SUMMARY

1. Eggs of Crocodilia and Chelonia, like those of birds, have a pair of egg membranes separating a thick layer of albumen from the calcareous shell. In contrast, eggs of oviparous Lepidosauria have only a single shell membrane, upon which relatively small amounts of calcium carbonate are deposited; and the volume of albumen in eggs is extraordinarily small at the time of oviposition.



2. With the possible exception of certain geckos and some chelonians, eggs of oviparous reptiles seem always to absorb water from the substrate during the course of normal incubation. In so far as the rate of water absorption exceeds the rate of water loss by transpiration from exposed surfaces, the eggs swell during incubation. The term ‘cleidoic’ cannot be used to describe eggs of this type.



3. Embryos of lizards and snakes influence the water potential of extra-embryonic fluids contained within their eggs, thereby maintaining or increasing the gradient in water potential that drives water absorption.



4. Embryos of Crocodilia and Chelonia obtain a substantial portion of the calcium used in ossification of skeletal elements from the inner surfaces of the eggshell. In contrast, embryonic lizards and snakes draw upon extensive reserves of calcium present in the yolk, and obtain little (if any) calcium from the eggshell.



5. All reptilian embryos seem to produce substantial quantities of urea as a detoxification product of protein catabolism. Contrary to expectation, uricotelism may not be common among reptilian embryos, even in those few instances where development takes place within a hard, calcareous egg.



6. In eggs of Crocodilia and Chelonia, respiratory gases seem to pass by diffusion through pores in the calcareous eggshell and through spaces between the fibres of the pair of egg membranes. No pores have been observed in the shell of lepidosaurian eggs, and so gases presumably diffuse between the fibres of the single (multilayered) shell membrane.



7. Metabolism of reptilian embryos is temperature-dependent, as is true for most ectothermic organisms. For each species, there appears to be a particular temperature at which embryonic development proceeds optimally, and departures from this optimum elicit increases in developmental anomalies and/or embryonic mortality.



8. Viviparity has evolved on numerous occasions among species of the Squamata, but seemingly never among Crocodilia or Chelonia. Since the evolution of viviparity entails a progressive reduction in the eggshell, only those organisms whose embryos do not depend upon the eggshell as a source of calcium may have the evolutionary potential to become viviparous.



9. Evolutionary transitions from oviparity to viviparity could have been driven by selection related to (i) thermal benefits to embryos consequent upon retention of eggs within the body of a parent capable of behavioural thermoregulation; (ii) protection of the eggs from nest predators and/or soil microbes; and (iii) more effective exploitation of a seasonal food resource by early emerging young.

The physiological ecology of reptilian eggs and embryos. and the evolution of viviparity within the class reptilia

Over the past 3 decades, the status of sea turtles and the need for their protection to aid population recovery have increasingly captured the interest of government agencies, non-governmental organisations (NGOs) and the general public worldwide. This interest has been matched by increased research attention, focusing on a wide variety of topics relating to sea turtle biology and ecology, together with the interrelations of sea turtles with the physical and natural environments. Although sea turtles have been better studied than most other marine fauna, management actions and their evaluation are often hindered by the lack of data on turtle biology, human–turtle interactions, turtle population status and threats. In an effort to inform effective sea turtle conservation a list of priority research questions was assembled based on the opinions of 35 sea turtle researchers from 13 nations working in fields related to turtle biology and/or conservation. The combined experience of the contributing researchers spanned the globe as well as many relevant disciplines involved in conservation research. An initial list of more than 200 questions gathered from respondents was condensed into 20 metaquestions and classified under 5 categories: reproductive biology, biogeography, population ecology, threats and conservation strategies.

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Global research priorities for sea turtles: informing management and conservation in the 21st century

