET AL.: GROWTH OF SKULL 203Ln Ln) ~ . CICf 0

z L6oo o~ C C- C Lo 0-C\- C\ OLO O CLO ---IJ --I~ ~ CO CO- Cj~

(CI LC) C- Col CO --I CM c

CMi LC) 00 0) ~ ~ Ca-CM L) C --i Ln4 LCO Ci tL0-( C6 ~C:Mr ~ C-

LO LO LO (.0 - 0 )C-0 L -. \ C~ C~ C 0) (.0) . C)~ 0)(00 LC)' LO CM ~(0 (.C ( LO C\ .(- .( ~C0 .-(,-( ~~LC) C. C CO .-- CM C

U)) C\ c)cl 'n IOLO --'I LC\ OC-. C(. O.v C's ~ ~ ~ - C O M C

C\f CO -. CM C(.0 LjU 0 jU (.0 -4-- c~~ico co.~~ o-.- o cMQo ~CO~ 0CM~ 0(.0 ~6oo(.0 oCM COib .-)-) LO C-. CO '-1CO '-)-I CM C

b.00 CO (C- C-.j (OC CO(. CO- C--C C-.(. CO. C-'- C

Os~~~~~C CM~Oc.0~~~~~~~~~~~~~~~~~~~~0SO J C LO CrO r CO~ LC)Co L)-4 'Uj.- Uit ' EjULC(C-C a) (.0 CO CM '- CMf CM

CO CO CM(. 0) (.0 )

4() IC(0 j IC) CO CMCM C0~~~~~~~~~~~~~~~~~00 DO ---IC) OrC\--- .---C

(.0 (.0 CM CO 0) CO CO- ~ LEC) COCM n CO )O r)C0 C(. COIC COC COCO 'I COO)bl)

bi)

cC .~~.o *~Q .0 ~-'.. (C.0 Ci)U) 0 .0Cs CS.0UCi) CU O 0 C' Cs -. (C m 5 C' (C'

.74 r. bi)cCC M 0- bC) ~ 0CC b) 00 n)C C r0 U0 cubi c Q0 N Cs u



8/10

204 THE AMERICAN MIDLAND NATURALIST 78(1)00 C1n C& Co Io,l C'I& Co n nS

1\& C; CtC\l C\ + cli ce eI C ' ) L )a) C) 0c ' 6 6 o ( 6t m -C' 'o '' C' Co CO(Z C\ + cn cli c\n0 1 C" Cm ' CO C' C'M

t - 00 C, a; C CO+ + nn

C O C* I co coLr) ) cn o co Cm c~~~~~~~ ~ ~ ~ ~ ~~'~c cr COl CO \l 0I CoC c" C

4o 1n m h o + s o

-w l mc co c' CS 0'- O7 rX o +~~~~~~~~~~~~~L cn cn --bi Lr o Lq cl I- cl clIo c '' r C-

00 a) L cn0 CO~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0o l0~~~~~~~~~~~~~~~~~~~~0O= C' '- Cf) 6 o coC C' ' C'< ~ C~I t' *-0 *) 0o Co CoC) LO c' CO '\'0 - t~ '- o Oe C' C'.

I l ' Cs - C' 0- 0 c

CO LO) CO C" Co C"C' Co Co Co Co Co C -6r- -4 rZ: (6 l

l ce o ~~~~~~~~~~ ~~~~~clili Lr c\) Lf)-o (.0 C" C' '' L O C"

