RESEARCH NEWS & VIEWS challenges and opportunities. The data tell us that there are some ecological and demographic circumstances in which the benefits of lethal aggression exceed the costs for chimpanzees, nothing more. Humans are not destined to be warlike because chimpanzees sometimes kill their neighbours. For those who are persuaded by Wilson and colleagues’ evidence, a more interesting set of questions emerges. For example, how do chimpanzees overcome the collective-action problem? By eliminating rival males and infants sired by males from other communities, chimpanzees gain access to new territories and mating partners. But these benefits flow to the group as a whole, which creates opportunities for free-riding. Although the
imbalance-of-power hypothesis relies on the odds being in the aggressors’ favour, males forgo opportunities to mate and forage while they are on patrol, and run at least some risk of being injured in attacks. Do males that join patrols and lead attacks gain more benefits than those that remain in the security of their own community’s territory? What forces curtail free-riding? The answers to these questions will provide interesting insight into the selective forces that favour group-level cooperation in species without language, social institutions and systems for sanctioning free-riders. ■ Joan B. Silk is in the School of Human Evolution & Social Change, Arizona State
University, Tempe, Arizona 85287-2402, USA. e-mail: [email protected] 1. Themnér, L. & Wallensteen, P. J. Peace Res. 51, 541–554 (2014). 2. Choi, J.-K. & Bowles, S. Science 318, 636–640 (2007). 3. Fry, D. P. & Söderberg, P. Science 341, 270–273 (2013). 4. Wilson, M. L. et al. Nature 513, 414–417 (2014). 5. Goodall, J. et al. in The Great Apes (eds Hamburg, D. A. & McCown, E. R.) 13–53 (Benjamin/Cummings, 1979). 6. Crofoot, M. C. & Wrangham, R. W. in Mind the Gap: Tracing the Origins of Human Universals (eds Kappeler, P. M. & Silk, J. B.) 171–195 (Springer, 2010). 7. Power, M. The Egalitarians — Human and Chimpanzee: An Anthropological View of Social Organization (Cambridge Univ. Press, 2005). 8. Ferguson, R. B. in Origins of Altruism and Cooperation (eds Sussman, R. W. & Cloninger, C. R.) 249–270 (Springer, 2011).
Seth et al. ‘weighed’ the black hole by determining its gravitational influence on nearby stars orbiting it 5,6. To explain the observed stellar velocities and the distribution of light within M60-UCD1, they had to invoke the presence of a central black hole with a mass 21 million times that of our Sun. A black hole of that size would be expected to reside in a host galaxy with a mass of about 7 billion solar masses. However, Seth et al. An oversized, supermassive black hole has been discovered at the centre of a estimate that M60-UCD1 has a stellar mass of densely packed conglomeration of stars. The finding suggests that the system is only 120 million solar masses. the stripped nucleus of a once-larger galaxy. See Letter p.398 Although the discovery of an enormous black hole in such a small galaxy is surprisAMY E. REINES Seth and colleagues present the first clear case ing, recent work has uncovered a substantial that an individual ultra-compact dwarf is a number of black holes in other low-mass dwarf upermassive black holes, which have stripped-galaxy nucleus, because star clusters galaxies7. However, M60-UCD1 is clearly a masses millions or even billions of times do not host supermassive black holes. different beast from those — it is far more that of our Sun, reside at the centre compact and has a much more massive of almost every massive galaxy, includblack hole. The small black holes in other ing our Milky Way1. There seems to be low-mass dwarf galaxies are probably some connection between the evolution similar to the first ‘seeds’ of supermassive of galaxies and that of these black holes, black holes8. Over cosmic time, such seeds although the nature of the relationship is grow by swallowing gas and coalescnot well understood. What we do know is ing with other black holes during galaxy that, in general, bigger galaxies harbour mergers. With a mass 200 times that of bigger black holes at their centres, and the smallest nuclear black holes known, these black holes are typically about 0.5% M60-UCD1’s black hole seems to have of the total mass of a spheroidal galaxy’s already grown considerably. stars1. But on page 398 of this issue, Seth So how did such a big black hole get et al.2 report the detection of an overinto such a tiny galaxy? The answer may sized supermassive black hole that is a be related to M60-UCD1’s galactic neighwhopping 18% of the stellar mass of its bourhood. This ultra-compact dwarf unusual host. galaxy is right next door to the giant The dense stellar system in which the elliptical galaxy M60 (Fig. 1). Seth and coblack hole has been found, M60-UCD1 worke rs’ s i mu l at i ons show t h at (ref. 