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Evolution: Radiotracking Sexual Selection

Why are members of one sex bigger than those of the other? A new study in heavy (Figure 1). Males of different weta which male giant weta were radiotracked found that smaller, longer legged species exhibit a variety of mating males win the race to inseminate females. strategies indicative of sexual selection, including harem guarding, Gregory A. Sword dimorphism in the Cook Strait giant resource defence, and male weaponry and Stephen J. Simpson weta, Deinacrida rugosa (: used in male–male conflicts (elongated ). The researchers mandibles or ‘tusks’) [10]. D. rugosa Body size differences between the capitalised on the use of radiotelemetry males, however, do not engage in these sexes are widespread in . to monitor the movement and mating tactics, but rather rove about seeking Although the potential for sexual of individual weta in the field, providing out receptive females whilst they are selection to result in sexual size an unprecedented link between foraging at night on the ground. dimorphism has been recognised since male reproductive success and Working on Te Hoiere/Maud Island, Darwin [1], elucidating the mechanisms sexual selection for smaller, more a small scientific reserve, Kelly et al. [9] involved is still an active area of mobile males. first set out to assess the size and research in evolutionary biology [2,3]. Weta are a group of large, flightless mating status of D. rugosa individuals It is widely accepted that sexual and nocturnal related to on the island. Adult male and female selection and selection for increased crickets which are endemic to the New weta were collected, weighed and fecundity are two major forces driving Zealand archipelago. They occur in measurements of the hind tibia larger body sizes in males and females, a variety of different habitats and vary length and pronotum width taken as respectively. For example, larger males considerably among species in their indicators of body size. Mating status often achieve greater mating success distribution, ecology and behaviour. was determined by whether or not in the face of direct competition for Body size differences in D. rugosa an was observed to be in close mates, or as a result of females make it an exceptional candidate contact with a member of the opposite choosing larger mates. Large body size for studying female-biased sexual sex. A male giant weta typically in females, on the other hand, is often dimorphism, with females weighing remains in direct physical contact with associated with the production of in at a massive (for an insect) 20 g a newly discovered receptive female more or higher quality offspring. whereas males are roughly half as using either his legs or antenna and These benefits of larger body size are themselves counterbalanced by costs associated with becoming too large that may also vary between the sexes. Thus, selection pressures of different intensities, or even in different directions, on males versus females can result in the observed patterns of sexual size dimorphism within a given species [3]. Although males are typically the larger sex in sexually dimorphic birds and mammals, the opposite is true amongst the more numerous but less studied invertebrates, where female-biased size dimorphism is more common [4,5]. Several hypotheses have been put forth to explain why smaller males might have a reproductive advantage over larger males and thus be favoured by sexual selection [3]. Among these is the ‘mobility hypothesis’ [6–8], which holds that smaller, more mobile males should be more effective in courtship or mate finding under scramble competition with larger Figure 1. Body size dimorphism in male (w10 g) and female (w20 g) Cook Straight giant weta males. A recent study by Kelly et al. (Deinacrida rugosa). [9] investigated female-biased sexual The smaller male on the right is tagged with a radiotransmitter. (Photo: Luc Bussie` re.) Current Biology Vol 18 No 20 R956 follows her back to a diurnal refuge tusks used in male–male competition 7. Sze´ kely, T., Reynolds, J.D., and Figuerola, J. (2000). Sexual size dimorphism in shorebirds, where they mate, often repeatedly, for mates [15]. Thus, weta join other gulls, and alcids: the influence of sexual and throughout the next day [10,11]. recent studies in birds and mammals natural selection. Evolution 54, 1404–1413. Some weta were tagged with small, [16,17] suggesting that the intensity 8. Raihani, G., Sze´ kely, T., Serrano- Meneses, M.A., Pitra, C., and Goriup, P. (2006). 0.4 g radiotransmitters, which enabled of sexual selection for female-biased The influence of sexual selection and male individual insects reliably to be dimorphism can equal that observed in agility on sexual size dimorphism in bustards (Otididae). Anim. Behav. 71, 833–838. relocated and their movements male-biased species. 9. Kelly, C.D., Bussie` re, L.F., and Gwynne, D.T. tracked over successive days By directly linking small male body (2008). Sexual selection for male mobility in (Figure 1). To measure reproductive size with increased mobility and mating a giant insect with female-biased size dimorphism. Am. Nat. 172, 417–423. success, the investigators took success, these findings provide 10. McIntyre, M.E. (2001). The ecology of some advantage of the fact that weta, like field-based support for the sexual large weta species in New Zealand. In The Biology of Wetas, King Crickets and Their many other invertebrates, transfer selection mobility hypothesis in the Allies, L.H. Field, ed. (Wallingford: CAB sperm from males to females in small giant weta. This work also raises the International), pp. 225–242. packages called spermatophores [12]. possibility that similar selection 11. Richards, A.M. (1973). A comparative study of the biology of the giant wetas Deinacrida Female giant weta eject the emptied pressures may play a widespread role heteracantha and D. fallai (Orthoptera: spermatophores after mating, which in the evolution of female-biased Henicidae) from New Zealand. J. Zool. 169, 236. 