5. Barnes, B. M. Science 244, 1593–1595 (1989).6. Lyman, C. P., Willis, J. S., Malan, A. & Wang, L. C. H. Hibernation

and Torpor in Mammals and Birds (Academic, New York, 1982).7. Frerichs, K. U. et al. Proc. Natl Acad. Sci. USA 95, 14511–14516

(1998).8. Aloia, R. C. Fed. Proc. 39, 2974–2979 (1980).9. Geiser, F., Kenagy, G. J. & Wingfield, J. C. J. Comp. Physiol. B

167, 416–422 (1997).

entered torpor after a brief phase of activityin the evening (Fig. 2), became active againnear sunrise and frequently entered a sec-ond bout of torpor after they had flown totheir day roost. Dawn torpor was briefer(3.551.2 hours) than night torpor(7.051.2 hours) and was often terminatedby passive rewarming in the sun.

Our study shows that the large frog-mouth regularly enters torpor in winter. Weargue that the use of torpor enables the birdto survive in a wide range of habitats and toremain resident in its territory throughoutthe year7, despite feeding on arthropods.Because torpor occurs in this large bird andbecause birds are more diverse and smalleron average than mammals, we predict thatavian torpor is much more common than iscurrently believed.Gerhard Körtner, R. Mark Brigham*, Fritz GeiserSchool of Biological Sciences, University of NewEngland, Armidale 2351, Australiae-mail: fgeiser@metz.une.edu.au*Permanent address: Department of Biology,University of Regina, Regina, Saskatchewan S4S 0A2, Canada

1. Lyman, C. P., Willis, J. S., Malan, A. & Wang, L. C. H. Hibernation

and Torpor in Mammals and Birds (Academic, New York, 1982).

2. Geiser, F. & Ruf, T. Physiol. Zool. 68, 935–966 (1995).

3. Cossins, A. R. & Barnes, B. M. Nature 382, 582–583 (1996).

4. Brigham, R. M. Physiol. Zool. 65, 457–472 (1992).

5. Reintertsen, R. E. Polar Res. 1, 269–284 (1983).

6. Körtner, G. & Geiser, F. Oecologia 123, 350–357 (2000).

7. Körtner, G. & Geiser, F. J. Zool. Lond. 248, 501–507 (1999).

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other Communication Disorders, National Institutesof Health, Bethesda, Maryland 20892, USAe-mail: kacharb@nidcd.nih.gov1. Arav, A. et al. Cryobiology 33, 589–599 (1996).2. Crowe, J. H. et al. Cryobiology 38, 180–191 (1999).3. Williams, W. P. Phil. Trans. R. Soc. Lond. B 326, 555–570 (1990).4. Wada, H. et al. Nature 347, 200–203 (1990).

brief communications

Metabolism

Winter torpor in a large bird

Torpor is a natural state in which ani-mals show a substantial and controlledreduction of body temperature to con-

serve energy1,2. A few small birds (weighingless than 80 g) are known to use it as a sur-vival strategy in winter, but we have discov-ered that a large bird, the Australian tawnyfrogmouth, which weighs 500 g, can alsoenter this state. This surprising findingincreases the size of birds known to use nat-ural torpor by almost tenfold, suggestingthat avian torpor is more widespread thanis commonly believed, enabling birds tostay in their territory throughout the year.

Small endothermic birds and mammalshave enormous food requirements becauseof the fast metabolic rate that is necessary toregulate a high body temperature (Tb).When this high metabolic rate is unsustain-able, for example in periods of adverseweather and/or food shortage, many smallmammals (body mass 2–10,000 g) surviveby entering a state of torpor1–3. The study oftorpor in birds has so far been restricted tospecies weighing less than 80 g (refs 2,4,5),and rather than using torpor to overcome

adverse conditions, birds may migrate toavoid them. However, many birds are seden-tary and often rely on ephemeral, weather-dependent food sources — so how do theyovercome periodic energy bottlenecks?

To answer this question, we investigatedwhether the Australian tawny frogmouth(Podargus strigoides ; Fig. 1), a sedentarybird which feeds mainly on arthropods,uses torpor in the wild. The study wasconducted from the Australian autumn tosummer in an open woodland of Eucalyptusand Acacia at 1,000 m altitude in a cooltemperate area near Armidale, New SouthWales. We captured seven frogmouths andfitted them with temperature-sensitivetransmitters (calibrated to the nearest 0.1°C) weighing 3 g. All birds received anexternal backpack-style transmitter4 (longrange) to measure skin temperature (Tskin),and three birds had a second internal trans-mitter (short range), to measure core Tb

and to determine Tb1Tskin differentials,implanted under general anaesthesia.Transmitter signals were recorded at 10-min intervals for up to nine months6.

