What Factors Affect the Size and Composition of Monarch Spermatophores?
(See also Oberhauser 1988 and 1992)
Abstract | Background |
Methods | Results | Discussion
| References | Karen's Research
Questions
Abstract
During mating, male monarch butterflies transfer sperm and other substances packaged
in a gelatinous body called a spermatophore. This spermatophore represents a significant
material investment by males. I studied how male size, age and mating history affect
the size and composition of monarch spermatophores. Males transferred about 5-10%
of their body mass during mating. Spermatophores increased in size with male age,
but just after a mating, males produced very small spermatophores. Spermatophores
are composed mainly of water and protein, with protein comprising about 21% and
65% of their wet and dry mass, respectively.
Background
Male monarchs, like all Lepidoptera, produce spermatophores during mating. These
spermatophores consist of a sperm sac embedded into a gelatinous body formed from
male accessory gland sections. Research by Boggs and Gilbert (1979) demonstrated
that lepidopteran spermatophores contain nutrients used by females in egg production,
and my early research focused on quantifying the investment that male monarchs
made in spermatophores.
Spermatophore content is also important. Previous research (Marshall 1982) has shown
that lepidopteran spermatophores are made up of water, protein, and small amounts
of other molecules such as fats and sterols (cholesterol is an example of a kind
of sterol, although it has not been found in monarch spermatophores). The component
that is most likely to affect both male and female fitness is protein, since adult
monarchs obtain very little protein in their diet. The only source of protein for
males is the food they eat as larvae, but females can use the protein that comes
in spermatophores. This may increase the number of eggs they can lay.
Methods
Unmated females were put into outdoor screen cages (2m x 2m x 2m) and allowed to
mate with males of different ages, sizes, and mating histories. Soon after pairs
separated (just after daybreak), I removed some mated females from the mating cage
and dissected them in insect saline, a solution similar in composition to the intracellular
fluids in insects. I removed spermatophores from the bursa copulatrix, blotted them
on tissue paper, and weighed them to the nearest 0.01 mg on an analytical balance.
I then froze the spermatophores for later analysis of water and nitrogen content.
I obtained water content by placing spermatophores in a 65°C drying oven overnight,
removing them to a desiccator, and allowing them to come to room temperature. Dried
spermatophores were then weighed to the nearest 0.01 mg, and their dry mass compared
to their wet mass to measure water content. I used a technique called a micro-Kjeldahl
analysis to determine nitrogen content.
Results
Spermatophore Mass
Age was the most important predictor of spermatophore mass from males that had never
mated before (figure 1). Older males transferred larger spermatophores,
and spermatophore size continued to increase as males get older. The time that had
elapsed since their last mating was most important for males that had mated previously
(figure 1). Males that had just mated transferred very small
spermatophores, and size increased as males waited longer between matings. Other
factors that had significant effects on spermatophore size were mass at eclosion
for unmated males, and both mass at eclosion and the number of previous matings
for males that had already mated. Table 1 shows regression equations
that illustrate the effects of all of these factors; larger males tended to transfer
larger spermatophores, and males that had mated more times in the past transferred
smaller spermatophores.
Figure 1. Spermatophore masses plotted against their most important predictors.
For unmated males, the most important predictor of spermatophore mass was the age
of the male. For previously-mated males, the most important predictor was the time
that had elapsed since the last mating of the male.
Table 1a: Predictors of Spermatophore Mass - Unmated Males
|
Variable |
Coefficient |
P |
|
Constant |
-5.47 |
0.420 |
|
Age |
1.86 |
0.000
|
|
Mass at Eclosion |
0.048
|
0.001
|
|
N = 67, R2 = 0.722, P < 0.001 |
Table 1b: Predictors of Spermatophore Mass - Previously-Mated Males
|
Variable |
Coefficient |
P |
|
Constant |
-4.87 |
0.339 |
|
Log of Time Since Last Mating |
26.8 |
0.000 |
|
Times Mated |
-1.48 |
0.000 |
|
Mass at Eclosion |
0.032 |
0.000 |
|
N = 62, R2 = 0.858, P < 0.001 |
Spermatophore Water and Nitrogen Content
I measured the proportion of water and nitrogen in spermatophores transferred by
all five male groups. By looking up the approximate proportion of nitrogen in the
common amino acids in spermatophores, I could estimate how much protein is in spermatophores.
