Behavioural development in wild Western lowland gorillas (gorilla gorilla gorilla)
AuthorsNowell, Angela A.
AdvisorsFletcher, Alison W.
MetadataShow full item record
AbstractBehavioural development has received little attention in primates, despite having important influences on infant mortality, interbirth intervals, and therefore, growth of populations. Gorillas have long developmental periods, exhibit strong maternal bonds and integrate into intricate social systems, making them an ideal species in which to investigate non-human primate development. Gorillas exist across a range of habitats, and differences in behaviour, both within and between species reflect socioecological differences, for example, in the availability and distribution of food. Consequently, by using gorillas as a model, opportunities also exist to investigate environmental constraints on the development of independence. This study provides the first detailed analysis, with reference to ecological factors, of the development of behavioural skills and relationships in wild western lowland gorillas (Gorilla gorilla gorilla). Behavioural development of western lowland gorillas is then compared with published accounts of development in mountain gorillas (Gorilla beringei beringei) to determine the extent to which differing ecology influences behaviour. The study was conducted at Mbeli Bai in the Republic of Congo, a large, marshy clearing, visited by gorillas predominantly for feeding purposes. Data were collected using scan, focal, all-occurrence and ad libitum sampling methods from 58 gorillas below 8 years of age. Spatial relationships, suckling, and the nature of interactions involving immature individuals were analysed. The distribution of time between different behaviours by immatures, and the development of independent feeding and travelling behaviour was also investigated, and all were tested for differences as a result of immature age, sex and social group, or the mother's parity. Towards the end of infancy, individuals showed competent feeding behaviour in the bai. However, western lowland gorillas were not weaned until the juvenile period, and until this time, close association was common between mothers and offspring. With increasing independence from the mother there was limited investment in relationships with other individuals, and instead, a greater emphasis was placed on developing skills through play, alloparenting and agonistic interactions. When results were compared with those of mountain gorillas, there was evidence of increased investment in relationships, particularly with the silverback, by immature mountain gorillas, which was assumed to reflect lower rates of natal dispersal by mountain gorillas, and the greater likelihood that relationships with individuals in the natal group could prove useful in the future. Suckling and close proximity to the mother continued until later ages in western lowland gorillas, resulting in clear differences between them mountain gorillas in the duration of investment by mothers. More frugivorous western lowland gorillas required increased levels of investment by the mother before independence could be achieved, demonstrating the effect that resource availability can have on behavioural development in species where resources are widely and unpredictably dispersed.
TypeThesis or dissertation
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Food transfers in immature wild western lowland gorillas (Gorilla gorilla gorilla)Nowell, Angela A.; Fletcher, Alison W.; University of Chester (Springer Verlag, 2006-10)This article discusses how the transfer of food items between primates serves an informative purpose in addition to supplementing the diet of immature individuals. Food transfers amongst immature western lowland gorillas (Gorilla gorilla gorilla), at Mbeli Bai, Republic of Congo weew observed.
Development of independence from the mother in Gorilla gorilla gorillaNowell, Angela A.; Fletcher, Alison W.; University of Chester (Springer Verlag, 2007-04)This article investigates the development of independence in a population of wild western gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Nouabalé-Ndoki National Park, Republic of Congo.
Maternal investment in mountain gorillas (Gorilla Beringei Beringei)Fletcher, Alison W.; Eckardt, Winnie (University of Liverpool (University of Chester), 2010-02)Investigating maternal investment (Ml) and mother-offspring relationships during the period of infant dependency is critically important to furthering the understanding of female reproductive strategies in primates. Infant primates are completely dependent upon their mothers. The way in which a mother allocates her resources therefore is crucial for infant survival, but is balanced Against her need to invest in subsequent offspring. One approach to examining how mothers might invest in their offspring stems from the Trivers & Willard hypothesis (TWH, 1973), which predicts that mothers in good condition should bias their investment towards sons and whereas mothers in poorer condition should bias investment toward daughters. Long-term demographic records on birth sex ratio and inter-birth interval suggest that female mountain gorillas (Gorilla beringei beringei) do not bias investment prenatally, but they may adjust postnatal Ml according to the TWH. This study investigated Ml and mother-offspring relationships in wild mountain gorillas, using behavioural correlates of Ml, including suckling, weaned age, physical contact, "transport, and grooming to redress the lack of understanding about Ml in this species. The appropriateness of TWH was investigated, integrating different indicators of maternal condition. Important determinants of Ml and mother-offspring relationships were considered, such as offspring age, parity, presence of siblings and maternal relatives, group size and lastly, personality, which has been largely neglected in nonhuman primates. The extent, to which the offspring influenced Ml patterns, was examined using the parent-offspring conflict theory (Trivers, 1972) as a theoretical framework. During 2006-2007, 38 mother-offspring dyads were observed in the Virunga massif, resulting in 1210 hours of direct behavioural observation. Additional field data from the previous four decades were integrated into the dataset for the analysis of suckling and weaned age. Gorilla personality was assessed through the Hominoid Personality Questionnaire. Findings relating to suckling frequency, weaned age, and maternal feeding activities were consistent with the TWH: sons suckled more often than daughters when they had mothers in good condition, whereas the reverse sex-pattern occurred in offspring with mothers in poorer condition. In addition, daughters were weaned at an earlier age than sons when mothers were in better condition, although this sex-difference reduced in older mothers that were categorised as being in good condition. Maternal feeding time and feeding efficiency revealed that mothers in poorer condition spent more time ingesting food when they had daughters, whereas mothers in better .condition spent more time ingesting food when they had sons. Furthermore, group size affected lactation duration with offspring in small groups being weaned earlier than offspring in large groups. Behavioural conflicts over Ml showed that the mother and offspring influenced Ml patterns during the period of dependency. Finally, six personality dimensions were identified, of which five revealed effects maternal behaviour, such as maternal retrieval, responsiveness and rejection, although their relative importance varied between those behaviours. In general, mother and offspring personality effects were complex due to their interactions with the developmental stage of offspring. In conclusion, my thesis research has made several novel contributions to furthering the understanding of female reproductive strategies in the highly endangered mountain gorilla. I presented the first evidence using behavioural data that females bias their postnatal investment towards the sex with the greatest fitness return as predicted by the TWH. My findings are discussed in the light of alternative Ml strategies, such as the local resource competition and enhancement model. My research has highlighted the importance of integrating anthropometric and physiological measures and demographic long-term data into future Ml studies to assess direct costs and benefits of Ml. The examination of mother-offspring behavioural conflicts showed that offspring have a strong impact on the level of Ml they receive. I have also examined the personality of a wild mountain gorilla population for the first time. My findings demonstrate that personality-parenting links are evident in several respects and I have demonstrated the great potential of personality as a determinant of maternal behaviour and mother-offspring relationships.