Studies in population physiology. VI. The effect of differentially conditioned flour upon the fecundity and fertility of Tribolium confusum Duval

STUDIES IN POPULATION PHYSIOLOGY

VI. THE EFFECT OF DIFFERENTIALLY CONDITIONED FLOUB UPON THE FECUNDITY AND FERTILITY OF

TRIBOLIUM CONFUSUM DUVAL

THOMAS PARK' Department of Biology, School of Hygiene and Public Health,

Johna Hopbins University

INTRODUOTION

As populations grow and maintain themselves they must of necessity modify their environment. Conversely, it is logi- cal to suppose that their environment must modify them. The biologist is well aware of this reversible relationship in a general, qualitative sort of way but frequently finds it difficult, largely for technical reasons, to get a more precise picture of the process. There has been a heightening of interest in this and closely related problems of late and notable contributions have been made, along with others, by Allee ( '31), Gause ( '34), Prenant ( '34), Pearl ( '30), Chapman ( '33), Elton ( '27), Johnson ( '33), Volterra and D'Ancona ('35), and Nicholson ('33). In recent papers (Park, '34, '35, '36; Stanley, '34), an attempt has been made to approach this problem by analyzing the effects of conditioned flour on the beetle, Tribolium confusum Duval. As Tribolium populations grow, the flour, in which the beetles spend their entire life period, becomes altered through the activity of the forms themselves. By using experimentally

* My best thanks are due to Prof. Raymond Pearl, of the Johns Hopkins University, for reading the manuscript. I am also greatly indebted to Miss Nancy Woollcott for assistance in making the fecundity and fertility counts.

393

FHE JOURNAL OP EXPERIMENTAL 206I&GY, VOL. 73, NO. 3

394 THOMAS PARK

this altered or ‘conditioned’ flour it has been possible to test some of the effects of a biologically modified medium upon the organisms causing that modification. More specifically, the effects of conditioned flour on fecundity, fertility and metamorphosis of Tribolium have been explored. I t was found that the beetles in conditioned flour laid fewer eggs and metamorphosed more slowly and with a higher mortality than did those in fresh flour. The fertility of the eggs, on the other hand, was not significantly altered. In short, conditioning has adverse effects upon certain processes related to productivity and, for this reason, is an important factor in understanding the growth, maintenance and decline of Tri- bolium populations. Since the flour becomes more conditioned as the populations become more concentrated, and, since conditioned flour has inhibitory effects upon fecundity and metamorphosis, it appears obvious that, for this case at least, environmental conditioning is a patent factor in ex- plaining population decline.

In previous investigations only the effects of heavily conditioned flour, i.e., flour taken from densely populated cultures, have been studied. Since conditioning is a gradual process related to the aging and increasing density of the population it would seem of interest to analyze the effects of graded or differentially conditioned flour upon productivity. Such an analysis is of importance in understanding population trends by suggesting how soon conditioning of the medium by the beetles becomes an influential factor in limiting their population growth.

In the present investigation, records of Tribolium fecundity and fertility have been obtained for beetles living in flour in five distinct stages of conditioning. These different environments were prepared by diluting heavily conditioned flour with known amounts of fresh flour and thoroughly mixing the two elements to form a homogeneous medium. The following five series were prepared : All fresh flour (a control) ; three-fourths fresh flour to one-fourth conditioned flour ; one-half fresh flour to one-half conditioned flour ; one-f ourth

STUDIES IN POPULATION PHYSIOLOGY. VI 395

fresh flour to three-fourths conditioned flour, and all-conditioned flour. The conditicned flour used, both in the diluted and in the all-conditioned cultures, was of the type studied in previous investigations and was taken from an old and waning Tribolium population. The fresh flour was ‘ Ceresota,’ an unbleached patent flour.

The experiment was set up so that there were forty bottles for each flour series or a total of 200 bottles for all five series. Each bottle contained 32 gm. of its respective flour and two pairs of Tribolium confusum from the same fresh stock culture. Egg counts were made every 5 days by sifting the infected flour through fine silk bolting cloth which separates the eggs from their medium. After counting, the eggs were isolated into fresh flour and allowed to hatch into larvae so that fertility data could be obtained. The beetles from all of the conditioned environments were returned to their respective flour at each 5-day period. The fresh flour, however, was renewed at the time of each count so that the control series could be kept, within 5-day limits, in an unconditioned state. In order to avoid adverse handling effects the beetles were always removed gently from their flour before it was sifted. The experiment was run in two series of 100 bottles each (twenty bottles for each type of flour) designated in the paper as series ‘A’ and series ‘B.’ Series B was started after A had terminated. Each series was run 30 days or until six 5-day egg counts had been made. All bottles were maintained at a temperature of 28°C. and at a relative humidity of approximately 40 %. An account of the general biology of Tribolium confusum has been published by Park (’34 a) and may be consulted for further information about the beetle.

