One aspect of this debate concerns the targeting of elephant bulls with large tusks in hunting, and whether or not this is an acceptable loss to elephant population genetics. With large-tusked elephants in decline, what does the science say on the subject?
Proponents of the trophy hunting of these elephant bulls generally either argue that the individuals selected are at an age where they have already had time to contribute to the gene pool or are no longer reproducing. Those against the practice argue that hunting these “genetically gifted” elephants amounts to the “surgical removal of Africa’s remaining large-tusked elephants”.
SUMMARY
Populations across Africa may differ in terms of the effect of ageing on reproductive potential. Extrapolations from one area may not apply with absolute certainty to another.
Bull elephants exhibit indeterminate growth, and their tusk growth rate increases exponentially throughout their lifetime.
Bulls can and do sire offspring from around 25 years of age, but their reproductive success increases until they reach a peak roughly between 45 and 49 years of age.
On average, their reproductive success decreases slightly from the age of 50 to 54 and more significantly from 55 to 61 years of age.
Individuals have been recorded in musth (suggesting they can reproduce) at the age of 63.
Senescence may result in breeding cessation when a bull can no longer maintain the necessary physical condition to compete and mate – the age when this occurs is likely to be highly variable.
Factors other than age play a role in reproductive success, including the timing of musth cycles.
Given the difficulty of accurately ageing an elephant by sight and their capacity for reproduction into old age, the margin for error is relatively narrow.
We do not fully understand the genetic mechanisms of tusk size, which impact its heritability. This will play a role in determining whether a large-tusked elephant can be considered to have contributed “sufficiently” to elephant genetics before his death.
This article focuses on summarising the available science on growth, tusk size and breeding as a function of age in adult male African savannah elephants. However, at the outset, it is essential to clarify that there will always be individual physiological differences within any population. The ageing process (and its effects on reproductive potential) will never be the same for two individual male elephants. This is also true of populations of elephants living in different parts of the continent, exposed to various environmental factors and, most importantly, consuming slightly different diets.
HOW DO ELEPHANTS AGE?
Elephants feed almost constantly throughout the day to meet the metabolic requirements needed to sustain their massive frames. Elephants are equipped with four enormous molars (two on the top jaw and two on the bottom) that wear down due to continuous chewing. These molars are replaced five times during their lifetime (equating to six sets of teeth). When the final set wears down entirely, the elephant can no longer chew properly and will eventually die of malnutrition or related complications.
Bull elephants have a maximum life expectancy of around 60-65 years, and they continue to grow throughout their lives (indiscriminate growth). However, experts estimate that only around half of all male elephants survive to peak competitive age, and relatively few (approximately 10%) live longer than around 50 years old (Moss 2001, Poole et al. 2013).
Tusk growth in African savannah elephants (Loxodonta africana) is sexually dimorphic, with the tusks of the males growing thicker and longer than those of the females. Research suggests that not only do these tusks grow throughout the elephant’s life, but the growth pattern appears to be exponential rather than linear. In other words, their tusks grow at an accelerating rate as they get older – particularly in terms of increasing mass and circumference (Spinage 1994, White and Hall-Martin 2014).
Longevity, indeterminate growth, and increasing tusk size point towards sexual competition favouring larger and older male elephants. As dominance between bulls is determined by body size (Poole 1989b), the natural conclusion is that older elephants will have increased mating success, borne out by the available research.
MUSTH AND THE REPRODUCTIVE LIFESPAN OF A BULL ELEPHANT
Bulls reach sexual maturity (are physically capable of mating and producing offspring) at around the age of 15, but under natural conditions, it may be several years before they can secure the opportunity to mate. Around the age of 25 to 30 years, male elephants start entering into musth cycles characterised by a substantial increase in testosterone concentrations and several behavioural, physiological and physical changes.
The importance of musth in elephants cannot be understated. Males in musth are more aggressive towards competitors, engage in mate guarding and reduce foraging time to prioritise the search for oestrus females (Rasmussen et al. 2008, Taylor et al. 2019). Females receptive to mating (in oestrus) also show a preference for musth bulls (Poole 1989). However, this does not mean that elephant bulls not in musth do not sire offspring. Opportunistic mating does occur, and non-musth bulls are responsible for a constant, low percentage of the paternity of calves (Hollister-Smith 2007, Rasmussen 2008). However, musth bulls will dominate mating opportunities, even those smaller than non-musth competitors and most conceptions will result from a bull in musth (Hollister-Smith 2007, Poole et al. 2013). When two musth bulls compete for access to females, body size generally determines dominance – tusk size does not seem to play a role (Poole 1989b). This suggests that large-tusked elephants are no more likely to be successful in sexual competition than a counterpart of similar age.
Mali’s elephants show how people and nature can share space in a complex world
Thus, elephant bulls over the age of 30 rely on musth as their primary reproductive strategy. Older bulls with longer musth cycles will inevitably have more mating opportunities than younger individuals. Research indicates that the “median duration of musth increases from two days for males aged 16–25 years, to 13 days for males aged 26–35 years, to 52 days for males aged 36–40 years, to 69 days for males aged 41–45, to 81 days for males aged 46–50 years, and then declines again to 54 days for males aged 51–60 years of age” (Rasmussen 2008, Poole et al. 2013). Males over the age of 35 are generally only in musth once a year (Poole 2013). Elephant bulls up to 63 years old have been observed exhibiting musth, suggesting that for some individuals, at least, almost lifelong reproduction is possible.
