Animal Behaviour 86 (2013) 1207e1214

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Animal Behaviour

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The role of tusks, musth and body size in maleemale competition among Asian elephants, Elephas maximus

Karpagam Chelliah, Raman Sukumar*

Centre for Ecological Sciences, Indian Institute of Science, Bangalore, India article info The evolution of sexually dimorphic, elaborate male traits that are seemingly maladaptive may be driven e Article history: by sexual selection (male male competition and or female mate choice). Tusk possession in the Asian Received 30 April 2013 elephant is sexually dimorphic and exaggerated but its role in maleemale competition has not yet been Initial acceptance 18 June 2013 determined. We examined the role of the tusks in establishing dominance along with two other known Final acceptance 3 September 2013 maleemale signals, namely, body size and musth (a temporary physiologically heightened sexual state) Available online 10 October 2013 in an Asian elephant population in northeastern India with equal proportions of tusked and tuskless MS. number: 13-00362R males. We observed 116 agonistic interactions with clear dominance outcomes between adult (>15 years) males during 458 field days in the dry season months of 2008e2011. A generalized linear mixed- Keywords: effects model was used to predict the probability of winning as a function of body size, tusk possession Elephas maximus and musth status relative to the opponent. A hierarchy of the three maleemale signals emerged from this maleemale competition analysis, with musth overriding body size and body size overriding tusk possession. In this elephant male secondary character e sexual selection population tusk possession thus plays a relatively minor role in male male competition. An important tuskless male elephant implication of musth and body size being stronger determinants of dominance than tusk possession is that it could facilitate rapid evolution of tuskless males in the population under artificial selection against tusked individuals, which are poached for ivory. Ó 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Extravagant, ornamental and seemingly useless male traits Antlers are elaborately branched, the tips are often curved back and abound in nature across many taxa, extant and extinct, examples of hardly effective in striking an injurious blow to the opponent the latter being the highly curved tusks of adult mammoths and the (Barrette 1977). Alternatively, weapons and badges of dominance immense antlers of the Irish elk (Gould 1974). In addition to the could have evolved originally in the context of maleemale combat theory of natural selection, Darwin (1871) proposed the mecha- and, subsequently, into ornamental forms through female choice nism of sexual selection to account for the evolution of such (Berglund et al. 1996). exaggerated male traits. The two mechanisms of sexual selection as The evolution and function of tusks in elephants (the African originally proposed by Darwin are maleemale competition and savannah elephant, Loxodonta africana, the African forest elephant, female mate choice. In the former, males compete with each other Loxodonta cyclotis, and the Asian elephant) pose similar challenges to mate with females and the male trait may function as a weapon to biologists as do mammalian horns and antlers. Tusks are elon- or as a signal of fighting ability between males. In the latter, females gated second upper incisors and have been the norm in the pro- actively choose the most splendidly ornamented male with which boscidean fossil record (Osborn 1936, 1942). Male and female to mate. These two mechanisms could act on the same trait and African elephants possess long tusks whereas female Asian ele- affect the strength and direction of selection (Hunt et al. 2009). phants are tuskless or with vestigial tusks that are barely visible. The best-studied examples in mammals of male secondary traits Male elephants may also be tuskless; although extremely rare in as possible weapons in maleemale combat are the horns of bovids male African elephants, the percentage of tuskless males (called (Geist 1966; Bro-Jørgenson 2007) and antlers of cervids (Clutton- makhnas) in Asian elephants varies from about 5% to over 90% Brock 1982, 1987). A fundamental problem in accepting that ant- across different populations (Sukumar 1989; Kurt et al. 1995). lers are weapons in maleemale combat is their inefficient design. The functional role of tusks in the elephant has not been empirically determined but there are anecdotal observations of elephants using their tusks for digging mud for minerals and debarking trees. Such functions, however, could be merely by- * Correspondence: R. Sukumar, Centre for Ecological Sciences, Indian Institute of Science, Bangalore 560012, India. products of, and not the driver of, tusk evolution. No trait acts in E-mail address: [email protected] (R. Sukumar). isolation in interanimal interactions (Hoem et al. 2007) and this is

