spin quantum numbers 19/2 and 17/2. It isquite easy to detect atoms in different spinstates separately, and as there were no 17/2atoms present in the original mixture (only19/2 and 15/2), counting the number ofsuch atoms determines the number of mol-ecules that had been present. By measuringthe response of the molecules to different fieldfrequencies,Regal et al. were also able to mea-sure the extremely small binding energy of themolecules — of the order of only a nanoelec-tronvolt, and in excellent agreement with cal-culations based on the known properties oflow-energy collisions of potassium atoms.

This experiment opens up severalpromising avenues for development. Themolecules should have a temperature com-parable to that of the atoms from which theywere made — about 150 nanokelvins. Astudy of the momentum distribution and thecoherence properties of such a molecular gasis clearly in order, to see whether it mightactually be a molecular Bose–Einstein con-densate. There are also challenges to theory,such as understanding why only half of theatoms were converted to molecules.Anothermajor experimental goal is to demonstratethe existence of superfluidity in quantumdegenerate fermionic gases, which mightarise through the pairing of fermions in different spin states, in the same way thatCooper pairing of electrons is the basis ofsuperconductivity. Theory even suggeststhat the very strong coupling associated witha Feshbach-resonance state should make

such an effect much easier to observe4.A new era of research into ultracold mol-

ecular gases may be about to begin. There ismuch to do. For example, little is knownabout the collisions or chemical reactivity ofultracold molecules, which will determinethe lifetime of such gases. Ultracold mole-cules also have potential uses in precise timeand frequency measurements, in the searchfor an electron dipole moment15 and inquantum computing16. ■

Paul S. Julienne is in the Atomic Physics Division,National Institute of Standards and Technology,100 Bureau Drive, Gaithersburg,Maryland 20899-8432, USA.e-mail: paul.julienne@nist.gov

1. Nature Insight on Ultracold Matter Nature 416,

205–246 (2002).

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433–439 (1999).

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Walser, R. Phys. Rev. Lett. 87, 120406 (2001).

5. Regal, C. A., Ticknor, C., Bohn, J. L. & Jin, D. S. Nature 424,

47–50 (2003).

6. Julienne, P. S., Burnett, K., Band, Y. B. & Stwalley, W. C.

Phys. Rev. A 58, R797–R800 (1998).

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Mixture of atomsand molecules

Fermionicatom mixture

Ene

rgy Resonance

energy

Bindingenergy

Time

Figure 1 Making cold molecules. Regal et al.5 havesucceeded in creating molecules in a mixture ofcold gas atoms. The atoms exist in two differentspin states (represented by red and blue dots) at acertain energy, but can be made to pair up througha Feshbach resonance. Varying the magnetic fieldapplied to the system changes the energy of theresonance. When the resonance energy becomesthe same as the energy level of the atomic mixture,colliding atoms can be converted to resonant-statemolecules. As the resonance energy decreasesfurther, the molecules finally arrive in a lower-energy state, lower than the atomic state by theamount of molecular binding energy.

One of the unique features of mam-mals is the parental nursing ofyoung and suckling of milk from the

mother1. On page 68 of this issue2, Schaal etal. describe a major step towards gaining adeeper understanding of the nursing–suck-ling relationship, and of pheromones.Pheromones elicit specific behavioural orphysiological activity between members ofthe same species3, and their effectivenessdoes not depend on learning. In mammals,most research has centred on pheromonalinfluences on reproductive behaviour, andone such aspect has been tackled by Schaaland colleagues. They have identified a sub-stance — 2-methylbut-2-enal (2MB2) —that is synthesized in the mammary gland ofrabbit mothers and is released in milk toelicit the complex behavioural sequence thatculminates in the attachment of a pup to themother’s nipple, and suckling.

The report will be viewed as a benchmarkstudy in behavioural ecology and neuro-science. The authors combined expertise ininvestigation of the chemical senses, and indevelopmental psychobiology and analyticalchemistry, and started by fractionating rabbit milk by gas chromatography. Thebehavioural potency of each of the resultingfractions was assessed by presenting them topups through a port on the gas chroma-tograph or on a glass rod. The prelude tosuckling involves a specific ‘head-scanning’behaviour, which culminates in a pup’s

identifying and locking on to the nipple.Schaal et al. found that pup responses to 2MB2were indistinguishable from those induced bymilk. Both elicited head scanning and, when2MB2 was presented on a rod, seizing ofthe rod end; none of the 20 other fractions isolated from milk triggered the sequence.

