Behavioural ecology of Asian elephants in southern India : The role of offspring sex and demography

Abstract

Life history characteristics may be broadly grouped into (i) age specific schedules of birth and death, and (ii) developmental attributes. Characteristics involving reproductive parameters (e.g., gestation period, inter birth intervals, magnitude of parental investment, and age at first reproduction) and developmental patterns (e.g., size at birth, growth rate, and adult body size) are often strongly influenced by mating systems. In polygynous mating systems, theory predicts male-female differences in reproductive strategies that can lead to sex differences in body size, development rates, mortality rates, parental investment, and a range of behavioural traits. Although such sex differences are expected under polygyny, ecological variation-environmental, social, or demographic-may amplify or reduce these differences. For example, in polygynous species, demographic variation affecting male dispersal conditions or the intensity of male-male competition may influence age at first reproduction or parental investment patterns. Behavioural mechanisms, known for their plasticity, often mediate phenotypic responses to such differences. Studies of behavioural ontogeny and ecological (proximate) influences on these patterns have played an important role in behavioural ecology and evolutionary biology. This study examines patterns in the development of independence in Asian elephant (Elephas maximus Linn.) sons and daughters, as reflected by behavioural ontogeny (changes in activities and interactions with age), mother-offspring spatial relationships, maternal investment patterns, and behaviours related to dispersal and reproduction. It compares sons and daughters, and contrasts these patterns across two populations with distinctly different population and operational sex ratios. Proximate causes underlying between sex and between population differences are analysed. Asian elephants are polygynous and sexually dimorphic. Males-the predominantly dispersing sex-are larger and heavier than females and may possess tusks, unlike most females. Intense male-male competition for access to receptive females is common. Adult males are typically solitary (forming temporary bachelor groups occasionally), whereas adult females and juveniles live in herds with strong social bonds. Two populations were studied: • Mudumalai Wildlife Sanctuary, and • Periyar Tiger Reserve, southern India. Between February 1993 and June 1995, demographic data and over 600 hours of behavioural observations were collected from each population. Observations focused primarily on 117 and 67 mother-offspring dyads from Mudumalai and Periyar, respectively. Additional individuals (not in dyads) were observed for pre dispersal and reproductive behaviour. The adult sex ratio was moderately skewed in Mudumalai but extremely skewed in Periyar due to extensive ivory poaching that nearly eliminated adult males. • Mudumalai adult sex ratio (>15 years): 1 male : 11 females • Periyar adult sex ratio: 1 male : 101 females Operational sex ratio (males available per oestrous female per day): • Mudumalai: 6.7 : 1 • Periyar: 0.31 : 1 This demographic difference also affected birth rates: • Mudumalai: 0.29 female¹ year¹ • Periyar: 0.075 female¹ year¹ ________________________________________ Major Results 1. Ontogeny and Sex Differences (Both Populations) • Young sons and daughters spent similar amounts of time in various activities but diverged as they aged. • Sons invested more time in movement and exploration of peripheral herd areas than daughters. • Sons increasingly wandered away from natal herds; daughters did not. Behavioural synchrony: • Offspring under two years showed little active synchrony with mothers. • Above this age, daughters showed higher synchrony with mothers than expected by chance; sons did not. Social interactions: • Very young calves showed no sex biased preference in choosing interaction partners. • With age, calves increasingly interacted with same sex partners. • Daughters continued this pattern; sons later shifted towards interacting more with females. Spatial relationships: • Sons were, on average, farther from their mothers than daughters. • Mothers discouraged proximity of sons more than daughters, as seen in approach and leave behaviours. Maternal investment: • Sons received more milk (via suckling) than daughters. • Findings align with expectations from polygyny, where sons require greater investment due to future competition for mates, whereas daughters remain in natal herds and build social bonds. ________________________________________ 2. Between Population Comparisons Differences in operational sex ratio significantly affected male-male competition, female choice, and age distribution of mating opportunities: • Mudumalai: 94% of courtship events occurred with more than one male present versus 4% in Periyar. • 62% of courtship in Mudumalai involved male-male aggression; none was recorded in Periyar. • Periyar females were more likely to accept courtship due to lack of competing males. Developmental differences in sons: • Sons in Periyar were discouraged more strongly by mothers from staying close. • They spent more time exploring away from natal herds and courting adult females than sons in Mudumalai. • Maternal investment was more strongly biased toward sons in Periyar. • Sons aged 3-5 years received disproportionately high investment (Mudumalai ratio 1.5; Periyar ratio 2). Maternal age effects: • Young mothers (15-20 years) did not show investment bias in either population. • Mothers aged 20-35 (Mudumalai) and 20-40 (Periyar) biased investment toward sons. • Oldest mothers in Mudumalai (>40 years) biased investment toward daughters; those in Periyar showed no bias. Although male biased investment in Mudumalai aligns with the Trivers-Willard hypothesis, the strong male bias in Periyar is harder to explain through current theory. One hypothesis is that increased maternal investment may support the energetic costs of early dispersal, which is feasible in Periyar due to reduced aggression from adult males. Dispersal behaviour: • In Mudumalai, 32-86% of pre dispersal exploratory trips by sons were aborted due to aggressive adult male encounters. • In Periyar, no such aggression occurred. • Consequently, 95% of observed courtship in Periyar was initiated by males 15 years old, compared to 22% in Mudumalai. • Mean age of first courtship was lower in Periyar due to decreased male-male competition and higher acceptance by females. ________________________________________ Conclusion Many aspects of Asian elephant behavioural ecology arise from sex specific selective pressures during development and adulthood. Population level demography profoundly influences male dispersal, mating opportunities, maternal investment, and behavioural development.