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Current Biology Vol 15 No 16 R624

kinesin-14 family members? In the 1029–1041. are microtubule end-stimulated 3. Marsh, L., and Rose, M. The Pathway of ATPases. Mol. Cell 11, 286–288. case of NCD, in vivo observations Cell and Nuclear Fusion during Mating in 13. Hunter, A.W., Caplow, M., Coy, D.L., have not revealed a role for S. cerevisiae. In The Molecular and Hancock, W.O., Diez, S., Wordeman, L., depolymerase activity. NCD is Cellular Biology of the Yeast and Howard, J. (2003). The kinesin- Saccharomyces, ed. J.R. Pringle, J.R. related protein MCAK is a microtubule required for spindle assembly and Broach, and E.W. Jones. Vol. 3. 1997, depolymerase that forms an ATP- maintenance of bipolarity in the Cold Spring Harbor Laboratory Press: hydrolyzing complex at microtubule Cold Spring Harbor. 827-888. ends. Mol. Cell 11, 445–457. Drosophila early embryo, and 4. Maddox, P.S., Bloom, K.S., and Salmon, 14. Moores, C.A., Yu, M., Guo, J., Beraud, chromosome segregation in E.D. (2000). The polarity and dynamics of C., Sakowicz, R., and Milligan, R.A. meiosis [16,18]. It is not clear at microtubule assembly in the budding (2002). A mechanism for microtubule yeast Saccharomyces cerevisiae. Nat. depolymerization by KinI kinesins. Mol. this time how these activities might Cell Biol. 2, 36–41. Cell 9, 903–909. be linked to plus end 5. Maddox, P., Chin, E., Mallavarapu, A., 15. Niederstrasser, H., Salehi-Had, H., Gan, depolymerase activity, however Yeh, E., Salmon, E.D., and Bloom, K. E.C., Walczak, C., and Nogales, E. (1999). Microtubule dynamics from (2002). XKCM1 acts on a single minus end directed motility has mating through the first zygotic division protofilament and requires the C been implemented in these in the budding yeast Saccharomyces terminus of tubulin. J. Mol. Biol. 316, cerevisiae. J. Cell Biol. 144, 977–987. 817–828. functions. Testing the role of NCD 6. Cleveland, D.W., Mao, Y., and Sullivan, 16. Endow, S.A., Henikoff, S., and Soler- depolymerase in vivo will likely K.F. (2003). Centromeres and Niedziela, L. (1990). Mediation of meiotic prove challenging due to the kinetochores: from epigenetics to mitotic and early mitotic chromosome checkpoint signaling. Cell 112, 407–421. segregation in Drosophila by a protein complex nature of the mitotic 7. Maddox, P.S., Stemple, J.K., Satterwhite, related to kinesin. Nature 345, 81–83. spindle. Nevertheless, similar L., Salmon, E.D., and Bloom, K. (2003). 17. Walker, R.A., Salmon, E.D., and Endow, The minus end-directed motor Kar3 is S.A. (1990). The Drosophila claret studies on other kinesin-14 required for coupling dynamic segregation protein is a minus-end proteins will help shed light on the microtubule plus ends to the cortical directed motor molecule. Nature 347, biological mechanism of this shmoo tip in budding yeast. Curr. Biol. 780–782. 13, 1423–1428. 18. Sharp, D.J., Brown, H.M., Kwon, M., interesting protein family. 8. Endow, S.A., Kang, S.J., Satterwhite, Rogers, G.C., Holland, G., and Scholey, Given our extensive knowledge L.L., Rose, M.D., Skeen, V.P., and J.M. (2000). Functional coordination of of Kar3p function during mating, Salmon, E.D. (1994). Yeast Kar3 is a three mitotic motors in Drosophila minus-end microtubule motor protein embryos. Mol. Biol. Cell 11, 241–253. what can we say about its that destabilizes microtubules 19. Tanaka, K., Mukae, N., Dewar, H., van mechanistic role during mitosis? preferentially at the minus ends. EMBO Breugel, M., James, E.K., Prescott, A.R., J. 13, 2708–2713. Antony, C., and Tanaka, T.U. (2005). Recently, Tanaka et al. [19] 9. Saunders, W., Hornack, D., Lengyel, V., Molecular mechanisms of kinetochore showed that, in mitosis, the minus and Deng, C. (1997). The capture by spindle microtubules. Nature end directed motor activity of Saccharomyces cerevisiae Kinesin- 434, 987–994. related motor Kar3p acts at preanaphase 20. Manning, B.D., Barrett, J.G., Wallace, Kar3p contributes to spindle poles to limit the number and J.A., Granok, H., and Snyder, M. (1999). bi-orientation of chromosomes on length of cytoplasmic microtubules. J. Differential regulation of the Kar3p the spindle. This study did not, Cell Biol. 137, 417–432. kinesin-related protein by two 10. Huyett, A., Kahana, J., Silver, P., Zeng, associated proteins, Cik1p and Vik1p. J. however, present specific X., and Saunders, W.S. (1998). The