Sex-determining mechanisms in reptiles can be divided into two convenient classifications: genotypic (GSD) and environmental (ESD). While a number of types of GSD have been identified in a wide variety of reptilian taxa, the expression of ESD in the form of temperature-dependent sex determination (TSD) in three of the five major reptilian lineages has drawn considerable attention to this area of research. Increasing interest in sex-determining mechanisms in reptiles has resulted in many data, but much of this information is scattered throughout the literature and consequently difficult to interpret. It is known, however, that distinct sex chromosomes are absent in the tuatara and crocodilians, rare in amphisbaenians (worm lizards) and turtles, and common in lizards and snakes (but less than 20% of all species of living reptiles have been karyotyped). With less than 2 percent of all reptilian species examined, TSD apparently is absent in the tuatara, amphisbaenians and snakes; rare in lizards, frequent in turtles, and ubiquitous in crocodilians. Despite considerable inter- and intraspecific variation in the threshold temperature (temperature producing a 1:1 sex ratio) of gonadal sex determination, this variation cannot confidently be assigned a genetic basis owing to uncontrolled environmental factors or to differences in experimental protocol among studies. Laboratory studies have identified the critical period of development during which gonadal sex determination occurs for at least a dozen species. There are striking similarities in this period among the major taxa with TSD. Examination of TSD in the field indicates that sex ratios of hatchlings are affected by location of the nests, because some nests produce both sexes whereas the majority produce only one sex. Still, more information is needed on how TSD operates under natural conditions in order to fully understand its ecological and conservation implications. TSD may be the ancestral sex-determining condition in reptiles, but this result remains tentative. Physiological investigations of TSD have clarified the roles of steroid hormones, various enzymes, and H-Y antigen in sexual differentiation, whereas molecular studies have identified several plausible candidates for sex-determining genes in species with TSD. This area of research promises to elucidate the mechanism of TSD in reptiles and will have obvious implications for understanding the basis of sex determination in other vertebrates. Experimental and comparative investigations of the potential adaptive significance of TSD appear equally promising, although much work remains to be performed. The distribution of TSD within and among the major reptilian lineages may be related to the life span of individuals of a species and to the biogeography of these species.(ABSTRACT TRUNCATED AT 400 WORDS)

Environmental sex determination in reptiles: ecology, evolution, and experimental design

Scientific results of an inquiry into the ecology and economic status of the Nile Crocodile (Crocodilus niloticus) in Uganda and Northern Rhodesia

The seven species of sea turtles occupy a diversity of niches, and have a history tracing back over 100 million years, yet all share basic life-history features, including exceptional navigation skills and periodic migrations from feeding to breeding habitats. Here, we review the biogeographic, behavioural, and ecological factors that shape the distribution of genetic diversity in sea turtles. Natal homing, wherein turtles return to their region of origin for mating and nesting, has been demonstrated with mtDNA sequences. These maternally inherited markers show strong population structure among nesting colonies while nuclear loci reveal a contrasting pattern of male-mediated gene flow, a phenomenon termed ‘complex population structure’. Mixed-stock analyses indicate that multiple nesting colonies can contribute to feeding aggregates, such that exploitation of turtles in these habitats can reduce breeding populations across the region. The mtDNA data also demonstrate migrations across entire ocean basins, some of the longest movements of marine vertebrates. Multiple paternity occurs at reported rates of 0–100%, and can vary by as much as 9–100% within species. Hybridization in almost every combination among members of the Cheloniidae has been documented but the frequency and ultimate ramifications of hybridization are not clear. The global phylogeography of sea turtles reveals a gradient based on habitat preference and thermal regime. The cold-tolerant leatherback turtle (Dermochelys coriacea) shows no evolutionary partitions between Indo-Pacific and Atlantic populations, while the tropical green (Chelonia mydas), hawksbill (Eretmochelys imbricata), and ridleys (Lepidochelys olivacea vs. L. kempi) have ancient separations between oceans. Ridleys and loggerhead (Caretta caretta) also show more recent colonization between ocean basins, probably mediated by warm-water gyres that occasionally traverse the frigid upwelling zone in southern Africa. These rare events may be sufficient to prevent allopatric speciation under contemporary geographic and climatic conditions. Genetic studies have advanced our understanding of marine turtle biology and evolution, but significant gaps persist and provide challenges for the next generation of sea turtle geneticists.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-294X.2007.03542.x

Population genetics and phylogeography of sea turtles.

SUMMARY
A four-year study was conducted on breeding populations of Green Sea Turtles, Chelonia mydas (Linn.), in Malaya and Sarawak. The area of study was confined to the South China Sea (Sunda Shelf) coasts of the two countries. The history of the exploitation of sea turtle populations for eggs in the Muslim areas of Malaysia is reviewed; compared with other parts of the world where turtles are killed for flesh and fat, the populations in Malaya and Sarawak appear to have suffered little reduction in size. The general aspects of the marine environment and the different types of sea turtle breeding beaches in the area are described. Chelonia mydas tends to build up large breeding concentrations around a small number of beaches, with only limited use of other beaches in the region. Possible causes and adaptive features of the observed situation are discussed. The type of beach utilised by large breeding concentrations of Green Turtles is described, and several particular beaches are described in detail.

