news and views the curve of ejection fraction versus velocity major step forward in sorting out the history 2. Ahrens, T. J. Annu. Rev. Earth Planet. Sci. 21, 525–555 (1993). and so makes a big difference: Chen and of the air we breathe. ■ 3. Melosh, H. J. & Vickery, A. M. Eos 69, 388 (1988). Ahrens found that “almost all atmosphere” H. J. Melosh is at the Lunar and Planetary 4. Chen,G.Q.& Ahrens,T.J.Phys. Earth Planet. Inter. 100, ǁ 21–26 (1997). is lost at 8 km s 1, but Genda and Abe find Laboratory, University of Arizona, Tucson, 5. Cameron, A. G. W. Lunar Planet. Sci. Conf. XXIII, 199–200 that less than 30% of the atmosphere is lost Arizona 85721, USA. (1992). ǁ1 at 6 km s . e-mail: [email protected] 6. Pepin, R. O. Icarus 126, 148–156 (1997). 1. Genda, H. & Abe, Y. Icarus 164, 149–162 (2003). 7. Pepin, R. O. Icarus 92, 2–79 (1991) Why is this result important? Although . no one has yet found a convincing and general explanation for how the terrestrial planets acquired their atmospheres, the Evolutionary biology present abundances, particularly of Earth’s heavy noble gases (neon, argon, xenon and krypton), seem hard to reconcile with a Polygamy and parenting nearly complete loss of atmosphere after the Mark Pagel Earth was assembled6. The abundances of these gases and their isotopes in the present In most animal groups, females put more effort into rearing children, and atmospheres of Earth, Mars and Venus males compete for female attention. But what about seahorses and (insofar as we know them) differ substan- pipefish, in which males invest the most in offspring? tially from their abundances either in the Sun’s atmosphere or in meteorites. These odern men who think that they can differences have led atmospheric scientists to attract women by being good with postulate a wide variety of mechanisms by Mchildren may wish to read a study by R. KUITER which such gases may be acquired, partially Wilson and colleagues1 that has just appeared lost and isotopically fractionated7. But the in the journal Evolution. In seahorses and mechanical ejection of gases that was previ- pipefish at least, females compete for males ously imagined for the giant impact is no only when they — the females — have the help at all with this problem: in the impact time to, and not, it seems, according to scenario, the gases are ejected wholesale, the effort the males put into looking after without separation or fractionation. offspring. If Earth’s primordial atmosphere had Evolutionary theory suggests that the rel- been completely lost in the impact, then a ative amount of effort, or ‘investment’, that new inventory of gases would have had to males and females put into rearing their off- have been acquired later. It is unclear where spring determines which sex competes for such a secondary atmosphere would come the other2. In most animal species, females from. It seems nearly certain that it would contribute substantially more to the off- have a very different composition from that spring than males, and it is the males that of either Earth’s original atmosphere or the compete for mates. This leads to a form of original atmospheres of the other planets. biological evolution called sexual selection, Total atmospheric loss thus adds a major in which traits are favoured that make males Figure 1 Breaking the mould: wild card to the already highly uncertain mix good at competing with other males syngnathid species. Males of these of constraints on atmospheric evolution. (such as big antlers or sharp teeth) species put more effort By showing that total atmospheric loss in or more attractive to discerning than females into a giant impact is unlikely, Genda and Abe1 females (such as colourful tails or com- rearing offspring, as have permitted us to think what once seemed plex songs and displays). pictured here. Above, the pot-bellied unthinkable: in spite of the violence of the Pipefish and seahorses of the seahorse (Hyppocampus abdominalis), Moon-forming giant impact, our primor- family Syngnathidae (Fig. 1) have showing its enclosed brood pouch. Left, the dial atmosphere may have survived all of the some rather different habits, how- weedy seadragon (Phyllopteryx taeniolatus), vicissitudes of Earth’s turbulent birth.Going ever. Many species appear to be with its unprotected egg-compartments. even further, the authors argue that much of monogamous (a rarity in nature), the Mars-size impactor’s atmosphere would and all male syngnathids have these unconventional animals to investigate also have survived,merging with Earth’s own some form of specialized egg- whether the normal patterns of sexual selec- gas envelope. brooding structure located on tion acting on males get reversed when it is Much remains to be done before we can the abdomen or tail, into which the males that make most of the parental understand how Earth’s atmosphere evolved females deposit their