Cuckoos are infamous for laying theireggs in the nests of other bird species:the unfortunate hosts raise the parasitic

chicks, reducing or even nullifying their ownreproductive output. It is less well knownthat ‘brood parasitism’ among females of thesame species is common in certain fishes,insects and birds1. In this conspecific broodparasitism, where females parasitize othersof their own species, the host parents willbear a high cost when they feed chicks thatare not their own.

Ways of reducing these costs are thereforeexpected to be favoured by natural selection.On page 495 of this issue, Bruce Lyon2

describes how an aquatic bird, the Americancoot Fulica americana (Fig. 1), uses severalremarkable behavioural tactics to this end.In this species, a parasitic chick that survivesusually does so at the cost of a host chick. Butthe host parents can often identify the eggs ofparasites, and reject or otherwise disadvan-tage many of them.

With some exceptions3–6, discriminationagainst parasitic eggs seems to be a raredefence in conspecific brood parasitism. Inmany species, hosts may not be able to tell thedifference between their own and alien eggs1.Coot eggs vary considerably in colour, how-ever, and Lyon finds that coots can not onlyoften distinguish their own eggs from those ofa parasite, but — remarkably — can countthem, disregarding the number of parasiticeggs in the clutch. This surprising cognitivefeat makes good adaptive sense. Like manyother birds, coots are indeterminate layers7;that is, they regulate egg production usingexternal stimuli, such as the number or surface area of the eggs already in the nest, tocontrol egg development and final clutch size.If the parent cannot distinguish between itsown and parasitic eggs, the latter can cause itto stop egg production too early, reducing thefinal clutch size below that necessary for itsown maximum reproductive success.

As with most birds’ eggs, there is little dif-ference in surface texture between eggs laid bydifferent female coots. So although responsesto a tactile stimulus cannot be completelyruled out, it seems unlikely that this is the distinguishing cue. By contrast, to the humaneye, the coloration of coot eggs — a con-spicuous pattern of dark spots on a brownbackground — varies considerably amongfemales (Fig. 1, inset). Visual cues thereforeseem to offer ample scope for egg discrimi-nation. Lyon2 indeed found that the visual

differences between eggs of host and parasitewere larger in nests where parasitic eggs wererejected than where they were accepted. Forthe host to be able to reject a parasitic egg,therefore, it seems that there must be a largeenough difference in the eggs’ appearance.

Lyon compared coot pairs that acceptedparasitic eggs (‘acceptors’) with pairs thatrejected them (‘rejectors’). He found thatacceptor females produced a lower final num-ber of their own eggs. Rejector females, how-ever, did not reduce their own final clutch size.Evidently they recognized and counted theirown eggs, avoiding a maladaptive reductionin the number of their own eggs in response to the added parasite eggs. Later on, rejectorhosts discarded parasitic eggs by buryingthem under nest material, or moved them toperipheral positions in the clutch. The conse-quence was to prevent or delay the hatching of parasitic chicks, so reducing competitionwith the host’s true offspring. On average,such behaviour halved the effect of parasitismon the reproductive success of the hosts.

Some aspects of this defensive behaviourraise more questions. Why do coots eject

cracked or rotten eggs from the nest, butbury parasitic eggs? And why do they movesome parasitic eggs to the periphery of theclutch rather than reject them altogether?Lyon suggests that the latter behaviour mayrepresent an intermediate form of defence,used when the bird is uncertain whether theegg is parasitic.

From an ecological perspective, Lyon’sresults are striking in what they tell us aboutthe costs of brood parasitism, and defencesagainst it. They also shed light on cognitionand counting by animals, in a natural contextwhere reproductive success is at stake. Thereis suggestive evidence8 that counting may beimportant in such circumstances. But it israrely as clear as here.

Usually, the number of items counted innatural situations is small, as is the case forcoots counting their eggs. With properexperimental training, however, pigeons andrats have made surprisingly accurate choicesin complex tasks that involve counting to upto several dozen9. Many animals apparentlyhave a brain wiring that in the right circum-stances can support competent countingwithout verbal symbolic representation ofnumbers. Lyon’s findings provide a fasci-nating example of how this capacity is put to good use in the wild. Who would haveexpected that from a birdbrain?

