research reports g

FIGURE 1

Underwater photograph of swimming red kangaroo showing alternate hindlimb movement,and movement in unison of ipsilateral fore and hind limbs.

How Kangaroos Swim

GEORGE R. WILSON*

During extensive flooding in easternAustralia in ]974, many kangarooswere marooned on islands. A]ong theDarling River and in the BullooOverflow, groups of up to 150 red. andgrey kangaroos were sighted from theair. When frightened, they often tookto the water and one grey kangaroowas seen to swim over 300 metres.

While Troughton (1967) and Breedenand Breeden (1966) describedkangaroos entering water to defendthemselves when chased by dogs andGould (1863) reported a 2-mile swimby a kangaroo during a hunt, therehave been no studies of the details ofthe manner of swimming, or thecapacity of kangaroos to swim overextended distances.

To elucidate the swimming action ofkangaroos, underwater photographswere taken of two male red kangaroos,Mega/eia ruta, in a swimming pool,They were 3 and 4 years of age andweighed 25 and 3] kg respectively.Both had been bred in captivity andpreviously had not swum. Camerasused were a Bo]ex ]6-mm in a perspexhousing and a Nikonos 35-JTlm.

When first submerged in water thekangaroos showed a diving reflex;they did not inhale water, and thenmaintained their heads and necksabove the surface. Initially theyattempted to hop, but after] 0-15seconds they began stroking smoothlyin an instinctive manner.

The forelimbs and hindlimbs were

moved ipsilaterally. TIlis is similar topacing, which is an unusual gait onland; the camel and the giraffe are theonly species to use it without training(Muybridge 1899).

TIle forelimbs of the swimmingkangaroo alternately make a posteriorpower stroke, with the digits extendedand the manus at its maximum size.

During the movement anteriorly themanus is repositioned in a flexedcondition and close to the body.

Although hindlimbs are also moved

* Dept of the Environment and Conservatiun,Canberra.

alternately, they appear to be moredirected to treading water for uprightstability and maintaining buoyancy ,rather than contributing much toforward motion. The need for buoyancywould be greatest at the hindquartersbecause on land this is the site of the

kangaroo's centre of gravity (Badoux,]965).

The tail was flexed horizontally, to theside of the rearward hindlimb (incontrast to the usual vertical motion of

the kangaroo tail). Although some ofthis swishing tail movement may be areaction to hindlimb movement, itnevertheless appears to help to drivethe kangaroo forward by sweepingfrom side to side while presenting aninclined surface in the water. This

action is similar to that of aquaticanimals as described by Gray (1957),Tricker (1966) and Tarasoff et al .(1972).

The speed at which the red kangarooswere swimming was estimated at 1msec-1 by timing them in a 12-m pool.The frequency of their limb and tailcontraction was 2.1-2.6 Hz. TIlis

frequency agrees well with the hoppingfrequencies reported for both redkangaroos (Dawson and Taylor, 1973),and the grey kangaroos (Stewart andSetchell ]974). It would seem that

there is a preferred frequency of thecycle of limb movement, whether itbe the simul taneous motion of bothlegs in hopping or their alternatemovement in swimming.

DiscussionKangaroos swim so well that it raisesquestions of whether their ability is anadaption to environment, or anindication of phylogeny, or acoincidence due to a morphologyamenable to the dynamics of swim­ming.

Any animal swimming forward meetsdrag or resistance which is a complexfunction of shape, size and speed, andto transport itself it must supply aforce, by propelling water backward,equal to this drag. To this end, swim­ming animals (like digging animals)have short powerful limbs, in contrastto the long fast-moving limbs ofcursorial animals (Hilderbrand 1960).The kangaroo has short forearms andlong hindlimbs, of which the formerare used in swimming and diggingwhile the latter are essentiallycursorial in type and function. Thetail motion resembles the tail motionsof fish, and it has been establishedthat fish (as would be expected) aremore efficient swimmers in energytemlS than are land animals paddlingat the surface (Schmidt-Nielson,]972); the difference would be largelydue to the turbulence generated,which in case of the ka~jgaroo tailwould be relatively low.

598 Search Vol. 5 No. 11-12, Nov-Dec 1974

N 5rlJtrOV:Jfl5UR51Tb51J~t51QBzi 5grlJever 1 es I s Ile 01 th se. or 0-

I 1!J5Nt5lw t~flgrn5v,omdf(1j~o t~'{:Joglc a nDues Wlllcn ass].';!ilie reak fl Od 0 flOUOITIJp05flBJ1U.fTU25f115d

angaroo In 'f.ater" e occaslOnion. Bf115u; ] ':J 10 IU 0 wwhIch they need t9rfWfo/,ar~~n requentin an environnltfurvwrn~ rarely floodsand then only along the limited areaof wit er courses. It is interesting tonote, however, that Euros, whosenormal habitat is mountain ranges,were seen in Kinchega National Parkto be poor swimmers and flounderawkwardly in water. Possibly thekangaroo's ability to swim developedas an adaptation to predators; there isanecdotal evidence of the use to whichkangaroos will put water whenpursued.

