Research Project

How Big are Monarch Eggs?

(see also Oberhauser 1997)

Abstract

As part of a study of monarch fecundity and lifespan in monarch butterflies (Oberhauser 1997, What factors affect the number of eggs that female monarchs lay?), I measured the mass of eggs laid by females throughout their lives. Females in this study received varying amounts of spermatophore material from males. The mass of individual eggs decreased over the female lifespan, and was positively correlated with female size. The total mass of eggs laid by females over their lives will be a function of both the number and size of eggs laid. The amount of nutrients that females received during mating from males appeared to affect the number of eggs laid, but not their mass. These results suggest that females utilize nutrients from different sources differently in egg production; nutrients received during the larval period affect egg size but not egg number, while nutrients received from males affect egg number but not egg size.

Background

Reproductive success is determined not only by the quantity of offspring that individuals produce, but also the quality of these offspring. Producing large numbers of offspring that do not survive to reproduce themselves would not result in an individual’s genes being passed on to the next generation. Many biologists assume that there is a tradeoff (for a discussion of tradeoffs, see Overview of Karen's research) between the size of offspring produced and the fitness of these offspring. Given the fact that organisms have limited amounts of resources to allocate to reproduction and survival, it seems apparent that this tradeoff must exist. However, Christer Wiklund and his colleagues in Sweden have looked for a correlation between offspring fitness and egg size, and have not been able to demonstrate one (Wiklund & Persson 1983, Karlsson & Wiklund 1984, 1985). It is possible that the method of reproduction in butterflies, in which females deposit eggs on a host plant and the offspring fend for themselves, makes such a relationship unlikely. Newly-hatched larvae are exposed to many kinds of mortality that may not be affected much by variation in size, at least within the ranges of egg sizes that females can produce. Thus, female butterflies may maximize their reproductive success by laying as many eggs as possible, as long as these eggs are above a certain minimum size.

Previous studies of lepidopteran egg mass have looked for effects of three general factors. First, Christer Wiklund and his collaborators in Sweden have studied many species to see if larger butterflies tend to lay larger eggs. They have found that egg size does increase with body size across several satyrid species (Wiklund & Karlsson 1984 and Wiklund et al. 1987). Second, many researchers have looked to see if larger females lay larger eggs than smaller females of the same species. They have not found a clear relationship between female size and egg size. Jones et al. (1982) found a negative correlation in the cabbage white, Boggs (1986) a positive correlation in Mormon fritillaries, and Wiklund & Karlsson (1984) no correlation in 10 satyrid butterflies. This suggests that while small butterfly species tend to lay small eggs and large butterfly species lay large eggs, this relationship does not usually hold within species (Wiklund & Karlsson 1984). Third, researchers have weighed eggs over females’ lives to see if egg mass changes with female age. In all of these studies, egg mass decreased with time (Jones et al. 1982; Murphy et al. 1983; Wiklund & Persson 1983; Karlsson & Wiklund 1984, 1985; Wiklund & Karlsson 1984; Boggs 1986; Svärd & Wiklund 1988).

Across many groups of animals, offspring size varies with female body mass (Peters 1986). This relationship does not have to be a result of natural selection, it is just an example of a common allometric relationship (a relationship between body size and some other characteristic of a species). Wiklund et al. (1987) argue that when fecundity is limited by the number of eggs females can actually lay, and not by the number they can produce, there will be nonadaptive scaling of egg size to body size. This means that larger species will tend to produce larger eggs, not because larger eggs are better, but just because this kind of relationship tends to exist in all groups of species. However, when fecundity is limited by egg production, there should be selection on females to produce the smallest eggs possible, within limits posed by viability constraints. Since female monarchs do not seem to live long enough to lay all of the eggs they do produce (see Oberhauser 1997 and What factors affect the number of eggs that female monarchs lay?), a relationship between egg size and female body size in monarchs could support Wiklund’s argument, and this is one reason that I measured monarch egg mass.

A knowledge of the mass of the eggs that females produce, in addition to just the number of eggs, is also important in determining the total investment that females make in their offspring. I tested how this investment varies when females receive varying amounts of nutrients from males.