The authors’ further studies were charac-terized by their perspective and thorough-ness. For example, in what will be seen as atutorial on how to explore the intersectionbetween analytical chemistry and behaviour,Schaal et al. showed an exactly paralleldecline, due to compound volatility, in theeffectiveness of milk and 2MB2 in elicitingresponses: after 90 minutes of exposure toair, both had lost their behavioural potency.Applying the powerful logic of counter-experiment, favoured by the renowned nineteenth-century physiologist ClaudeBernard, the authors found that when milkwas deactivated it could no longer elicit thestereotyped responses, but that thoseresponses were reinstated by adding 2MB2 at the concentration found in rabbit milk.

Other experiments showed that freshmilk or colostrum from several differentspecies — including cattle, sheep, rodents,humans and horses — failed to elicit thestereotyped nipple-seeking behaviour in the breed of rabbit used. Conversely, 2MB2elicited such patterns in six other breeds, butwas not effective in a distant relative ofrabbits, or in rodents or kittens. The authors

NATURE | VOL 424 | 3 JULY 2003 | www.nature.com/nature 25

Reproductive biology

Mammary messagesElliott M. Blass

Identification of a pheromone that induces suckling in newborn rabbitssets a standard for studies on other mammals, and should primeinvestigations of the neurobiological basis of this behaviour.

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© 2003 Nature Publishing Group

also provide strong evidence that 2MB2 is ofmammary origin, in that it is present in milkbut not blood plasma or amniotic fluid. Thepower of 2MB2 to elicit suckling did notdepend on the mother’s diet during or afterpregnancy: pups of does that had been fed ontotally different foods during pregnancy andafter delivery reacted identically. Moreover,2MB2 successfully elicited the initial suck-ling in rabbits that had been delivered byCaesarean section and those that had beendelivered vaginally and captured beforecoming into contact with the mother’s nip-ple region. This is further evidence that theeffects of 2MB2 are not dependent on experi-ence — that is, they are not learned.

Rabbits have an unusual mothering style,in that they visit and nurse their young for5–7 minutes in a single daily visit4. This dif-fers markedly from almost all other com-monly studied mammals. Rat mothers, forexample, are essentially in continuous con-tact with their young during the first weekafter delivery, leaving the nest only verybriefly to eat and drink5. They also differ inbeing omnivores, not herbivores. The initialnipple attachment of rat young is controlledby their matching the odours of amnioticfluid detected prenatally with those detectedpostnatally6. The olfactory properties of theamniotic fluid reflect the mother’s diet,entering the circulation and being filteredthrough the blood plasma during the fluid’sformation.In rabbits,nipple attachment wasnot influenced by substances that had proba-bly filtered into these fluids, even thoughpups can sense and respond to odours thatare detected in amniotic fluid or milk.

A further difference between the speciesis that, whereas in rats the elicitors ofsuckling change, in rabbits 2MB2 continuedto elicit nipple attachment at four days ofage. In rats, by this stage, pup behaviour isprobably based on the olfactory properties of the maternal diets, which the pups detectthrough the mother’s milk (also a plasma filtrate)7.The same holds for humans — who,like rats, are omnivores, and also have pro-tracted contact between the mother andinfant8. For both species maternal diet,through amniotic fluid and milk filtrates,influences diet choice at the time of weaningand attraction to the mother9.

In their research with rabbits, Schaal etal.2 have provided an anchor for futureanalyses of chemosensory influences on thedevelopment of animals with different feed-ing styles, reproductive strategies and ‘nesthistory’. More immediate questions are notonly how 2MB2 elicits the first attachment ofpups, but how it signals the daily pattern ofmeals, given that milk instability and 2MB2volatility render them ineffective within anhour, and that a doe visits her young onlyonce daily. Perhaps a doe detects the activityof her pups as she approaches the subter-ranean nest through its connecting tunnel,

and perhaps — as occurs in human mothersin different circumstances — a modest leakage of milk is triggered that elicits thesequence reported by Schaal and co-workers.