EFFECT OF DIFFERENTIAL CONDITIONING ON FECUNDITY

It should be emphasized at the start that the egg counts presented here are not sensu strict0 records of absolute fecundity since Tribolium, as pointed out elsewhere (Chap- man, ’28; Park, ’32, ’33), eat their own eggs. This means

396 THOMAS PARK

that all fecundity records obtained from beetle population are records, not of the total number of eggs laid, but of thc number of eggs which escape being eaten. In a previou: experiment, Park ('35) showed that beetles in conditionec flour consumed fewer eggs than did those in fresh flour Thus conditioning, as far as cannibalism is concerned, i s favorable to population growth since it permits more eggs to get through to the larval stage. Despite this effect, however, the fresh flour cultures are still much more productive than the conditioned due to their higher fecundity. For this reason, in analyzing the effects of conditioned flour upon egg production it is possible to ignore the differential influence of cannibalism since the latter selects against the actual results. It might be possible by frequently moving the beetles from one culture to a new one to get a closer approximation of the absolute fecundity rate. From the point of view of the population student, however, such data would not seem to be as interesting as those obtained from natural cultures where oviposition and cannibalism are in their normal relationship. As a matter of actual fact, previous results indicate that egg production rates are so much higher than egg eating rates that the latter effect probably does not appreciably alter the fecundity of small, short-lived, experimental cultures.

The fecundity data are summarized in table 1 where the mean number of eggs produced per bottle for each 5-day period is given for series A, B, and A + B. The data for the combined series are represented graphically in figure 1. I n the first place, it is obvious that the oviposition of the beetles is affected by Conditioning of as short a duration as 5 days. For the 5-day reading of the A + B series the fresh flour bottles are producing significantly more eggs than all of the other groups. Also, the differences between the conditioned series, though not great enough to have statistical reliability, are consistent enough with later figures to show that differential conditioning effects have started during the first 5 days of the experiment.


TABLE 1

Mean number of eggs produced per bottle per &day period b y

mPm OF FLOUE

Conditioned

TYPI OF FMUB

Conditioned

8

El

teries A, B, and A + B

7 20 DAY8

[OD - A + B

4 1 . l f 0.69

- -

24.6-C 0.58

25 .42 , 0.55

,20.82 ' 0.50

16.32 055

398 THOMAS PARK

As the experiment progresses there is a gradual yet signifi- cant lowering of the rate of oviposition for all five flour series. This same type of effect has been previously reported (Park, ’34) and it is thought to be due primarily to aging by the beetles although it is quite conceivable that handling effects,

Zme h ~ Q Y S Fig.1 Mean egg production for all five flour seriea at 5-day intervale for

30 days.

even though minimized as much as possible, may be a con- tributing cause. The fecundity differences between the conditioned groups become more marked with time. At the 5-day period none of the means for the A + B series differ significantly. At the 30-day period all of the means differ significantly from each other with the exception of the three-fourths fresh : one-fourth conditioned and the one-half fresh : one-half conditioned environments which are not significantly different.


Since all the series maintain about the same relative relationship to each other for the entire 30-day period the3 may be adequately compared by contrasting the five means of the A + B series based on the total experimental run. Such a comparison has the advantage of allowing one figure (the mean of the total) to represent each flour series instead of six figures, and of increasing the number of cases from forty to 240. The last column of table 1 presents these data, and, in table 2, the relation of the difference between the various means to the probable error of that difference is summarized. It is striking to note the dissimilarity in fecundity between

TABLE 2

Statistical comparisons between means baaed on the total period ( A + B series)

DIFP'IEDNClE

DIFPZ-NOE THE COYPAUXBON I P.E.

Fresh with 8 fresh: 4 conditioned Fresh with 3 fresh: f conditioned Fresh with 4 fresh: 9 conditioned Fresh with all-conditioned 9 fresh: 4 conditioned with 4 fresh: f conditioned 9 fresh: 3 conditioned with 3 fresh: 8 conditioned 8 fresh : 3 conditioned with all-conditioned 4 fresh : f conditioned with f fresh : 3 conditioned f fresh: f conditioned with all-conditioned 4 fresh: 4 conditioned with all-conditioned

18.3 XP.E. 19.6XP.E. 25.3 XP.E. 31.0XP.E. 1.1XP.E. 5.4XP.E.

11.8 X P.E. 6.5 X P.E.

13.0 XP.E. 6.4 XP.E.

DIFPFEBENOB IN PAVOE or

Fresh Fresh Fresh Fresh 3 fresh: 3 conditioned 9 fresh: 3 conditioned 8 fresh: 8 conditioned f fresh: 3 conditioned f fresh : 3 conditioned 3 fresh: 9 conditioned

the fresh and any or all of the conditioned cultures. The former is producing consistently from two to three times more eggs than the latter and this difference in fecundity has high statistical reliability. There is considerably more difference between the fresh and the most lightly conditioned culture than there is between all four sets of conditioned series. This immediately raises the interesting point that environmental conditioning, whether it be quantitatively drastic o r moderate, is an important factor in lowering egg production. It is obvious from these data that experiments, testing the effects of very slight conditioning ranging between the fresh and the three-fourths fresh : one-fourth conditioned media, are indicated, and such investigations are underway.

