The myology of the manus of microtine rodents

A study of the muscles of the manus of 13 genera and subgenera of microtine rodents revealed considerable variation which could be organized into 23 primitive-derived evolutionary hypotheses (transmutation series). The results, when compared with those for other mammals, produced functional/ecological hypotheses which were tested by comparing their predictions with the literature. The microtine manus is morphologically distinguished by nine characters and is adapted for a terrestrial/semi-fossorial niche. In particular, Dicrostonyx and Lemmus (distinguished by eight characters) possess the most highly fossorial manus, while the manus of Arborimus (distinguished by eight characters) contains a combination of primitive characters and derived characters which give the Tree vole the most highly developed 'clasping' ability among the microtines studied. The study has revealed characters which have potential systematic value but it is probable that parallelism has occurred in the microtine manus. When the manual characters are combined with characters from other functional complexes, their systematic value can be more clearly determined.     

Why spacing behavior does not stabilize density in cyclic populations of microtine rodents

Summary There are several published hypotheses that consider spacing behavior to be a significant factor causing the multiannual density fluctuations characteristic of some microtine rodent populations. Recent modeling efforts have concluded, however, that spacing behavior should have a stabilizing rather than a destabilizing effect on population dynamics. Why doesn't spacing behavior stabilize these cyclic populations? We argue that while spacing behavior does have a stabilizing influence on population dynamics by limiting the number of breeding individuals, reproduction continues and population size is not limited in an asymptotic manner. Rather, microtine social organization produces demographic changes within a population that allow density cycles to occur under certain conditions. Using a simulation model, we demonstrate that in a strongly seasonal environment populations with low density dependence in reproduction will cycle whereas populations with high density dependence in reproduction will have relatively stable densities. Given such complicating factors as the annual species nature of microtine rodents, occasionally intense predation, and the tendency for territoriality to break down during the non-breeding season, individuals with low density dependence in reproduction will always be able to invade and eventually dominate populations with high density dependence in reproduction, regardless of the resulting destabilization of population dynamics.     

Habitat and life history differences between two species of Gambusia

Life history strategies reflect trade-offs that tend to maximize fitness, such as investment in a few large or many small offspring. We compared life histories of two temperate livebearing fishes Gambusia affinis and G. nobilis, an endangered species which is virtually unstudied. The two species persist in environments that differ widely in abiotic and biotic factors in the same local area. Gambusia affinis were typically found in habitats with high productivity and wide fluctuations in temperature, salinity and dissolved oxygen, whereas G. nobilis occurred in more stable spring-fed habitats. We collected data on life-history traits: embryo mass, brood size (number of embryos), total maternal reproductive effort, population sex ratios, and size (mass and length) distributions of adults and juveniles. There was no difference between species in reproductive effort per brood, but they differed in investment strategy. Gambusia affinis females produced large broods with small embryos, whereas G. nobilis females produced broods of fewer, larger embryos. These differences in life history strategies reflect a tradeoff between individual productivity and differential mortality rates in different environments. At our field site G. affinis persists as an annual species with relatively high growth rates and corresponding reproductive patterns, whereas G. nobilis females have a slower reproductive tempo and may live multiple years.     

Natural selection and sex differences in morbidity and mortality in early life

Both morbidity and mortality are consistently reported to be higher in males than in females in early life, but no explanation for these findings has been offered. This paper argues that the sex difference in early vulnerability can be attributed to the natural selection of optimal maternal strategies for maximizing lifetime reproductive success, as modelled previously by Trivers and Willard. These authors theorized that males and females offer different returns on parental investment depending on the state of the environment. Natural selection has therefore favoured maternal ability to manipulate offspring sex in response to environmental conditions in early life, as shown in variation in the sex ratio at birth. This argument can be extended to the whole period of parental investment until weaning. Male vulnerability in response to environmental stress in early life is predicted to have been favoured by natural selection. This vulnerability is most evident in the harsh conditions resulting from pre-term birth, but can also be seen in term infants, and manifests as greater morbidity and mortality persisting into early childhood. Malnutrition, interacting with infection after birth, is suggested as the fundamental trigger mechanism. The model suggests that whatever improvements are made in medical care, any environmental stress will always affect males more severely than females in early life.     

Development of food intake controls: Neuroendocrine and environmental regulation of food intake during early life

This article is part of a Special Issue "Energy Balance".     

