ASSORTATIVE MATING IN SOLDIER BEETLES (after Brown & Downhower 1988)


INTRODUCTION


This lab is an observational study designed to test hypotheses on the mating behavior of a common species of beetle. First, some background on the theoretical framework regarding differential mating behavior...


Sexual selection occurs when higher mating success allows some individuals produce more offspring than other members of the same sex. This is especially interesting because mating behavior can lead to evolution of characteristics that are independent of physiological vigor or success in avoiding predators, etc. Like natural selection, sexual selection can occur in several different patterns. For example, if the largest individuals in a population are more successful in obtaining mates, then directional selection will occur, but higher success of "average" individuals leads to stabilizing selection.


Sexual selection depends on mating that is non-random. Such assortative or non-random mating has been demonstrated in many vertebrates and invertebrates. In some cases, it may be attributable to simple physical constraints; for example, a small beetle may not be able to mate with any male much larger than her. In other cases, assortative mating may be due to either female choice of males possessing particular characteristics, or to interactions among individuals that restrict access to mates. In this exercise we will examine mating patterns in the cantherid soldier beetle Chauliognathus pennsylvanicus. In general, we are asking whether mating in soldier beetles is random. From this general statement emerge the following hypothesis and predictions:

  1. Ho: mating is random with respect to size
    Ha: mating is assortative with respect to size
    Prediction: if mating is random and we measure pairs of soldier beetles found copulating, there will be no correlation between size of male and size of females. Alternatively, there will be a (positive?) correlation between sizes of mated soldier beetles.
  2. Ho: probability that a male will mate is not related to size
    Ha: size influences the probability of mating in male soldier beetles
    Prediction: if probability of mating is not related to size, then males (females) found mating will not differ in size from males (females) found by alone (not copulating). Alternatively, mating individuals will be significantly different in terms of size than solitary individuals.

Obviously, there are many other hypotheses and predictions that can be formulated. For this lab, you will test these predictions from these hypotheses. As you are doing your work, think about what other predictions arise from these hypotheses and how you would test them. Your lab notebook should include these other possible predictions and tests.


THE STUDY ORGANISMS


Chauliognathus pennsylvanicus, the soldier beetle, is commonly found on yellow flowers in late summer or fall. The beetle uses the flowers as a source of food (pollen), but the flowers also serve as a "social focus" for interactions between individual beetles. Goldenrod (Solidago) is a favored food plant for adults, as are other late-summer flowering plants.

Soldier Beetles copulating on goldenrod flower


This species has been the subject of several analyses of assortative mating. In summary, there are many open questions. Some studies have demonstrated directional selection for larger body size by showing that mated individuals are larger than non-mated ones. Other analyses have shown that selection occurs only on some, specific characteristics (not body size per se).


METHODS


We will work in pairs. The lab has two parts. The first is a 30-minute observation period during which we will locate soldier beetles on goldenrod (or other yellow composites) in the butterfly garden and nearby fields. These observations will help you better understand beetle behavior and may help you interpret the results of the analyses you will do to assess assortative mating in this species. What activities can you describe? These detailed observations of interactions between individual beetles should be in your notebook. Compare activities of individual beetles to those of pairs. When interactions are seen, distinguish the behavior of "initiators" (an individual that approaches another within 1 cm) and "recipients" (the approached beetle). In your lab notebook, record the behavior, the sex of each individual, and which individual ended the encounter. Attempt to observe male-female, male-male, and female-female encounters.


During the second part of the lab, each pair will capture and measure a total of at least 40 beetles in the following distribution: ten mating pairs (10 males, 10 females), 10 solitary males and 10 solitary females. As you capture the beetles, you will record the (1) sex, (2) width of thorax, and (3) length of the elytra. Measure the thorax and elytra as indicated below (Figure 1), to the nearest 0.05 mm using Vernier calipers.

Measuring width of thorax.


The paper data sheet is double-sided, with one side for recording single beetles and the other for recording mated pairs. The second (Measure Pair of Beetles) you should use when you have caught a copulating pair of beetles. Place each beetle you have measured in a small vial or plastic bag to ensure that no beetles are measured more than once (Why do we do this?). At the end of the lab, we will release all beetles back to the wild (to the tune of “Born Free”).

Measuring length of elytra


SUGGESTED ANALYSES


We are primarily interested in the following questions: (Hypothesis 1) Is there any significant correlation between male and female sizes for beetles captured while mating? (Hypothesis 2) Were sizes of copulating male beetles different from non-copulating males? Use Excel to do the following:

Analyses: Hypothesis 1

Analyses: Hypothesis 2

On the web, you can find instructions on how to use Excel to do a Scatterplot and Bar Charts, or for Excel Instructions, in general


In your Lab Notebook, think about the following questions: What would a correlation between the sizes of copulating males and females mean? Was there a correlation with your data? How large was the correlation? What were the behavioral interactions between and within the sexes, and what do they tell us about female choice or male-male competition, etc? What does it mean if copulaing males are larger on average than non-copulating males? What other tests could you do to address these hypotheses?

 

A few references:

Rahman N.; Dunham D.W.; Govind C.K. 2002. SIZE-ASSORTATIVE PAIRING IN THE BIG-CLAWED SNAPPING SHRIMP, ALPHEUS HETEROCHELIS. Behaviour. 139:1443-1468.

Hingle A, Fowler K, Pomiankowski A. 2001. Size-dependent mate preference in the stalk-eyed fly Cyrtodiopsis dalmanni. ANIMAL BEHAVIOUR 61: 589-595.