Methods

For details on the rearing of experimental butterflies, see What factors affect the number of eggs that female monarchs lay?. Briefly, the day after eclosion, I weighed 47 females to the nearest 0.01 mg, and measured their forewings to the nearest 0.1 mm. Female mass and mean forewing length ranged from 288 to 624 mg and 43 to 55 mm, respectively, reflecting a wide range of sizes. Butterflies were kept in glassine envelopes and fed a 20% honey solution ad libitum every other day until they were ready to mate.

I used five different mating treatments in which the amount of spermatophore nutrients females received varied (see What factors affect the size and composition of monarch spermatophores?). Females mated with a) two unmated males, b) an unmated male followed by a mated male, c) a mated male followed by an unmated male, d) two mated males, or e) one mated male. Every day, I fed females and counted the number of eggs that they laid. Ten eggs from each female, in batches of five, were weighed to the nearest 0.01 mg every day of laying.

Results

Egg mass

The mass of a single monarch egg ranged from 0.242 mg to 0.588 mg, with a mean mass of 0.460 mg. This mean represents approximately 1/1000 of the mass of a female.

Several factors affected egg mass. The most important factor was female age, with older females tending to lay lighter eggs. Figure 1 shows the relationship between egg mass and the number of days that had elapsed since females started laying eggs. Egg mass decreased over the period of egg laying. The shape of the plot of egg mass on day of egg laying is concave, and using the log of time as a predictor significantly improved the regression over using untransformed data. When time is controlled, there is an effect of female mass on egg mass; larger females laid larger eggs. In addition, females in treatments 'b' and 'e' laid smaller eggs than other females (table 1a).

Table 1. Predictors of egg mass.
Predictor	Coefficient (s.e.)	Student's T	P
1a. Individual Egg Mass
Constant	0.620 (0.221)	2.80	0.0052
Log TSM	-0.280 (0.024)	-11.67	0.000
Female size	0.036 (0.004)	8.44	0.000
Trt 'e'	-0.0176 (0.005)	-3.53	0.000
Trt 'b'	-0.0161 (0.004)	-4.13	0.000
N = 614, Adj. R² = 0.231 (no effect of interactions between treatments and TSM, or first spermatophore size)
2b. Total Egg Mass Laid by Females
Predictor	Coefficient (s.e.)	Student's T	P
Constant	-84.12 (107.3)	-0.78	0.438
Large First Spermatophore	50.24 (24.07)	2.09	0.043
Female mass at eclosion	0.5313 (0.2145)	2.48	0.018
Egg-laying lifespan	8.471 (1.834)	4.62	0.000
N = 44, Adj. R² = 0.0.500

Total reproductive effort

I measured females’ reproductive output in two ways. First, I counted the total number of eggs each female laid. Next, I calculated the total egg mass produced by each female by multiplying the average egg mass by the number of eggs laid for each day, and summing this over the female’s whole egg-laying lifespan. Table 2 summarizes means for both of these measures in each mating treatment.

Table 2. Treatment means for lifetime fecundity and total egg mass
Treatment	N	Mean fecundity	Total egg mass
a. large-large	11	711^ab	321^ab
b. large-small	9	936^a	413^a
c. small-large	12	596^b	275^b
d. small-small	9	700^ab	317^ab
e. small	6	664^ab	302^b
TOTAL	47	715	323

Means followed by the same letter are not significantly different at the 0.05 level of confidence (Tukey LSD comparisons).

The total number of eggs produced by females ranged from 290-1179, with an overall mean of 715. There was a treatment effect on total fecundity, with females that received a large first spermatophore tending to lay more eggs (table 2, see also What factors affect the number of eggs that female monarchs lay?). The total egg mass laid by females ranged from 129 mg to 510 mg, with an overall mean of 323 mg. Again, females that received a larger first spermatophore tended to lay a great total mass of eggs (table 2). In addition, female mass at eclosion and the total amount of time over which she laid eggs affected total egg mass. Females that laid eggs over a longer period of time, and those that weighed more at eclosion tended to lay a great total mass of eggs (table 1b; figure 2 illustrates the effect of female mass alone on total egg mass).

