Summary 1.Seasonal migration has evolved in many taxa as a response to predictable spatial and temporal variation in the environment. Individual traits, physiology and social state interact with environmental factors to increase the complexity of migratory systems. Despite a huge body of research, the ultimate causes of migration remain unclear. 2. A relatively simple, tractable system -giant tortoises on Santa Cruz Island, Galapagos, was studied to elucidate the roles of environmental variation and individual traits in a partial migratory system. Specifically, we asked: (i) do Galapagos tortoises undergo long-distance seasonal migrations? (ii) is tortoise migration ultimately driven by gradients in forage quality or temperature; and (iii) how do sex and body size influence migration patterns? 3. We recorded the daily locations of 17 GPS-tagged tortoises and walked a monthly survey along the altitudinal gradient to characterize the movements and distribution of tortoises of different sizes and sexes. Monthly temperature and rainfall data were obtained from weather stations deployed at various altitudes, and the Normalized Difference Vegetation Index was used as a proxy for forage quality. 4. Analyses using net displacement or daily movement characteristics did not agree on assigning individuals as either migratory or non-migratory; however, both methods suggested that some individuals were migratory. Adult tortoises of both sexes move up and down an altitudinal gradient in response to changes in vegetation dynamics, not temperature. The largest tagged individuals all moved, whereas only some mid-sized individuals moved, and the smallest individuals never left lowland areas. The timing of movements varied with body size: large individuals moved upward (as lowland forage quality declined) earlier in the year than did mid-sized individuals, while the timing of downward movements was unrelated to body size and occurred as lowland vegetation productivity peaked. 5. Giant tortoises are unlikely candidates for forage-driven migration as they are well buffered against environmental fluctuations by large body size and a slow metabolism. Notably the largest, and presumably most dominant, individuals were most likely to migrate. This characteristic and the lack of sex-based differences in movement behaviour distinguish Galapagos tortoise movement from previously described partial migratory systems.

The Galapagos Islands are among the most renowned natural sites in the world. Unlike other oceanic archipelagos, the ecological and evolutionary processes characteristic of Galapagos have been minimally affected by human activities, and the archipelago still retains most of its original, unique biodiversity. However, several recent reports suggest that the development model has turned unsustainable and that the unique values of the archipelago might be seriously at risk. In response to international concern, UNESCO added Galapagos to the list of World Heritage in Danger in 2007. Our goal was to provide new insights into the origins of the present-day crisis and suggest possible management alternatives. To this end, we reexamined the Galapagos situation from a broad systems perspective, conceptualizing the archipelago as a complex social-ecological system. Past, present, and possible future trends were explored using the resilience theory as a perspective for understanding the dynamics of the system. Four major historical periods were characterized and analyzed using Holling's adaptive cycle metaphor. The current Galapagos situation was characterized as a prolonged series of crisis events followed by renewal attempts that have not yet been completed. Three plausible future scenarios were identified, with tourism acting as the primary driver of change. The current tourism model reduces the system's resilience through its effects on the economy, population growth, resource consumption, invasive species arrival, and lifestyle of the island residents. Opportunities to reorganize and maintain a desirable state do exist. However, strong political and management decisions are urgently needed to avoid an irreversible shift to a socially and environmentally undesirable regime. Key measures to achieve a new sustainability paradigm for Galapagos include modifying traditional practices to produce a more adaptive resilience-based co-management model, adopting a more comprehensive approach to territorial planning, strengthening participative approaches and institutional networks, and promoting transdisciplinary research at the frontiers of social and biophysical sciences.

Although many classic radiations on islands are thought to be the result of repeated lineage splitting, the role of past fusion is rarely known because during these events, purebreds are rapidly replaced by a swarm of admixed individuals. Here, we capture lineage fusion in action in a Galápagos giant tortoise species, Chelonoidis becki, from Wolf Volcano (Isabela Island). The long generation time of Galápagos tortoises and dense sampling (841 individuals) of genetic and demographic data were integral in detecting and characterizing this phenomenon. In C. becki, we identified two genetically distinct, morphologically cryptic lineages. Historical reconstructions show that they colonized Wolf Volcano from Santiago Island in two temporally separated events, the first estimated to have occurred ~199 000 years ago. Following arrival of the second wave of colonists, both lineages coexisted for approximately ~53 000 years. Within that time, they began fusing back together, as microsatellite data reveal widespread introgressive hybridization. Interestingly, greater mate selectivity seems to be exhibited by purebred females of one of the lineages. Forward-in-time simulations predict rapid extinction of the early arriving lineage. This study provides a rare example of reticulate evolution in action and underscores the power of population genetics for understanding the past, present and future consequences of evolutionary phenomena associated with lineage fusion.

Giant tortoises are amongst the longest-lived vertebrate animals and as such provide an excellent model to study traits like longevity and age-related diseases. However, genomic and molecular evolutionary information on giant tortoises is scarce. Here, we describe a global analysis of the genomes of Lonesome George, the iconic last member of Chelonoidis abingdonii , and the Aldabra giant tortoise ( Aldabrachelys gigantea ). The comparison of these genomes to those of related species, using both unsupervised and supervised analyses, led us to detect lineage-specific variants affecting DNA repair genes, inflammatory mediators and genes related to cancer development. Our study also hints at specific evolutionary strategies linked to increased lifespan and expands our understanding of the genomic determinants of ageing. These new genome sequences also provide important resources to help the efforts for restoration of giant tortoise populations.

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