Review Article
The ecology of overwintering among turtles: where turtles overwinter and its consequences
- Gordon R. Ultsch
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- Published online by Cambridge University Press:
- 15 May 2006, pp. 339-367
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Turtles are a small taxon that has nevertheless attracted much attention from biologists for centuries. However, a major portion of their life cycle has received relatively little attention until recently – namely what turtles are doing, and how they are doing it, during the winter. In the northern parts of their ranges in North America, turtles may spend more than half of their lives in an overwintering state. In this review, I emphasise the ecological aspects of overwintering among turtles, and consider how overwintering stresses affect the physiology, behaviour, distributions, and life histories of various species.
Sea turtles are the only group of turtles that migrate extensively, and can therefore avoid northern winters. Nevertheless, each year a number of turtles, largely juveniles, are killed when trapped by cold fronts before they move to safer waters. Evidently this risk is an acceptable trade-off for the benefits to a population of inhabiting northern developmental habitats during the summer.
Terrestrial turtles pass the winter underground, either in burrows that they excavate or that are preformed. These refugia must provide protection against desiccation and lethal freezing levels. Some burrows are extensive (tortoise genus Gopherus), while others are shallow, or the turtles may simply dig into the ground to a safe depth (turtle genus Terrapene). In the latter genus, freeze tolerance may play an adaptive role.
Most non-marine aquatic turtles overwinter underwater, although Clemmys (Actinemys) marmorata routinely overwinters on land when it occurs in riverine habitats, Kinosternon subrubrum often overwinters on land, and several others may overwinter terrestrially on occasion, especially in more southern climates. For northern species that overwinter underwater, there are two physiological groupings, those that are anoxia-tolerant and those that are relatively anoxia-intolerant. All species fare well physiologically in water with a high partial pressure of oxygen (PO2). A lack of anoxia tolerance limits the types of habitats that a freshwater turtle may live in, since unlike sea turtles, they cannot travel long distances to hibernate.
Hatchlings of some species of turtles spend their first winter in or below the nest cavity, while hatchlings of other species in the same area, including northern areas, emerge in the autumn and presumably hibernate underwater. All hatchlings are relatively anoxia-intolerant, and there are no studies to date of where hatchling turtles that do not overwinter in or below the nest cavity spend their first winter. Equally little is known of the ontogeny of anoxia tolerance, other than that adults of all species are more anoxia-tolerant than their hatchlings, probably because of their better ossified shells, which provide adults with more buffer reserves and a larger site in which to sequester lactate. The northern limits of turtles are most likely determined by reproductive limitations (time for egg-laying, incubation, and hatching) than by the rigors of hibernation.
Mortality is typically lower in turtle populations during hibernation than it is during their active periods. However, episodic mortality events do occur during hibernation, due to freezing, prolonged anoxia, or predation.
The origin of human pathogens: evaluating the role of agriculture and domestic animals in the evolution of human disease
- Jessica M. C. Pearce-Duvet
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- Published online by Cambridge University Press:
- 04 May 2006, pp. 369-382
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Many significant diseases of human civilization are thought to have arisen concurrently with the advent of agriculture in human society. It has been hypothesised that the food produced by farming increased population sizes to allow the maintenance of virulent pathogens, i.e. civilization pathogens, while domestic animals provided sources of disease to humans. To determine the relationship between pathogens in humans and domestic animals, I examined phylogenetic data for several human pathogens that are commonly evolutionarily linked to domestic animals: measles, pertussis, smallpox, tuberculosis, taenid worms, and falciparal malaria. The majority are civilization pathogens, although I have included others whose evolutionary origins have traditionally been ascribed to domestic animals. The strongest evidence for a domestic-animal origin exists for measles and pertussis, although the data do not exclude a non-domestic origin. As for the other pathogens, the evidence currently available makes it difficult to determine if the domestic-origin hypothesis is supported or refuted; in fact, intriguing data for tuberculosis and taenid worms suggests that transmission may occur as easily from humans to domestic animals. These findings do not abrogate the importance of agriculture in disease transmission; rather, if anything, they suggest an alternative, more complex series of effects than previously elucidated. Rather than domestication, the broader force for human pathogen evolution could be ecological change, namely anthropogenic modification of the environment. This is supported by evidence that many current emerging infectious diseases are associated with human modification of the environment. Agriculture may have changed the transmission ecology of pre-existing human pathogens, increased the success of pre-existing pathogen vectors, resulted in novel interactions between humans and wildlife, and, through the domestication of animals, provided a stable conduit for human infection by wildlife diseases.
