Post by Infinity Blade on Jun 21, 2023 4:55:15 GMT 5
This post reviews and details the mechanics of elephant intraspecific combat, from both a behavioral and biomechanical perspective. It will go over what factors are most relevant in determining victory, the method in which elephants fight, and the frequency of severe consequences from combat (i.e. death).
For those looking for a brief summary, it is as follows: musth trumps body size, and body size trumps tusk possession. Fatal fights between elephants are rare. Therefore, despite appearances, tusks do not appear to function primarily as lethal weapons.
Musth:
Musth is a condition in male elephants that occurs periodically; they experience a surge in hormone levels and significantly increased aggression. Testosterone levels in temporal gland secretions during musth reaches 0.5 mg/mL (in the serum it is approximately one tenth of this) (Rajaram, 2006).
Multiple studies have found that musth status is the most important factor in determining victory in agonistic encounters between bull elephants, overriding both body size and tusk status/possession. A study in northeastern India observing 116 agonistic interactions during 458 field days between 2008 and 2011 found that in Asian elephant bulls, musth clearly trumped discrepancies in body size and tusk possession (Chelliah & Sukumar, 2013).
The evolution of sexually dimorphic, elaborate male traits that are seemingly maladaptive may be driven by sexual selection (male–male competition and or female mate choice). Tusk possession in the Asian elephant is sexually dimorphic and exaggerated but its role in male–male competition has not yet been determined. We examined the role of the tusks in establishing dominance along with two other known male–male signals, namely, body size and musth (a temporary physiologically heightened sexual state) in an Asian elephant population in northeastern India with equal proportions of tusked and tuskless 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 2008–2011. A generalized linear mixed-effects model was used to predict the probability of winning as a function of body size, tusk possession and musth status relative to the opponent. A hierarchy of the three male–male signals emerged from this analysis, with musth overriding body size and body size overriding tusk possession. In this elephant population tusk possession thus plays a relatively minor role in male–male competition. An important 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.
Males with a musth advantage won irrespective of relative tusk status or body size except on two of the 86 occasions on which one of the males in the interacting pair was in musth.
We investigated the role of tusk status, musth and body size in male–male 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 male–male signal of 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.
Studies conducted on African bush elephants (Loxodonta africana) have found that the same is true for them as well.
Poole (1989)
Predictions derived from game theory suggest that animals should not signal their intentions during conflict situations. However, during the period of musth, male elephants, Loxodonta africana, announce a state of heightened aggression with signals that are unbluffable. Since smaller musth males in poor condition are able to dominate larger, normally higher-ranking, non-musth males in good condition, musth provides a useful system with which to examine the possibility of honest signalling of motivation, rather than of fighting ability. Despite the highly aggressive state of males in musth, escalated contests are extremely rare. The behaviour of musth and non-musth males suggests that opponents are able to estimate their often rapidly changing roles in the asymmetries with relative accuracy. Since, unlike most other rutting mammals, elephants have asynchronous sexually active periods, resource value varies both with age and the fluctuating sexual state of a particular individual. It is suggested that musth may be a case where information about resource value is conveyed.
However, during aggressive interactions between males where one of a pair was in musth and the other was not, the winner was determined by musth, rather than by body size. For example, examining only the cases where the smaller males was in musth and the larger male was not in musth, in 42 (86%) different pairs the smaller musth male won, while the larger non-musth male won in only seven different pairs (χ2=25, df=1, P<0.001; number of different pairs=49; number of interaction-days=53).