3), is called an ultra-compact dwarf M60-UCD1 may have formerly been a galaxy, and marks a previously unknown more massive galaxy than it is now (with environment for supermassive black holes. a proportionally sized black hole), but lost M60-UCD1 Ultra-compact dwarf galaxies are densely most of its stars in a gravitational tug of packed spherical conglomerations of war while orbiting its giant neighbour. stars4. For years, astronomers have debated What is left today is the dense stellar Figure 1 | Dwarfed by its neighbour. This composite the nature of these objects — are they image, constructed from data from NASA’s Chandra nucleus and central supermassive black extremely massive star clusters, or are they X-ray Observatory and Hubble Space Telescope, hole from the larger progenitor galaxy. the nuclei of galaxies that have had their depicts the massive elliptical galaxy M60 and the nearby The evidence for a supermassive black outer layers stripped off through gravi- ultra-compact dwarf galaxy M60-UCD1. Seth et al.2 report hole in M60-UCD1 is strong, but it is tational interactions with other galaxies? that M60-UCD1 contains a supermassive black hole. not the only possible explanation for the AST ROPHYSICS
S
3 2 2 | NAT U R E | VO L 5 1 3 | 1 8 S E P T E M B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
X-RAY: NASA/CXC/MSU/J. STRADER ET AL.; OPTICAL: NASA/STSCI
Giant black hole in a stripped galaxy
NEWS & VIEWS RESEARCH data. Although seemingly unlikely, we cannot definitively rule out the existence of a popu lation of low-mass stars or dead stellar remnants at the galaxy’s centre that do not produce much visible light. An X-ray source that might be produced by a supermassive black hole has been detected at the heart of M60-UCD1, but this radiation could also be generated by the dead remnant of a star3. Follow-up observations at radio wavelengths could distinguish between these possi bilities9,10 and provide further support for the presence of a supermassive black hole. Seth and colleagues’ discovery is an important step towards understanding the nature of ultra-compact dwarf galaxies. Many other ultra-compact dwarfs show tantalizing hints that they, too, harbour supermassive black holes and are therefore stripped galaxy nuclei, but direct evidence is lacking. The authors are participating in ongoing observing programmes that may provide conclusive evidence for supermassive black holes in four other ultra-compact dwarfs. But at present, detecting the gravitational pull of a black hole on surrounding stars is feasible for only the brightest and closest systems. Attempting to detect the direct gravitational signatures of black holes in a large population of ultracompact dwarfs must therefore wait for the next generation of telescopes.
If supermassive black holes are indeed commonplace in ultra-compact dwarfs, this would have major implications for the demographics of such black holes — Seth et al. estimate that there could be more than double the number of supermassive black holes in the local Universe than is presently thought. Although this is possible, it is far from certain. Future studies will tell us whether M60-UCD1 is a fluke, or whether other ultra-compact dwarfs are also stripped galactic nuclei that host black holes. ■ Amy E. Reines is in the Department of Astronomy, University of Michigan, Ann Arbor, Michigan 48109-1107, USA. e-mail: [email protected] 1. Kormendy, J. & Ho, L. C. Annu. Rev. Astron. Astrophys. 51, 511–653 (2013). 2. Seth, A. C. et al. Nature 513, 398–400 (2014). 3. Strader, J. et al. Astrophys. J. 775, L6 (2013). 4. Brodie, J. P., Romanowsky, A. J., Strader, J. & Forbes, D. A. Astron. J. 142, 199 (2011). 5. Schwarzschild, M. Astrophys. J. 232, 236–247 (1979). 6. van den Bosch, R. C. E. & de Zeeuw, P. T. Mon. Not. R. Astron. Soc. 401, 1770–1780 (2010). 7. Reines, A. E., Greene, J. E. & Geha, M. Astrophys. J. 775, 116 (2013). 8. Volonteri, M. Astron. Astrophys. Rev. 18, 279–315 (2010). 9. Gültekin, K., Cackett, E. M., King, A. L., Miller, J. M. & Pinkney, J. Astrophys. J. 788, L22 (2014). 10. Merloni, A., Heinz, S. & Di Matteo, T. Mon. Not. R. Astron. Soc. 345, 1057–1076 (2003).
NE URO SCIENCE
Shedding light on a change of mind
W
e often believe that our memories are accurate, but in fact they can be malleable, changing over time as recollections become less precise or as events that never happened are falsely remembered1. There is also another way in which memory can change. The memory of a romantic first meal out with a partner may take on a different mood when the relationship falters. That of a favourite family beach in summer may be destroyed by witnessing a swimming tragedy there. In these cases, memory of the place remains accurate, but the positive associations with that place are lost. On page 426 of this issue, Redondo et al.2 investigate the neural basis of this selective change.