0003-3472/$38.00 Ó 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anbehav.2013.09.022 1208 K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214 clearly evident in maleemale competition in the African elephant using morphological attributes mentioned above to facilitate (L. africana) population of Amboseli, Kenya (Poole 1989a; Hollister- searching and identifying individuals in the database. A comput- Smith et al. 2007). Sexually active males ranging in age from 15 to erized version of the database was implemented in R (R 55 years engage in agonistic interactions to establish dominance. Development Core Team 2013), with Graphical User Interface Two traits, body size and the state of musth, are important de- provided by the package ‘tctltk’. terminants of dominance. Generally, the larger male in a contest Shoulder height was used as a surrogate for age up to 20 years for wins except when the opponent is in a state of musth (Poole 1987, males according to an ageeheight relationship derived from captive 1989a, b; Briffa et al. 2013). Musth is a temporary but intense sexual elephants of known ages and suitably corrected for wild elephants state, expressed by both the African and the Asian elephant (Sukumar et al. 1988; Sukumar 1989). Shoulder heights were (Sukumar 2003), physiologically characterized by sharply elevated measured using three different methods described elsewhere (see plasma testosterone levels and signalled through pheromone- Appendix). Elephants older than 15 years of age were considered as laden secretions from an active temporal gland and pungent adults and classified with some degree of subjectivity into five broad urine dribble (Jainudeen et al. 1972; Poole 1982, 1987; Rasmussen age classes (15e20, 20e30, 30e40, 40e50 and 50þ years) based on a et al. 1984; Ganswindt et al. 2005; Hollister-Smith et al. 2008). combination of features such as ear fold, wrinkled skin, temporal Being in a state of musth is positively associated with high domi- depression and size of the cranium relative to the body. nance status and high reproductive success in the male African elephant (Poole 1989a, b; Hollister-Smith et al. 2007; Rasmussen et al. 2007). Behavioural Observations The role of the tusks in establishing dominance and conse- fi e quently improving male reproductive success, however, has not We de ned competitive male male interactions between a pair ’ been investigated in elephants. It would be difficult to delineate the of males as follows: both males are apparently aware of each other s ’ effects of body size/age and tusk possession as these traits are presence or at least one of the two males is aware of the other s positively correlated with each other. An ideal study population presence and one or more of the behavioural repertoire listed in would have both tusked and tuskless adult males and body size and Table 1 is observed. If two adult males were encountered within tusk length would be uncorrelated. Such populations are found in about 500 m of each other, engaged in activities such as feeding, northeastern India where roughly equal proportions of the two drinking, resting, wallowing in water or mud bathing, they were phenotypes can be found (Sukumar 1989). We therefore examined the correlation between dominance outcome of adult maleemale Table 1 e agonistic interactions and the three presumed male male signals, Behavioural repertoire of adult maleemale agonistic interactions in Asian elephants namely tusks, musth and body size in an Asian elephant population Behaviour Definition at Kaziranga in northeastern India. Nontactile dominant behaviour METHODS Approach An individual pauses its current activity and moves towards another individual Charge An individual suddenly breaks into a run towards the other Study Site and Elephant Population with ears spread out Follow An individual follows another that is walking or running Kaziranga National Park (KNP; 26610e26720N, 93190e away Chase An individual runs behind another individual that is walking 93480E; 993 km2), Assam, India, is located along the floodplains of or running away the Brahmaputra, and comprises mainly riverine habitat, tall and Circular head Vigorous shake of the head from side to side in a plane short grassland, water bodies and patches of tropical semi- shake perpendicular to the ground evergreen forest (Kushwaha 2008). KNP has an annual rainfall of Head raise An individual raises his head and thrusts it forward with the 1500e2500 mm across a westeeast gradient while temperatures trunk curled in and held tense Parallel walk The males walk parallel to each other for hundreds of metres on an average range from 25 Cto35 C during the summer (March back and forth. At times there is a barrier such as a water to October), and from 10 Cto25 C during the winter (November body, dry stream bed or vegetation between them to February; Kushwaha 2008). The population size of the Asian Trunk raise An individual raises his trunk high above and holds it there elephant estimated by the KNP forest department was 1293 in 2008 for a few seconds before either placing his trunk on the opponent or dropping it down and 1165 in 2011, with an adult (15 years and above) male:female Redirected An individual exhibits physical aggression towards inanimate ratio of 1:2.3, and adult tusked to tuskless male ratio of 1:1.5 (KNP aggression objects such as vegetation or a fallen log Forest Department 2008, 2011). Stare An individual stands in front of the other looking in that KNP is accessible for research during the dry season from direction with no other activity such as feeding. This is in November to May the following year, the rest of the time being contrast to turning away and standing fl Sniff posterior One male sniffs the urogenital region of the opponent male usually under ood waters. The present study was conducted over which stands very still three dry seasons (a total of 18 months): November 2008eMarch Tactile dominant behaviour 2009, November 2009eMay 2010 and November 2010eApril 2011. Touch with Trunk or tusk on head, upper lip or body of the opponent; Direct observations of elephants were conducted opportunistically trunk/tusks trunk on tusk of the opponent Fence with tusks A pair of tusked males clashing their tusks without any other along road networks and at water holes and watchtowers between body contact between them fi 0800 hours and sunset on 458 eld days over this period. Push Push gently or vigorously with head Other Poke or gore with tusk; sparring; fighting (Poole 1982); kick; Individual Identification and Age Structure grab tail or bite tail Subdominant behaviour fi Freeze Standing still Adult male elephants (132) were individually identi ed based Retreat Turn, walk or run away; walk backwards on morphology of the ear (degree of ear fold, cuts in the ear margin, Vocalize Roar or roar with circular head shake; trumpet; chirp holes in the ear and vein pattern) and photographed using an Other Penis unsheathed; hide in tall grass or vegetation; present in Olympus C740 UZ digital camera. A photographic database was the periphery of the herd when opponent is in the midst of the herd guarding oestrous female created with a unique numerical ID for each individual and indexed K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214 1209 observed for a minimum of 1 h. If a competitive interaction was musth status and tusk status using a logistic regression model initiated within the hour, the pair was observed for a maximum of (Hardy & Field 1998). The 116 maleemale agonistic interactions 2 h from the start of the interaction until one or both the