Life in a changing environment
The glaring heat of mid-day in the Amboseli basin is followed by a 20°C temperature drop during the cool night. A few months of lush grasses, hordes of insect larvae, and tree blossoms are quickly replaced by a long dry season of dust, bare earth, and grass stubble. Renewal occurs with the rains but the rains fail, unpredictably, approximately one year out of five. These short-term changes occur against a backdrop of larger scale ecological changes that accumulate over
decades – once thick woodland becomes open grassland, daily temperatures increase, the water table rises, and the ice caps shrink on Mt. Kilimanjaro, the mountain that dominates Amboseli's landscape.
Understanding how organisms adapt to environmental change of this sort is crucial for biological conservation, because many parts of the world are now experiencing rapid anthropogenic climate change. It is also crucial for gaining general insight into population processes, because environmental change has probably been experienced by most or all organisms at some point in their evolutionary history. Our goal is to document in detail the response of the Amboseli
baboons to the environmental change they are experiencing.
Over the past half century, the number and size of swamps and ponds in the Amobseli basin have increased, previously widespread Acacia woodlands have been dramatically reduced and replaced by grassland, halophytic vegetation, and swamps [135; see also Struhsaker 1973, 1976, Western and van Praet 1973, Hauser et al. 1986, Isbell et al. 1990, Behrensmeyer 1993]. Equally importantly, daily temperatures have increased by more than 5 degrees C . The shrinking ice caps on Kilimanjaro [Hastenrath & Greischar 1997, Thompson et al. 2002] play an as-yet unknown role in the local changes occurring in Amboseli.
The baboon population experienced a dramatic decline in the 1960's at the onset of woodland die-off. However, the population recovered even as the woodland die-off continued, and population size has continued to increase moderately over the past several decades (although not without fluctuations) [64, 155]. The baboons' success is in striking contrast
to the failure of Amboseli's vervet monkeys to adapt. Vervet monkeys, like baboons, are widespread savanna-dwelling monkeys, and the two species show considerable overlap in habitat and diet. However, the vervet population has undergone dramatic decline, including local extinction in some locales, as a consequence of environmental change [Struhsaker 1973, 1976, Hauser et al. 1986, Isbell et al. 1990].
Our behavioral data indicate that environmental change in Amboseli has profoundly influenced the time budgets, social lives, diet, and habitat use of baboons. During periods of woodland die-off, baboons experienced both low fertility and high infant mortality . They also devoted nearly 80% of daylight hours to foraging and dramatically reduced their social time, in spite of both theoretical and empirical evidence indicating that they attempt (and often succeed in) conserving social time as a means of servicing their crucial social relationships . A widely accepted and influential model of baboon ecology predicted that social groups will lose cohesion and either fission or go extinct under extreme environmental stress [Dunbar 1992], our data do not support this model. Rather, the baboons modified their diet by increasing the diversity of food items; they also moved to completely new home ranges in areas of Amboseli with intact woodland. Survival and fertility increased following these behavioral changes [159, 167].
We are currently working to delineate behavioral, physiological, and demographic responses by the Amboseli baboons to the extensive environmental change they have experienced. We aim to gather detailed information on how environmental change affects fitness components and related traits. We also aim to elucidate how different individuals are differentially affected by, and respond to, environmental change. Baboons exhibit substantial interindividual variation in behavior, and our focus on individual differences will provide important insight into traits that confer an adaptive advantage in the face of environmental change.
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