There are plenty of open questions. Forexample, what is the genetic basis and heri-tability of the visual appearance of eggs, anddoes this have consequences for parasitisminvolving relatives? Does parasitism create

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NATURE | VOL 422 | 3 APRIL 2003 | www.nature.com/nature 483

Behavioural ecology

Coots countMalte Andersson

American coots distinguish their own eggs from the eggs that other femalecoots lay in the same nest. They use a variety of tactics to minimize theadverse reproductive effects of the parasitic behaviour.

Figure 1 Coots in a flap. These birds fight over territorial borders, territory size setting the limit forthe number of chicks that can be successfully fed by the parents. Hence the evolution of broodparasitism to bypass this constraint. The inset shows a parasitized nest — the two darker eggs are theones laid by a brood parasite.

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selection for increased variability in egg coloration, making it easier to detect parasiticeggs5? A combination of Lyon’s incisive fieldtechniques with genetics, and with moleculardetermination of parasitism and parent-age10,11, seems likely to provide further insightsinto the cognitive and tactical aspects of broodparasitism and reproductive behaviour. ■

Malte Andersson is in the Department of Zoology,University of Gothenburg, Box 463, SE 405 30 Göteborg, Sweden.e-mail: malte.andersson@zool.gu.se

On Earth, elements heavier than urani-um do not exist in easily measurablequantities, because they become

increasingly unstable against radioactivedecay. Some heavier elements can be artifi-cially created through the collision and fusionof other nuclei, and over the past 60 yearsabout 20 new elements have been added to theperiodic table — up to an atomic number of at least 110. But fusion becomes less successfulwhen the projectile and target nuclei are chosen to form a product with nucleon num-ber, A, greater than 220 (more than 90 protonsand 130 neutrons). For example, in someexperiments1 only about one out of 1018 nucleiincident on a target leads to the creation anddetection of the desired new element.

In Physical Review Letters, Hinde, Das-gupta and Mukherjee2 present a detailedanalysis of this inhibition of fusion near A4220. They made a careful comparison of fusion cross-sections (or probabilities)from their own experiment on the reaction16O&204Pb with other experiments on40Ar&180Hf, 48Ca&172Yb, 82Se&138Ba and124Sn&96Zr, all of which lead to the samecompound system, 220Th. The results showthat it is much more difficult to make 220Thwith more symmetric combinations of target and projectile than with the mostasymmetric combination (16O&204Pb) —specifically about ten times more difficult.Hinde et al. propose that this is due to com-petition with the process of ‘quasi-fission’.Colliding nuclei form a composite at theonset of the fusion process, but the compositemay break up, or undergo fission, beforefusion is complete. True fission occurs afterthe formation of an equilibrated compoundnucleus; quasi-fission results from the muchfaster breakup of a partially fused composite.

Today’s theories of heavy-ion collisionsare mainly macroscopic; the energy of the

colliding system can be written as a sum ofthe repulsive electrostatic energy and theattractive nuclear energy. After the collidingnuclei touch, these energies must be calcu-lated for the combined system as functions ofits shape. The resulting ‘energy landscape’ (amulti-dimensional potential-energy surfacein deformation space) has a strong influenceon the dynamics of the fusion process.

In macroscopic models, the energy land-scape changes relatively slowly with protonand neutron number and with deformationof the shape of the nucleus away from a sim-ple sphere. But microscopic effects, whicharise because the protons and neutrons in thenuclei obey quantum-mechanical laws, varymuch more rapidly as the neutron and pro-ton numbers and the shape change, some-times producing large differences betweenthe behaviour of systems with only slightlydifferent nucleon number. Microscopiceffects are not often included in theoreticalstudies of nuclear collisions, but we believethat they should be considered more carefully.There are other factors, too: how dissipationconverts the kinetic energy of the projectileinto internal excitation energy of the fusingsystem; and the effect of the relative orienta-tion of target and projectile if one or both ofthem are deformed (around 50% of stablenuclei are not spherical).