Finally, here is a topic that invites the further involvement of neuroscientists.Giventhe possibility of identifying the configura-tions of protein receptors in the olfactorymucosa10,and of tracing the nerve inputs intothe olfactory bulb and cortex in the brain, itshould be possible to trace the neural pathwaybetween the detection of 2MB2 and nippleattachment. And as the behavioural analysesprogress, so too can the neurology. In parti-cular, the linkage of food odours with 2MB2might provide an understanding of post-weaning diet choice in rabbits, and mightadvance similar studies in animals with different life histories, including humans5. ■

Elliott M. Blass is in the Department of Pediatrics,

Boston Medical Center, Boston, Massachusetts, andthe Department of Psychology and the Division ofNeuroscience and Behavior, University ofMassachusetts, Amherst, Massachusetts 01003, USA.e-mail: eblass@medusa.sbs.umass.edu1. Blass, E. M. & Teicher, M. H. Science 210, 15–22 (1980).

2. Schaal, B. et al. Nature 424, 68–72 (2003).

3. Beauchamp, G. K., Doty, R. L., Moulton, D. G. & Mugford, R. A.

in Olfaction, Behavior, and Mammalian Reproduction (ed. Doty,

R. L.) 143–160 (Academic, New York, 1976).

4. Coureaud, G., Schaal, B., Hudson, R. & Orgeur, P.

Dev. Psychobiol. 40, 372–390 (2002).

5. Johnson, B. A. & Leon, M. in Handbook of Behavioral

Neurobiology: Developmental Psychobiology Vol. 13 (ed. Blass,

E. M.) 53–80 (Kluwer/Plenum, New York, 2001).

6. Pedersen, P. E. & Blass, E. M. Dev. Psychobiol. 15, 349–355

(1982).

7. Pedersen, P. E., Williams, C. L. & Blass, E. M. J. Exp. Psychol.:

Anim. Behav. Processes 8, 329–341 (1982).

8. Menella, J. A., Johnson, A. & Beauchamp, G. K. Chem. Senses 20,

207–209 (1995).

9. Galef, B. G. Jr & Henderson, P. W. J. Comp. Physiol. Psychol. 78,

213–219 (1972).

10.Buck, L. B. & Axel, R. Cell 65, 175–187 (1991).

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The subterranean life of plants is noteasily documented. Take, for instance,the question of how root activities

may determine the composition and diver-sity of plant communities. Above groundthere is an observable struggle for light, butwhat goes on in the soil may be of greaterconsequence in determining diversity. Suchis the conclusion reached by Tara Rajaniemiand colleagues from experiments that theydescribe in the Journal of Ecology1.

Understanding the factors that controldiversity is high on the agenda for ecologistsand conservationists, and different factorsevidently operate at different scales. Forexample, at a continental scale the areas thathave the greatest diversity of species are often those with the highest levels of primaryproduction. The productive forests of theequatorial regions support more speciesthan do less productive systems in the cooltemperate zone. In North America, the areasthat are richest in tree species correspond tothose regions (such as the south-east) whereforest productivity is highest2. But at thehabitat scale this correlation does not apply.A reed bed may be highly productive, but it is relatively non-diverse, and productivegrasslands are usually lower in plant diversitythan less fertile ones3. Competition seems tolie at the heart of this problem, with robustand productive plants eliminating those thatare less able to acquire the resources that they need. The most obvious of these aresunlight, nutrient elements and water, the

first being tapped by shoots, the latter two by roots.

Ecological research tends to concentrateon above-ground processes,because they aremore straightforward to study. Rajaniemiand colleagues1, however, have attempted tomanipulate shoot and root competition separately so as to analyse the influences onecosystem diversity. They conducted theirexperiments in an ‘old field’ habitat in Michigan, selecting seven of the ten mostdominant plant species — including twograsses and five forbs (broad-leaved herbs).They set up plots in which each of thesespecies was grown in monoculture andexposed to or protected from either theshoots or roots of other species. Protectionwas achieved by using nets to exclude shading from neighbouring shoots, andtrenches to eliminate root competition from surrounding plants.Control plots wereexposed to both shoot and root competition.Half of the plots were then fertilized to boosttheir productivity.

Thus, each of the seven species was sub-jected to either root or shoot competitionfrom other species while in monoculture, orexposed to both of these in the control plots,and each treatment had a replicate that hadbeen given fertilizer. Maintaining shoots, yetrestraining them with nets, means that rootsystems below ground remain intact andactive even though the shoots above groundare not responsible for any shading of the testplants.

26 NATURE | VOL 424 | 3 JULY 2003 | www.nature.com/nature

Ecology

Roots of diversityPeter D. Moore

Competition between plants is in part responsible for the diversity ofvegetation in different ecosystems. Diversity studies have to take intoaccount what is happening beneath the soil as well as above it.

© 2003 Nature Publishing Group