DDOES OES FFREE REE FFEMALE EMALE MMATE ATE CCHOICE HOICE AAFFECT FFECT FFEMALE EMALE FFECUNDITY ECUNDITY IIN N TTHE HE CCONFUSED ONFUSED FFLOUR LOUR BBEETLE? EETLE?

JAY NAGTALON†, LEE DRICKAMER* & STEPHEN SHUSTER*

†Department of Integrative Biology, University of California at Berkeley*Department of Biological Sciences, Northern Arizona University

ABSTRACTABSTRACT

Although mate choice is presumed to affect individual fitness, the effect of mate choice on offspring numbers is seldom investigated. To test whether female flour beetles, Tribolium confusum, actively choose males, and whether those choices affect their fecundity, we allowed virgin females to select mates, and then bred them with virgin males that females preferred or did not prefer. As a control we also mated virgin females to virgin males at random. Using ANOVA, we compared the 40 day productivities of all females by mating class (Ncrosses=30, Nprogeny=3,500). Our results showed that while females actively chose males, the number of offspring produced by females bred either with preferred, non-preferred, or random males, did not differ significantly between groups (F[2,29]=0.86, p=0.44). Thus, contrary to current “good genes” predictions, free female mate choice in T. confusum showed no effect on female fecundity.

INTRODUCTIONINTRODUCTIONThe “good genes” model of sexual selection predicts that females will

choose mates based on phenotypic traits which reflect male genetic quality. According to this model, females indirectly benefit by producing high-quality offspring; thus female choice and offspring viability are presumed to be positively correlated (Zahavi 1975, Andersson 1982, Hamilton & Zuk 1982). Although studies presenting evidence of increased offspring viability as a result of female choice are now common (Petrie 1994, Drickamer et al. 2000, 2003, Gowaty et al. 2003), parental and offspring fitnesses are confounded in estimates of offspring viability. Predictions about the adaptive outcomes of mate choice are therefore difficult or impossible (Wolf & Wade 2001). As a measure of parental fitness, fecundity is unconfounded by offspring fitness. Thus, in assessing the direct effects of mate choice on parental fitness, fecundity has a significant advantage over offspring viability (Shuster & Wade 2003).

Although the “good genes” model does not predict how female mate choice will affect female fecundity, if female choice improves offspring performance, choosy females are expected to be more fecund. Studies show that the number of progeny produced, a component of breeder fitness, does not differ significantly among mating pairs that preferred each other versus mating pairs that do not prefer each other (Petrie 1994; Drickamer et al. 2000, 2003, Gowaty et al. 2003). Thus, the aims of our study were to determine: 1) whether or not T. confusum females show mate preferences, and if so, 2) how female mate choice affects fecundity. As our measure of fecundity, we measured beetle productivity, which we defined as the number of progeny produced per given number of females in a given interval (Sokoloff 1977). Consistent with the “good genes” hypothesis, we predicted that a female who mated with a male she preferred would produce significantly more offspring than a female who mated with a male she did not prefer.

RESULTSRESULTS

Results from choice tests (N=20)We asked three questions:

1. Did females walk randomly? One female (*R) spent equal amounts of time in the ‘y’ arms and in the neutral arm of the maze (Table 1).2. Did females actively avoid males? Fifteen females showed a choice, but five females

(*A) showed no choice (as they did not meet our criterion) (Table 2). 3. Did females show no preference for either male? Of the fifteen females that showed a

choice, one female (*NC) spent equal amounts of time with both males (Table 3).

Results from breeding pairs (N=30)Females that showed no choice were grouped under Treatment Group 3: female

mated with a random male, ♀- R♂ . Forty day productivities were analyzed using one way ANOVA. The average number of progeny produced did not differ significantly among all three mating types: Group 1(♀-P♂)=115.4±15.55; Group 2(♀-NP♂)

=101.9±14.54; Group 3(♀- R♂)= 125.3.±10.62 (F[2,29]=0.86, p=0.44) (Figure 1).

METHODSMETHODSSelection of Experimental Animals (See Pictures)1. Beetle pupae were sexed.2. Pupae were placed in individual flour-filled vials and were allowed to develop into adults.

Choice Tests3. Mate choice tests were performed by allowing individual virgin females to associate with individual males isolated behind a fine mesh net in the arms of a y-maze. Each trial lasted 30 minutes (1800 seconds). The amount of time the female spent with each male was recorded to determine which male she preferred and which male she did not prefer. In detail, durations in seconds were recorded for each arm designated "short" duration, "long" duration, and neutral. Our first criterion for a choice by females was met if a female spent significantly greater than 2/3 of the trial time (67%) in the “male” side of the maze (i.e., in either of the arms of the ‘y’). Thus, trials with nonsignificant values for a G-test with df=1 were eliminated as “no choice,” as were significant trials with >600 seconds in the neutral zone (“avoiders”). To prevent an experiment-wise type I error, we adjusted our level of significance to

α=.05/20 trials=.0025; thus Gsignificant=10.83, df=1. Our second criterion for a choice by

females eliminated as “no choice” trials those in which females spent a significant amount of time on the male side of the maze, but did not spend significantly more than ½ of the trial time in one arm of the maze. Because our first criterion eliminated 5/20 trials (see

Results), we adjusted our level of significance to α=.05/15 trials=.003; thus Gsignificant=9.0,

df=1.