in- involved only 82 adult males as some males were observed in dividuals disappeared from the observer’s view for more than several contests. This pseudoreplication in the data was addressed 10 min. If an adult male was encountered within 500 m of a female- by incorporating individual identity of the males as a random effect led herd, the observer initiated a focal animal sample of the male to and the explanatory variables relative musth status, tusk status and document the male’s interactions with adult and subadult females body size as fixed effects (Briffa et al. 2013). Relative body size was in the herd for as long as the animals were in view (duration: coded as shoulder height (cm) of the focal individual minus the 15 min to 6 h). If another adult male or males arrived at the loca- shoulder height of the opponent. Asymmetry in musth status was tion, focal group sampling of all the males was initiated and all coded as 1 when the focal individual was in musth and its opponent adult maleemale and maleefemale interactions were recorded. not in musth, 0.5 when the focal individual’s musth secretion was When a solitary adult male was encountered, a 10 min focal sample distinctly more intense than the opponent’s, 0.5 when the focal was initiated. If another adult male arrived within 500 m of the individual’s musth secretion was distinctly less intense than the focal male, the two individuals were observed for a minimum of opponent’s, 1 when the focal individual was not in musth but the 1 h. This sampling strategy was required to understand the context opponent was in musth, and 0 when both were not in musth. in which maleemale interactions occurred. Of the 468 h of Asymmetry in tusk status was coded as 1 when the focal individual behaviour sampling, the actual duration of maleemale interactions had tusks and its opponent did not, 0.5 when the focal individual’s according to our definition was only 37 h. The number of samples of tusks were visibly longer (to the observer by at least one-fifth) than maleemale interactions with a clear outcome in dominance used in the opponent’s, 0.5 when the focal individual’s tusks were visibly the statistical model described below was 116. shorter (by at least one-fifth) than the opponent’s, 1 when the Distance between the observers and the animals ranged from 10 focal individual did not have tusks but the opponent had tusks, and to 400 m. The majority of the observations were from either tall 0 when both did not have tusks. Elephants with a single tusk watchtowers or a vehicle and very few while on foot; observer (congenitally single or one tusk fractured) were considered as effect on the elephants was therefore minimal. The elephants tusked; there were only four such individuals in our sample. Sta- tended to be wary if they could smell but not see the observers. In tistical analyses were performed with R version 3.0.1 (R contrast, the elephants quickly habituated to observers’ presence if Development Core Team 2013). Function ‘glmer’ from package the observers stayed still and silent but visually conspicuous, at lme4 version 0.999999-2 was used for the generalized linear distances of 40e50 m. Focal group samples (Altmann 1974) of all mixed-effects model. adult maleemale interactions were recorded with a digital video A generalized linear mixed-effects model with binomial errors camera (Panasonic HDC HS 20 and/or Sony handycam HDR-SR10E). and logit link function was used to predict the probability of win- At times two video cameras were used if a single camera did not ning (P) for an individual (focal animal) in a paired interaction as a pan all the adult males. All occurrences of known dominance- function of body size (b), musth status (m) and tusk status (t) relative related behaviour events and states (Table 1) were quantified to the opponent. Our global model involving interaction terms was from video recordings, using behaviour-scoring software, designed of the form P w b þ m þ t þ b m þ b t þ m t þ random (focal and developed by the authors, similar to JWatcher (Blumstein et al. individual) þ random (opponent). An individual within each dyadic 2006) but customized, to meet our specific requirements. interaction was randomly chosen as the focal animal. As the best-fit In some studies of maleemale competition, sparring (often statistical model may vary with the configuration of the data owing called play fighting) is distinguished from injurious fighting but to this random assignment of focal animal, the analysis was per- both could function to establish dominance (Poole 1982; Barrette & formed 1000 times and the coefficients of the explanatory values Vandal 1990). Therefore, in our study we explored the dominance along with their significance levels were recorded. Those variables outcome of sparring contests as well as fights. Males could assess whose 95% confidence intervals of the coefficients (slopes) included relative fighting ability through noninjurious sparring and decide zero were dropped from the model. The proportion of times that to fight or retreat just as they would through nontactile displays. A each explanatory variable emerged as significant was also recorded. male that exhibits subdominant behaviour at the end of the inter- action is deemed the ‘loser’ and the opponent the ‘winner’. The RESULTS outcome of an interaction may be unresolved if both males exhibit both dominant and subdominant behaviours with almost equal Effect of Age, Tusks and Musth Phenotypes frequency during the course of an interaction. Adult male African elephants interact to establish dominance throughout their lifetime We observed 116 agonistic interactions with a clear outcome and even in the absence of female-led herds in their proximity involving 82 different adult males. The outcome was unclear in 15 (Poole 1982, 1989a). Whether male Asian elephants do the same is interactions as both males exhibited dominant and subdominant not known; therefore, the presence or absence of female-led herds behaviours almost equally. In 99 (85%) of these 116 interactions, and oestrous females was also noted. The interactions were broadly female-led herds were present within 100 m of the interacting classified into three intensities: (1) nontactile displays, (2) mild males while in at least 58 (59%) of the cases an oestrous female was tactile (involves noninjurious physical contact such as trunk on also present in the herd. The durations of 91 (78%) of the in- head/body, gentle pushing, etc.), (3) intense tactile (involves teractions were less than or equal to 10 min (Appendix Fig. A1). vigorous pushing with head and/or thrusting with tusk, fencing Overall, the numbers of tusked males and tuskless males involved with tusks and biting tail). Elephants in the dyadic interaction were in these agonistic interactions were almost the same (Table 2). also photographed for individual identification, documenting Eighty-six of the 116 agonistic interactions involved at least one relative tusk length, musth status and body size. tusked male. Intense tactile contact occurred in only 19 (22%) of these 86 interactions (Table A1). In none of the 19 contests with Statistical Models and Analyses intense tactile contact did goring or fencing with tusks occur; however, tusks may have clashed when two tusked males head Our aim was to explore the probability of a male emerging as butted or wrestled with their trunks. Seven males (four tuskless dominant over its opponent as a function of relative body size, and three tusked) with puncture wounds, presumably caused by 1210 K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214