Journey to the Jade Sea. By John Hillaby — Books by writers who go to Africa in search of their souls are always interesting to us who went there in search of wages; this book is fascinating. “Essentially, I walked into the N.F.D. for the hell of it” (p. 2); Mr. Hillaby took his own hell by using a small string of unhealthy camels for transport instead of a lorry or Land-Rover, as do other people in that part of Kenya. “Perhaps all safaris start this way. Somewhat despondently …” (p. 7); they do not, but I should have been despondent if I had started with that collection of provisions … in old cardboard boxes … Messrs. Constable have published a most interesting book. They might also publish an interesting one by the Warden, for he no doubt would tell us more about the animals and plants. From Nature 19 September 1964
100 Years Ago
Sophisticated genetic tools that make brain cells responsive to light have now been used in mice to trigger a memory connected with a particular place, and to switch its association from negative to positive, or vice versa. See Letter p.426 T O M O N O R I TA K E U C H I & R I C H A R D G . M . M O R R I S
50 Years Ago
Our memories are representations of past experiences that are believed to be encoded in networks of neurons that fire together or in sequence. The representation of a particular place — a ‘where’ memory — is encoded in a brain structure called the hippocampal formation, which is embedded within the medial temporal lobe. A separate representation in the amygdala of the brain encodes a ‘what’ memory, which recalls whether one feels good about a place (a positive valence) or has marked it off as dangerous (a negative valence). These two representations are thought to become connected during learning. The amygdala also has direct downstream connections to the action and endocrine systems that are involved in approach and avoidance3. Redondo and colleagues investigated the separate representations of ‘where’ and
Let us consider lastly a disease which collects the last toll from one-seventh of humanity, and debilitates and enfeebles the lives of many whom it does not entirely destroy … How are we organizing our campaign against tuberculosis? Bacteriology has taught us that it is an infectious disease and has isolated the organism … all over the civilized world the total deathroll of human kind annually from tuberculosis probably does not fall short of a million souls … This disease must be stopped at its source as well as dealt with on its course. No disease has ever been eradicated from a community by discovering cures for it, and none ever will; many diseases have disappeared because their sources have been cut off. Let us be scientific, let us search out the truth; having found it, let us act upon it, and let us conceal nothing that is true. From Nature 17 September 1914
1 8 S E P T E M B E R 2 0 1 4 | VO L 5 1 3 | NAT U R E | 3 2 3
© 2014 Macmillan Publishers Limited. All rights reserved
imbalance-of-power hypothesis relies on the odds being in the aggressors’ favour, males forgo opportunities to mate and forage while they are on patrol, and run at least some risk of being injured in attacks. Do males that join patrols and lead attacks gain more benefits than those that remain in the security of their own community’s territory? What forces curtail free-riding? The answers to these questions will provide interesting insight into the selective forces that favour group-level cooperation in species without language, social institutions and systems for sanctioning free-riders. ■ Joan B. Silk is in the School of Human Evolution & Social Change, Arizona State
University, Tempe, Arizona 85287-2402, USA. e-mail: [email protected] 1. Themnér, L. & Wallensteen, P. J. Peace Res. 51, 541–554 (2014). 2. Choi, J.-K. & Bowles, S. Science 318, 636–640 (2007). 3. Fry, D. P. & Söderberg, P. Science 341, 270–273 (2013). 4. Wilson, M. L. et al. Nature 513, 414–417 (2014). 5. Goodall, J. et al. in The Great Apes (eds Hamburg, D. A. & McCown, E. R.) 13–53 (Benjamin/Cummings, 1979). 6. Crofoot, M. C. & Wrangham, R. W. in Mind the Gap: Tracing the Origins of Human Universals (eds Kappeler, P. M. & Silk, J. B.) 171–195 (Springer, 2010). 7. Power, M. The Egalitarians — Human and Chimpanzee: An Anthropological View of Social Organization (Cambridge Univ. Press, 2005). 8. Ferguson, R. B. in Origins of Altruism and Cooperation (eds Sussman, R. W. & Cloninger, C. R.) 249–270 (Springer, 2011).
