Santa Cruz Giant Tortoises — Seasonal Elevational Migration

Chelonoidis porteri (western) and C. donfaustoi (eastern) · adults shuttle between humid highland refuge and arid lowland forage

Migration is seasonal in direction but partial and facultative — not every adult migrates, and departure dates shift by more than a month between years.

TRACKED INDIVIDUALS La Reserva cluster (west) — C. porteri LR‑1 LR‑2 LR‑3 LR‑4 Cerro Fatal cluster (east) — C. donfaustoi CF‑1 CF‑2 Real GPS tracks of individual tortoises, digitised from “Migration routes of individual Galapagos tortoises on Santa Cruz Island” (Fig. 1). Each line is one animal, not a generalised corridor. Both tracked clusters lie on the southern flank; they are not a complete census of the island’s migration routes.
Elevation map of Santa Cruz Island, Galápagos (0–850 m) HUMID ZONE inner dotted ring · dry-season refuge AGRICULTURAL ZONE outer dashed ring · farmland belt crossed by migrants
ELEVATION · AS ON THE MAP 0 m200 m600 m850 m ~400 m highland ~50 m lowland ADULT ELEVATIONAL RANGE The bar is the island’s elevation, coloured exactly as the map. The marker tracks the modelled population-mean; the block is the spread. Recorded range 6–423 m (Blake et al. 2013).
SEASONAL CYCLE — JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC HOT–WET SEASON COOL–DRY SEASON (GARÚA) downslope ↓ DOWNSLOPE TO LOWLANDS · DEC–MAY upslope ↑ UPSLOPE TO HIGHLANDS · JUL–NOV departures highland lowland modelled population-mean elevation — band shows spread between tracks (schematic) Season limits (hot–wet Jan–May, cool–dry Jun–Dec) are verbatim from Blake et al. 2024. Migration windows are not the same thing: downslope Dec–May is verbatim from Bastille-Rousseau et al. 2019. The upslope window Jul–Nov comes from secondary reporting on Blake et al. 2024, not from the primary paper — flag this if you cite it. Sources differ on the cue: Blake et al. describe migrants “following the green” as lowland forage declines; Bastille-Rousseau et al. 2019 found timing tracks ~7-year averages, not current-year conditions. Departure ticks are schematic, illustrating documented staggering (largest adults upslope from ~Jul, mid-sized from ~Oct); they are not per-corridor published data. Downslope timing is not size-dependent. Because each one-way journey takes 2–4 weeks and departures are staggered, peak highland occupancy trails the Jul–Nov migration window by about a month. Partial migration: 6 of 25 tracked individuals (24%) were year-round residents (Blake et al. 2015). Almost all adult males migrate, while among adult females some migrate and others do not (Blake et al. 2024, eastern Santa Cruz); the migratory fraction of the population is unknown. Sources: Blake et al. 2013, J. Anim. Ecol. 82:310–321 · Yackulic et al. 2017, J. Anim. Ecol. 86:972–982 · Bastille-Rousseau et al. 2019, Ecology 100(6):e02658 · Blake et al. 2021, Ch.13 in Galápagos Giant Tortoises · Blake et al. 2024, Ecology and Evolution 14:e10994 · Poulakakis et al. 2015, PLOS ONE 10(10):e0138779 (C. donfaustoi).