Table 2 these losses were to opponents with a musth advantage and eight The age structure and tusk phenotype of adult male elephants observed in 116 to opponents with a body size advantage (Fig. 1a). agonistic maleemale interactions The role of tusk status in dominance is best determined from Age class in years Tusked males Tuskless males Total contests in which two males are comparable in body size and 15e20 20 11 31 musth status but not in tusk status. Of the 116 contests, 39 involved 20e30 7 10 17 a pair of males with the same musth status (Fig. 1b). If we define a 30e40 6 8 14 pair of males as being comparable in body size when the size dif- e 40 50 5 4 9 ference is 10 cm, only 24 contests with males of comparable size 50þ 11011 Total 39 43 82 and musth status were observed. Of these contests only 14 were asymmetrical with respect to tusk status, and in 11 of these 14 contests the elephant with a tusk advantage won (Fig. 1b). In the 13 contests with a body size advantage of more than 10 cm and tusk tusks of an opponent, were observed in the population during the asymmetry, the larger individual won irrespective of tusk status; study period of 458 field days (during which time at least one adult tuskless males won seven and tusked males six of these 13 contests male was sighted every day). All these males were older than 40 (Fig. 1b). years and in musth, except for one tuskless musth male in the 30e 40 years age class. Males with a musth advantage won irrespective of relative tusk Outcome of Models status or body size except on two of the 86 occasions on which one of the males in the interacting pair was in musth. On the other The interaction terms from the global model were dropped as hand, males with a tusk advantage did not always win: in 82 of 116 the 95% CI of their slopes included zero (Table 3). In 1000 config- interactions a male had a tusk advantage over the opponent urations of the data with random assignment of the focal male the (Fig. 1a) and such males won 54 times and lost 28 times. However, interaction terms were seldom significant; while body size and 36 of these wins were associated with a musth advantage and 12 musth emerged significant in 90% of the trials, tusk status was with a body size advantage, and thus only six wins could be significant in only 66.6% of the trials (Table 3). The statistical model attributed to a tusk advantage alone. The 28 losses show a clear that best describes our data is P w b þ m þ t with musth having a pattern with respect to musth and body size disadvantage: 18 of greater influence on dominance outcome than tusk status (Fig. 2a, b). The positive change in probability of winning from having a musth disadvantage to having a musth advantage (Fig. 2b) is much greater than the corresponding change from having a tusk disad- (a) Tusk advantage vantage to having a tusk advantage (Fig. 2a) for a given body size N = 82 asymmetry.