Kar3p Cell Biol. 144, 1219–1233. observations that are consistent and Kip2p motors function antagonistically at the spindle poles to with plus end depolymerase influence cytoplasmic microtubule Ludwig Institute for Cancer Research, activity for Kar3p in the mitotic numbers. J. Cell Sci. 111, 295–301. Department of Cell and Molecular spindle. Interestingly, Kar3p in the 11. Page, B.D., Satterwhite, L.L., Rose, M.D., Medicine, CMM East, Room 3071, and Snyder, M. (1994). Localization of nucleus is thought to interact with G9500 Gilman Drive, La Jolla, California the Kar3 kinesin heavy chain-related 92093-0653, USA. a different light chain (Vik1p) than protein requires the Cik1 interacting protein. J. Cell Biol. 124, 507–519. that in the cytoplasm (Cik1p [20]). 12. Walczak, C.E. (2003). The Kin I kinesins DOI: 10.1016/j.cub.2005.08.008 It is possible that differential light chain binding can bias Kar3p toward either motility (Vik1p) or depolymerase activity (Cik1). Brain Evolution: Getting Better All Future studies on the depolymerase activity of the Time? Kar3p/Vik1p complex should yield interesting results and help to better understand this important Recent studies on bats, goats and hominids suggest that some motor molecule. In any case, this mammalian brains may have undergone dramatic evolutionary fascinating bi-functional motor reductions in size. These studies emphasise the importance of protein will continue to provide selective pressures upon mammalian brain evolution and the need to insight into the ubiquitous problem integrate studies of neuroanatomy, neurophysiology and behaviour. of microtubule based force production throughout biology. Jeremy E. Niven reduce the selective advantage of a particular trait, potentially leading References possess traits that convey to its reduction or loss. Vision, for 1. Sproul, L.R., Anderson, D.J., Mackey, A.T., Saunders, W.S., and Gilbert. S.P. a selective advantage but which example, is vital for predator and (2005). Cik1 targets the minus-end are often costly in terms of energy prey detection, conspecific kinesin depolymerase Kar3 to and resources, resulting in a trade- recognition and navigation, but in microtubule plus-ends. Curr. Biol. 15, 1420–1427. off between costs and benefits [1]. low light environments such as 2. Meluh, P.B., and Rose, M.D. (1990). Changes in the ecology or caves it has frequently been lost. KAR3, a kinesin-related gene required for yeast nuclear fusion. Cell 60, behaviour of the may The brain is subject to the same Dispatch R625

selective pressures as other traits. Pteropodidae Therefore, when energy and Rhinolophidae resources are limiting and Hipposideridae demands on neural processing are reduced, brains would be Megadermatidae expected to get smaller. Indeed, Nycteridae the exceptionally high energetic Craseonycteridae Ancestor cost of the brain suggests that it Rhinopomatidae would be under strong pressure to Emballonuridae reduce cost. There is certainly evidence to Molossidae support the reduction of specific Vespertilionidae brain regions such as the visual Noctilionidae systems of cave fish. Examples of Mormoopidae brain size reduction under Phyllostomidae domestication also exist [2] and, although this is artificial selection, Current Biology they clearly show that such Figure 1. A phylogenetic tree of Chiropteran families showing changes in relative brain changes are possible. Yet mass. reduction in brain size has received Branches containing some species showing brain size reduction in blue, expansion in little attention during over 100 red and a mixture in yellow. All families contain some species possessing the ancestral of comparative brain analysis brain size. Lineages that evolved in Gondwana are highlighted by a grey box. (Data: in vertebrates [3,4]. Most studies of Kamran Safi, Zoologisches Institut, Universität Zürich). the vertebrate brain have focussed exclusively upon expansion of the Gallogoral meneghini, whose (Figure 1). Both reduction and brain or specific brain regions relative brain mass is similar to that expansion are evident in families [5–8], emphasising their expansion of other fossil and living bovids, thought to have evolved in relation to behavioural confirming the dramatic brain size independently on Laurasia and specializations [7,8]. For example, reduction in Myotragus [9]. Gondwana, suggesting the degree of specialization