12. Gwynne, D.T. (2001). Katydids and then fall to the ground or sometimes sexual size dimorphism in many other Bush-Crickets: Reproductive Behavior and remain stuck to their bodies. As species. The ability to radiotrack and Evolution of the Tettigoniidae (Ithaca: Cornell a result, by relocating a radiotagged reliably relocate individual insects was University Press). 13. Promislow, D.E., Montgomerie, L.R., and male paired with a female in their refuge a critical component of this and other Martin, T.E. (1992). Mortality costs of sexual before the couple emerge at night, recent studies (for example [18,19]). dimorphism in birds. Proc. Roy. Soc. Lond. B 250, 143–150. a count of the ejected spermatophores Indeed, as individual-based tracking 14. Moore, S.L., and Wilson, K. (2002). Parasites as in the refuge served as an indicator of technologies such as radiotelemetry a viability cost of sexual selection in natural the amount of sperm transferred by become increasingly applicable to populations of mammals. Science 297, 2015–2018. that particular male while mating smaller animals like insects [20] we 15. Kelly, C.D. (2008). Identifying a causal agent during the day. Using their unique can expect further innovative field of sexual selection on weaponry in an insect. Behav. Ecol. 19, 184–192. combination of individual mobility studies that move beyond descriptions 16. Sze´ kely, T., Freckleton, R.P., and data, insemination success and body of individual movement patterns to test Reynolds, J.D. (2004). Sexual selection explains size information, Kelly et al. [9] clearly general hypotheses about underlying Rensch’s rule of size dimorphism in shorebirds. Proc. Nat. Acad. Sci. USA 101, 12224–12227. showed that those males that were behavioural, ecological and 17. Rossiter, S.J., Ransome, R.D., Faulkes, C.G., more mobile were also more successful evolutionary processes. Dawson, D.A., and Jones, G. (2006). Long-term paternity skew and the opportunity for in finding and inseminating females. selection in a mammal with reversed sexual Importantly, they linked increased References size dimorphism. Mol. Ecol. 15, 3035–3043. mobility with longer legs and 1. Darwin, C. (1871). The Descent of Man, and 18. Sword, G.A., Lorch, P.D., and Gwynne, D.T. Selection in Relation to Sex (London: J. Murray). (2005). Migratory bands give crickets smaller bodies in males, whereas no 2. Shuster, S.M., and Wade, M.J. (2003). Mating protection. Nature 433, 703. phenotypic traits were found to be Systems and Strategies (Princeton: Princeton 19. Wikelski, M., Moskowitz, D., Adelman, J.S., University Press). Cochran, J., Wilcove, D.S., and May, M.L. associated with either mobility or 3. Blanckenhorn, W.U. (2005). Behavioral causes (2006). Simple rules guide dragonfly migration. insemination success in females. and consequences of sexual size dimorphism. Biol. Lett. 22, 325–329. In addition to finding evidence for Ethology 111, 977–1016. 20. Holland, R.A., Wikelski, M., and Wilcove, D.S. 4. Andersson, M. (1994). Sexual Selection (2006). How and why do insects migrate? sexual selection favouring smaller, (Princeton: Princeton University Press). Science 313, 794–796. longer legged males, Kelly et al. [9] 5. Abouheif, E., and Fairbairn, D.J. (1997). A comparative analysis of allometry for sexual tested a related hypothesis about the size dimorphism: assessing Rensch’s rule. Am. School of Biological Sciences, The University underlying intensity of selection. It has Nat. 149, 540–652. of Sydney, Sydney, New South Wales 2006, been suggested that the intensity of 6. Andersson, M., and Norberg, R.A. (1981). Australia. Evolution of reversed sexual size dimorphism E-mail: [email protected] sexual selection is weaker in species and role partitioning among predatory birds, exhibiting female-biased as opposed with a size scaling of flight performance. Biol. J. to male-biased dimorphism [13,14].To Linn. Soc. 15, 105–130. DOI: 10.1016/j.cub.2008.08.055 test this, the authors used a statistical measure of the strength of sexual selection known as the ‘opportunity for sexual selection’ (Imates) [2]. In the case Human Cortex: Reflections of Mirror of the giant weta, because males mate with only a single female per day, Neurons this statistic reduces simply to the proportion of mated to unmated males Claims to have identified mirror neurons in human cortex have been that were observed by the investigators controversial. A recent study has applied an fMRI adaptation protocol to the while sampling the insects for the problem and come up with novel evidence for the existence of movement- study. They found that the intensity selective mirror neurons in human cortex. of sexual selection on giant weta males was similar to that found in another co-occurring weta species llan Dinstein premotor area F5 of the macaque under sexual selection (Hemideina monkey responded not only when the crassidens), characterised by In 1996, Gallese et al. [1] discovered monkey executed a particular male-biased dimorphism in mandibular that about 10% of neurons in ventral movement — as expected in this