DISCUSSION

Wins = 54 Losses = 28 We investigated the role of tusk status, musth and body size in maleemale competition in a wild population of the Asian elephant that is possibly unique in having roughly equal proportions of tusked and tuskless adult males. Musth and body size combined clearly overrode tusk status as an adult maleemale signal of +m 36 em 17−m 1 +m 0 em 10 −m 18 dominance in this population. Asymmetry in musth status was present in 80% of all contests; therefore, tusk status played a role only in the remaining 20% of the contests. When males were of comparable body size and musth status, elephants with a tusk +b 12 eb 3 −b 2 +b 1 eb 1 −b 8

Table 3 (b) Even musth 39 Statistical model for the probability (P) of the focal male elephant winning in an agonistic maleemale interaction

Variable 95% CI Proportion significant

Lower Upper b ≤ 10 cm : 24 b > 10 cm : 15 b 0.18 2.51 0.902 m 3.40 26.90 0.902 +t 14 +t 13 t 1.11 17.32 0.666 bm 0.77 0.820 0.001 bt 0.36 0.306 0.000 mt 4.15 3.830 0.000

P w b þ m þ t þ b m þ b t þ m t, where b ¼ difference in body size (cm) Win 11 Loss 3 Win 6 Loss 7 relative to the opponent, m ¼ musth status relative to the opponent, t ¼ tusk status relative to the opponent: m ¼ 1 when the focal elephant is in musth and the Figure 1. (a) Number of wins and losses for elephants with a tusk advantage in 82 opponent not in musth; m ¼ 0.5 when the focal elephant has a more intense musth maleemale agonistic interactions with asymmetry in tusk status. þm: musth advan- secretion than the opponent; m ¼ 0 when both elephants are not in musth; m ¼1 tage; em: musth status the same (both not in musth or both in musth); m: musth when the focal elephant is not in musth and the opponent is or when the opponent disadvantage; þb: body size advantage; eb: body size the same; b: body size has a more intense musth secretion. The lower and upper confidence intervals are disadvantage; b: difference in body size in cm; þt: tusk advantage. Numbers indicate for the coefficients of the variables from 1000 analyses in which the focal elephant the number of samples of each type of contest. (b) Number of wins and losses for males was chosen at random. Proportion significant is the proportion of 1000 runs in with a tusk advantage over an opponent of comparable musth status and body size. which the coefficient of a variable was significant. K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214 1211

(a) 1

0.8 Tusk advantage Same 0.6 Tusk disadvantage

0.4 Tusk advantage 0.2 Same Tusk disadvantage 0

−40 −20 0 20 40

(b) 1

0.8 Musth advantage Probability that focal male wins Same 0.6 Musth disadvantage

0.4 Musth advantage 0.2 Same Musth disadvantage 0

−40 −20 0 20 40 Difference in body size (cm)