for What factors led to the reduction independent reduction of relative food hoarding in birds is highly of Myotragus’s brain? One key brain size (Figure 1). correlated with the relative size of factor is likely to be the isolation of One explanation for brain size their hippocampus [7], whilst Myotragus on an island. Islands reduction in bats could be their neocortical volume in is often have a depleted fauna foraging strategies and habitiat correlated with deception rate [8]. compared to the mainland and complexity [10,12]. Habitat Three recent studies [9–11] show Majorca lacked large mammalian complexity correlates with relative that brain size reduction may have predators. The impoverishment of brain size for animal-eating bats occurred in several mammalian the island ecosystem and the but not for plant-eating bats, lineages under different conditions. release from predation pressure which all tend to have relatively These studies show the reduction may have reduced the requirement large brains, suggesting that of brain mass relative to the body for Myotragus to support a large habitat and foraging strategy mass, an important consideration brain. Resource limitation may also affect brain size [10]. Bats with as brain mass scales allometrically have played an important role. smaller brains hunt in open, making absolute brain size a Foraging opportunities are often uncluttered environments, which potentially misleading measure. limited on islands and, in the presumably place lower demands The first study documents brain absence of predation, Myotragus on neural processing whilst those size reduction in an extinct bovid populations may have expanded, with larger brains hunt in cluttered, , Myotragus [9]. Myotragus increasing pressure on energetic complex environments requiring fossils are found on two islands, expenditure. high levels of neural processing. Majorca and Menorca, and are Is brain size reduction limited to Energetic constraints are also an related to the chamois goat islands? A second study on bats important consideration for flying ( rupicapra). Having suggests that it may be far more animals. Excessively large brains crossed from the mainland to these wide-ranging [10]. Reconstruction not only consume energy but also islands during a drop in sea level, of the ancestor of modern bats require large amounts of energy to Myotragus was subsequently suggests that it had intermediate carry. Therefore, as in Myotragus, isolated and underwent a wing area, body mass and brain brain reduction has occurred when substantial reduction in brain mass: mass, implying that some modern demands on neural processing are 50% compared to living bovids of bat species may have undergone reduced and energy is limiting. similar body mass. To be certain brain size reduction whilst others The recent discovery of a small- Myotragus underwent brain size have expanded their brains [10]. brained fossil hominid, Homo reduction, however, it is essential Brain size reduction seems to floresiensis, on the island of Flores to know the brain size of its have occurred in some species of in Indonesia [11,13] raises many continental ancestor; the primary at least eight families, whilst important questions about brain candidate is another fossil bovid, expansion occurred in six families size [14]. Although the brain of H. Current Biology Vol 15 No 16 R626 floresiensis is smaller in absolute particular time. The relative 8. Byrne, R.W., and Corp, N. (2004). Neocortex size predicts deception rate in terms than that of its putative reduction or expansion of brains primates. Proc. Biol. Sci. 271, ancestor, H. erectus, the reduction would alter the number of neurons 1693–1699. in relative brain size may be small and neural circuits but not the 9. Köhler, M., and Moyá-Solá, S. (2004). Reduction of brain and sense organs in depending upon the exact body density of neural activity, the fossil insular bovid Myotragus. Brain mass estimate. H. floresiensis has producing fewer or more neurons Behav. Evol. 63, 125–140. 10. Safi, K., Seid, M., and Dechmann, D.K.N. a brain 2.5–4.6 times larger than active simultaneously. Therefore, (2005). Bigger is not always better: when expected for an ‘average’ the relationships between brains get smaller. Biol. Lett. doi: of equivalent body mass, whereas numbers of neurons, their activity 10.1098/rsbl.2005.0333. 