Discussion

Even though individual eggs are small (weighing about 1/1000 as much as the female herself), female monarchs invest a large proportion of their mass in eggs. If we divide the total egg mass produced by each female by her own body mass, values range from 0.302 to 1.14, with a mean of 0.702. This means that females can lay more than their own mass in eggs, with the average female laying 70% of her mass in eggs. This is equivalent to a 120 pound human female having 12 seven pound babies over the course of her life!

Female monarch butterflies lay smaller eggs as they age (figure 1 and table 1a), and larger females tend to lay larger eggs (table 1a), and a greater total mass of eggs (figure 2). While the amount of nutrients that females receive from males affects their total egg mass, it does not appear to affect the mass of individual eggs. These results have implications in our understanding of how females utilize available nutrients in egg production. Since larger females tend to lay larger eggs, it appears that nutrients that females obtain as larvae affect egg mass. This effect is interesting, because female mass does not affect the total number of eggs that females lay when the amount of time over which females lay eggs is controlled (Oberhauser 1997 and What factors affect the number of eggs that female monarchs lay?). Nutrients that females receive from males do not affect egg mass in a predictable way; there were statistically significant negative effects of two mating treatments, but these treatments varied a great deal in the amount of spermatophore material received. In addition, as egg mass dropped most rapidly (days three to seven of egg laying, figure 1), most females were still breaking down spermatophores (see What factors affect the size and composition of monarch spermatophores?), and thus still receiving these nutrients.

Wiklund et al (1987) suggested that when fecundity is limited by the number of eggs females can actually lay instead of by the number of eggs they can produce, there should be a scaling of egg size to body size. When fecundity is limited by the number of eggs that females can produce, females should manufacture the smallest eggs possible in order to maximize their fecundity. I have suggested elsewhere that monarch fecundity is limited by a female’s egg-laying lifespan, and not the number of eggs she can produce (Oberhauser 1997 and What factors affect the number of eggs that female monarchs lay?). Thus, my results appear to support the hypothesis of Wiklund et al. Larger female monarchs lay larger eggs, rather than all females producing eggs of some minimum viable size.

Acknowledgements

I thank De Cansler, Ann Feitl, Rachel Hampton, Brenda Jenson and Christine Jessup for all of their help counting and weighing 30,000 eggs. This study was funded by the National Science Foundation (DEB-9220829 and DEB-9442165).

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References

Boggs, C. L. (1986) Reproductive strategies of female butterflies: variation in and constraints on fecundity. Ecological Entomology 11, 7-15.

Jones, R.E., Hart, J.R. & Bull, G.D. (1982) Temperature, size and egg production in the Cabbage Butterfly (Pieris rapae L.). Australian Journal of Zoology 30, 223-232.

Karlsson, B. & Wiklund, C. (1984) Egg weight variation and lack of correlation between egg weight and offspring fitness in the wall brown butterfly Lasiommata megera. Oikos 43, 376-385.

Karlsson, B. & Wiklund, C. (1985) Egg weight variation in relation to egg mortality and starvation endurance of newly hatched larvae in some satyrid butterflies. Ecological Entomology 10, 205-211.

Murphy, D.D., Launer, A.E. & Ehrlich, P.R. (1983) The role of adult feeding in egg production and population dynamics of the checkerspot butterfly Euphydryas editha. Oecologia, 56, 257-263.

Oberhauser, K. S. 1997. Fecundity and egg mass of monarch butterflies: effects of age, female size and mating history. Functional Ecology 11(2): 166-175.

Peters, R.H. 1986. The ecological implications of body size. Cambridge University Press.

Svärd, L. & Wiklund, C. (1988) Fecundity, egg weight, and longevity in relation to multiple matings in females of the monarch butterfly. Behavioral Ecology and Sociobiology 23, 39-43.

Wiklund, C. & Karlsson, B. (1984) Egg size variation in satyrid butterflies: adaptive vs historical "Bauplan", and mechanistic explanations. Oikos 43, 391-400.

Wiklund, C., Karlsson, B., & Forsberg, J. (1987) Adaptive versus constraint explanations for egg-to-body size relationships in two butterfly families. American Naturalist 130, 828-838.

Wiklund, C. & Persson, A. (1983) Fecundity, and the relation of egg weight variation to offspring fitness in the speckled wood butterfly Pararge aegeria, or why don't female butterflies lay more eggs? Oikos 40, 53-63.

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