Figure 2. Association between dominance outcome and asymmetries in body size, tusk status and musth status of male elephants. Symbols indicating observed outcome are plotted with some noise to avoid overplotting. Lines depict predicted outcome from the statistical model P w b þ m þ t: (a) opponents with the same musth status; (b) opponents with the same tusk status. P is the probability that the focal male wins; b is the difference in body size (cm) between the focal male and the opponent; m is the musth status and t the tusk status of the focal male relative to the opponent. The equation of the statistical model plotted for one random configuration of data is log (P/1 P) ¼ 0.036 þ 0.17b þ 3.68m þ 1.44t.

advantage won in the majority of cases; however, the observed bamboo plants is almost twice that of ivory of the Asian elephant frequency of such contests in this population was low (ca. 12%). (110 MPa; Rajaram 1986). Elephants can effortlessly snap bamboo Musth overriding body size as a maleemale signal is well known culms with their trunk and body weight, and it is possible that they in the African savannah elephant (Poole 1989a; Briffa et al. 2013) can do the same to the tusks of an opponent, especially if they are and, therefore, not surprising. On the other hand, musth overriding long. We have observed bull elephants draping their trunks over an tusk status could initially be puzzling because it is unclear how a opponent’s tusk and pressing down with their body weight. Many tuskless male could clearly prevail over a tusked male of larger wild male Asian elephants have been observed with fractured tusks body size when challenged. In the majority of the contests (Krishnan 1972; Sukumar 1994). Poole (1989a) recorded only 20 involving intense tactile contact, high-impact head butting and fights in male African savannah elephants in 14 years that resulted wrestling were more common than ‘fencing’ or goring with tusks. in puncture wounds and tusk fractures. We recorded three males Tusks are probably used to gore the opponent only after he is that fractured their tusk (cause unknown) during our study period already rendered vulnerable (Poole 1989a), perhaps through in- at Kaziranga. ternal injuries caused by high-impact head butting. The majority Only a few elephants were observed with puncture wounds, and (60%) of male Soay sheep, Ovis aries, that died in maleemale almost all of them were musth bulls over 40 years. Elephants are combat had sustained fractures of the cervical vertebrae (Clutton- capable of inflicting puncture wounds with their tusks but the Brock et al. 1990, cited in Preston et al. 2005) implying that inter- moderate tensile strength of tusks and their susceptibility to frac- nal injuries were more often fatal than puncture wounds caused by ture suggest that they are fit for use as a weapon only sparingly. If a horns. The nature of internal injuries that may be sustained by male young adult male elephant wins a particular contest at the cost of elephants in maleemale combat is currently unknown. However, it fracturing a tusk, he would no longer possess tusks suitable for is not unreasonable to expect tuskless males of larger body size to future contests. Theory predicts that in long-lived species with high have an advantage over a tusked male in a fight involving high- potential future reproductive success, noninjurious fighting stra- impact head butting. tegies (‘limited war strategies’) can evolve and be evolutionarily The tensile strength of the tusk of the Asian elephant is low, in stable against invasion by ‘injurious total war strategies’ (Maynard fact, lower than that of antler and bovine femur bone (Rajaram Smith & Price 1973). Total war strategies in a species are expected 1986). The ultimate tensile strength of raw bamboo culms of only when they have just one breeding opportunity in their entire different species ranges from 111 to 219 MPa (Naik 2000, cited in lifetime, or among individuals of the older age class of a long-lived Verma et al. 2012), that is, the tensile strength of some species of species just prior to reproductive senescence. It is highly unlikely 1212 K. Chelliah, R. 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Plasma testosterone levels in relation to musth and sexual activity in the male Asiatic elephant, Elephas ride the advantage of possessing tusks. maximus. Journal of Reproduction and Fertility, 29,99e103. KNP Forest Department 2008. Elephant Census, Kaziranga National Park. Guwahati: Acknowledgments Assam Forest Department. KNP Forest Department 2011. Elephant Census, Kaziranga National Park. Guwahati: Assam Forest Department. We thank the Assam Forest Department for research permits Krishnan, M. 1972. An ecological survey of the larger mammals of peninsular India: and their staff at Kaziranga National Park for their unstinted sup- the Indian elephant. Journal of the Bombay Natural History Society, 69,297e315. port during our field work. We thank Rufford Small Grants for Kurt, F., Hartl, G. & Tiedemann, R. 1995. History and population genetics of a man- made phenomenon. Acta Theriologica, Supplement, 3,125e143. Nature Conservation, U.K., the Department of Science and Tech- Kushwaha, S. P. 2008. Mapping of Kaziranga Conservation Area, Assam. Dehra Dun: nology, New Delhi (through the J.C. Bose National Fellowship to Wildlife Institute of India. R.S.), and the Ministry of Environment and Forests, New Delhi, for Maynard Smith, J. & Price, G. R. 1973. The logic of animal conflict. Nature, 246,15e fi 18. funding part of our eld work. We thank Dr T. Chelliah and Mrs Osborn, H. F. 1936. Proboscidea: A Monograph of the Discovery, Evolution, Migration Lakshmi Chelliah for sponsoring the field vehicle and field equip- and Extinction of the Mastodonts and Elephants of the World. Vol. 1: Moerither- ment (digital still cameras, video cameras, tripods and binoculars). ioidea, Deinotherioidea, Mastodontoidea. New York: American Museum Press. fi Osborn, H. F. 1942. Proboscidea: A Monograph of the Discovery, Evolution, Migration We are indebted to our eld assistants, Mr Deepak Saikia and Mr and Extinction of the Mastodonts and Elephants of the World. Vol. 2: Steg- Diganta Das, without whom data collection at Kaziranga would odontoidea, Elephantoidea. New York: American Museum Press. have been extremely difficult. We thank Sandeep Pulla, Dr Kavita Poole, J. H. 1982. Musth and maleemale competition in the African elephant. Ph.D. Isvaran and Nandita Mondal for their help with the statistical an- thesis. University of Cambridge. Poole, J. H. 1987. Rutting behaviour in African elephants: the phenomenon of alyses. We thank Dr Priya Iyer, Professor Raghavendra Gadagkar, musth. Behaviour, 102, 283e316. Nishant M.S. and Nitin Sekar for their valuable feedback on our Poole, J. H. 1989a. Announcing intent: the aggressive state of musth in African e manuscript. We are grateful to our two anonymous referees and the elephants. Animal Behaviour, 37,140 152. Poole, J. H. 1989b. Mate guarding, reproductive success and female choice in editor for their suggestions, which considerably improved the African elephants. Animal Behaviour, 37, 842e849. quality of our manuscript. Preston, B. T., Stevenson, I. R., Pemberton, J. M., Coltman, D. W. & Wilson, K. 2005. Male mate choice influences female promiscuity in Soay sheep. Pro- ceedings of the Royal Society B, 272, 365e373. References Rajaram, A. 1986. Tensile properties and fracture of ivory. Journal of Materials Sci- ence Letters, 5, 1077e1080. Altmann, J. 1974. Observational study of behaviour: sampling methods. Behaviour, Rasmussen, L. E. L. 1995. Evidence for long-term chemical memory in elephants. 49,227e267. Chemical Senses, 20,762. K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214 1213