The process of nesting is described at length, drawing on extensive field notes made over a four-year period. Particular attention is paid to behavioural aspects of the nesting process and to their possible adaptive significance. Female turtles approaching the beach for nesting are very shy while still in the water and still able to make a rapid escape; they show a clear set of reactions to visual stimuli. With progression up onto the land and isolation from the aquatic environment, the turtles show progressively less response to alarm stimuli. Ihe mechanics of terrestrial locomotion and of the various nesting activities are described, along with discussions of inhibitory and initiatory stimuli which appear to govern the different stages in the process. A table is presented showing the timing of the nesting process in three selected observations on individual females.

Courtship and copulation in Chelonia mydas are described as they were observed in Malaya and Sarawak.

2,720 adult female turtles were tagged on three islands off Sarawak, where the Sarawak Government operates a turtle egg industry. Through the cooperation of the industry, statistical studies were carried out. Methods of tagging are described and pertinent features of the statistical work are discussed The recorded annual egg production from the three islands averages slightly less than two million eggs a year, with a recorded minimum of 700,000 and a maximum in excess of three million eggs.

While there is no non-breeding season in Sarawak, there is a pronounced annual cyclical variation in the intensity of breeding, with a marked heavy season around June, July, and August. The absence of a non-breeding season is considered to be related to the virtual lack of any marked annual variation in temperature. The existence of a definite peak breeding season is thought to relate to the adaptive advantage of a habit of synchronous breeding aggregation in a widely ranging species. The timing of the peak breeding season appears to relate to the period of the Northeast Monsoon, which brings relatively inclement weather from November through February.

The development and decline of the breeding season as shown by egg production figures from the industry shows a remarkable consistency through “good” years and “bad”. While presumably located in time to occur when monsoon effects are minimal, it does not show correspondence to yearly fluctuations in the timing of the monsoon. The per cent contribution of each calendar month to the annual total egg production tends to remain constant regardless of whether the monsoon periods preceding or following are early or late.

Individual tagged turtles returned repeatedly to the same beach to nest. Of 5,748 records of returning tagged females only 3–7 per cent failed to return to the same island previously used, despite the fact that two of the islands were only about 500 metres apart.

Female turtles nested six or seven times in a season at intervals of about 10 days. Records showed a maximum of 11 successful nests made by a single individual. 8,147 clutches of eggs averaged 104-7 eggs per clutch, with a Standard Deviation of 1–31.

The return in 1956 of numbers of the turtles tagged in 1953 indicates a three-year breeding cycle in the Sarawak turtles, and provides evidence that individuals are able to make their way back to the same nesting grounds after the long absence.

The environment of the incubating nest is described and the hatchery procedures practised on the Sarawak islands are outlined. A special series of nests was transplanted to the industry's hatchery enclosure for studies on incubation. Although surface sand temperatures underwent a diurnal fluctuation of as much as 23o C, at the depths where eggs were deposited the measured temperature never fluctuated more than about 2o C. Incubating nests showed a progressive temperature rise due to the metabolic heat produced by the developing embryos, reaching temperatures as high as 35o C. by the time of hatching. Records of 354 hatchery nests showed an average of 47 per cent of the transplanted eggs resulting in hatchlings which emerged to the surface. Recorded intervals between oviposition and time of emergence of young turtles to the surface varied seasonally from an average of about 70 days during the latter part of February to an average of about 54 days during the period June to October.

The process of emergence of the hatchling turtles from their subterranean chamber and their movement to the sea is described.

The available data on growth rate and age of maturation is assembled and discussed. It is concluded that Asian Green Turtles probably mature in from four to six years. The carapace measurements of 200 adult females from Sarawak are plotted on a graph; the weights and carapace measurements of ten females are given in a table.

Predation upon incubating nests and upon turtles at all stages is discussed. Ghost Crabs (Ocypoda sp.) were the most serious observed menace to incubating nests in the areas studied. The heaviest predation observed on hatchling turtles was by Ghost Crabs and small sharks. There is little predation by sea birds on the Malaysian beaches. Several large Tiger Sharks (Galeocerdo sp.) were caught off the Sarawak islands; two of these had the remains of adult sea turtles in their stomachs.

The various parasites and commensals found on turtles during the study are reported; observations are made on the burrowing parasitic barnacle, Stephanolepas muricata, and its apparent tumour-inducing proclivities.

The paper concludes with an analysis of the problem of scientific management of wild populations of Green Turtles. It is concluded that exploitation for eggs is more logical, more profitable, and more amenable to management techniques than is exploitation for flesh and fat.

John R. Hendrickson

Papers

The green sea turtle, chelonia mydas (linn.) in malaya and sarawak