These are out-of-phase lattice vibrations in the plane of the thin sample, arising when neigh-bouring atoms in the lattice have different charge or mass.

Until now, lattice vibrations were something electron microscopists have had to worry about only in terms of the sample damage that they induce, or when matching experimental images to simulations7. However, the main point of Krivanek and colleagues’ work is that optical phonons are key signatures of chemical bonds, particularly those involving light ele-ments such as hydrogen, as is well established by the techniques of infrared and Raman (opti-cal) spectroscopy. The implication is, there-fore, that STEM–EELS may provide a route for the direct mapping of chemical bonding, including that associated with light elements, at near-atomic resolution.

This achievement would present tremendous benefits in a number of highly topical areas of research into new advanced materials and devices. The improvements in overall energy resolution of EELS will undoubtedly aid the study of the local spatial variation of energy bandgaps in semiconducting structures, and the identification of localized collective oscil-lation of electrons in ‘plasmonic’ structures for light capture. The ability to detect and map light elements, including hydrogen, could extend the existing capability of analytical electron micro-scopy to the study of organic materials such as polymers and pharmaceuticals, as well as energy-storage materials — if issues associated with electron-beam-induced damage can be addressed. Directly measuring phonons could potentially help to identify chemical reactions involving the surfaces of nanoscale hetero-geneous catalyst particles, and could aid the investigation of the transmission of lattice vibra-tions across micro- and nanostructural features, such as interfaces and defects in thermal and optical materials. As with the emergence of any new technique, many additional research areas may ultimately prove to be most fruitful.

Existing theory suggests that electrons that have undergone phonon scattering would be scattered through large angles and the result-ing phonon signal would be spatially highly de localized, preventing atomic-resolution analysis. However, Krivanek and colleagues present some initial findings which, together with recent theoretical predictions8, suggest that under appropriate conditions the phonon signal may be sufficiently localized for the study of vibrations at a spatial resolution better than that achieved by scanning probe tip-enhanced vibrational spectroscopies9. The authors observed an exponential delocalization of the phonon signal as an electron probe is moved away from the surface of a sample and into the surrounding vacuum. However, there seems to be a more localized component of the signal that peaks in intensity close to the surface itself, and the researchers discuss a possible experimen-tal geometry for signal collection that would

A N I M A L B E H AV I O U R

Incipient tradition in wild chimpanzeesThe adoption of a new form of tool use has been observed to spread along social-network pathways in a chimpanzee community. The finding offers the first direct evidence of cultural diffusion in these animals in the wild.

enhance this more localized contribution. Furthermore, if the probe is inside the sample, it seems that the delocalization could be screened at the interface between two mater ials with different electrical properties.

The authors also demonstrate a method for remotely exciting such phonons at a surface using the inherent delocalization of the signal; here, the beam is located in the vacuum close to the edge of a sample, potentially helping to mitigate electron-beam-induced damage of radiation-sensitive samples. These investiga-tions of the spatial resolution of the phonon signal represent a clear example of experiment leading theory in terms of the interpretation of the results, and is indicative of the new experi-mental landscape that this development in instrumentation unfolds. Undoubtedly, many more exciting experiments with this technol-ogy will follow, aided by the delivery, later in 2014, of a third-generation instrument to a shared user facility: the Engineering and Physical Sciences Research Council National

Facility for Aberration-Corrected STEM, or SuperSTEM10, in Daresbury, UK. I look forward to the community charting this new frontier of research. ■

Rik Brydson is at the Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK.e-mail: r.m.drummond-brydson@leeds.ac.uk

1. Krivanek, O. L. et al. Nature 514, 209–212 (2014).2. Krivanek, O. L. et al. Phil. Trans. R. Soc. A 367,

3683–3697 (2009).3. Krivanek, O. L. et al. Microscopy 62, 3–21 (2013).4. Bleloch, A. & Ramasse, Q. in Aberration-Corrected

Analytical Transmission Electron Microscopy (ed. Brydson, R.) Ch. 4, 55–87 (Wiley, 2011).

5. Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope 3rd edn (Springer, 2011).

