Predator‐induced life history changes in amphibians: egg predation induces hatching

Abstract

The timing of transitions between life history stages should be affected by factors that influence survival and growth of organisms in adjacent life history stages. In a series of laboratory experiments, we examined the influence of predation risk as a cue to trigger a life history switch in amphibians. In the Oregon Cascade Mountains, some populations of Pacific treefrogs (Hyla regilla) and Cascades frogs (Ranacascadae) are under intense egg predation by predatory leeches (families Glossiphonidae and Erpobdellidae). We document that both treefrogs and Cascades frogs show plasticity in hatching characteristics in response to the threat of egg predation. Pacific treefrogs hatch sooner and at an earlier developmental stage when either predatory leeches or non‐predatory earthworms are allowed direct contact with the developing egg mass. The same response is elicited even without direct contact. Chemical cues of predatory leeches and chemicals released from injured eggs appear to elicit the same early hatching response in treefrogs. For Cascades frogs, cues of leeches, but not those of injured eggs, elicit an early hatching response. Hatching early in response to egg predators may reduce predation. Plasticity of hatching characteristics has rarely been examined. However, we suspect that it may be common, particularly in populations or species that experience high variability in predation pressure between years.

Egg-Killing Fungus Induces Early Hatching of Red-Eyed Treefrog Eggs

“…regilla and Rana cascadae hatch early in response to egg-eating leeches (Chivers et al 2001), and Hyperolius cinnamomeoventris hatches at a smaller size when exposed to egg-eating fly larvae (Vonesh 2000). Embryos also respond to abiotic factors affecting egg and larval survival.…”

“…Predator effects on hatching stage have now been documented in six species of amphibians (Sih and Moore 1993, Warkentin 1995, Vonesh 2000, Chivers et al 2001G. Schalk, personal communication), and have been suggested in crustaceans (Blaustein 1997 Knowing that some embryos respond to mortality risks by altering their hatching stage, an important question is: what determines which species will respond to which risks?…”

“…First, embryos might respond to a chemical released either by the fungus itself or by other embryos being killed. Other amphibian embryos respond to chemical cues from predators or predation events by delaying hatching (Ambystoma barbouri; Moore 1993, Moore et al 1996) or accelerating hatching (Hyla regilla and Rana cascade; Chivers et al 2001). Second, the fungus could reduce oxygen availability to embryos either by reducing the exposed egg surface area available for oxygen diffusion, or by directly competing with embryos for oxygen within egg capsules.…”

Social learning in a high-risk environment: incomplete disregard for the ‘minnow that cried pike’ results in culturally transmitted neophobia

“…[3,4]). Some prey alter life-history traits such as age or size of sexual maturation [5] or the timing of life-history switches such as metamorphosis, to decrease their risk of predation at a particular life stage [6,7]. However, behavioural responses to risky cues are generally the first lines of defence, as they are more plastic, not so costly and are expressed on a much shorter timescale than other alternatives [8].…”

Development of red-eyed treefrog eggs affects efficiency and choices of egg-foraging wasps

“…Like the resource value of eggs, these defences may change little over the course of embryonic development, providing little reason for egg predators to forage selectively. However, it has become clear that some amphibians, fish and invertebrates also have a developmentally variable egg defence: during later stages of embryonic development, they are capable of hatching prematurely to escape from egg-stage risks (Warkentin 1995;Chivers et al 2001;Li 2002;Wedekind 2002). In such species, less developed embryos that are not yet capable of hatching may be more secure prey for egg predators.…”