Moss (1983)
1. During the course of an eight year field study of African elephants in Amboseli National Park, Kenya, from September, 1972 to December, 1980, females were seen exhibiting oestrous behaviour on 154 occasions, during which 43 copulations were witnessed. 2. Five categories of oestrous behaviour are described: a) wariness, b) the oestrous walk, c) the chase, d) mounting, and e) consort behaviour. 3. The oestrous behaviour as described was found to be temporally associated with ovulation, as judged by conceptions, in 70.7% of 58 cases. 4. The duration of oestrous periods is estimated at 2-6 days. 5. On average Amboseli females conceived once every 5 years and for each of these conceptions the female may only have been in oestrus once. 6. Evidence from the Amboseli study suggests that some females may exercise choice in mating partners. For example, females were able to elude their pursuers in 69.4% of all chases (n = 134). Observation of female behaviour during oestrus suggested that some females preferred to mate with males in the largest size class, particularly those in musth. 7. Two possible short-term advantages to females exercising choice in mating partners are suggested: a) avoidance of harassment from other bulls; and b) Large males in musth may be more likely to impregnate a female. 8. A possible long-term advantage to mating with a Large, and therefore older, male could be his ability to pass on a trait for longevity. 9. Although females may be exercising choice among the size/age classes, male-male competition among the Large males may override female choice on the individual level.
It is worth noting here that musth is not a reflection of a male elephant’s actual fighting ability. It rather appears to be an honest signal of a male’s motivation to mate with females. African elephant bulls in peak condition but not in musth have been recorded being chased off by much smaller, musth males in visibly poorer condition (Poole, 1989). In fact, the evolutionary advantage of musth is to advertise sexual activeness while reducing the likelihood of fatal fights (Reardon, 2021).
The evolutionary advantage of musth is that it identifies sexually active males to non-musth males and normally reduces the likelihood of fatal fights over rare oestrous females - cows can mate successfully only during a brief 48-72-hour period. Playback experiments found that the calls of musth males cause an aggressive reaction in other musth bulls and avoidance of the area by non-musth males.
Body size:
When musth status is equal, body size is the next most important factor determining dominance/victory in agonistic encounters.
Chelliah & Sukumar (2013)
The evolution of sexually dimorphic, elaborate male traits that are seemingly maladaptive may be driven by sexual selection (male–male competition and or female mate choice). Tusk possession in the Asian elephant is sexually dimorphic and exaggerated but its role in male–male competition has not yet been determined. We examined the role of the tusks in establishing dominance along with two other known male–male signals, namely, body size and musth (a temporary physiologically heightened sexual state) in an Asian elephant population in northeastern India with equal proportions of tusked and tuskless 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 2008–2011. A generalized linear mixed-effects model was used to predict the probability of winning as a function of body size, tusk possession and musth status relative to the opponent. A hierarchy of the three male–male signals emerged from this analysis, with musth overriding body size and body size overriding tusk possession. In this elephant population tusk possession thus plays a relatively minor role in male–male competition. An important 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.
Males with a musth advantage won irrespective of relative tusk 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 hand, males with a tusk advantage did not always win: in 82 of 116 interactions a male had a tusk advantage over the opponent (Fig. 1a) and such males won 54 times and lost 28 times. However, 36 of these wins were associated with a musth advantage and 12 with a body size advantage, and thus only six wins could be attributed to a tusk advantage alone. The 28 losses show a clear pattern with respect to musth and body size disadvantage: 18 of these losses were to opponents with a musth advantage and eight to opponents with a body size advantage (Fig. 1a). The role of tusk status in dominance is best determined from contests in which two males are comparable in body size and musth status but not in tusk status. Of the 116 contests, 39 involved a pair of males with the same musth status (Fig. 1b). If we define a pair of males as being comparable in body size when the size difference is 10 cm, only 24 contests with males of comparable size 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 asymmetry, the larger individual won irrespective of tusk status; tuskless males won seven and tusked males six of these 13 contests (Fig. 1b).
As with musth, this also applies to African elephants.
Poole (1989)
The factor determining dominance rank between non-musth males was body size (Spearman rank correlation rs=0.99, N=37, P<0.01; Fig. 1), with larger males ranking above smaller males in dyadic interactions (number of interaction-days=574). When both of a pair of males were in musth, the winner of an interaction was also usually determined by body size. During interactions between 75 different pairs of musth males the interactions were won by the larger male in 70 (93%) pairs (χ2=56.4, df=1, P<0.001; number of interaction-days=129).
