Hatching

Abstract: The factors controlling sexual differentiation in crocodilians are unknown, but heteromorphic sex chromosomes are absent from all species1. Nichols and Chabreck2 speculated that the sex of Alligator mississippiensis was not rigidly determined at the time of hatching but could be influenced by the post-hatching environment. They presented little evidence to support their hypothesis3 (no histological sections of hatchling gonads, no indication of the sex ratio at hatching), and their study failed to take account of habitat preferences of adult male and female alligators4. Here we demonstrate by laboratory and field experiments, that in A. mississippiensis: (1) Sex is fully determined at the time of hatching and naturally irreversible thereafter, and depends on the temperature of egg incubation, temperatures ⩽30 °C producing all females, ⩾34 °C yielding all males. (2) The temperature-sensitive period is between 7 and 21 days of incubation. (3) Natural nests constructed on levees are hotter (34 °C) than those constructed on wet marsh (30 °C), thus the former hatch males and the latter females. (4) The natural sex ratio at hatching is five females to 1 male. (5) Females hatched from eggs incubated at 30 °C weigh significantly more than males hatched from eggs incubated at 34 °C. This weight difference constitutes a possible selective evolutionary advantage of temperature-dependent sex determination (TSD) in alligators in that females become large and sexually mature as early as possible. The occurrence of TSD in alligators has wide-ranging implications for embryological, teratological, molecular, evolutionary, conservation and farming studies as well as for theories relating to the extinction of other Archosaurs.

Abstract: Abstract This is the first re-appraisal in 50 years of concepts of development made in birds. This book is a case study in evolutionary diversification of life histories. Although birds have a rather uniform body plan and physiology, they exhibit marked variation in development type, parental care, and rate of growth. Altricial birds are fully dependent on their parents for warmth and nutrition and begin posthatching life in a more or less embryonic condition. At the other extreme, such superprecocial species as the megapodes are independent of all parental care from hatching, and the neonate, able to fly, resembles an adult bird. This book thus attempts to present an integrative perspective of organism biology, ecology, and evolution.

Abstract: IN many gull species, the third laid egg of the typical three egg clutch is distinctly smaller than the first two1,2. In the herring gull (Larus argentatus), the chick hatching from this third egg suffers a much higher mortality than either of its siblings3, although the hatching success is the same for all three eggs. In a series of egg transfer experiments carried out on the Isle of May, Scotland, it has been possible to demonstrate at least two factors contributing to this differential mortality, namely the size disadvantage and the sequence of hatching, because third laid eggs normally hatch last. Although the survival of chicks from third eggs increased when they were the first to hatch, it was still lower than that of its siblings.

Abstract: Experiments monitoring the egg and larval production of captive northwest Atlantic cod Gadus morhua indicated that first-time spawners perform poorly compared to second-time spawners. They breed for a shorter period, produce fewer egg batches, exhibit lower fecundity, and produce smaller eggs with lower fertilization and hatching rates; moreover, their larvae are less likely to hatch in environmental conditions favorable for survival. Batch-average egg diameters of first-time spawners underwent a continuous rapid decline between initial and terminal batches, whereas second-time spawners commonly exhibited a parabolic curve (slight increase followed by a decrease). Seasonal composite hatching rates of all eggs spawned by first- and second-time spawners were 13 and 62%, respectively. Larval production per maternal gram was an order of magnitude greater for second-time than for first-time spawners (200 versus 20 larvae/g). Batch-average egg diameter declined 11% and larval dry weight declined 29% fr...