6. Mahan, G. D. Condensed Matter in a Nutshell (Princeton Univ. Press, 2010).

7. Williams, D. B. & Carter, C. B. Transmission Electron Microscopy 2nd edn (Springer, 2009).

8. Dwyer, C. Phys. Rev. B 89, 054103 (2014).9. Hermann, P. et al. Analyst 136, 1148–1152 (2011).10. www.superstem.org

A N D R E W W H I T E N

Social learning — learning from others — is one of the fastest-expanding research fields in animal behaviour1,2. At the

fundamental level of evolutionary biology, social learning provides a high-speed ‘second inheritance system’ that interacts with genetic inheritance to enrich behavioural evolution2. From a more anthropocentric perspective, ani-mal social learning casts light on the evolution-ary foundations of the cultural capacities that make our own species so successful. Studies of putative cultural variations in wild chim-panzees2–4, the primates with which (together with bonobos) humans last shared a common ancestor, have been particularly influential in our understanding of behavioural evolu-tion. But these observed regional behavioural differences have displayed little change, mak-ing it difficult to investigate the workings of social learning. Now, writing in PLoS Biology, Hobaiter et al.5 describe a new form of tool use in the Sonso community of chimpanzees of the Budongo Forest in Uganda, and present a sta-tistical technique for tracing the social trans-mission of this innovation.

Chimpanzees at Sonso fold wads of leaves in their mouths to fashion a ‘leaf sponge’ that they dip into tree holes to extract water to drink. In 2011, the researchers observed the dominant male of the group, Nick, creating a sponge of moss gathered from a tree trunk and using it to drink from a small flooded waterhole — a behaviour not previously recorded in the 20-year research programme at the site. He was watched by the dominant female, Nambi. Over the next six days of intensive and often competitive use of the waterhole (which the researchers suspect contained unusual densi-ties of minerals or other desirable content), Nambi and six other chimpanzees began to display the moss-sponging technique (Fig. 1a). More than 20 other individuals drank at the hole or in puddles around it, but either directly with their mouths or using leaf sponges rather than moss sponges.

To establish whether this behavioural spread was due to social learning, the researchers developed a form of network-based diffusion analysis (NBDA). This statistical technique quantifies the extent to which the spread of a new behaviour is consistent with the prediction that it will follow the social network — a

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numerical representation of who associates most closely with whom in the community. An impressive recent example of NBDA6 used more than 73,000 observations of 653 hump-back whales during the 27-year spread of a ‘lobtail’ prey-capture technique, which dif-fused as predicted by the social network, implying social learning. However, NBDA studies have so far used only a static, summary quantification of the social network. Hobaiter et al. took this approach to a new level, which they describe as dynamic NBDA, by incor-porating repeatedly updated information on whom each individual was likely to have watched (those within 1 metre of and facing the current sponger). They found strong evi-dence consistent with social transmission, with an estimated 15-fold enhancement of moss-sponging behaviour for each time a novice observed an existing moss-sponger.

Of course, NBDA and other purely statistical approaches to analysing observational data are essentially correlational and thus do not nec-essarily imply cause. To interpret such data, one has to try to rule out alternative potential explanations — in this case, for example, that the order of acquisition observed in the chim-panzees resulted not from social learning but from rank-based queuing to gain access to the waterhole, with each lower-ranked individ-ual happening to watch the previous higher ranker before they got their turn. It seems that this possibility can be excluded, because the ‘lower rankers’ were often offspring of the higher-ranked earlier moss-spongers and so had simultaneous access to the waterhole, yet started to use moss only after watching their mothers. However, caution is still warranted in interpreting these findings in case some subtle alternative factor explains the observed puta-tive evidence for social learning.

Experimental approaches can provide more-robust tests of causality. Indisputable cases of social learning have already been seen

in captive primates, including chimpanzees, in studies in which alternative techniques for using tools or otherwise manipulating foraging tasks are seeded and subsequently spread in different groups7. Such approaches are inher-ently difficult to engineer in the field, but a few attempts have been made (see ref. 8 for a review). Unfortunately, this method has yet to be successfully implemented with wild chimp-anzees, which are surprisingly neophobic.