Seth et al. ‘weighed’ the black hole by determining its gravitational influence on nearby stars orbiting it 5,6. To explain the observed stellar velocities and the distribution of light within M60-UCD1, they had to invoke the presence of a central black hole with a mass 21 million times that of our Sun. A black hole of that size would be expected to reside in a host galaxy with a mass of about 7 billion solar masses. However, Seth et al. An oversized, supermassive black hole has been discovered at the centre of a estimate that M60-UCD1 has a stellar mass of densely packed conglomeration of stars. The finding suggests that the system is only 120 million solar masses. the stripped nucleus of a once-larger galaxy. See Letter p.398 Although the discovery of an enormous black hole in such a small galaxy is surprisAMY E. REINES Seth and colleagues present the first clear case ing, recent work has uncovered a substantial that an individual ultra-compact dwarf is a number of black holes in other low-mass dwarf upermassive black holes, which have stripped-galaxy nucleus, because star clusters galaxies7. However, M60-UCD1 is clearly a masses millions or even billions of times do not host supermassive black holes. different beast from those — it is far more that of our Sun, reside at the centre compact and has a much more massive of almost every massive galaxy, includblack hole. The small black holes in other ing our Milky Way1. There seems to be low-mass dwarf galaxies are probably some connection between the evolution similar to the first ‘seeds’ of supermassive of galaxies and that of these black holes, black holes8. Over cosmic time, such seeds although the nature of the relationship is grow by swallowing gas and coalescnot well understood. What we do know is ing with other black holes during galaxy that, in general, bigger galaxies harbour mergers. With a mass 200 times that of bigger black holes at their centres, and the smallest nuclear black holes known, these black holes are typically about 0.5% M60-UCD1’s black hole seems to have of the total mass of a spheroidal galaxy’s already grown considerably. stars1. But on page 398 of this issue, Seth So how did such a big black hole get et al.2 report the detection of an overinto such a tiny galaxy? The answer may sized supermassive black hole that is a be related to M60-UCD1’s galactic neighwhopping 18% of the stellar mass of its bourhood. This ultra-compact dwarf unusual host. galaxy is right next door to the giant The dense stellar system in which the elliptical galaxy M60 (Fig. 1). Seth and coblack hole has been found, M60-UCD1 worke rs’ s i mu l at i ons show t h at (ref. 3), is called an ultra-compact dwarf M60-UCD1 may have formerly been a galaxy, and marks a previously unknown more massive galaxy than it is now (with environment for supermassive black holes. a proportionally sized black hole), but lost M60-UCD1 Ultra-compact dwarf galaxies are densely most of its stars in a gravitational tug of packed spherical conglomerations of war while orbiting its giant neighbour. stars4. For years, astronomers have debated What is left today is the dense stellar Figure 1 | Dwarfed by its neighbour. This composite the nature of these objects — are they image, constructed from data from NASA’s Chandra nucleus and central supermassive black extremely massive star clusters, or are they X-ray Observatory and Hubble Space Telescope, hole from the larger progenitor galaxy. the nuclei of galaxies that have had their depicts the massive elliptical galaxy M60 and the nearby The evidence for a supermassive black outer layers stripped off through gravi- ultra-compact dwarf galaxy M60-UCD1. Seth et al.2 report hole in M60-UCD1 is strong, but it is tational interactions with other galaxies? that M60-UCD1 contains a supermassive black hole. not the only possible explanation for the AST ROPHYSICS
S
3 2 2 | NAT U R E | VO L 5 1 3 | 1 8 S E P T E M B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
X-RAY: NASA/CXC/MSU/J. STRADER ET AL.; OPTICAL: NASA/STSCI
Giant black hole in a stripped galaxy
NEWS & VIEWS RESEARCH data. Although seemingly unlikely, we cannot definitively rule out the existence of a popu lation of low-mass stars or dead stellar remnants at the galaxy’s centre that do not produce much visible light. An X-ray source that might be produced by a supermassive black hole has been detected at the heart of M60-UCD1, but this radiation could also be generated by the dead remnant of a star3. Follow-up observations at radio wavelengths could distinguish between these possi bilities9,10 and provide further support for the presence of a supermassive black hole. Seth and colleagues’ discovery is an important step towards understanding the nature of ultra-compact dwarf galaxies. Many other ultra-compact dwarfs show tantalizing hints that they, too, harbour supermassive black holes and are therefore stripped galaxy nuclei, but direct evidence is lacking. The authors are participating in ongoing observing programmes that may provide conclusive evidence for supermassive black holes in four other ultra-compact dwarfs. But at present, detecting the gravitational pull of a black hole on surrounding stars is feasible for only the brightest and closest systems. Attempting to detect the direct gravitational signatures of black holes in a large population of ultracompact dwarfs must therefore wait for the next generation of telescopes.