400 THOMAS PARK

The question may be asked: Is there a direct and pro- portionate relationship between the amount of conditioning and the fecundity of the beetles? I n other words, does egg production fall o,ff regularly as the medium becomes more conditioned? The data show that this is the case for certain of the flour series but not for others. For example, the two least conditioned environments, the three-fourths fresh : one- fourth conditioned and the one-half fresh : one-half conditioned, are not differentiated significantly from each other in their egg production. On the average, there is a slight tendency for the latter to be more productive than the former for the entire 30-day period but this difference is not a sig- nificant one. These data suggest that Tribolium fecundity is not highly sensitive to changes in conditioning in this particular central range. On the other hand, the one-fourth fresh : three-fourths conditioned and the all-conditioned flours have had a differential effect upon the beetles’ oviposition. The Tribolium in the former environment produced significantly more eggs than those in the all-conditioned medium but less than the beetles in the one-half fresh: one-half conditioned or in the three-fourths fresh : one-fourth conditioned series. The conclusion to be drawn from these results seems to be that, with the exception of the two least conditioned series which are not unlike in their egg production, there is a grading effect in fecundity roughly comparable to the grading effect in conditioning. This effect, however, is not a symmetrically proportional one between the flour series since egg production diminishes unequally with conditioning.

A word about the variability of the data might be instruc- tive. The dispersal constants, the standard deviation and the coefficients of variation, were calculated for the A, B, and A + B series for every 5-day period. These statistics suggest the following points: First, the variation in egg production, both absolute and relative, is greater during the early stages of the experiment than during the later stages ; second, the beetles in the more conditioned environments had a greater variation in fecundity than those in the less conditioned and fresh environments, and, third, the two series

STUDIES IN POPULATION PHYSIOLOGY. 401

A and B, which were not run simultaneously, had about the same amount of variation in egg production although series A produced consistently more eggs than did series B. This last point is brought out graphically in figure 2 where the mean total fecundity rate for each of the five flour groups is plotted for series A, B, and A + B.

so -

A

B MeQR orA+B

I I I I

f / o w Serres Fresh V4F A% *c c

Fig. 2 Mean of the total number of eggs produced by series A, B, and A + B, for the entire 30-day period.

EFFECT OF DIFFERENTIAL CONDITIONING UPON FERTILITY

The influence of conditioned flour on fertility has never been as easy to interpret as the effect of such flour on fecundity. In general, it has not been possible to show that egg fertility is definitely affected by conditioning. In other words, if an egg is once laid it has just about as good a chance of hatching into a larva if the beetle lived in some type of conditioned flour as it has if the beetle was raised in fresh flour. Pearl ('32) has reached a similar conclusion for Drosophila finding that the most important influence of increasing population density upon the productivity of the flies is a lowering of their fecundity.

402 THOMAS PARK

30DAYS

% 65.4 56.0 61.9 72.5 69.8 65.1

In the present experiments eggs were always placed, after each count, into fresh flour. The eggs were isolated so that fertility records could be obtained on the A, B, and A + B series for each type of medium and every 5-day period. The fertility percentages (the percentage of eggs hatching out of the total number laid) are recorded in table 3. Since the fertility data were essentially similar for both series A and B the two were combined. The mean fertility percentage for the entire experiment was 70.3 % which compares quite closely with the 71.4 % and the 65.6 % obtained in an earlier investigation (Park, '35). If the actual number of eggs developing into larvae for the entire experiment is contrasted with a calculated number (obtained by taking 70.3 % of the

XEANS

% 68.5 68.1 72.4 75.8 66.7 70.3

TABLE 3

Percentage 01

Fresh 3 fresh : 3 conditioned 3 f reah : + conditioned 3 fresh: 2 conditioned Conditioned Means

egg8 deueloping into larvae (fertility pt

62.5

number of eggs produced for each experimental condition) by the x2 test the difference is large enough to have statistical significance. This means that the egg fertility is deviating from a theoretically expected value more than can be antici- pated on a chance basis. For this reason, caution must be observed in drawing conclusions as to the effect of conditioning on fertility. However, since there are no very defhite trends discernible in the fertility data it seems safe to con- clude within the limits of the present experiments that, at the most, egg fertility is but slightly affected by flour conditioning.

