Headstarting turtles to larger body sizes for multiple years increases survivorship but with diminishing returns

Abstract

Headstarting is a conservation tool that assumes raising turtles in protected ex situ environments to larger body sizes, then releasing them back into the wild, increases their survivorship compared to wild, non-headstarted turtles. Our goal was to quantitatively test this fundamental assumption by comparing somatic growth and survivorship among 3 age classes of headstarted juvenile wood turtles (Glyptemys insculpta) monitored for 3 years (2016)(2017)(2018) in Ontario, Canada. Our age classes were turtles headstarted for 2 years (n = 15), turtles headstarted for 1 year (n = 30), and turtles incubated and hatched ex situ, then released (i.e., no headstarting; hatchlings, n = 30). Both age classes of headstarted turtles were radiotracked for 1 year after release. We released hatchlings in August and radio-tracked them for 1 month. All cohorts exhibited positive somatic growth after release. One-month post-release survival of hatchlings was 70%. Turtles headstarted for 2 years had slightly higher 1-year post-release survivorship (67%) than turtles headstarted for 1 year (47%), but there was little to no evidence for a difference (P = 0.17).Modeling these survivorship results with real-life constraints of only 100 hatchlings available for headstarting each year (biological constraint from source population), and a 100-turtle

Effects of release method on the survival, somatic growth, and body condition of headstarted turtles

“…Headstarting typically involves collection of eggs from the wild or collection of juveniles from ex situ breeding operations, then rearing individuals in human care for 1–4 years prior to releasing them into the wild (Heppell et al 1996, Burke 2015, Ross et al 2022). Headstarted animals in human care attain faster growth rates and larger body sizes compared to wild conspecifics of similar age, which makes headstarted animals less vulnerable to predation and other sources of mortality (Heppell et al 1996, Sacerdote‐Velat et al 2014, Mullin et al 2023). Increased survivorship of juveniles is predicted to result in a greater proportion of them reaching sexual maturity which, in turn, should lead to a larger adult breeding population (Alberts 2007).…”

“…Headstarted animals in human care attain faster growth rates and larger body sizes compared to wild conspecifics of similar age, which makes headstarted animals less vulnerable to predation and other sources of mortality (Heppell et al 1996, Sacerdote-Velat et al 2014, Mullin et al 2023. Increased survivorship of juveniles is predicted to result in a greater proportion of them reaching sexual maturity which, in turn, should lead to a larger adult breeding population (Alberts 2007).…”