Compensatory growth acceleration enhances survival of early-hatched salamanders

Hatching at an underdeveloped stage of growth and development is widespread across taxa and may occur either adaptively in response to embryonic threats such as egg predation, or passively as a consequence of damage to egg capsules or membranes. Because hatchlings are inherently small, physiologically fragile, and have limited locomotor ability, such early hatching places them in an extremely risky condition. A plausible strategy to mitigate this vulnerability is to accelerate post-hatching growth and development, yet this hypothesis has rarely been tested. Our study here, using the Japanese salamander Hynobius retardatus, in which intense cannibalism frequently occurs immediately after hatching, shows that: both naturally and artificially earlier-hatched individuals attained larger body sizes and more advanced developmental stages than later-hatched siblings in a natural pond; this growth acceleration occurred even in the complete isolation without any prey animals in the laboratory, demonstrating that the response was probably driven by rapid yolk absorption; and early-hatched individuals survived significantly longer than late-hatched ones in laboratory cannibalism assays, showing that growth acceleration enhances survival under cannibalistic interactions. Our findings provide the first experimental evidence that forced early hatching can trigger compensatory growth and developmental acceleration in salamanders, and that this plastic response confers an adaptive advantage.