eggs dur- investment. They expected that females to its present state. Genda and Abe’s com- ing mating. The males’ brood would compete for the males (a pattern they putation could be augmented by a better pouches vary in complexity among species, call sex-role reversal) in the species with the equation of state for atmospheric gases at from simple sticky patches to complex uterus- most complex brood pouches.But they don’t. the high temperatures reached in the shock like structures with placenta-like features for Instead, sex-role reversal in syngnathids propagating upwards from the surface. nourishing the eggs:a mere transvestite seems tends to arise in those species with polyga- A better understanding of how the solid unadventurous by comparison. By adopting mous mating systems — those in which both Earth would respond to the shock of a giant these normally female traits and habits,males males and females can have multiple mating impact would inspire more confidence in can be sure of their paternity of the fertilized partners.More tellingly,it is often the females the correctness of the surface boundary eggs.But they achieve this by investing consid- of these species, and not the males, that conditions. And we are still a long way from erably more parental effort than females — attempt to have more than one partner, a understanding the processes that affect the and, it is assumed3, increasingly so as the mating system known as polyandry. So it composition of the atmosphere. Never- brood pouch gets more complex. appears that, in seahorses, natural selection theless, Genda and Abe’s paper represents a Wilson and colleagues1 took advantage of for sexually exuberant female mating habits

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© 2003 Nature Publishing Group news and views drives sexual selection for female–female gametes and ‘females’ evolving sperm-like seahorses, with their occasional sex-role competition, independently of males’ pater- sex cells? Were females to make smaller eggs, reversal and would-be-female males, show nal investment. Selection for female poly- they could produce more gametes in total to us one reason why. ■ gamy also favours other traits that are assist them in their efforts to mate with many Mark Pagel is in the School of Animal and normally associated with sexual selection in males. Males in turn might be selected to Microbial Sciences, University of Reading, males of different animal groups: some increase the size of their sperm to ensure that Reading RG6 6AJ, UK. female seahorses are vividly coloured. the fertilized eggs were still well provisioned. e-mail: [email protected] Contrary to expectations, then, the assid- The syngnathid sexual circle would then be 1. Wilson, A. B., Ahnesjö, I., Vincent, A. C. J. & Meyer, A. Evolution uousness of syngnathid males does not pre- complete, as girls became boys and boys 57, 1374–1386 (2003). dict sex-role reversal. But why? The answer is became girls. 2. Trivers, R. L. in Sexual Selection and the Descent of Man (ed. Campbell, B.) 136–179 (Heinemann, London, 1972). probably that about half of the species with Has this ever happened? Whatever the 3. Bergland, A., Rosenqvist, G. & Svensson, I. Mar. Ecol. Prog. Ser. complex brood pouches are monogamous, answer, the inverted world of seahorses 29, 209–215 (1986). and that complicates any attempt at predic- shows that simple gamete-based definitions 4. Møller, A. P. in The Encyclopedia of Evolution (ed. Pagel, M.) tion, because females of monogamous of ‘male’ and ‘female’ can be misleading if 347–349 (Oxford Univ. Press, 2002). species are generally just too busy to compete taken to indicate a typical sex-role. Indeed, 5. Kaplan, H. S. & Lancaster, J. B. in Offspring (eds Wachter, K. et al.) 170–223 (Natl Academies Press, Washington DC, 2003). for other matings. Far from being the expres- the genetic sex-determining architecture is 6. Bull,J.J.in The Evolution of Sex and its Consequences (ed. 6 sion of undying mutual commitment and intriguingly variable in animals .Maybe Stearns, S. C.) 93–115 (Birkhäuser, Basel, 1987). affection heralded by church and state, monogamy,if it even exists4,is a sort of evolu- tionary last resort: it arises only when Condensed-matter physics both partners’full efforts are required to raise offspring successfully. Under these circum- stances, both partners will be selected to Really cool molecules evolve whatever adaptations will improve the Paul S. Julienne offspring’s survival.This may explain why the males of some seahorse species have evolved Ultracold molecules have been made by applying a changing magnetic such elaborate brood pouches. It could also field to a quantum gas of ‘fermionic’ atoms. This raises the prospect of explain why female syngnathids are rarely creating novel superfluids and molecular Bose–Einstein condensates. sex-role-reversed in the monogamous