Massive evidence has now accumulated which indicates that the pituitary-adrenal cortical system responds sensitively to psychological influences. This evidence represents converging and reinforcing findings from diverse studies of many species, including man. It involved the use of many methods of e

A review of psychoendocrine research on the pituitary-adrenal cortical system.

Though it has been widely predicted that in animals in which reproductive value declines with age, reproductive effort should increase toward the end of the lifespan, analysis of changes in reproductive effort are impeded by fundamental difficulties in measuring the costs of reproduction. Energetic measures may not reflect the effects of breeding on subsequent survival and breeding success, especially in organisms in which body size increases with age, while attempts to estimate reproductive costs directly are complicated by positive correlations between breeding success and parental survival. Though the long-lived birds and mammals are among the most promising organisms on which to test the theory that reproductive effort increases with age, measures of fecundity commonly decline with increasing maternal age. Some recent evidence suggests, however, that offspring survival may improve toward the end of the lifespan.

Reproductive Effort and Terminal Investment in Iteroparous Animals

Publisher Summary This chapter summarizes the current information on population cycles in small rodents It first looks at the general questions about cycles, and then discusses the demographic machinery which drives the changes in numbers And finally, analyzes the current theories which explain population cycles in rodents Population cycles in voles and lemmings are accompanied by a series of changes A few of them include fluctuations occurring in a variety of genera and species from arctic to temperate areas, from Mediterranean to continental climates, from snowy areas to snow-free areas Populations living in a wide variety of plant communities in a small geographic area all fluctuate in the same way, often in phase Survival of adult males fluctuates independently of that of adult females, when viewed on a weekly time scale Males may suffer heavy losses in the decline for a few weeks when females are surviving very well, and vice versa

Population Cycles in Small Mammals

The relationship between previously published values of "the intrinsic rate of natural increase" (r m ) and body weight is studied. When organisms covering a wide range of body weights are compared, a correlation is found which can be described by the equation r m =aW n where r m is the intrinsic rate of natural increase per day and W is the average body weight in grams; a is a constant which takes three different values for unicellular organisms and heterotherm and homoiotherm animals respectively. The constant n has a value of about-0.275 for all three groups. This result is compared to the previously found relationship between the metabolic rate per unit weight and body size. It is shown that r m can be inter-preted as the productivity of an exponentially growing population and thus must correlate with metabolic rate. The values of the constants a and n, however, show that for each of the three groups, unicellular organisms, heterotherms and homoiotherms the ratio of energy used for maintenance to that used for production increases with increasing body size and that the evolution from protozoa to metazoa and the evolution from heterotherm to homoiotherm animals in both cases resulted in not only an increased metabolic rate, as shown previously, but also in a decreased population growth efficiency. It is shown that the increase in reproductive potential of homoiotherms relative to that of heterotherms is due to a shorter prereproductive period in the former group.Previous estimates of r m for different species and comparisons between these values in relation to their ecology are discussed in context with the found "r m -body weight" relationship. Attempts to show that such comparisons will be more meaningful when body size is included in the considerations are made. It is suggested that the found relationship may represent the maximum values r m can take rather than average values for all species, since it is likely that the species used for laboratory population-studies are biased in favor of species with high reproductive potentials.

Intrinsic rate of natural increase: The relationship with body size

Recent advances of three different kinds are driving a change in the way that modelling Is being done in ecology. First, the theory of chaos tells us that short-term predictions of nonlinear systems will be difficult, and long-term predictions will be impossible. The grave Implications this has for ecology are only just beginning to be understood. Second, ecologists have started to recognize the importance of local interactions between individuals in ecological systems. And third, improvements in computer power and software are making computers more inviting as a primary tool for modelling. The combination of these factors may have far-reaching consequences for ecological theory.

The rise of the individual-based model in ecology

The estimation of population parameters from data obtained by means of the capture-recapture method: iii. an example of the practical applications of the method

The estimation of population parameters from data obtained by means of the capture-recaptutre methodthe maximum likelihood equations for estimating the death-rate

SUMMARY. 

1
An analysis has been made of some data relating to the longevity and fertility of the Orkney vole, M. orcadensis, as observed in the laboratory.

2
The probability of surviving the first three weeks of life, based on a total of 1,358 live-born young, of which 1,250 were still alive at weaning, was 0·9205.

3
A total of 131 young of both sexes was observed between the ages of three and nine weeks, and of these 117 were alive at six weeks, and 114 at nine weeks of age. Combining the information for infant and juvenile mortality, some 20 per cent of live-born young, taking both sexes together, died before reaching the age of nine weeks.

4
The adult expectation of further life at the age of nine weeks, based on the life-histories of sixty male and sixty female voles, was for males, 90·3±2·95 weeks, and for females, 97·3 ±4·34 weeks. Taking both sexes together, the average expectation of life at birth was about 1·6 years.

5
. A total of 508 litters, consisting of 1,358 live young, waa recorded, giving a mean number of 2·67 young per litter. The largest number observed in any one litter was eight young. The mean number of young per litter, however, varied according to the age of the mother.

6
The observed span of reproductive life for females lay between the ages of nine and 123 weeks. A female, if she lived throughout this span, might be expected on the average to give birth to a total of 12·9 littens, consisting of 17·0 daughters, if an equal sex-ratio at birth is assumed.

7
. The relation of fertility to the age of the mother was similar to that observed in other mammalian and some insect species, namely an initial risc to a peak, followed by a gradual fall as the age of the mother increased.

8
The intrinsic rate of increase of a population subject to the observed life table and age-specific fertility rates, was estimated to be, r=0·072 per head of population per week. A population increasing at this rate would double in numbers in 9·6 weeks.

9
The longevity and intrinsic rate of increase of the Orkneg vole is compared with that of the mainland vole, M. agrestis. Judging from the available evidence, it appears that in the laboratory the Orkney vole is a longer lived animal, and its intrinsic rate of increase is less than that of agreatis. This difference is due mainly to the fact that the Orkney vole has fewer young per litter, and because the females reach their peak of productivity at a more advanced age.

10
The expenses of this investigation were partly paid for hy a grant froin the Agricultural Research Council.

The longevity and fertility of the Orkney vole, Microtus orcadensis, as observed in the laboratory.

Changes in the relative size of the nucleus in the intervertebral discs of stressed orkney voles (Microtus orcadensis)

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The estimation of population parameters from data obtained by means of the capture-recapture method: iii. an example of the practical applications of the method

The estimation of population parameters from data obtained by means of the capture-recaptutre methodthe maximum likelihood equations for estimating the death-rate

The longevity and fertility of the Orkney vole, Microtus orcadensis, as observed in the laboratory.

Changes in the relative size of the nucleus in the intervertebral discs of stressed orkney voles (Microtus orcadensis)