Fellow:
Erin Heydenreich
Department
of Biology
Mate location in moths is accomplished by long
distance detection of low concentrations of pheromones emitted by females upon
emergence. Pheromone based mating
systems have been suggested to be under strong sexual selection (Phelan,
1997). There are several aspects of moth
biology that suggest that larger body size and larger antennae will be selected
for in males. In this study I predicted
that polyphemus (Antheraea plyphemus:
Family Saturniidae) males able to successfully locate
females would have proportionately larger antennae than males sampled from the population
at random. Captive, reared males represented the base population and males
caught with captive females represented the “successful”, or sexually selected,
population. Wild males were captured
using female-baited traps in Mclay Flats of the Blue
Mountain Recreation Area, the Rattlesnake Recreation Area, and Rock Creek, in
Key phrases: sexual selection, antenna
morphology, sex pheromones, moth mating systems, and saturniidae.
Fellow:
Erin Heydenreich
Department
of Biology
Mate location in moths is accomplished by long distance
detection of low concentrations of pheromones emitted by females upon
emergence. Pheromone based mating
systems have been suggested to be under strong sexual selection (Phelan, 1997). There are several aspects of moth biology
that suggest that larger body size and larger antennae will be selected for in
males. In this study I predicted that polyphemus (Antheraea plyphemus: Family Saturniidae)
males able to successfully locate females would have proportionately larger
antennae than males sampled from the population at random. Captive, reared
males represented the base population and males caught with captive females
represented the “successful”, or sexually selected, population. Wild males were captured using female-baited
traps in Mclay Flats of the Blue Mountain Recreation
Area, the Rattlesnake Recreation Area, and Rock Creek, in
Key phrases: sexual selection, antenna
morphology, sex pheromones, moth mating systems, and saturniidae.
Most moth
species use pheromone based mating systems.
Females remain stationary upon emergence and attract males by the use of
chemical pheromones (Phelan, 1997).
Males are equipped with large plumed antennae that are highly sensitive
to very low levels of pheromone (females generally emit picogram
to nanogram concentrations of pheromones) and have
been known to fly long distances to locate females (Hogue, 1993; Phelan,1997).
Most moths
are sexually dimorphic for antennae size and morphology: males generally have
longer and wider antennae then females, and many more chemoreceptors. The intensity of selection on male ability to
find mates is reflected by these strong sexual dimorphisms in the number and
sensitivity of chemoreceptors. For example, in one
species of saturniids, male antennae contain over
64,000 sensory hairs (females have less than half that), 80 percent of these
are specific for sex pheromone detection (Evans, 1984). It has been suggested that low release of
female pheromone is a form of sexual selection in which females select for
strong searchers (Phelan, 1997).
There are
several features of moth biology that suggest that pheromone-based mate
location will select preferentially for the largest males. First, sensory
receptors are located on the sensory hairs of the antennae. Large antennae may
contain more hairs with more sensory receptors, and therefore have a more acute
sensitivity to female pheromone. Second, antennae size scales allometricaly
with body size in most saturniids (Heydenreich,
unpublished data) as it does in other insects (Hogue, 1993), resulting in males
with disproportionately large antenna. Third, both flight capacity and flight
speed have been related to body size in several moth species (Gu, and
Danthanarayana, 1992; Kuenen and Carde, 1993).
This suggests that larger individuals may not only be more sensitive to
low pheromone concentrations, but they may be able to fly longer distances, and
perhaps under a wider range of weather conditions.
Based on
these features of moth biology, I predicted that male saturniids
able to successfully locate females, will either have proportionately larger
antenna for a given body size, or will be larger over all in both body and
antennae size than the average male. In this study I will compare two
populations of males to test this prediction.
The relationship between antennae size and body size will be compared between
wild males caught with female bated traps and a base population of males raised
in captivity.
Although
sexual selection has often been suggested as an important force during the
evolution of moth pheromones, relatively few moth species have been studied
with respect to pheromone use and behaviors associated with mate attraction
(Phelan, 1997). Studies of evolution of mate-signaling systems and speciation
in insect pheromones are rare in general, and therefore can pose interesting
and unanswered questions (Phelan, 1997).
Besides scientific curiosity, use of moth pheromones is an important
tool in detecting and controlling pest populations in agriculture (Barnes et
al., 1992). Although few saturiniids are
pests, monitoring populations is still important for conservation reasons. A number moth species, especially saturniids, are becoming endangered in the
Polyphemus
moths (Antheraea polyphemus)
of the family Saturniidae occur throughout the
Polyphemus moths are large and easy to handle, therefore they can be measured alive released. All saturniids are convenient to study because they require no feeding (in adults the proboscus is reduced and they are unable to feed). Also caged virgin female moths have been found to attract and trap mates for many moth families, including Saturniids (Martin et al., 1992).
Seventy-five Polyphemus cocoons
were raised in an enclosed outdoor tent consisting of a rainproof roof and
mosquito-netted walls. The cocoons were
placed in trays of peat moss and sprayed with water daily until hatching. Upon emergence, males were measured using
millimeter scale metal calipers.