The role of early experience and emotionality in social facilitation of pecking in chickens

Three questions were examined relevant to consideration of three possible theoretical analyses of the social facilitative mechanism and its ontogeny. First, the effects of rearing, testing and their interaction on pecking during the first eight days of life were assessed, with a view to examining the role of learning in social facilitation. An attempt was made to resolve several procedural complexities associated with previous results. Second, the effects of these variables on emotionality were considered. Some correspondence might be predicted between behavioural and emotional effects in accordance with a postulated fear-reduction hypothesis. Third, also relevant to examination of the postulated causal relationship between level of fear or arousal induced by the test situation and inhibition of the dominant response, the effects on behaviour of independent manipulation of emotionality were assessed. Results provided some support for the postulated role of emotionality as one of possibly several factors in social facilitation, and also indicated the importance of previous experience in facilitation both as a single factor determining peck rates, and via its interaction with testing conditions. The difficulties posed by these and previous results for the three theoretical analyses described were noted.     

The influence of early experience with vanillin on food preference later in life.

A study with 133 adults, who had been breast-fed or bottle-fed after birth, shows that neonatal experience with vanilla influences preferences for other foods in later life.     

The role of cell contact in the life cycle of some dinoflagellate species

It is suggested, based on published and experimental data, thatinter- and intra-specific cell contact plays an important rolein the life cycle of some dinoflagellate species.     

Expression of Na+-D-glucose cotransporter SGLT2 in rodents is kidney-specific and exhibits sex and species differences

With a novel antibody against the rat Na(+)-D-glucose cotransporter SGLT2 (rSGLT2-Ab), which does not cross-react with rSGLT1 or rSGLT3, the ～75-kDa rSGLT2 protein was localized to the brush-border membrane (BBM) of the renal proximal tubule S1 and S2 segments (S1 > S2) with female-dominant expression in adult rats, whereas rSglt2 mRNA expression was similar in both sexes. Castration of adult males increased the abundance of rSGLT2 protein; this increase was further enhanced by estradiol and prevented by testosterone treatment. In the renal BBM vesicles, the rSGLT1-independent uptake of [(14)C]-α-methyl-D-glucopyranoside was similar in females and males, suggesting functional contribution of another Na(+)-D-glucose cotransporter to glucose reabsorption. Since immunoreactivity of rSGLT2-Ab could not be detected with certainty in rat extrarenal organs, the SGLT2 protein was immunocharacterized with the same antibody in wild-type (WT) mice, with SGLT2-deficient (Sglt2 knockout) mice as negative control. In WT mice, renal localization of mSGLT2 protein was similar to that in rats, whereas in extrarenal organs neither mSGLT2 protein nor mSglt2 mRNA expression was detected. At variance to the findings in rats, the abundance of mSGLT2 protein in the mouse kidneys was male dominant, whereas the expression of mSglt2 mRNA was female dominant. Our results indicate that in rodents the expression of SGLT2 is kidney-specific and point to distinct sex and species differences in SGLT2 protein expression that cannot be explained by differences in mRNA.     

The genetics of species differences

Species are separated by reproductive isolation as well as by more 'ordinary' differences in morphology and behavior that play no necessary role in blocking gene flow. Although a great deal is now known about the genetics of reproductive isolation, we are only beginning to understand the genetic basis of ordinary phenotypic differences between species. I review what is known about the number of genes involved in such differences, as well as about the role of major genes and epistasis in the evolution of these differences. I also compare and contrast these findings with those on the genetics of reproductive isolation.     

The influence of differences in social experience on the development of species recognition in zebra finch males

Zebra finch males were first raised by zebra finch parents and then placed in a group of Bengalese finches between the ages of 30 and 60 days. A higher number of aggressive as well as non-aggressive initiatives by Bengalese finches towards young zebra finch males during this period was correlated with a more Bengalese-finch-directed sexual preference when these males were given a choice between a zebra finch and a Bengalese finch female as adults. Experiments in which a zebra finch male was exposed to Bengalese finches behind a wire screen or to Bengalese finch models gave corresponding results. The study shows that, in contrast to earlier findings, zebra finch males raised by their parents for 31 days can still develop a preference for Bengalese finches. Short term changes in preference are discussed. The results indicate that the behaviour shown by stimulus birds in studies on ‘sexual imprinting’ is important for the development of sexual preferences.     

Comparative morphology and scale formation in four species ofOncorhynchus during early life

Morphological divergence including scale formation in alevin and juvenile stages of masu (Oncorhynchus mason), coho (O. kisutch), chum (O. keta) and pink (O. gorbuscha) salmons reared in freshwater was investigated. Chum and coho salmons developed faster than the other species when raised at constant water tempertaure. Growth decreased suddenly at emergence from the gravel bed in all the four species. Morphology changed significantly from hatch through emergence to squamation. Two distinct morphological types were identified: the rotund type (masu and coho), and the streamlined type (chum and pink salmons). The former had well-developed parr marks, a deeper body and caudal peduncle, and higher relative growth coefficients, while the latter had parr marks which appeared only temporarily or never appeared, a slimmer body and caudal peduncle, and a more gradual and continuous change in morphology. Coho and chum developed squamation and formed circuli on scales considerably earlier than masu and pink salmons.