Tusks:
You would think that elephant tusks, which are long and pointed structures that can be used to gore, would be critical in determining dominance between elephants. In fact, as already indicated by some of the data presented above, tusks are subordinate to musth and body size.
In fights between bull Asian elephants, tuskers are not necessarily victorious to tuskless bulls. Tuskless bulls often make up for their lack of tusks with a size advantage (>10 cm difference in shoulder height), a more robust body, a larger skull, and more developed musculature of the trunk (Sukumar, 2003). In fact, during intense tactile combat, high-impact head butting and wrestling are more common than fencing or goring with tusks (Chelliah & Sukumar, 2013).
Several other questions on sexual selection in elephants come to mind. Tuskless male elephants (called makhnas) obviously follow a different strategy in attracting a female. In populations in which both tuskers and makhnas are found in roughly equal numbers, as in northeastern India, the makhnas seem on average to be larger and more robustly built. Presumably, selection would have favored traits such as larger body size, better musculature of the trunk, and development of the skull in makhnas to compensate for lack of tusks. Even among tusked males, those with longer tusks may not necessarily be more robust in physique; in fact, the opposite may be true. The bulls Ahmed of Kenya and Cross Tusks Jr. of Mudumulai, with exceptionally long tusks, were relatively poorly built. No doubt, such subjective comparisons will have to be quantified with more rigor.
The point I am trying to make here is that male elephants may follow alternative stable strategies. Madhav Gadgil has argued that dimorphism among males of many animal groups, from insects to mammals, may arise from selection for alternative strategies on cost-benefit considerations. One form may be superior in combat, but pay the costs of higher expenditure of energy to maintain the needed weaponry, while the other may gain in other ways (such as longevity) by conserving this energy. “This costly arms race comes to an end when those investing in weaponry are just as well off as those which have totally opted out of such investment” (1972, p. 580). With elephants, the situation may be similar, but with the additional role of female choice. Could the tuskless elephants or makhnas gain mating advantage through their robust build and intense expression of musth, while other males divert resources into growth by which they gain a reproductive advantage through female choice for tusk size? Even among the tusked males, those with short tusks better suited for actual combat may be able to dominate in male-male competition or expend more energy for musth, while those with oversize tusks may gain from female choice for this handicap.
The point I am trying to make here is that male elephants may follow alternative stable strategies. Madhav Gadgil has argued that dimorphism among males of many animal groups, from insects to mammals, may arise from selection for alternative strategies on cost-benefit considerations. One form may be superior in combat, but pay the costs of higher expenditure of energy to maintain the needed weaponry, while the other may gain in other ways (such as longevity) by conserving this energy. “This costly arms race comes to an end when those investing in weaponry are just as well off as those which have totally opted out of such investment” (1972, p. 580). With elephants, the situation may be similar, but with the additional role of female choice. Could the tuskless elephants or makhnas gain mating advantage through their robust build and intense expression of musth, while other males divert resources into growth by which they gain a reproductive advantage through female choice for tusk size? Even among the tusked males, those with short tusks better suited for actual combat may be able to dominate in male-male competition or expend more energy for musth, while those with oversize tusks may gain from female choice for this handicap.
Musth overriding body size as a male–male signal is well known in the African savannah elephant (Poole 1989a; Briffa et al. 2013) and, therefore, not surprising. On the other hand, musth overriding tusk status could initially be puzzling because it is unclear how a tuskless male could clearly prevail over a tusked male of larger body size when challenged. In the majority of the contests involving intense tactile contact, high-impact head butting and wrestling were more common than ‘fencing’ or goring with tusks. Tusks are probably used to gore the opponent only after he is already rendered vulnerable (Poole 1989a), perhaps through internal injuries caused by high-impact head butting. The majority (60%) of male Soay sheep, Ovis aries, that died in male–male combat had sustained fractures of the cervical vertebrae (CluttonBrock et al. 1990, cited in Preston et al. 2005) implying that internal injuries were more often fatal than puncture wounds caused by horns. The nature of internal injuries that may be sustained by male elephants in male–male combat is currently unknown. However, it is not unreasonable to expect tuskless males of larger body size to have an advantage over a tusked male in a fight involving high-impact head butting.