The ability of macropods to swimmust have an effect upon migrationand dispersion, and its significance inthe formation of subspecies on islandsis a subject warranting furtherattention. One kangaroo marked byBailey (197 I) was recorded on theother side of the Darling River,presumably having swum across;Main (196 I), describing the distribu­tion of macropods on offshore islandsin Western Australia, attributed theirpresence to movement over landbridges, not to swimming.

It seems unlikely that the 'pacing'swimming action of kangaroos hasany phylogenetic significance such asthat attributed to human infants.Under 4 months of age human infantsshow a reflex swimming behaviourwhich bears striking testimony to thephylogenesis of man (McGraw, 1939).ll1eir trotting action is comparableto that of most other animals such asthe horse and the dog which Kolb(I 962) described as the most apt ofdomestic animal swimmers.

ll1e separate-hindlimbs action ofswimming kangaroos is reminiscent ofphalangerids from whose aborealprogression Marshall (I 974) hasreviewed the development of thekangaroo's cursorial specializationsand the evolution of hopping. Windsorand Dagg (I 97 I) found that the walkof the tree kangaroo dendrolagus wasthe only gait of macropods in whichpairs of limbs were not usedsynchronously, and represen ted areturn to aboreal habitats by a wallaby.The separate hind-leg action of theswimming red kangaroo may thereforerepresent a reversion to earlier times,and the phylogenesis of the animal.

Nevertheless, the only aquatic

Search Vo!. 5 No. 11-12, Nov-Dec 1974

a-··-rsu is still the South America

opossum Chironectes minimus,and although kangaroos are com­petent swimmers they swim onlyunder unusual circumstances.

AcknowledgementsThe National Parks and Wildlife

Service of NSW employed the authorduring the period in which the investi­gation was carried out.

Mr G. Steer photographed and editedthe film which was provided by Mr L.Hudson. Mr K. Coles allowed us to

use his swimming pool. The kangarooswere housed in facilities provided bythe Faculty of Veterinary Science,University of Sydney and they wereattended by Mr G. Robertson andMiss C. Coles from the National Parksand Wildlife Service of NSW.

FIGURE 2

Below: Kangaroo's tail exhibits a wave-likemotion similar to that of a fish; right: alater stage in the cycle of motion of the tail.

References

BADOUX, D.M (1965) Acta anat. 62418.BAILEY, P.T. (1971) J. CSIRO Wildl. Res.16 11.

BREEDEN, S. and BREEDEN, K. (1966)The Life of the Kangaroo. Sydney: Angus& Robertson.

DAWSON, T.F. and TAYLOR, C.R. (1973)Nature 246 313.

GOULD, J. (1863) (1973) Kangaroos.Melbourne: Macmillan.

GRAY, J. (1957) Scient. Am. 197 248.

HILDEBRAND, M. (1960) Scient. Am.202 148.

KOLB, E. (1962) Lehrubuch derPhysiologie der Haustiere. Jena: GustavFischer.

MAIN, A.R. (1961) J. Roy. Soc. W. Aust.44 84.

MARSHALL. loG. (1974) Nature 248174.

MUYBRIDGE, E. (1899) Animals inmotion. London: Chapman & Hall 1957.

McG RAW, M.B. (1939) J. Pediatrics 15485.SCHMIDT-NIELSEN, K. (1972) Science177 222.

Continued

599

~... ,."'. ~:'''>••,

''110'- , !."~~ ",.~,

FIGURE 3 .Red kangaroo swimming past a diver with an underwater camera.

STEWART. I.C.F. and SETCHELL, P.J.(1974) Search 5107.TARASOFF. F.J .• BISAILLON. A.,PIERARD. J. and WHITT, A.P. (1972) Can.J. Zool. 50915.TRICKER, R.A.R. (1966) The Science ofMovement. London: Mills & Boon Ltd.

TROUGHTON. E. (1967) Furred Animalsof Australia. Sydney: Halstead Press.

WINDSOR, D.E. and DAGG, A.1. (1971)J. Zool .• Lond. 163 165.

Submitted 29 Mav. 1974

600 Search Vol. 5 No. 11-12, Nov-Dec 1974