DISCUSSION AND CONCLUSIONS

Fecundity and fertility, though in themselves physiological processes, are admittedly very important aspects of popula-


tion phenomena since they constitute the factor of natality which is measured by the birth rate. If the fecundity and fertility of its components are high, the population, providing the death rate is not as great as the birth rate, increases in size. Conversely, if the fecundity and fertility are lower than the death rate the population declines. In previous work it has been shown that the fecundity of the flour beetle, Tribolium confusum, is much lower in heavily conditioned flour than it is in fresh flour (unconditioned flour). The beetles condition or alter their medium by simply living in it. Accordingly, as the populations increase in size the flour becomes more conditioned. Conditioning then is really an ex- pression of population density and, providing the flour medium is not changed or renewed, is a normal and inevitable result of population growth. The present experiments have emphasized the relation of fecundity and fertility to flour in intermediate stages of conditioning. The question behind this work has been, is the conditioning effect previously described confined to old and decadent cultures in which the flour is highly conditioned, or, do younger populations also condition their environment to a great enough degree to decrease their fecundity? The data showed that a flour medium composed of 25 % conditioned flour was sufficiently altered to lower oviposition significantly below the level of a fresh flour control. No appreciable difference in fecundity could be detected between an environment composed of 25 "/o conditioned flour and one composed of 50 % conditioned flour. Tribolium living in a medium of 75 % conditioned flour, however, produced significantly fewer eggs than did the beetles in the fresh and more lightly conditioned environments. Beetles living in the all or most heavily conditioned flour had the lowest rate of egg production. No distinct effects of conditioning on fertility could be demonstrated.

These data suggest that self-conditioning of the environment by the beetles is a factor which contributes to the decline of their populations considerably before the culture has reached a decadent state.

404 THOMAS PARK

LITERATURE CITED

A m , W. C. 1931 Animal aggregations. A study in general sociology, 431 pp.

CHAPMAN, R. N. 1928 Quantitative analysis of environmental factors. Ecology,

1933 The causes of fluctuations of populations of insects. Proc. Ham. Ent. Soc., vol. 8, pp. 279-292.

ELTON, CHARLES 1927 Animal ecology, 207 pp. Macmillan Company, New Pork.

GAUSE, G. F. 1934 The struggle for existence, 163 pp, Williams and Wilkina Company, Baltimore.

JOHNSON, W. H. 1933 Effects of population density on the rate of repro- duction in Oxytricha.

NICHOLSON, A. J. 1933 The balance of animal populations. J. Anim. Ecology, vol. 2, pp. 132-178 (supplement).

PARK, THOMAS 1932 Studies in population physiology. The relation of numbers to initial population growth in the flour beetle Tribolium confusum Duval. Ecology, vol. 13, pp. 172-181. 1933 Studies in population physiology. 11. Factors regulating

initial growth of Tribolium confusum populations. J. Exp. Zool., vol. 65, pp. 17-42. 1934 Studies in population phyaiology. 111. The effect of condi-

tioned flour upon the productivity and population decline of Tribolium confusum. Ibid., vol. 68, pp. 167-182. 1934a Observations on the general biology of the flour beetle,

Tribolium confusum. Quart. Rev. Biol., vol. 9, pp. 36-54. 1935 Studies in population physiology. IV. Some physiological

effects of conditioned flour upon Tnbolium confusum Duval and its populations. Physiol. Zool., vol. 8, pp. 91-115. 1936 Studies in population physiology. V. The oxygen consump-

tion of the flour beetle, Tribolium confuaum Duval. J. Cell. and Comp. Physiol., vol. 7, pp. 313-323.

PEARL, RAYMOND 1930 The biology of population growth, 260 pp. Knopf, New Pork.

The influence of density of population upon egg production in Drosophila melanogaster.

PRENANT, MARCEL 1934 Adaptation kologie et biocoenotique. Actualit& Sci- entiiiques et Industrielles, no. 103, 59 pp. Hermann & Cie, Pans.

STANLEY, JOHN 1934 A mathematical theory of the growth of populations of the flour beetle, Tribolium confusum, Dud. 111. The effect upon the early stages of population growth of changes in the nutritive value, palatability and density of packing of the flour medium. Canad. J. Res., vol. 11, pp. 728-732.

VOL-, VITO AND UMBERTO D 'ANCONA 1935 Lee associations biologiques au point de vue math6matique. Actualit& Scientifiques et Industrielles, no. 243, 96 pp. Hermann & Cie, Paris.

The University of Chicago Press, Chicago.

vol. 9, pp. 111-122.

Physiol. ZoSl., vol. 6, pp. 22-54.

1932 J. Exp. Zool., vol. 63, pp. 57-84.

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Studies in population physiology. VI. The effect of differentially conditioned flour upon the fecundity and fertility of Tribolium confusum Duval