species, even in those with the most complex t the quantum level, any particle can field, to lie close to that of two separated brood pouches — these females are probably also be considered to be a wave, its atoms. By ramping the magnetic field so that too encumbered helping to rear offspring to A momentum corresponding to a the energy of the resonance state moved from fight among themselves for males. wavelength. In ultracold atomic gases, at being above to being below the energy of two Is the picture that emerges from syn- temperatures much lower than a millionth separated atoms,colliding pairs of atoms were gnathids — that sexual selection is determined of a degree above absolute zero, the wave- induced to form a lower-energy molecule that not by relative amounts of parental effort, but length of the atoms becomes larger than the is bound with respect to the separated atoms rather by mating habits — likely to be general? mean distance between them, giving rise (Fig. 1). Regal et al. were able to convert up to The unusual propensity of seahorses and to some remarkable quantum mechanical half of the atoms to diatomic molecules. pipefish for monogamy may make their case properties that are at the forefront of con- Producing a molecular gas in this ultra- exceptional. But it shows us that for relative temporary physics research1. The properties cold regime has proved to be a challenge. degrees of parental investment to influence of such cold quantum gases depend on There have been several proposals to make sexual selection, the sex that invests the least whether the atoms are bosons or fermions. molecules, and possibly a molecular must have some flexibility in its mating habits. Bosonic atoms have integer values of the Bose–Einstein condensate, by combining In support of this,some evidence from human quantum number known as spin, and can bosonic atoms in a Bose–Einstein condensate groups suggests that as the strictures of form a Bose–Einstein condensate in which using a molecular resonance state6–10.Experi- monogamy are relaxed, men will take advan- the trapped atoms occupy the single ground ments have come tantalizingly close. Wynar tage of women who invest in children at high state of the system. Fermionic atoms, on the et al.11 were the first to infer the formation of levels, by proportionately reducing their own other hand, have half-integer spin values molecules nearly at rest,by observing the loss contribution5. This, then, may become the and each identical fermion must occupy a of condensed rubidium-87 atoms as they incipient state of sexual selection for different quantum level. But if two fermions were photoassociated into molecules using a male–male competition (and likewise perhaps paired up to make a bosonic molecule, an two-colour pulse of light. Donley et al.12 have for female–female competition in the poly- exotic kind of superfluidity — flow without also given evidence for coherent atom–mol- gamous seahorses). resistance — could be the result2–4. On page ecule interconversion13,14 in experiments with Finally, what are we to make of seahorse 47 of this issue, Regal et al.5 describe an a rubidium-85 condensate that involved and pipefish males, especially those in the important step in this direction. manipulating the energy of a Feshbach-reso- sex-role-reversed species: in what sense are Their experiment began with a quantum nance state using a sequence of magnetic- they ‘male’? Evolutionary biology prosaically gas of fermionic potassium atoms, a mixture field pulses. However, neither experiment defines maleness and femaleness by the size of equal numbers of atoms with spin quan- could detect any molecules directly: the of one’s gametes. Females make the large, tum numbers ǁ9/2 and ǁ5/2. The different imaging methods used for atoms do not work well-provisioned and energetically costly spin states are necessary for the fermions to for molecules, because they have very differ- gametes called eggs,and males make the tiny, undergo collisions with each other with low ent light-scattering properties. unprovisioned sperm. Given that male syn- collision energy. To create molecules from Part of the beauty of the new work by Regal gnathids have found themselves in most these atoms, Regal et al. took advantage of a et al.5 is that they have conclusively demon- other respects in the role of females, and vice special molecular state known as a Feshbach strated the presence of cold molecules. They versa, what prevents them from being not resonance, which may be thought of as a exposed the molecules to a radio-frequency just sex-role-reversed but instead fully weakly bound pair of atoms. The energy of electromagnetic field of the right frequency to sex-reversed, with ‘males’ evolving egg-like such a state can be tuned, using a magnetic make them fall apart into single atoms with

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