Rasmussen, L. E. L., Buss, I. O., Hess, D. L. & Schmidt, M. J. 1984. Testosterone and Table A1 dihydrotestosterone concentrations in elephant serum and temporal gland Intensity of agonistic maleemale interactions in relation to the tusk and musth secretions. Biology of Reproduction, 30, 352e362. status of the males Rasmussen, H. B., Okello, J. B. A., Wittemyer, G., Siegismund, H. R., Arctander, P., Vollrath, F. & Douglas-Hamilton, I. 2007. Age- and tactic- Interaction Nontactile Mild tactile Intense tactile Total related paternity success in male African elephants. Behavioral Ecology, 19, type (N¼67) (N¼27) (N¼22) (N¼116) 9e15. Naik, N. K. 2000. Report on Mechanical and Physic-chemical Properties of Bamboo. Tusk status * Bombay: Indian Institute of Technology. TT 17 2 7 26 R Development Core Team 2013. R: A Language and Environment for Statistical TL 33 15 12 60* Computing, Vienna, Austria. LL 17 10 3 30 Sukumar, R. 1985. Ecology of the Asian elephant and its interaction with man in South Musth India. Ph.D. thesis. Indian Institute of Science. MM 11 1 3 15* Sukumar, R. 1989. The Asian Elephant: Ecology and Management. Cambridge: MN 51 14 6 71* Cambridge University Press. NN 5 12 13 30 Sukumar, R. 1994. Elephant Days and Nights: Ten Years with the Indian Elephant. Delhi: Oxford University Press. Status of males: TT: tusked versus tusked; TL: tusked versus tuskless; LL: tuskless Sukumar, R. 2003. The Living Elephants: Evolutionary Ecology, Behavior and Con- versus tuskless; NN: nonmusth versus nonmusth; MM: musth versus musth; MN: servation. New York: Oxford University Press. musth versus nonmusth. Sukumar, R., Joshi, N. V. & Krishnamurthy, V. 1988. Growth in the Asian * Total number of interactions involving at least one tusked male or one musth elephant. Proceedings of the Indian Academy of Sciences (Animal Science), 97, male. 561e571. Verma, C., Chariar, C. & Purohit, R. 2012. Tensile strength analysis of bamboo and layered laminate bamboo composites. International Journal of Engineering Research and Application, 2, 1253e1264. By the property of similar triangles, the ratio of object size to Watve, M. G. & Sukumar, R. 1997. Asian elephants with longer tusks have lower object distance from the lens is equal to the ratio of image size to parasite loads. Current Science, 72, 885e889. image distance from the lens: Zahavi, A. 1975. Mate selection: a selection for a handicap. Journal of Theoretical Biology, 53, 205e214. o=u ¼ i=v

