Allometric and temporal scaling of movement characteristics in Galapagos tortoises

Abstract

1. Understanding how individual movement scales with body size is of fundamental importance in predicting ecological relationships for diverse species. One-dimensional movement metrics scale consistently with body size yet vary over different temporal scales. Knowing how temporal scale influences the relationship between animal body size and movement would better inform hypotheses about the efficiency of foraging behaviour, the ontogeny of energy budgets, and numerous life-history trade-offs. 2. We investigated how the temporal scaling of allometric patterns in movement varies over the course of a year, specifically during periods of motivated (directional and fast movement) and unmotivated (stationary and tortuous movement) behaviour. We focused on a recently diverged group of species that displays wide variation in movement behaviourgiant Galapagos tortoises (Chelonoidis spp.)to test how movement metrics estimated on a monthly basis scaled with body size. 3. We used state-space modelling to estimate seven different movement metrics of Galapagos tortoises. We used log-log regression of the power law to evaluate allometric scaling for these movement metrics and contrasted relationships by species and sex. 4. Allometric scaling of movement was more apparent during motivated periods of movement. During this period, allometry was revealed at multiple temporal intervals (hourly, daily and monthly), with values observed at daily and monthly intervals corresponding most closely to the expected one-fourth scaling coefficient, albeit with wide credible intervals. We further detected differences in the magnitude of scaling among taxa uncoupled from observed differences in the temporal structuring of their movement rates. 5.Our results indicate that the definition of temporal scales is fundamental to the detection of allometry of movement and should be given more attention in movement studies. Our approach not only provides new conceptual insights into temporal attributes in one-dimensional scaling of movement, but also generates valuable insights into the movement ecology of iconic yet poorly understood Galapagos giant tortoises.

Benefits of the destinations, not costs of the journeys, shape partial migration patterns

“…We calculated daily movement rates, D x , as: where the scaling relationship is derived from free‐ranging Galapagos tortoises in the process of migrating (Bastille‐Rousseau et al . ) and adjusted by the associated activity multiplier for that day. For a tortoise, leaving on day x , we calculated the travel time between the highlands and lowlands, χ x (assumed to be the same in either direction) by determining the minimum days required to cover 7 km, the approximate average distance between highland and lowland habitats on Santa Cruz (Blake et al .…”

“…We modelled migration speed based on previously published relationships between average daily speeds during migration and body size (Bastille‐Rousseau et al . ). Tortoises are capable of faster rates, but often stop and forage as they migrate.…”

“…(b) The model then calculates the number of days that would be required to migrate between habitats based on a relationship between body size and daily movement rates during migration derived from GPS ‐tagged tortoises (Bastille‐Rousseau et al . ), modified by the estimated internal temperature and assuming that habitats are separated by seven kilometres. (c) Using calculations of estimated net energy gains in the lowlands and highlands on each day of the year, the number of days required for migration and the energy gains or losses associated with migration, we then determine the optimal movement tactic over the course of the year, and the annual energy gains (or losses) associated with this tactic using deterministic dynamic programming (a commonly used method for finding the optimal sequence of decisions).…”

Altitudinal migration: ecological drivers, knowledge gaps, and conservation implications

“…However, the migration patterns were opposite to those predicted based on capacity to tolerate cold temperatures and low food availability. Specifically, larger individuals moved longer distances than smaller individuals, consistent with the food‐limitation hypotheses discussed in Section III.1 a (Blake et al ., ; Bastille‐Rousseau et al ., ). Thus, at least for these large‐bodied ectotherms, the cost of a reduced food intake by remaining when food production is low outweighs the energetic cost of moving (Yackulic et al ., ).…”

Comparison of the Effect of Three Food Diets Provided to Giant Tortoises (Chelonoidis guntheri and Chelonoidis vicina) Bred in Captivity at Isabela Island – Galápagos

“…In this regard, [17] indicate that resource distribution and food selection are fundamental to ecology, life history, physiology, population dynamics, and conservation of animals; so in Galapagos introduced plants are changing the feeding dynamics of herbivores in many ecosystems often with unknown consequences; Giant tortoises, like many herbivores, undertake migrations along elevation gradients dependent on variability in vegetationandcourse of a year, specifically during periods of motivated [18].…”