11. Brown, P., Sutikna, T., Morwood, M.J., the value for H. erectus is 3.3–4.4 and their energy consumption Soejono, R.P., Jatmiko, Saptomo E.W times, suggesting that their relative may be key to understanding links and Due, R.A (2004). A new small-bodied brain size may been similar [11]. between relative brain size and hominin from the Late of Flores, Indonesia. Nature 431, Any brain size reduction that has behavioural complexity. Studies 1055–1061. occurred may have been due to the on Myotragus, bats and H. 12. Hutcheon, J.M., Kirsch, J.A.W., and Garland, T. (2002). A comparative same factors as in Myotragus and floresienses demonstrate the analysis of brain size in relation to the Chiroptera: reduced demands potential effects of selective foraging ecology and phylogeny in the on neural processing and an pressures on relative brain size. Chiroptera. Brain Behav. Evol. 60, 165–180. increased pressure on energetic To understand the implications of 13. Falk, D., Hildebolt, C., Smith, K., expenditure. If reduction has changes in relative brain size, Morwood, M.J., Sutikna, T., Brown, P., occurred, this suggests that H. however, neurophysiology and Jatmiko, Saptomo E.W, Brunsden, B., and Prior, F. (2005). The brain of LB1, floresiensis may not have been behaviour are also essential. Homo floresiensis. Science 308, capable of the behavioural 242–245. References 14. Mirazón Lahr, M., and Foley, R. (2004). complexity observed in modern Human evolution writ small. Nature 431, 1. Darwin, C.R. (1859). On the Origin of humans. However, archaeological 1043–1044. Species. (London: John Murray). 15. Morwood, M.J., Soejono, R.P., Roberts, evidence found near H. floresiensis 2. Kruska, D.C.T. (2005). On the R.G., Sutikna, T., Turney, C.S., suggests it was capable of tool use evolutionary significance of encephalization in some eutherian Westaway, K.E., Rink, W.J., Zhao, J.-x., and of possibly harnessing fire [15]. : effects of adaptive radiation, van den Bergh, G.D., Due, R.A. et al. The discovery of H. floresiensis domestication and feralization. Brain (2004). Archaeology and age of a new hominin from Flores in eastern emphasises that the relationship Behav. Evol. 65, 73–108. 3. Jerison, H.J. (1973). Evolution of the Indonesia. Nature 431, 1087–1091. between brain size and Brain and Intelligence. (New York: 16. Martin, R.D. (1981). Relative brain size behavioural complexity remains Academic Press) and basal metabolic rate in terrestrial 4. Striedter, G.F. (2005). Brain Evolution. vertebrates. Nature 293, 57–60. unclear. Brain size affects the (Sunderland: Sinauer Associates Inc. US) 17. Attwell, D. and Laughlin, S. (2001). An number of neurons in the brain 5. 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DOI: 10.1016/j.cub.2005.08.007 However, absolute brain size does not appear to be as important as relative brain size in determining behavioural complexity, because Sexual Selection: Copycat Mating whales and elephants have larger brains than humans [4]. Relative in Birds brain size appears to be linked to resting metabolic rate [16]. Energy places limits on both the total Female zebra finches may be influenced by the choices of other numbers of neurons — because of females when selecting mates, challenging the view that mate-choice maintenance costs — and the copying should not occur in species with biparental care. density of neural activity. Calculations suggest that in both Gillian R. Brown1 and males, or same kind of males, as the rat and the human brains only Tim W. Fawcett2 they do. a small proportion of neurons are For species in which males do active at any particular time, Choosing a mate is not a simple not help care for their young, such because of the high energetic business. A female needs a male mate-choice copying might work costs of neural signalling [17,18]. in order to reproduce, but how very well. Females want the One possibility is that, although does she know which one to pick? healthiest, most attractive males, larger animals have greater How can she find a good mate and the behaviour of other numbers of neurons, their brains without wasting too much time females might guide them to also have lower mass specific and effort? One answer is to these males. However, if males metabolic rates, suggesting lower watch what other females are help to care for their young and densities of neural activity at any doing and choose the same females are searching for a good