In gorillas, the mother takes full responsibility for carrying and feeding offspring, and lactational amenorrhea prevents her from investing in another offspring while suckling. Therefore, mothers should encourage immature gorillas to become gradually more independent until they are physically able to acquire sufficient resources, travel independently, and manage social relationships unaided. We investigated the development of independence in a population of wild western gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Nouabalé-Ndoki National Park, Republic of Congo. We observed the spatial proximity, the development of travel, patterns of suckling and weaning, and the nature of mother-offspring interactions for 42 immature gorillas to investigate the nature of the relationship between mother and offspring. Though mother-offspring distance and independent travel increased during immaturity, with few active interactions between the dyads, offspring still continued to suckle until a median age of 4.6 yr, longer than in mountain gorillas. The tolerance of the mother toward suckling by older offspring may reflect the importance of continued nutritional investment: the temporal nature of the diet of western gorillas means that succulent fruits-likely to be preferred weaning foodsare not always available. We propose that to buffer against potential fruit shortage, mothers continue to nurse their offspring to decrease the likelihood of death from care-dependent factors and to improve an offspring's chances of survival.

The transfer of food items between individuals has been described in primates as serving an informative purpose in addition to supplementing the diet of immature individuals. This behaviour has yet to be described in western lowland gorillas (Gorilla gorilla gorilla), and results are presented here of observations of food transfers in immature gorillas at Mbeli Bai, Republic of Congo. The frequency of food transfers decreased with increasing immature age, while the frequency of independent feeding and processing of food increased. Transfers between mothers and infants were the most frequent, with infants attempting to take items from the mother. These attempts were not always successful and the item was relinquished on less than 50% of attempts. Mothers also took items from their offspring. The results point to the functional significance of food transfers in western lowland gorillas being informational. In a bai environment, where one species forms the majority of a visiting gorilla's diet despite other species being available, the initiation of food transfers by immatures is proposed to serve the purpose of familiarising them with which species, and which parts of those species, may be eaten.

Before being weaned, young primates must learn which plant species are suitable to eat, and how to process and combine them to meet their nutritional needs. This study provides a detailed investigation of the development of feeding behaviour in a wild population of western lowland gorillas observed in a bai habitat. The behaviour of 56 known immature individuals aged between 0 and 8 years was recorded at Mbeli Bai, Republic of Congo, using focal, scan and all-occurrence sampling over a period of 16 months. Results indicated that independent feeding skills were acquired by the end of infancy and did not undergo significant change beyond this point. However, suckling continued into the juvenile period, suggesting that some continued investment by the mother was necessary until the strength and skills of immatures developed, perhaps to enable self-sufficiency through periods of succulent fruit (assumed to be the most suitable weaning food) scarcity. This was supported by observations of an increase in feeding-related behaviour with age, particularly during infancy. Additionally, age-related differences in the species and plant parts selected by different age classes, as well as in levels of food processing, were apparent. Play feeding, suggested to be a precursor to independent feeding, was most prevalent in infancy, decreasing as feeding skills developed. Thus, it would appear, given that food transfer also occurs (Nowell & Fletcher, 2006), that feeding skills in western lowland gorillas are learned by a combination of exposure to and manipulation of food items, and probably also through the observation of conspecifics. Observations in a forest habitat are needed to improve our understanding of feeding development in this subspecies.