Natural Mortality of Eggs and Larvae of Ambystoma T. Tigrinum

All the eggs deposited by an isolated breeding population of Ambystoma t. tigrinum were located. Egg deposition was not synchronous. Mortality was determined in the field by counting living and dead eggs until hatching. After hatching the larval population size was estimated at intervals until metamorphosis. Gastrulation, hatching, and metamorphosis were critical stages in the field. Between these stages survivorship curves are hyperbolic. Small egg masses incurred heavier mortality than larger masses. Heavy mortality within an egg mass did not influence survival of remaining eggs as it does in the laboratory. The environment acts on individual eggs rather than on the mass as aunit. Low temperature was probably the major source of embryonic mortality although freezing, predation, and low pH may be important. Predation was probably the most important source of hatchling mortality. Free—swimming larvae suffered negligible mortality, but this may vary from year to year depending upon embryonic and hatchling mortality. Survival from egg deposition to metamorphosis averaged 3.3%, varying in three ponds from 0% to 8.7%. This high mortality may be characteristic of pond—dwelling amphibians of the temperate region.