The evolution of egg colour and patterning in birds
- R. M. Kilner
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- Published online by Cambridge University Press:
- 02 June 2006, pp. 383-406
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Avian eggs differ so much in their colour and patterning from species to species that any attempt to account for this diversity might initially seem doomed to failure. Here I present a critical review of the literature which, when combined with the results of some comparative analyses, suggests that just a few selective agents can explain much of the variation in egg appearance. Ancestrally, bird eggs were probably white and immaculate. Ancient diversification in nest location, and hence in the clutch's vulnerability to attack by predators, can explain basic differences between bird families in egg appearance. The ancestral white egg has been retained by species whose nests are safe from attack by predators, while those that have moved to a more vulnerable nest site are now more likely to lay brown eggs, covered in speckles, just as Wallace hypothesized more than a century ago. Even blue eggs might be cryptic in a subset of nests built in vegetation. It is possible that some species have subsequently turned these ancient adaptations to new functions, for example to signal female quality, to protect eggs from damaging solar radiation, or to add structural strength to shells when calcium is in short supply. The threat of predation, together with the use of varying nest sites, appears to have increased the diversity of egg colouring seen among species within families, and among clutches within species. Brood parasites and their hosts have probably secondarily influenced the diversity of egg appearance. Each drives the evolution of the other's egg colour and patterning, as hosts attempt to avoid exploitation by rejecting odd-looking eggs from their nests, and parasites attempt to outwit their hosts by laying eggs that will escape detection. This co-evolutionary arms race has increased variation in egg appearance both within and between species, in parasites and in hosts, sometimes resulting in the evolution of egg colour polymorphisms. It has also reduced variation in egg appearance within host clutches, although the benefit thus gained by hosts is not clear.
Positive selection in the evolution of cancer
- Bernard J. Crespi, Kyle Summers
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- Published online by Cambridge University Press:
- 09 June 2006, pp. 407-424
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We hypothesize that forms of antagonistic coevolution have forged strong links between positive selection at the molecular level and increased cancer risk. By this hypothesis, evolutionary conflict between males and females, mothers and foetuses, hosts and parasites, and other parties with divergent fitness interests has led to rapid evolution of genetic systems involved in control over fertilization and cellular resources. The genes involved in such systems promote cancer risk as a secondary effect of their roles in antagonistic coevolution, which generates evolutionary disequilibrium and maladaptation. Evidence from two sources: (1) studies on specific genes, including SPANX cancer/testis antigen genes, several Y-linked genes, the pem homebox gene, centromeric histone genes, the breast cancer gene BRCA1, the angiogenesis gene ANG, cadherin genes, cytochrome P450 genes, and viral oncogenes; and (2) large-scale database studies of selection on different functional categories of genes, supports our hypothesis. These results have important implications for understanding the evolutionary underpinnings of cancer and the dynamics of antagonistically-coevolving molecular systems.
Human cell type diversity, evolution, development, and classification with special reference to cells derived from the neural crest
- Matthew K. Vickaryous, Brian K. Hall
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- Published online by Cambridge University Press:
- 22 June 2006, pp. 425-455
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Metazoans are composed of a finite number of recognisable cell types. Similar to the relationship between species and ecosystems, knowledge of cell type diversity contributes to studies of complexity and evolution. However, as with other units of evolution, the cell type often resists definition. This review proposes guidelines for characterising cell types and discusses cell homology and the various developmental pathways by which cell types arise, including germ layers, blastemata (secondary development/neurulation), stem cells, and transdifferentiation. An updated list of cell types is presented for a familiar, albeit overlooked model taxon, adult Homo sapiens, with 411 cell types, including 145 types of neurons, recognised. Two methods for organising these cell types are explored. One is the artificial classification technique, clustering cells using commonly accepted criteria of similarity. The second approach, an empirical method modeled after cladistics, resolves the classification in terms of shared features rather than overall similarity. While the results of each scheme differ, both methods address important questions. The artificial classification provides compelling (and independent) support for the neural crest as the fourth germ layer, while the cladistic approach permits the evaluation of cell type evolution. Using the cladistic approach we observe a correlation between the developmental and evolutionary origin of a cell, suggesting that this method is useful for predicting which cell types share common (multipotential) progenitors. Whereas the current effort is restricted by the availability of phenotypic details for most cell types, the present study demonstrates that a comprehensive cladistic classification is practical, attainable, and warranted. The use of cell types and cell type comparative classification schemes has the potential to offer new and alternative models for therapeutic evaluation.