The image distance to the lens can be approximated to the focal length of the lens, when the object distance is much greater than Appendix the image distance; the approximation is shown below. Focal length of a lens is related to the image and object distances as:

0.8 1=f ¼ 1=u þ 1=v;

f ¼ u v=ðu þ vÞ; 0.6 when u >> v,

f ¼ u v=u;

0.4 f ¼ v; Proportion The distance between the camera and the object was measured using a Leica (DISTO A3) laser distance meter, with an accuracy of 0.2 2 cm. Dimensions of the image-producing area of the sensor of this camera (in mm) were 7.182 (diagonal), 5.760 (width) and 4.290 (height). Dimensions of the digital photographs in pixels were 3648 2736; therefore one pixel represented 0.00168037 mm 0 horizontally and 0.00168129 mm vertically on the sensor. Shoulder 10 20 30 40 50 height measured in pixels was converted to image size in mm on Duration of interactions (min) the sensor using the above pixel to mm scaling.

Figure A1. Frequency distribution of contest durations of maleemale agonistic in- teractions in elephants at Kaziranga. N ¼ 116. Method 2: Using Elephant Footprints

When the footprints of an elephant were clearly discerned on a thin layer of sand, the circumference of the front foot was measured Shoulder heights of elephants were estimated using one or more using a tape. The shoulder height of the elephants was taken to be of the three methods described below. twice the front foot circumference (Sukumar et al. 1988).

Method 1: Measuring Object Size from Digital Photographs Method 3: Using Known Shoulder Height of Other Elephants Shoulder heights were measured from digital photographs captured with an Olympus C740 UZ, at known distances between When an elephant whose shoulder height had been measured the camera and the elephant. The object size (shoulder height of the using either method 1 or method 2 stood next to another elephant elephant, also termed ‘height at withers’) was calculated using the of unknown height, the elephants were either photographed or formula: object size (o) ¼ object distance from the lens (u) image captured on video. From the photograph or a snap shot of the video size (i)/focal length (f)(Sukumar 1985), all measured in mm. frame when the elephants were standing next to each other, the Derivation of the formula is as follows. ratio of their shoulder heights in pixels was measured. As the height 1214 K. Chelliah, R. Sukumar / Animal Behaviour 86 (2013) 1207e1214 of one elephant was known the other could be calculated from this (Arivazhagan & Sukumar 2008). The height of females was assumed pixel ratio. At times the height of a male elephant was estimated to be 210 cm for the 10e15 years age class, 226 cm for the 15e20 relative to an adult (15þ years) or subadult (10e15 years age class) years age class, 233 cm for the 20e30 years age class and 235 cm female that he may have been standing next to. Age of the female for females older than 30 years, these heights representing the was estimated qualitatively based on external morphological mean heights for these age classes, respectively (Sukumar et al. characteristics such as the degree of ear fold and depigmentation 1988; Sukumar 1989).