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

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

A general formula for the sensitivity of population growth rate to changes in life history parameters.

The deermouse, Peromyscus maniculatus (Wagner), of North America has been studied frequently, but there has been little attempt to determine the mechanisms of population control of the species. Both Blair (1948) and Howard (1949) assumed that predation was the most important factor, although Howard considered that cold weather combined with food shortage was also a possible mechanism. McCabe & Blanchard (1950) and Brant (1962) documented predator raids on their study areas and believed that P. maniculatus populations can be decimated or eliminated by predators. However the same authors present data which show that in geographically diverse areas of the United States there is a fairly regular annual cycle in numbers of deermice (Blair 1940, p. 279; Howard 1949, p. 33; McCabe & Blanchard 1950, p. 108; Brant 1962, p. 162). In these studies numbers were at their lowest in spring or early summer. They increased only slightly until autumn when there was a sudden rise to a winter high. After this there is a gradual decrease to the spring low. This annual cycle suggests that there is some mechanism other than predation which affects the numbers of deermice. Since the pattern of annual changes occurs in areas with different predators, it seems unlikely that predation can be a common factor, although it is theoretically possible for different predators to cause similar levels of predation pressure. McCabe & Blanchard (1950, p. 127) have suggested that some intrinsic intraspecific mechanism is important in the population regulation of Peromyscus. Recent work (Eisenberg 1962) on the behaviour of the species has shown that P. maniculatus has a wide behavioural repertoire which includes a considerable amount of agonistic activity. Seasonal changes similar to those in Peromyscus populations have been described by Chitty (1955) for Microtis agrestis and he suggested that the severe juvenile mortality in the early breeding season was an effect of adult hostility. The present paper describes an investigation into the effects of agonistic behaviour on population phenomena in Peromyscus maniculatus.

The Relationship between Agonistic Behaviour and Population Changes in the Deermouse, Peromyscus maniculatus (Wagner)

The intrinsic rate of natural increase, rm, was calculated for 44 mammalian species using the Cole (1954) equation and life history data from the literature. Values of rm so calculated were plotted as log10rm versus log10 body mass revealing a linear relationship with a slope of-0.2622. The equation of the regression line fitting these data was then used to correct rm for body mass so that interspecific comparisons with respect to rm and basal metabolic rate could be made to determine if a positive relationship exists between these two parameters. Basal metabolic rate correlates positively with rm, and apparently is one of many factors operating in the evolution of rm. Implications of these conclusions with respect to food habits, resource limitations, and the possible existence of a trade-off between maintenance and reproduction in certain environments is discussed.

Relationship among body mass, metabolic rate and the intrinsic rate of natural increase in mammals.

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.

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