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PubMed2001

Caudal vertebral development and morphology in three salamanders with complex life cycles (Ambystoma jeffersonianum, Hemidactylium scutatum, and Desmognathus ocoee).

We describe caudosacral and caudal vertebral morphology across life history stages in three caudate amphibians: Ambystoma jeffersonianum (Ambystomatidae), Desmognathus ocoee (Plethodontidae: Desmognathinae), and Hemidactylium scutatum (Plethodontidae: Plethodontinae). All three species have aquatic larvae, but adults differ in habitat and predator defense strategy. Predator defense includes tail autotomy in D. ocoee and H. scutatum but not A. jeffersonianum. Of the species that autotomize, H. scutatum has a specialized constriction site at the tail base. We investigated whether aquatic larvae exhibit vertebral features similar to those previously described for aquatic adults and examined the effect of metamorphosis, if any, on vertebral morphology and the ontogeny of specialized vertebral features associated with tail autotomy. Interspecific comparisons of cleared-and-stained specimens indicate that vertebral morphology differs dramatically at hatching and that caudosacral and caudal vertebrae undergo continuous ontogenetic change throughout larval, metamorphic, and juvenile periods. Larvae and juveniles of H. scutatum do not exhibit adult vertebral features associated with constricted-base tail autotomy. The pond-type larvae of A. jeffersonianum and H. scutatum have tapering centrum lengths posterior to the sacrum. This pattern is functionally associated with aquatic locomotion. The stream-type larvae of D. ocoee undergo enhanced regional growth in the anterior tail such that the anterior caudal centra become longer than the preceding caudosacral centra. With the exception of the first two caudal vertebrae, a similar growth pattern occurs in H. scutatum adults. We hypothesize that enhanced growth of the anterior caudal segments is associated with tail elongation and autotomy.

Ambystoma↗
PubMed2001

Low seasonal temperatures promote life cycle synchronization.

In this paper we discuss how seasonal temperature variation and life-stage specific developmental thresholds that cause quiescence can synchronize the seasonal development of exothermic organisms. Using a simple aging model it is shown that minimal seasonal temperature variation and periods of quiescence during extreme temperature conditions are sufficient to establish stable, univoltine ovipositional cycles. Quiescence induced by life-stage specific developmental thresholds, in fact, promotes synchronous oviposition and emergence. The mountain pine beetle, an important insect living in extreme temperature conditions and showing no evidence of diapause, invites direct application of this model. Simulations using mountain pine beetle parameters are used to determine temperature regimes for which stable ovipositional cycles exist.

Animals↗
PubMed1988

Counterurbanization: spatial division of labour and regional life-cycles in Canada.

"After a discussion of major concepts of counterurbanization, a narrow definition is proposed. Various demographic and socio-economic dimensions are analysed at different scales with special regard to the Canadian heartland. The basic hypothesis suggests a temporary weakening of urban growth dynamics: decentralization, thus, is a result of locational adjustment strategies, emerging during revolutions in basic technology. The corresponding 'filtering down' processes create a selective, interregional spread of labour. However, shift analytical time series indicate that the development capacity of core regions oscillates in a life-cyclical rhythm. Counterurbanization is expected to fade out with regional adjustment to basic technological transformation."

Americas↗