H r > D C ; oX O D -q he. glq ~~~~~~~~~~~-4



9/10

1967 HOFFMEISTER ET AL.: GROWTH OF SKULL 205breadth, and parietal breadth (Table 2). This corresponds to theage-changes in Citellus beecheyi as found by Hall (1926). In Neo-toma micropus (Allen, 1894) and Peromnyscus ruei (Hoffmeister,1951), there is a relative decrease in breadth of braincase and post-orbital breadth. The only relative increase in Sylvilagus is in thelength of nasals. This corresponds rather well to the change in C.beecheyi,N. micropus, and P. truei. Length of the molar series andpalatal bridge show no distinct relative change but remain fairlystable.Specimens in the four age-groups, as determined by condition ofthe exoccip,ital-supraoccipital uture, fit in the pattern of growth ofthe skull as noted by the divisionsof the graphs (Fig. 2). Skulls andhind feet in animals in group 4, beginningwith 170 days, are fairlyuniformfor they have attained 96.5% or more of adult-size.Group 3animals are still growing,and everymeasurementshows an increase,with the percent of increase varyingfrom 3 to 12 and averaging 6.5.It seems inadvisable to regard groups 3 and 4 as a single, uniformgroup of "adults," for there is too much growth-change n group 3.Actually, about half-way throuughroup 3, at 140 days, animals morenearly attain adult-size, and most growth-curveshave leveled orbegin to level off. Very young animals, younger than group 1, showthe greatestchanges in size; group 1 shows the next greatestchangebut group 2 shows nearly the same.

AGINGOF SKULLSSkulls with partially erupted cheek teethare less than 21 days old.By employingthe criteria for closure of the exoccipital-supraoccipitalsuture (Fig. 1) and erupted cheek teeth, skulls can be placed in 4groups whose ages are approximately: 22-92, 92-105, 105-170,171-days and older. By positioningseven cranial measurements withtheirrespectivecurves on Fig. 2, we can furtherrefine the age for agiven skull. To check this, we have run skullsthroughthis procedure.The accuracy of age-determinationn days is greatest up to the 170thday; after this it is difficultto give age in days for growth is soslight. The reliabilitybelow 170 days is not especially great becauseof individual variation and variation in lens-weight. For example,the determined age of a skull may prove too,old in four of sevenmeasurements and too young in the other three. Furthermore,forsome measurements, one can only say the specimen falls between120-150, or 110-140 days. To test ourselves,we checked a specimencaught in Piatt County, Illinois (no. 21789). We determinedfromskull measurements and the exoccipital-supraoccipital suture that itwas between 80 and 92 days old. Later we found that the lens,hadbeen weighed as 100 mg and the specimen was determined to, be 3months old (see Casteel, 1961: 541). This estimationwould seemto indicate that our criteria for aging are fairly reliable, but weshould emphasize the variability n older animals for increasing error.This variability may even prevent any accuracy in aging of skulls in



10/10

206 THE AMERICAN MIDLAND NATURALIST 78(1)group 3, and one would simply ndicate that the specimenwas "early"or "late" in development n this group.

REFERENCE SALLEN, J. A. 1894. Cranial variations n Neotoma micropusdue to growthandindividual differentiation. ull. Amer. Mus. Natur. Hist., 6:233-246.CASTEEL, D. A. 1961. A white cottontail rabbit. J. Mammal., 42:541.DUDZINSKI, M. L. AND R. MYKYTOWYCZ.1961. The eye lens as an indicator ofage in the wild rabbit in Australia. C.S.I.R.O. Wildl. Res., 6:156-159.HALL, E. R. 1926. Changes during growth in the skull of the rodent Oto-spermophilusgrammurusbeecheyi. Univ. California Publ. Zool., 21:355-404.HOFFMEISTER, D. F. 1951. A taxonomic and evolutionary tudy of the pifionmouse, Peromyscus truei. Illinois Biol. Monogr., 21: ix + 104 p.

AND M. R. LEE. 1963. Revision of the desert cottontail, Sylvilagusaudubonii, in the Southwest. J. Mammal., 44:501-518.LORD, R. D., JR. 1959. The lens as an indicator of age in cottontailrabbits.J. Wildl. Mgmt., 23:358-360.-. 1963. The cottontail rabbit in Illinois. Illinois Dept. Cons. Tech.Bull., No. 3: xi + 94 p.ORR, R. T. 1940. The rabbits of California. Occas. Pap. California Acad. Sci.,19:1-227.PETRIDES, G. A. 1951. The determinationof sex and age ratios in the cotton-

tail rabbit. Amer. Midl. Natur., 46:312-336.RONGSTAD, 0. J. 1966. A cottontail rabbit lens-growth urve from southernWisconsin. J. Wildl. Mgmt., 30:114-121.SUBMITTED 13 MAY 1966 ACCEPTED 8 JUNE 1966

craneo sylvilagus

Documents

Transcript of craneo sylvilagus