Measurements were taken of the wing length from the base of the wing to
the tip; body length from between the antennae to the tip of the thorax; width
of the thorax measured between the wings in the under side; antennae length and
width. The males remained in the tent
until death and were separated from the females by a mosquito-net curtain. These males will be termed “base males” and
represent the general (i.e. unselected) population.
To
measure sexual selection in these moths, I placed un-mated females in the field
and recorded the phenotypes of the subset of males that were successful in
locating them. By comparing these males
to the base population, I could test whether the antenna morphology of the
“successful” males differed from that of the males in the base population. In other words, to measure
whether variation in antenna size contributed to successful mate location.
Female Polyphemus were measured in the
same manner as the males with the exception of antennae width which is
negligible. Females were then placed in
traps, to prevent escape, and set one of the selected field sites. Traps consisted of a cylindrical wire frame
one-foot in diameter and about a foot and a half tall. The entrance to the trap was an inverted cone
three-inches in diameter, which prevented the moths
from escaping. The frame was covered in a mesh cloth and hung from a tree
branch with fishing line.
Traps were placed in Mclay flats in
the
To
supplement the sample of base population males (and attempt to include wild
males in the sample representing the base population), I used black-lights hung
in front of a white sheet between trees were used to attract males, and the
same collection and measurement procedure was employed. Black lights were used in the same three
study sights on nights when no female bated traps were out. Light traps were set on June 9th,
15th, 20th, and July 5th.
Scaling relationships between
antennae length and width and antennae length and body size for males and
females were determined using regression analysis. Analyses of covariance were used to test for
differences in the slope and intercept of the regressions for antennae length and
body size between the base males and the baited males.
Ten out of
the thirteen females used in traps attracted males. The average number of males caught per trap
was 7 (median 5). The most males
collected in one night were 15. Males
were rarely found inside the trap.
Instead, they tended to be piled on the outside of the trap on top of
one another, and directly above the female.
Only four males were successfully caught using the light trap at Rock
Creek, and these were included in the base population sample.
Sexually
selected males (i.e. males successful in locating a female) were not detectably
different from the base population. The
slope of the scaling relationship of baited males did not differ from that of
the control males (Figure 2; 1factor ANCOVA with body width as covariate and
antenna width as the dependent variable, f1, 74= 2.568;
p=0.1133). The intercepts of these
relationships also were not significantly different (1 factor ANCOVA with body
width as covariate and antenna width as the dependent variable, f1, 74=2.690;
p=0.1052).
There was
no significant difference in antenna size for males that find females in the
wild and males representing the general population in Polyphemus moths. This result has two major implications. First, that there may be little or no
selective pressure for larger antennae size in males. Second, some factor other than antennae size
may influence female-location success in males.
The first implication suggests that all males, regardless of size would do equally well in locating a female. One reason for this may be that females are emitting such a large concentration of pheromone that all males in the area are readily capable of detecting it. A study of pheromone sensitivity in blueberry leaftier moths (Tortricidae) showed that relatively low concentrations of pheromone result in antennal saturation (Ponder and Seabrook, 1991). This suggests that males are affected by very small amounts of pheromone, and therefore all males may be able to detect females regardless of antennae size.
The same
study tested female antenna sensitivity to her own
pheromone and found that there was no response.
This suggests that there is sexual dimorphism for chemoreceptor cells on
the antennae. Large antennae do indicate a greater number of receptors;
however, it may be the sensitivity of receptors, rather than the number, that
is important in mate location.
Sensitivity of receptor cells may be the important key factor in mate location in polyphemus moths and may have nothing to do with antennae size in males. Tests similar to those done by Ponder and Seabrook would have to be done with polyphemus to determine the role that sensors may play in differential mate location. However, no studies have been done to test sensitivity of antennae to pheromone with any saturniid. One reason may be that production of synthetic pheromones is very costly. Another reason may be that satuiniids are generally of little economical interest because very few are agricultural or forest pests.
A final
possibility is that male ability to locate a female may not be the primary
trait under selective pressure. An alternative involves male ability to
discriminate the correct species of female. It has been proposed that pheromone
mating systems are under strong stabilizing selection (Phelen, 1997). This means that males should be optimally
sensitive to the most common blend of pheromone in the population. Therefore males that are tuned to the right
blend for their species will be the ones selected for. In this case, sensitivity to a particular
blend of pheromone may have no correlation to antennae size, since a
qualitative mixture would be more important than a qualitative amount.
Given the
large p-values for the analysis of covariance for base verses baited males, it
is unlikely that the results are due to errors in experimental design. However it is possible that the base males
used were not representative of the general population for unseen reasons. Because the light traps were unsuccessful, it
was impossible to get a good sample of the base population in
In
conclusion, antenna size does not appear to be a factor that is selected for in
male Polyphemus moths. The results of
this study suggest that some other aspect of phenotype (i.e. sensitivity of
chemoreceptors) may be more important in mate location than antenna size and
therefore number of receptors. A study
manipulating concentrations of pheromones and a better sample of the base population, would be required to determine anything further.
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Acknowledgements:
This work was funded by an IBS-CORE Undergraduate Research Fellowship to Erin Heydenreich through a grant from the Howard Hughes Medical Institute to The University of Montana.