If you notice something in the excerpt from Chelliah & Sukumar (2013), you’ll see they say that “Tusks are probably used to gore the opponent only after he is already rendered vulnerable (Poole 1989a), perhaps through internal injuries caused by high-impact head butting”. Now note what Richard Estes says about fights between African elephants (Estes, 2012).
“When 2 equal-size bulls meet, they take each other’s measure by coming together with heads raised until their trunk bases and/or tusks are firmly engaged (fig. 17.8). This is also the position during combat. Each tries to be taller, to press downward on the other’s trunk. Or, if contact is made on the underside of their raised trunks, they may grapple for a hold. Tusk length is important, but so are height and weight, for these determine who can push whom around. The elephant that holds his head higher, whether because he is bigger or more confident, usually wins.”
“They entwine trunks, then try to twist or push each other out of line (fig. 17.9), while using the tusks as levers to force the opponent’s head down or sideways. When they break apart, the first to turn may be prodded gently in the flank by the victor. In a serious fight, this is the time and manner in which fatal goring may occur.”
Notice something? Height and weight are critical in determining which elephant can overpower the other (tusk length is also important, but in the sense that they serve as levers to facilitate wrestling). It’s only when the elephants break apart and the first one to turn/back down gets fatally gored (if it is a serious fight).
In African elephants, it is noted that tusk size is not even mentioned by previous studies (i.e. the ones I cited above) to be a factor affecting hierarchical position (Whyte & Hall-Martin, 2018).
In males, however, tusk size must be more important. It is difficult to imagine that very large tusks would be more useful as weapons or tools than smaller ones, but it may be that larger tusks are of symbolic value in dominance skirmishes or displays. Males with larger tusks might be expected to occupy higher positions in the hierarchical structure or to be favoured as mates by females. However work in Amboseli National Park, Kenya (Moss, 1983; Poole, 1989a) suggests that the phenomenon of ‘musth’ is far more important than body size in determining a male’s hierarchical position and its success in mating. The condition of musth is easily recognisable to other elephants and the position in the social hierarchy of a male in musth is immediately elevated above all non-musth males. Even when two large males are simultaneously in musth, Poole (1989b) found that the factor determining dominance rank in both musth and non-musth males was body size. Tusk size is not mentioned by Moss (1983) or Poole (1989b) as a factor affecting hierarchical position. The evolutionary reason why some elephant males have large tusks is therefore still unclear, and would be an interesting subject for further research.
(Another interesting thing worth mentioning is that bulls with super-long tusks are not necessarily better at combat than those with shorter, more normal-sized tusks. In fact, Sukumar (2003) and Whyte & Hall-Martin (2018) above both mention super-long tusks being less practical for actual combat.)
Why is this the case? Chelliah & Sukumar point out the rather low tensile strength of elephant ivory, and believe that tusks are better fit for use as weapons sparingly.
The tensile strength of the tusk of the Asian elephant is low, in fact, lower than that of antler and bovine femur bone (Rajaram 1986). The ultimate tensile strength of raw bamboo culms of different species ranges from 111 to 219 MPa (Naik 2000, cited in Verma et al. 2012), that is, the tensile strength of some species of bamboo plants is almost twice that of ivory of the Asian elephant (110 MPa; Rajaram 1986). Elephants can effortlessly snap bamboo culms with their trunk and body weight, and it is possible that they can do the same to the tusks of an opponent, especially if they are long. We have observed bull elephants draping their trunks over an opponent’s tusk and pressing down with their body weight. Many wild male Asian elephants have been observed with fractured tusks (Krishnan 1972; Sukumar 1994). Poole (1989a) recorded only 20 fights in male African savannah elephants in 14 years that resulted in puncture wounds and tusk fractures. We recorded three males that fractured their tusk (cause unknown) during our study period at Kaziranga.