To summarise: “The longer a male survives and the older he becomes, the more ‘successful’ he has the opportunity to be. Thus, a strong relationship exists between the number of years that a male has been seen to be in musth and the total number of conceptions that occurred during his musth periods” (Poole 2013).
PEAKS, DECLINES, AND THE CONTRIBUTION OF YOUNGER MALES
In keeping with the above conclusion, research from Amboseli shows increasing reproductive success with increasing age (Hollister-Smith 2007). Genetic samples were taken from 89 adult male elephants and 279 calves and their mothers. Their analysis shows a gradual increase in reproductive success (measured by calves sired) from around 30 to a peak from 45 and 53 and then decline to around the same level of a male in his early 40s. The oldest elephant that sired a calf was 59 years old, and four males sired 14 calves between them in their 50s. The three oldest males (aged 48, 53 and 58) were responsible for 30% of the paternity in question.
In the study, males in their mid-20s sired calves, but this was an uncommon occurrence. However, males under the age of 35 fathered 29% of the calves. So, while males do produce more offspring in their later years, they have likely contributed to the genetics of a population before that. However, whether this contribution is sufficient to ensure the continuity of the large-tusked phenotype is scientifically uncertain and depends at least in part on the heredity of the trait (more on this below).
HOW OLD IS THAT ELEPHANT?
Outside of known and extensively studied individuals, the most accurate way to age an elephant is to examine the condition of its molars. Naturally, this is difficult in the field and ageing an elephant by sight (particularly on foot) is inherently challenging. Experts rely on several physical features, including body condition (very old elephants have a gaunt appearance), posture and head shape.
A recent study from Kenya (Taylor et al. 2019) emphasised the importance of the effects of musth on elephant energy expenditure. Elephants in musth were found to walk faster and further than those not in an active reproductive state. This results in decreased feeding time, and in older bulls that maintain these musth cycles for months, the increased energy expenditure manifests as a significant loss of body condition (Poole 1989). As a result, when hunting an elephant, hunters could mistake a bull in musth for an older individual during a hunt.
Ageing an elephant by looking at the tusk size is also considered by experts to be problematic due to the considerable variation within individuals (Whyte and Hall-Martin 2014). While it is safe to assume that an elephant sporting large tusks is likely to be an older individual, whether or not they are within the “prime breeding” age bracket of between 45-49 is difficult to determine. Individual subjects of long-term studies whose ages are known, such as well-known tuskers Tim, Tolstoy and Satao (from Kenya) and Isilo (from South Africa), had substantial tusks well before their 50s. Given that tusk growth increases as the elephant ages, hunting young elephants before they exhibit signs of being a “tusker” may also result in the loss of large-tusk genetics.
HERITABILITY – THE NEXT BIG QUESTION?
It is apparent that tusk size is a heritable trait, but the extent of this heritability and its genetic basis are still not understood and likely to be complex. Recent genetic research on the elephants of Gorongosa identified some of the genes involved in tusk growth, including one linked to the X chromosome and one autosomal chromosome (Campbell-Staton et al., 2021). This research was based on exploring the phenomenon of tuskless elephants, and tusk size is probably determined by the effects of additional genes (and likely environmental factors). The dominant or recessive nature of the alleles (a variant form of a gene) will also affect physical manifestation of this trait.
The significance of this goes to the heart of the argument about whether or not large-tusked elephants killed over the age of fifty have contributed “sufficiently” to the genetics of a herd to preserve the large-tusk phenotype. Simply put, the more genes involved in producing large tusks, the smaller the chances of passing them along during one breeding event. Conversely, the more mating opportunities, the higher the chances of securing the future of large-tusk genetics in one or more offspring. (Of course, a corollary to this is that bulls with small tusks could still carry some alleles related to large tusk growth.)
FINAL THOUGHTS
The vast majority of the available research on the reproductive lives of male savannah elephants comes from long-term research conducted in Kenya, in the greater Amboseli ecosystem in particular. Exactly how this applies to elephant populations in other ecosystems is a matter for further study.
However, it is clear from existing research that claims of elephants over the age of 50 being “past their prime” are demonstrably false. These animals are still capable of mating and producing calves with greater success than when they were in their 20s and 30s. Furthermore, hunting a bull elephant between the ages of 45 and 49 under the mistaken impression that he was older will result in the loss of his most reproductively successful years. The margin for error here is a narrow one and emphasises the need for caution in aging prospective trophy bulls.
The significance of an elephant’s prior contribution to the tusk sizes of future elephant generations depends at least in part on the genetic basis of tusk size – a factor that has not yet been scientifically clarified. Without this knowledge, it is difficult to know with any certainty whether or not a trophy hunted bull has been given “sufficient” opportunity to pass on his large-tusked legacy before he is killed. Statistically, the more conceptions he contributes to, the greater the chance of genetic perpetuation. Thus the ethical question at hand becomes one of risk – are we willing to take that chance with large-tusked elephants?