If supermassive black holes are indeed commonplace in ultra-compact dwarfs, this would have major implications for the demographics of such black holes — Seth et al. estimate that there could be more than double the number of supermassive black holes in the local Universe than is presently thought. Although this is possible, it is far from certain. Future studies will tell us whether M60-UCD1 is a fluke, or whether other ultra-compact dwarfs are also stripped galactic nuclei that host black holes. ■ Amy E. Reines is in the Department of Astronomy, University of Michigan, Ann Arbor, Michigan 48109-1107, USA. e-mail: [email protected] 1. Kormendy, J. & Ho, L. C. Annu. Rev. Astron. Astrophys. 51, 511–653 (2013). 2. Seth, A. C. et al. Nature 513, 398–400 (2014). 3. Strader, J. et al. Astrophys. J. 775, L6 (2013). 4. Brodie, J. P., Romanowsky, A. J., Strader, J. & Forbes, D. A. Astron. J. 142, 199 (2011). 5. Schwarzschild, M. Astrophys. J. 232, 236–247 (1979). 6. van den Bosch, R. C. E. & de Zeeuw, P. T. Mon. Not. R. Astron. Soc. 401, 1770–1780 (2010). 7. Reines, A. E., Greene, J. E. & Geha, M. Astrophys. J. 775, 116 (2013). 8. Volonteri, M. Astron. Astrophys. Rev. 18, 279–315 (2010). 9. Gültekin, K., Cackett, E. M., King, A. L., Miller, J. M. & Pinkney, J. Astrophys. J. 788, L22 (2014). 10. Merloni, A., Heinz, S. & Di Matteo, T. Mon. Not. R. Astron. Soc. 345, 1057–1076 (2003).
NE URO SCIENCE
Shedding light on a change of mind
W
e often believe that our memories are accurate, but in fact they can be malleable, changing over time as recollections become less precise or as events that never happened are falsely remembered1. There is also another way in which memory can change. The memory of a romantic first meal out with a partner may take on a different mood when the relationship falters. That of a favourite family beach in summer may be destroyed by witnessing a swimming tragedy there. In these cases, memory of the place remains accurate, but the positive associations with that place are lost. On page 426 of this issue, Redondo et al.2 investigate the neural basis of this selective change.
Journey to the Jade Sea. By John Hillaby — Books by writers who go to Africa in search of their souls are always interesting to us who went there in search of wages; this book is fascinating. “Essentially, I walked into the N.F.D. for the hell of it” (p. 2); Mr. Hillaby took his own hell by using a small string of unhealthy camels for transport instead of a lorry or Land-Rover, as do other people in that part of Kenya. “Perhaps all safaris start this way. Somewhat despondently …” (p. 7); they do not, but I should have been despondent if I had started with that collection of provisions … in old cardboard boxes … Messrs. Constable have published a most interesting book. They might also publish an interesting one by the Warden, for he no doubt would tell us more about the animals and plants. From Nature 19 September 1964
100 Years Ago
Sophisticated genetic tools that make brain cells responsive to light have now been used in mice to trigger a memory connected with a particular place, and to switch its association from negative to positive, or vice versa. See Letter p.426 T O M O N O R I TA K E U C H I & R I C H A R D G . M . M O R R I S
50 Years Ago
Our memories are representations of past experiences that are believed to be encoded in networks of neurons that fire together or in sequence. The representation of a particular place — a ‘where’ memory — is encoded in a brain structure called the hippocampal formation, which is embedded within the medial temporal lobe. A separate representation in the amygdala of the brain encodes a ‘what’ memory, which recalls whether one feels good about a place (a positive valence) or has marked it off as dangerous (a negative valence). These two representations are thought to become connected during learning. The amygdala also has direct downstream connections to the action and endocrine systems that are involved in approach and avoidance3. Redondo and colleagues investigated the separate representations of ‘where’ and
Let us consider lastly a disease which collects the last toll from one-seventh of humanity, and debilitates and enfeebles the lives of many whom it does not entirely destroy … How are we organizing our campaign against tuberculosis? Bacteriology has taught us that it is an infectious disease and has isolated the organism … all over the civilized world the total deathroll of human kind annually from tuberculosis probably does not fall short of a million souls … This disease must be stopped at its source as well as dealt with on its course. No disease has ever been eradicated from a community by discovering cures for it, and none ever will; many diseases have disappeared because their sources have been cut off. Let us be scientific, let us search out the truth; having found it, let us act upon it, and let us conceal nothing that is true. From Nature 17 September 1914
1 8 S E P T E M B E R 2 0 1 4 | VO L 5 1 3 | NAT U R E | 3 2 3
© 2014 Macmillan Publishers Limited. All rights reserved