Nevertheless, non-interventional studies of natural behaviour, such as the one presented by Hobaiter et al., are vital to the field. Experi-mental studies make good sense only when they build on what has first been established in the wild. The innovation recorded by the authors was not dramatic — it was merely a modification of existing leaf-sponging exper-tise. But the findings are valuable as the first direct evidence of cultural diffusion in this key species, converging with observational evi-dence from the wild and rigorously controlled experiments in captive animals to consolidate a substantial case for the role of cultural trans-mission in such cases.

This study follows hot on the heels of another9 documenting the diffusion of a par-ticularly intriguing innovation — placing a blade of grass in the ear — in chimpanzees liv-ing in four large enclosures in an African sanc-tuary (Fig. 1b). That study is unusual because the behaviour seems to be functionless, and thus akin to human cultural phenomena such as fads and fashions. The grass-in-ear behav-iour spread from one apparent inventor in 2007 to seven others by 2012, in just one of the four groups. The lack of overt function makes any explanation other than social learning dif-ficult to accept, and underlines the potential potency of this form of learning in this species.

Researchers have also claimed the first documented case of successful transmission of a novel cultural behaviour — fishing for wood-boring ants using peeled bark or

other material — between wild chimpanzee communities10. And in other studies of the selection of materials for nut-cracking, researchers concluded that migrating female chimpanzees soon conform to the practices of the group they move into11. A major ques-tion for the future is thus what determines the outcome of such migrations between different local cultures. Why do migrants sometimes seed behaviours that diffuse in their new com-munity in the manner demonstrated in the moss-sponging study, whereas others instead abandon previous behaviours and conform to the new local norms? Investigating which factors throw this important switch will add considerably to our understanding of cultural transmission in animals. ■

Andrew Whiten is at the Centre for Social Learning and Cognitive Evolution, School of Psychology and Neuroscience, University of St Andrews, St Andrews KY16 9JP, UK.e-mail: a.whiten@st-andrews.ac.uk

1. Hoppitt, W. & Laland, K. N. Social Learning: An Introduction to Mechanisms, Methods, and Models (Princeton Univ. Press, 2013).

2. Whiten, A. Nature 437, 52–55 (2005).3. Nishida, T., Zamma, K., Matsusaka, T., Inaba, A. &

McGrew, W. C. Chimpanzee Behavior in the Wild: An Audio-Visual Encylopedia (Springer, 2010).

4. Boesch, C. Wild Cultures: A Comparison between Chimpanzee and Human Cultures (Cambridge Univ. Press, 2012).

5. Hobaiter, C., Poisot, T., Zuberbühler, K., Hoppitt, W. & Gruber, T. PLoS Biol. 12, e1001960 (2014).

6. Allen, J., Weinrich, M., Hoppitt, W. & Rendell, L. Science 340, 485–488 (2013).

7. Whiten, A. & Mesoudi, A. Phil. Trans. R. Soc. B 363, 3477–3488 (2008).

8. van de Waal, E., Borgeaud, C. & Whiten, A. Science 340, 483–485 (2013).

9. van Leeuwen, E. J. C., Cronin, K. A. & Haun, D. B. M. Anim. Cogn. http://dx.doi.org/10.1007/s10071-014-0766-8 (2014).

10. O’Malley, R. C., Wallauer, W., Murray, C. M. & Goodall, J. Curr. Anthropol. 53, 650–663 (2012).

11. Luncz, L. V. & Boesch, C. Am. J. Primatol. 76, 649–657 (2014).

This article was published online on 1 October 2014.

a b

Figure 1 | Tools and trends. a, A chimpanzee from the Sonso community using a moss sponge to drink from a waterhole. Hobaiter et al.5 observed that this new tool use spread from one animal to others along the community’s social network. b, Another study9 reports the spread of a new and seemingly useless behavioural ‘fad’ — sticking a blade of grass in the ear — among chimpanzees in a captive sanctuary community.

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