Only a few elephants were observed with puncture wounds, and almost all of them were musth bulls over 40 years. Elephants are capable of inflicting puncture wounds with their tusks but the moderate tensile strength of tusks and their susceptibility to fracture suggest that they are fit for use as a weapon only sparingly. If a young adult male elephant wins a particular contest at the cost of fracturing a tusk, he would no longer possess tusks suitable for future contests. Theory predicts that in long-lived species with high potential future reproductive success, noninjurious fighting strategies (‘limited war strategies’) can evolve and be evolutionarily stable against invasion by ‘injurious total war strategies’ (Maynard Smith & Price 1973). Total war strategies in a species are expected only when they have just one breeding opportunity in their entire lifetime, or among individuals of the older age class of a long-lived species just prior to reproductive senescence. It is highly unlikely that the injuries to the large, old musth males were caused by smaller, younger males because, as our own observations indicate, musth and body size override tusk status in fights. Therefore, the injuries are most likely to have been inflicted by an opponent in the older age classes. It is of course possible that juvenile and subadult elephants with short tusks that are less prone to fracture may use them as weapons to intimidate their tuskless peers (a speculation that can be empirically tested), thus gaining an advantage in establishing a more dominant position. As Asian elephants are a long-lived species with long-term memory of individuals (Rasmussen 1995), a dominance advantage gained during the juvenile stage and adolescence may continue at least into early adulthood before the onset of musth.
Only a few elephants were observed with puncture wounds, and almost all of them were musth bulls over 40 years. Elephants are capable of inflicting puncture wounds with their tusks but the moderate tensile strength of tusks and their susceptibility to fracture suggest that they are fit for use as a weapon only sparingly. If a young adult male elephant wins a particular contest at the cost of fracturing a tusk, he would no longer possess tusks suitable for future contests. Theory predicts that in long-lived species with high potential future reproductive success, noninjurious fighting strategies (‘limited war strategies’) can evolve and be evolutionarily stable against invasion by ‘injurious total war strategies’ (Maynard Smith & Price 1973). Total war strategies in a species are expected only when they have just one breeding opportunity in their entire lifetime, or among individuals of the older age class of a long-lived species just prior to reproductive senescence. It is highly unlikely that the injuries to the large, old musth males were caused by smaller, younger males because, as our own observations indicate, musth and body size override tusk status in fights. Therefore, the injuries are most likely to have been inflicted by an opponent in the older age classes. It is of course possible that juvenile and subadult elephants with short tusks that are less prone to fracture may use them as weapons to intimidate their tuskless peers (a speculation that can be empirically tested), thus gaining an advantage in establishing a more dominant position. As Asian elephants are a long-lived species with long-term memory of individuals (Rasmussen 1995), a dominance advantage gained during the juvenile stage and adolescence may continue at least into early adulthood before the onset of musth.
Obviously because elephants use their tusks as tools (or as weapons at all), they must have some strength to them. Longitudinal bending strength of African elephant and woolly mammoth ivory is impressive indeed, but bending strength and other mechanical properties in the transversal direction are far reduced (bending strengths <100 MPa) (Pfeifer et al., 2019).
Figures 4 and 5 and Table 2 show the results of the 3-point bending tests. Again, woolly mammoth and African elephant exhibit very similar properties. In the longitudinal direction Young’s Modulus of Elasticity, a measure for the stiffness of a material, is 10.1 ± 0.6 GPa for mammoth and 10.7 ± 0.6 GPa for elephant dentine. The bending strengths are 357.3 ± 26.1 MPa and 369.0 ± 21.8 MPa, respectively. The work of fracture, a measure of the damage tolerance and toughness of a material, is 22.3 ± 10.0 kJ/m² for mammoth and 23.8 ± 6.9 kJ/m² for elephant. In the transversal direction, the mechanical properties are significantly reduced and the Young’s modulus and work of fracture are different for mammoth and elephant. The Young’s modulus is 6.2 ± 0.3 GPa and 5.0 ± 0.5 GPa, and the work of fracture is 0.4 ± 0.2 kJ/m² and 1.1 ± 0.5 kJ/m², respectively. The bending strengths are similar and amount to 94.9 ± 10.7 MPa for mammoth and 97.0 ± 6.4 MPa for elephant (Figs 4a,b and 5a,b).