Figure 2 and Table 2 show the average proportions
of water, nitrogen and protein. Spermatophores are composed mainly of water (like
most components of living organisms), and most of their dry mass is protein. Spermatophores
from males that had mated one day previously had a higher proportion of water and
a lower proportion of nitrogen.
Table 2: Spermatophore Components
|
I measured:
|
I estimated:
|
|
In Most Spermatophores: |
|
|
70% water |
|
|
4.4% nitrogen (wet weight)* |
21% protein wet weight |
|
13.7% nitrogen (dry weight)* |
65% protein dry weight |
|
In Spermatophores From Males Mated One Day Previously: |
|
78% water |
|
|
2.5% nitrogen (wet weight)* |
12% protein wet weight |
|
12.8% nitrogen (dry weight)* |
61% protein dry weight |
*When biologists measure the components of substances, they often consider both
wet weight and dry weight. This is because most living tissue is composed mostly
of water. So we weigh a substance with the water in it, then dry it in a drying
oven to remove all of the water and reweigh it.
Figure 2. Proportions of water, protein and other components in spermatophores.
These proportions vary slightly for males that mated one day previously (see text).
Discussion
Two conclusions can be drawn from the data on spermatophore mass. First, male monarchs
appear to transfer all or almost all of the accessory gland material that is available
when they mate. Spermatophore mass from virgin males increased with male age, suggesting
that as more accessory gland material was produced, it was transferred to females
and not saved for future matings. In addition, the small size of spermatophores
from males that had just mated suggests that there was very little accessory gland
material available one day after mating. Second, male monarchs make a nontrivial
material investment in mating, producing spermatophores than can represent up to
10% of their body mass. It took four days after one mating until they were able
to produce spermatophores equal in mass to those produced by unmated males. A similar
pattern has been reported in other Lepidoptera (Rutowski 1984, Svärd 1985, Rutowski
and Gilchrist 1986, Svärd and Wiklund 1986).
Spermatophores transferred one day after a previous mating contain more water and
less nitrogen than other spermatophores. This suggests that water and non-protein
components are more readily available than protein for use in making spermatophores
right after a male has depleted his accessory glands by mating.
These results clarify possible benefits both sexes gain from large spermatophores,
and much of my subsequent research has been focused on quantifying these benefits.
I have shown that females that receive larger spermatophores wait longer to remate
(see Mating Frequencies in Male and Female Monarchs). Thus
males that transfer larger spermatophores will gain more offspring from a mating
because they will fertilize the female’s eggs over a longer time period. Large
spermatophores will provide the female with more nutrients, especially protein,
that can be used to produce more eggs (see What factors affect
the number of eggs that female monarchs lay?).
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References
Boggs, C. L. and L. E. Gilbert. 1979. Male contribution to egg production in butterflies:
evidence for transfer of nutrients at mating. Science 206:83-84.
Marshall L.D. 1982. Male nutrient investment in the Lepidoptera: what nutrients
should males invest? Amer Nat 120:273-279.
Oberhauser, K. S. 1988. Male monarch butterfly spermatophore mass and mating strategies.
Anim. Behav. 36:1384-1388.
Oberhauser, K. S. 1992. Rate of ejaculate breakdown and intermating intervals in
monarch butterflies. Behav. Ecol. and Sociobiol. 31:367-373.
Rutowski R.L. 1984. Production and use of secretions passed by males at copulation
in Pieris protodice (Lepidoptera, Pieridae). Psyche 91:141-152.
Rutowski R.L. and G.W. Gilchrist. 1986. Copulation in Colias eurytheme (Lepidoptera:
Pieridae): patterns and frequency. J Zool 207:115-124.
Svärd, L. 1985. Paternal investment in a monandrous butterfly, Pararge aegeria.
Oikos 45: 66-70.
Svärd, L. and C. Wiklund. 1986. Different ejaculate delivery strategies in first
versus subsequent matings in the swallowtail butterfly Papilio machaon L.
Behav. Ecol. Sociobiol. 18: 325-330.
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