Female mate choice in the Goldenrod soldier beetle, Chauliognathus Pennsylvanicus, between
two secondary sexual traits.
Henry Truong, Ela Godbole
Introduction:
The Goldenrod soldier beetle, Chauliognathus Pennsylvanicus, belongs to the coleopteran order
and is commonly found over much of eastern North America. These insects are pollen and nectar
feeders, therefore they are found in abundance during the late summer or early fall in open fields, on
Goldenrod plants, Solidago Canadensis (Mason, 1979). Mating occurs at this time when the beetles are
in abundance, and it is not uncommon to find several mating pairs on Goldenrod blossoms. The colour
patterns on the Goldenrod soldier beetle consist of a light yellow elytra (the hardened wing casing that
covers and protects the delicate wings), with a black oval shaped spot on the lower portion of the elytra.
Sexual dimorphism in male and female soldier beetles can be seen in many phenotypic aspects
such as differences in body size, elytra spot ratio and antennal scape width. These characteristic can also
be secondary sexual characteristic in males which vary among individuals that compete with each other
to attract member of the opposite sex. These visual cues can also a way by which females determine
male fitness that will influence her choice in choosing the prospect mate. (Alcock, 2009). This leads to
the topic of secondary sexual characteristics in males where the particular traits lead to fitness which
influences female mate choice in runaway selection hypothesis. Previous studies suggested that female
mate preference is dependent on secondary sexual traits such as an intermediate elytra length to spot
length ratio (McLain, 1988), and the antennal scape width, which is the enlarged basal stalk of the
antenna (Mason, 1979). A female may select for one trait over the other for the following reasons; a
larger antennal scape width may lead to better communication during mating (Mason, 1979), and the
intermediate elytra ratio is a good sign of symmetry and proportion in the male (McLain, 1988). This
reveals genetic stability in male that explain by the sexy-sons and good-genes hypothesis.
Sexual selection is well understood in evolutionary biology and has been observed in variety of
species. However multiple secondary sexual traits in coleopteran order are not very well known.
Multiple secondary sexual traits can be viewed as backup signals or multi-message signals that provide
an indication of male fitness levels, for mate choice selection that this proposed study hoping to
confirm. (Johnstone, 1996)
Questions:
When more than one secondary sexual trait exists in the species and both are known to
influence female mate choice selection. We want to investigate whether one particular secondary sexual
trait conveys a more important message in terms of fitness, than the other as in the multiple messages
hypothesis. (Johnstone, RA .1996).
Hypothesis:
When comparing between elytra spot ratio and antenna width we hypothesize that elytra spot
show a greater correlation toward female choice during mate selection.
Expected Result:
We expect females to have a stronger preference for an intermediate elytra-to-spot ratio as a
secondary sexual trait over the antenna scape width. A greater positive correlation is expected with the
elytra-to-spot ratio, this trait that will allow the female to determine the overall male’s fitness during
that mating season.
Method:
The sampling of the Goldenrod soldier beetle will start at Gatineau Park near Luskville area
where Goldenrod flower are common in the open field during late summer and early fall (August to
September). This is also the mating season for the soldier beetle during which time they feed on nectar
and pollen to get their body primed for the reproduction process. Four random transects will be setup
for soldier beetle collections, each transect will be 1-2 km away from each other. At each transect 10
mating pairs will be collected within 50 m radius using an insect net. The mating pair will then be
separated and the male beetle will be placed in a film canister with a lid and kept for analysis in the lab.
This process will be repeat 3 times for the next 3 continuous weeks. At the end of the sampling process
a total of 120 male mating soldier beetles as the sample size will be analysis during the mating season of
that month.
The male beetle will be freeze dry at the lab for ease of analysis, and the wings and the antenna
will be separated using a scalpel blade under the dissecting microscope. The elytra will be cut at the
attachment point to the body, and the antenna will be cut at the base. The procedure for measuring of
elytra spot ratio will be: the length of elytra spot/ the length of the elytra (McLain, 1988). Only one side
of the antenna and elytra will be used for analysis because they are symmetrical. Beetles with large
variation in symmetry will not be used in our analysis, although this is unlikely for mating male. We will
use a calliper to take measurements of elytra spot length ratio to the nearest 0.5mm (McLain 1988). We
will use the dissecting microscope to take images of the beetle antennae and measure the antennae
scape electronically using the scale bar on the imageJ program. Actual specimens length can be calculate
based on the scale bar and magnification used during measurement. This will prevent any breakage of
the antennae during handling for analysis and will give us a precise measurement of the sample.
The measurement of antenna scape width and elytron spot ratio of each male beetles will be
tabularize in excel by the date of collections for analysis. A multiple regressions analysis will be executed
in SPSS to determine which secondary sexual characteristic the female favor upon choosing her mate
during the mating season.
Implication:
This topic provides profound understanding in evolutionary biology and allows us to understand
the interaction of multiple secondary sexual traits as it appears more frequently in many taxa. This study
provides an important addition to the sexual selection concept that happens on a daily basis around us
as organism evolved multiple secondary sexual traits may be involved in the mate choice selection. In
addition, the understanding of multiple signal secondary sexual traits of Goldenrod soldier beetles
model allows potential research within the coleopteran order, which is extremely diverse and has a
huge impact on the ecosystem.
References:
Alcock J. 2009. Animal behavior 9th Edition, Evolution of reproductive behavior. Sinauer Associate Inc.
10: 360-376.
Mason L.G. 1980. Sexual selection and the evolution of pair bonding in soldier beetles. Evolution 34:771775
McLain, Denson Kelly. 1988. Male mating preferences and assortive mating in the soldier beetle.
Evolution, Vol. 42 (4): 729-735.
McLain, Denson Kelly. 2005. Female soldier beetles display a flexible preference for selectively favoured
male phenotypes. Evolution, 59(5):1085-1095.
Johnstone, RA .1996. Multiple display in animal communication: ‘backup signals’ and ‘multiple message’.
Philosophical transactions of the royal society of London series B-biological sciences, 351(1337):329-338