The takeaway here is that tusks are not the primary weapons of elephants in combat (this may be due to some limiting factors in their mechanical properties). Their weight is. The tusks come into play after the opponent has been overpowered or otherwise weakened by blunt trauma. The ability to do both of these things is, obviously, a function of weight and strength.
The frequency of death:
Elephants may sometimes be intuitively thought of as often causing death in intraspecific combat with their tusks. But in truth, with the exception of high-density populations (e.g. populations fenced in but grow at a high rate; this is unnatural), fatal combat is rare, something multiple sources on elephant behavior/biology mention.
Estes (2012), p. 265
Aggressive interactions between mature bulls are rare and usually low key, although serious and even fatal fights sometimes occur when 2 bulls of equal size compete over a cow in heat and/or are simultaneously in musth (32, 33).
Benyus (2014)
A tusk will occasionally pierce through hide, and if the momentum of a fast charge is behind it, the stabbing can be fatal. The fact that these deaths are rare, however, shows that fights are more ritualized than bloodthirsty.
Basappanavar, 2021
Fighting in nature is rarely fatal though, I knew many working males apparently, gored to death by wild bulls, with tusks that pierced into the bodies.
Reardon (2021)
Fencing the park undoubtedly benefited the elephant population as a whole, but between 1939 and 1948 the high rate of population growth was tempered by an increase in deaths among subadult bulls killed by senior bulls in musth. Studies have shown that high densities arising from high population growth within a confined space may affect social behavior in ways that until recently were little understood. In Addo one of the effects has been the unusually high levels of aggression between bulls. At least 70 per cent and up to 90 per cent of male deaths between 1977 and 1989 resulted from fighting. This is not considered natural elephant behavior: fighting to the death is rare except in high-density populations, where it is relatively common. ‘Addo bulls will kill for the right to mate,’ John Adendorff told me. ‘Sparring starts at a young age, and bull rivalries result in two or three deaths every year. At any one time only two or three bulls sire all of the offspring,’ he added.
Douglas-Hamilton (1975)
In later life when competitive situations occur over water or food, the dispute is usually settled by a mere threat display which may be no more than the extension of the ears or the wave of a trunk in the direction of the opponent. Serious fighting is extremely rare.
The extreme rarity of combat deaths between elephants seems to be corroborated by the fact that elephant skin is only about as thick as expected for an animal of their body size (unlike the disproportionately thick skin of a rhinoceros), and therefore not specialized as an armor to protect against tusks (Shadwick et al., 1992).
It is interesting to note that the largest terrestrial mammal, the african elephant, does not have the thickest skin. According to Harkness & Hardness (1965), elephant skin thickness varies from about 10 mm to 15 mm on the sides and back. This is not substantially different from the thickness predicted by the scaling model of figure 2, based on a body mass of 5000-6000 kg (Owen-Smith 1988). This suggests that elephant dermis may not be highly modified as an armour to protect against the tusks of conspecifics. Indeed, serious fighting between adult elephants is extremely rare (Douglas-Hamilton & Douglas-Hamilton 1975). More commonly, disputes between elephants are settled by threatening displays, although occasions where cows inflicted bloody wounds on calves have been observed (Douglas-Hamilton & Douglas-Hamilton 1975). Recently, Lillywhite & Stein (1987) have shown that elephant skin has a relatively thick epidermis and surface sculpturing that is designed to provide and effective barrier to water loss.
Key points:
- Musth is the most important factor determining dominance and victory in agonistic interactions between bull elephants
- When musth status is equal, body size trumps tusk size/possession.
- Tusks only become important factors when musth and body size are equal.
- Tusks are used after a foe has been overpowered or weakened by blunt trauma; this may be because of the mechanical properties of elephant ivory that may make them susceptible to fracture.
- Elephant combat resulting in death is rare, barring unnaturally high density populations.