In their paper, Hinde et al.2 consider vari-ous explanations for the inhibition of fusionseen in the data. First, they discuss the idea ofthe ‘extra push’ — a colourful misnomer usedto describe a dynamical threshold. For heavycompound systems, extra kinetic energy(more than is needed to bring the nuclei intocontact after overcoming their electrostaticrepulsion) is required for them actually to fuse and form a single nucleus. A good analogy is a skier crossing a mountain range,starting out with some initial energy. For

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NATURE | VOL 422 | 3 APRIL 2003 | www.nature.com/nature 485

Nuclear physics

Into the fission valleyPeter Möller and Arnold J. Sierk

Why are new elements difficult to make? Fusion of two nuclei to produceheavy elements seems to be hindered by a competing process of ‘quasi-fission’. New work builds a more complete picture.

100 YEARS AGOConcerning the recently discovered heatemission from radium, it is perhaps worthnoting that it appears to be connected with,and is probably an immediate consequenceof, the remarkable observation by Rutherfordthat radium emits massive positively-charged particles, which are probably atoms,with a velocity comparable to one-tenth ofthe speed of light… Because it is easy toreckon that the emission of a million heavyatoms per second, which is a small quantitybarely weighable in a moderate time such asa few weeks (being about the twentieth partof a milligramme per century), with a speedequal to one-tenth that of light, wouldrepresent an amount of energy equal to onethousand ergs per second; that is to say,would correspond to heat enough to melt amilligramme of ice every hour. And inasmuchas these atoms are not at all of a penetratingkind, but are easily stopped by obstacles,they would most of them be stopped by asmall thickness of air, and their energy wouldbe thus chiefly expended in the immediateproximity of the source, which source wouldthereby tend to be kept warm.From Nature 2 April 1903.

50 YEARS AGOBefore the War one could make materialsartificially radioactive by bombardment in big machines like cyclotrons, or by usingrelatively weak neutron sources. In thecyclotron, one can generally only use onetarget at a time and the irradiation istherefore costly. The weak neutron sourcesinduce only weak activities. Therefore only a few research workers profited from theradioisotopes which one could produce inthese ways. The situation changed suddenlywith the discovery of fission of uranium in1939. This discovery showed that chainreactions were possible in which moreneutrons are created than used. Acceleratedby war research, the first chain-reactingatomic pile was working on December 2,1942, in Chicago… New radioisotopes were quickly discovered and the chart ofradioactive isotopes started to expand. To-day, there are more than six hundredradioactive isotopes, of which, however, onlysome hundred can be made conveniently inan atomic pile. For most elements there is at least one usable radioactive isotope. Theonly notable exceptions are the two elementsnitrogen and oxygen for which no convenientradioactive isotope exists.From Nature 4 April 1953.

1. Davies, N. B. Cuckoos, Cowbirds and Other Cheats (Poyser,London, 2000).

2. Lyon, B. E. Nature 422, 495–499 (2003).3. Arnold, T. W. Condor 89, 675–676 (1987).4. Bertram, B. C. R. The Ostrich Communal Nesting System

(Princeton Univ. Press, 1992).5. Jackson, W. M. in Parasitic Birds and Their Hosts (eds Rothstein,

S. I. & Robinson, S. K.) 406–416 (Oxford Univ. Press, 1998).6. Jamieson, I. G., McRae, S. B., Simmons, R. E. & Trewby, M. Auk

117, 250–255 (2000).7. Haywood, S. Q. Rev. Biol. 68, 33–60 (1993).8. Seibt, U. Behav. Brain Sci. 11, 597–598 (1988).9. Hauser, M. D. Am. Sci. 88, 144–151 (2000).10.Andersson, M. & Åhlund, M. Ecology 82, 1433–1442 (2001).11.Birkhead, T. R., Burke, T., Zann, R., Hunter, F. M. & Krupa,

A. P. Behav. Ecol. Sociobiol. 27, 315–324 (1990).

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