Exp. Geront. Vol. 14, pp. 189-192. 0531-5565179/0801-0189502.00/0 ~L") Pergamon Press Ltd. 1979. Printed in Great Britain.

ASSOCIATIONS BETWEEN AGE SPECIFIC MORTALITY AND FECUNDITY RATES IN MAMMALS

JAMES T. GIESEL

Department of Zoology, University of Florida, Gainesville, FL 32611, U.S.A.

(Received 21 November 1978)

I N T R O D U C T I O N

RECENTLY David et al. (1975) were able to show that age specific schedules o f fecundi ty and fert i l i ty closely paral le l the curve o f age specific survivorship in Drosophi la melanogaster . Similar paral le ls were found between age specific dea th rates and age specific in terfemale var ia t ions in fecundi ty and fertility. I t was suggested tha t decreases in fecundi ty and ferti l i ty might signal age re la ted decreases in materna l vi tal i ty and tha t increases in the var iabi l i ty o f these pa ramete r s might be reflections o f age related decreases in materna l homeos ta t i c abi l i ty.

I t is the purpose o f this communica t ion to note tha t the basic life his tor ical re la t ions documen ted by Dav id et al. (1975) for Drosophi la app ly as well to small and, to a lesser extent, large mammals , and to suggest that here too changes in reproduct ive funct ions are funct ions of, and may presage, ageing.

M A T E R I A L S

Raw data suitable for the elucidation of age specific reproductive ability of Mus musculus, Sigmodon hispidus, Oryzomys palustris, Peromyscus californicus parasiticus and P. leueopus were provided by George A. Sacher. These data consisted of breeding records for laboratory colonies which were recent descendants of wild caught animals. The animals were raised from wild-caught founder stock or from animals received from other laboratories several generations from the wild. They were maintained in the animal facility at Argonne National Laboratory under conditions that are standard for laboratory mice, including 23°C constant temperature, 40 ~ humidity, 12[12 h light/dark cycle, rodent chow and water ad libitum and housed in the showbox cages, one pair per cage. Data were available for at least twenty females of each species and con- sisted of records of numbers of weaned young produced by females at each of many known ages. Using these data it was possible to compute maternal age specific mean and variation of offspring production. Data were not available on the incidence and age distribution of reproductive failure. Consequently esti- mates of mean and variation of fecundity are probably biased. Lack of "zero" data should raise estimates of mean reproduction and lower estimates of variation, particularly for callow and aged mothers. However, these biases should tend to strengthen the null hypothesis of no change in reproductive parameters with age. As seen below, cognizance of these biases strengthens available conclusions. Data on age specific survivor- ships were obtained from Sacher (1977) and were used to calculate age specific death rates

l(x) -- Itx+t) qx -- k l x

where k stands for time units in days).

R E S U L T S

D a t a compar ing the mate rna l age specific coefficients o f var ia t ion o f offspring p roduc t ion [Cv(~x) ] with age specific dea th rates for the five roden t species are presented in Fig. 1. The curves o f Cv(mx) are clearly re la ted to those o f dea th rate. In four o f the five cases, var ia t ion in reproduct ive funct ional i ty changes at a more rap id rate than dea th rate, the former seeming to presage the latter. The one except ional case (S igmodon hispidus) is a species

189

190 , T A M E S T . G I E S E L

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AGE (X)

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FIG. 1. Curves of death rates ( O ) and coefficients of variation of offspring production [Cv(mx)] ( x ) of several rodent species.

with a very high death rate and the curve of age specific variation in fecundity parallels the curve of age specific death rates.

Survivorship curves and age specific reproductive performances are compared in Fig. 2. In three cases (P. leucopus, P. cali/ornicus and S. hispidus) mean fecundity parallels prob- ability of survival for the entire expanse of life. In the remaining cases the parallelism develops only at advanced ages, following initial increases in age specific reproductive performance. These initial increases are probably not reflections of the ageing process, but the subsequent declines can be interpreted to be.

Data are also available for domestic sheep (Hickey, 1960) and for the That (Caughley, 1966). In the sheep, lambing percentage (i.e. fecundity) first increases and then decreases with maternal age. However, percentage of lambs surviving to weaning actually increases with maternal age. Since nursing is known to constitute a large drain on maternal energy reserves, and we would therefore expect to find decreases in this parameter among aged ewes, these data appear to be at variance with the phenomenon demonstrated by fruit flies and small mammals. Caughley's data on the Thar also indicate a late drop in fecundity.

DISCUSSION

According to David et al. (1975), age specific mean and variability of fecundity and fertility closely parallel and perhaps serve as predictors of age specific mortality in Dr•so-

A S S O C I A T I O N S B E T W E E N AGE S P E C I F I C M O R T A L I T Y A N D F E C U N D I T Y RATES IN M A M M A L S 191

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FIG. 2. Survivorship schedules ( • ) and fecundities ( :.:: ) of several rodent species.

phila melanogaster. The results presented here indicate that small mammals exhibit the same set of phenomena, but that the relationships are far less pronounced among large mammals, although of the same basic form. Perhaps the lower slope of fecundity vs death rate characteristic of the larger animals is due to greater homeostatic ability attendant on their larger body size and lower rates of metabolism (Sachet, 1977) and thus on their increased physiological buffering capacity (McNab, personal communication). Alter- natively, it is known that large mammals have low fecundity, and population biological theory (Giesel, 1976; Sachet and Staffeldt, 1974) suggests that natural selection should

192 JAMES T. Gn~SFL

favor increased effective reproductive life span in such species. In any event, David et al.'s

(1975) observation can be extended to include mammals. The seeming ubiquity of such parallelisms in light of the fact that schedules of death rate are usually Gompertzian (Sacher, 1977) lends weight to the theory that senescence is an ongoing process involving a general decrementation in vitality.

S U M M A R Y

As in Drosophila melanogaster, age specific fecundity and, among females, variation in fecundity, vary in parallel with age specific survivorship and death rate, respectively, in many small mammals. The seeming ubiquity of such parallelisms in the light of the fact that schedules of death rate are usually Gompertzian (Sacher, 1977) lends weight to the theory that senescence is an ongoing process involving a general decrementation in vitality.

Acknowledgement - - 1 am grateful to George A. Sacher for providing the rodent breeding records used in this study.

REFERENCES

CAUGHLEY, B. (1966) Ecology 47, 906. DAVID, J., COHET, Y. and FOUILLET, P. 0975) Exp. Geront. 10, 17. GIESEL, J. T. (1976) Ann. Rev. Ecol. Syst. 7, 57. HICKEY, F. (1960) N.Z. Jl agric. Res. 3, 332. SACHER, G. A. 0977) In: Handbook of the Biology of Aging (Edited by C. FINCH and L. HAYFLICK), p. 582.

Van Nostrand Reinhold, New York. SACHER, G. A. and E. F. STAFFELDT (1974) Am. Nat. 108, 593.