All individuals entered torpor in winter:Tb fluctuated around 38–40 °C duringactivity and fell to about 36 °C during therest phase, with a lower limit of 34 °C. Oncold (less than 7 °C) winter nights inJune–August, Tb fell below 34 °C on 202 of462 observation days (44%). The minimumTb recorded by an internal transmitter was29.1 °C, the lowest Tb calculated from Tskin

and the Tb1Tskin differential was 27.2 °C,and the mean minimum Tb for the sevenbirds was 29.051.0 °C. Birds usually

0

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Figure 1 Tawny frogmouth (Podargus strigoides ).

Figure 2 Fluctuations in body temperature of large Australian

tawny frogmouth (Tb, filled symbols, measured by an internal

transmitter) and in air temperature (Ta, solid line) over 4 days in

June (winter). Tb was high at the beginning of the night (black

bars), and then declined. The bird was aroused before dawn, after

which it changed its roost and re-entered torpor. Tb increased

from mid-morning to peak around sunset.

Ageing

Cloning of mice to sixgenerations

Mice have been cloned by nucleartransfer into enucleated oocytes1–3,and here we describe the reiterative

cloning of mice to four and six generationsin two independent lines. Successive gener-ations showed no signs of prematureageing, as judged by gross behaviouralparameters, and there was no evidence ofshortening of telomeres at the ends of chro-mosomes, normally an indicator of cellularsenescence — in fact, these appeared toincrease slightly in length. This increase issurprising, given that the number of mitoticdivisions greatly exceeds that of sexuallyproduced animals and that any deleteriouseffects of cloning might be expected to beamplified in sequentially cloned mice. Ourresults offer a new approach to the study oforganismal ageing.

Founder clones (G1) of two lines, A andB, were generated using cumulus cells ofadult female B6C3F1 mice (agouti coatcolour) as nucleus donors, and oocytesfrom adult B6D2F1 (black) females asnucleus recipients. The cloning procedure

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was repeated with cumulus cells from adultG1 mice as nucleus donors to produce thenext clonal generation, G2, and so on. Table1 summarizes the results obtained followingthe reconstruction of 3,920 enucleatedoocytes.

Previously, about 2% of enucleatedoocytes receiving a cumulus cell nucleusdeveloped to live-born pups1. This value iscomparable to the cloning efficiency for G1in lines A (1.5%) and B (4.2%). However,the success rate dropped in successivecloned generations: line A did not producea G5 clone from 670 reconstructed oocytes;in line B, the only live-born G6 clone wascannibalized by her foster mother, therebyterminating the line. Mouse lines A and Btherefore represent totals of 9 and 26 clonesfrom their respective donors. Placental sizewas consistently in the range previouslyreported for cloned mice2 and did notincrease in successive generations (data notshown).

Do sequentially cloned mice show signsof accelerated ageing? We assessed thebehaviour of these mice and determinedtelomere lengths to assess organismal andcellular measures of ageing, respectively. Weevaluated learning ability in the Morriswater maze and Krushinsky tests, as well asstrength and agility, and also used other

assays designed to monitor signs of prema-ture ageing, such as a decline in activity inthe home cage and loss of coordination4. Allcloned mice were, by these criteria, normalcompared with age-matched controls (datanot shown); the G5 mouse is alive andhealthy at 1.5 years.

We measured telomere length in periph-eral blood lymphocytes of clones G1–G6 bySouthern-blot analysis of terminal restric-tion-enzyme-digested fragments (Fig. 1)and found no evidence of shortened telom-eres in the cloned mice. In fact, our resultsshow that the telomeres lengthen with eachgeneration. As representative animals ofeach generation were sampled simultane-ously, we cannot rule out an age-relatedcontribution to this increase (with youngermice having longer telomeres). In addition,long telomeres in mice are optimally studiedby means of different assays such as quanti-tative fluorescence in situ hybridization5. Wehave detected telomerase activity in cumuluscells (data not shown); it is therefore possi-ble that telomeres in these cells are unusual-ly long, resulting in offspring withconcomitantly longer telomeres.