Productivity Analysis4. Female and male beetle pairs were mated in a flour + yeast-filled vial. There were three treatment groups as classified by the following type of paired mating:

1) a female mated with a male she preferred, ♀- P♂ (N=7) 2) a female mated with a male she did not prefer, ♀- NP♂ (N=8)3) a female mated with a random male, ♀- R♂ (N=15)

5. The total number of offspring produced after 40 days were counted for each mating pair.

CONCLUSIONSCONCLUSIONS

Our results indicate that T. confusum females actively choose between males and show clear mating preferences when allowed to do so. However, our productivity results also show that free female mate choice may not affect female fecundity. Although females chose mates, the average numbers of progeny produced by females mated with Preferred, NonPreferred and Random males were not significantly different from one another. Although our sample size was somewhat small (N=30 crosses, 3,500 total progeny), our finding of conspicuous mate choice without conspicuous fecundity returns suggests that adaptive explanations are not always necessary when evidence of mate choice is observed.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

This study was done through the Research Experiences for Undergraduates program funded by the National Science Foundation. Travel to the Animal Behavior Society conference was made possible by the Turner Award funded by the National Science Foundation. Special thanks to Eric Anderson for photography.

REFERENCESREFERENCES1. Andersson, M. 1982. Sexual selection, natural selection and quality advertisement. Biol. J. Linn. Soc. 17:375-393.2. Drickamer, L., Gowaty, P. & Holmes, C. Free female mate choice in house mice affects reproductive success and offspring viability and

performance. Animal Behaviour 59:371-378.3. Drickamer, L., Gowaty, D. & Wagner, D. 2003. Free mutual mate preferences in house mice affect reproductive success and offspring

performance. Animal Behaviour 65:105-114.4. Gowaty, P., Drickamer, L. & Schmid-Holmes, S. 2003. Male house mice produce fewer offspring with lower viability and poorer performance

when mated with females they do not prefer. Animal Behaviour 65:95-103.5. Hamilton, W. & Zuk, M. 1982. Heritable true fitness and bright birds: a role for parasites? Science 218:384-387.6. Petrie, M. 1994. Improved growth and survival of peacocks with more elaborate trains. Nature 371:598-599.7. Shuster, S. & Wade, M. 2003. Mating systems and strategies. Princeton University Press, Princeton.8. Sokoloff, A. 1977. The biology of Tribolium, vol. III. Oxford University Press, London. 9. Wolf, J. & Wade, M. 2001. On the assignment of fitness to parents and offspring: Whose fitness is it and when does it matter? J. Evol. Biol.

53:205-214.10. Zahavi, A. 1975. Mate selection: a selection for a handicap. J. Theor. Biol. 53:205-214.

 

Table 1: Test 1: Finding the random walkers. Durations in seconds were recorded for each arm of the y-maze designated "short" duration, "long" duration, and neutral. “*R” denotes a random walker.

test # short (sec) long (sec) neutral (sec) total (sec) G1 15 777 1008 1800 1336.952 14 1658 128 1800 2869.793 9 1761 30 1800 3536.834 655 959 186 1800 578.685 24 1303 473 1800 1641.446 445 813 542 1800 117.807 20 1767 13 1800 3581.438 628 1055 117 1800 865.57

9*R 572 657 571 1800 8.00*10 1 1780 19 1800 3727.2911 505 1242 53 1800 1375.9112 626 765 409 1800 111.3513 531 916 353 1800 270.8514 654 654 492 1800 30.1715 433 1176 191 1800 863.0316 281 1169 350 1800 755.7717 58 742 1000 1800 1065.8518 187 1265 348 1800 1071.9719 200 1512 88 1800 2017.6720 107 1050 643 1800 895.25

test # short+long (s) neutral (s) total (s) G 1*A 792 1008 1800 400.07*

2 1672 128 1800 699.603 1770 30 1800 1179.054 1614 186 1800 508.725 1327 473 1800 38.27

6*A 1258 542 1800 6.90*7 1787 13 1800 1306.038 1683 117 1800 741.60

9*A 1229 571 1800 1.34*10 1781 19 1800 1257.8911 1747 53 1800 1038.6912 1391 409 1800 91.7813 1447 353 1800 159.8314 1308 492 1800 27.0115 1609 191 1800 494.3416 1450 350 1800 164.08

17*A 800 1000 1800 385.03*18 1452 348 1800 166.9419 1712 88 1800 863.53

20*A 1157 643 1800 5.95*

Table 2: Test 2: Finding the avoiders. Durations in seconds were recorded for each arm of the y-maze designated "short" duration, "long" duration, and neutral. “*A” denotes an avoider.

Table 3: Test 3: Finding the preferences. Durations in seconds were recorded for each arm of the y-maze designated "short" duration, "long" duration, and neutral. “*NC” denotes a non-chooser.

Figure 1: Number of offspring produced by a mating pair in each treatment group. Red bars denote the means.

Experimental Protocol

male

female

1.

2.

4.

5.

test # short (s) long (s) total (s) G2 14 1658 1672 2156.093 9 1761 1770 2340.724 655 959 1614 57.605 24 1303 1327 1599.447 20 1767 1787 2257.838 628 1055 1683 109.5310 1 1780 1781 2452.0211 505 1242 1747 320.8612 626 765 1391 13.9113 531 916 1447 103.68

14*NC 654 654 1308 0.00*15 433 1176 1609 356.4716 281 1169 1450 584.2618 187 1265 1452 897.5419 200 1512 1712 1138.83

3.

Choice arm/

zone

Neutral arm/zone

Mesh net