Shortened6 and lengthened7 telomereshave been reported in cloned livestock but,unlike ours, those experiments involvedonly a single round of cloning. Our results

brief communications

NATURE | VOL 407 | 21 SEPTEMBER 2000 | www.nature.com 319

on sequentially cloned mice verify thattelomere shortening is not a necessary out-come of the cloning process8. However, asonly 1–2% of reconstructed oocytes yieldlive-born clones, the possibility of selectionfor donor nuclei with the longest telomerescannot be excluded. Further investigation isrequired into the consequences of nucleartransfer on telomere length and lifespan.Teruhiko Wakayama*†, Yoichi Shinkai‡,Kellie L. K. Tamashiro*, Hiroyuki Niida§, D. Caroline Blanchard||, Robert J.Blanchard||, Atsuo Ogura¶, KentaroTanemura¶, Makoto Tachibana‡, Anthony C. F. Perry#, Diana F. Colgan#,Peter Mombaerts#, Ryuzo Yanagimachi**Department of Anatomy and Reproductive Biology,John A. Burns School of Medicine, University ofHawaii, Honolulu, Hawaii 96822, USA‡Department of Cell Biology, Institute for VirusResearch, Kyoto University, Sakyo-Ku, Kyoto 606-8507, Japan§Life Sciences Division, Lawrence Berkeley NationalLaboratory, University of California, Berkeley,Cailfornia 94720, USA||Bekesy Laboratory of Neurobiology, University ofHawaii, Honolulu, Hawaii 96822, USA¶Department of Veterinary Science, NationalInstitute of Infectious Disease, Tokyo 162-8640,Japan#The Rockefeller University, 1230 York Avenue, New York, New York 10021, USA†Present address: The Rockefeller University, 1230York Avenue, New York, New York 10021, USAe-mail: wakayat@mail.rockefeller.edu1. Wakayama, T., Perry, A. C. F., Zuccotti, M., Johnson, K. R. &

Yanagimachi, R. Nature 394, 369–374 (1998).

2. Wakayama, T. & Yanagimachi, R. Nature Genet. 22, 127–128

(1999).

3. Wakayama, T., Rodriguez, I., Perry, A. C. F., Yanagimachi, R. &

Mombaerts, P. Proc. Natl Acad. Sci. USA 96, 14984–14989

(1999).

4. Tamashiro, K. L., Wakayama, T., Blanchard, R. J., Blanchard,

D. C. & Yanagimachi, R. Biol. Reprod. 63, 328–334 (2000).

5. Zijlmans, J. M. et al. Proc. Natl Acad. Sci. USA 94, 7423–7428

(1997).

6. Shiels, P. G. et al. Nature 399, 316–317 (1999).

7. Lanza, R. P. et al. Science 288, 665–669 (2000).

8. Wilmut, I., Clark, J. & Harley, C. B. Nature Biotech. 18, 599–600

(2000).

Gene expression

Total silencing by intron-spliced hairpin RNAs

Post-transcriptional gene silencing(PTGS), a sequence-specific RNAdegradation mechanism inherent in

many life-forms, can be induced in plantsby transforming them with either antisense1

or co-suppression2 constructs, but typicallythis results in only a small proportion ofsilenced individuals. Here we show thatgene constructs encoding intron-splicedRNA with a hairpin structure can inducePTGS with almost 100% efficiency whendirected against viruses or endogenous

48.5

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Figure 1 Telomere lengths in successive generations (G1–G5) of mice cloned from cumulus cells. Southern-blot analysis of terminal

restriction-enzyme-cut fragments in five sequential generations shows that telomeres do not undergo incremental erosion in successive

clonal generations. Genomic DNA isolated from peripheral-blood lymphocytes taken from representative animals from each generation

was digested with the restriction enzyme HinfI, resolved on a pulse field gel, transferred to a solid support and probed with a 58-32P-

labelled (T2AG3)3 oligonucleotide. Peripheral blood lymphocytes were sampled on the same day. Ages of mice (in months) were: in line A,

donor, 18; G1, 16; G2, 14; G3, 12; G4, 9; G5, 9; in line B, G1, 15.5; G2, 13; G3, 11; G4, 9; G5, 7. Suffix numbers (G4-1 and G4-2, for

example) identify different pups of each generation.

Table 1 Effect of sequential cloning on full-term development

Line G1 G2 G3 G4 G5 G6 Total

A 2/131 1/228 1/263 5/238 0/670 - 9/1,530(1.5) (0.4) (0.4) (2.1) (0) (0.6)

B 4/96 7/351 5/352 6/286 3/581 1/724 26/2,390(4.2) (2.0) (1.4) (2.1) (0.5) (0.1) (1.1)

Successive generations are represented as G1, G2 and so on for two independent mouse lines, A and B. The number of pups born live after cumulus-cellnuclear transfer is compared to successfully reconstructed oocytes (pups/oocytes), with the corresponding percentages in parentheses. Significant x 2

comparisons were derived for G4 and G5 from line A, G1 and G5, G6 from line B, and G2, G3, G4 versus G6 from line B (P*0.05).

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