Mating behaviour and spermatophore morphology: a comparative test of the female-choice hypothesis

Heather C. Proctor, Robert L. Baker, and Darryl T. Gwynne

Abstract: Complex, species-specific morphology of genitalia or "paragenitalia" such as spermatophores is traditionally considered an adaptation to prevent heterospecific matings. Eberhard argued against this lock-and-key hypothesis and suggested that elaborate male genitalia evolve through female preference for increased tactile stimulation. He found support for this argument in a cross-taxon comparison of spermatophore complexity among species with different degrees of contact between the sexes during spermatophore transfer. After expanding Eberhard's list of species, we tested the female-choice hypothesis with appropriate controls for phylogeny and using naive human subjects to rank spermatophore complexity. Our results uphold Eberhard's conclusion that the lock-and-key hypothesis does not explain the distribution of spermatophore complexity among taxa; however, they do not clearly support female choice as an alternative explanation. Other factors that may influence spermatophore complexity include environmental stress and whether spermatophores are deposited on a substrate. Because we seldom know what parts of a spermatophore are touched by females, or whether there are spermatophore-associated pheromones, human visual assessment may not always allow accurate judgement of realized spermatophore complexity.

RCsumC : La morphologie complexe, spkcifique h l'espkce, des genitalia ou des (( paragenitalia tels les spermatophores, est considkrke traditionnellement comme une adaptation propre h empecher les accouplements hetkrospkcifiques. Eberhard a rejete cette hypothkse de la clk et de la serrure et proposk comme hypothkse de rechange que les genitalia du m2le ont evolue par preference des femelles pour des stimulations tactiles plus intenses. I1 a appuye son argumentation en faisant une comparaison de la complexite des spermatophores d'un taxon h l'autre chez des espkces ayant divers degres de contact entre les sexes au cours du transfert des sperrnatophores. Aprks avoir augment6 la liste d'espkces

For personal use only. d'Eberhard, nous avons kprouvk l'hypothkse du choix des femelles au moyen d'espkces appropriees du point de vue phylogenetique et nous avons demande h des sujets humains non connaisseurs de classifier les spermatophores par rang de complexite. Nos resultats confirment la conclusion d'Eberhard selon laquelle l'hypothkse de la clk et de la serrure ne peut expliquer l'ordre de complexite des spermatophores chez les diffkrents taxons; ces resultats n'appuient cependant pas non plus clairement son hypothkse de la preference des femelles comme explication de rechange. Parmi les facteurs qui peuvent influencer la complexitk des spermatophores, il faut mentionner le stress dQ au milieu et le fait que les spermatophores peuvent 6tre deposks ou non sur un substrat. Comme on peut rarement savoir quelles parties du spermatophore viennent en contact avec les femelles, ou s'il existe des phkromones associees aux spermatophores, l'kvaluation visuelle de la complexitk rkelle des spermatophores n'est peut-6tre pas toujours exacte. [Traduit par la Redaction]

Introduction nation for this phenomenon is the lock-and-key hypothesis, which states thst genitalic complexity and specificity result Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 In many taxa, the male genitalia have complex and species- from selection to prevent interspecific hybridization; female specific mOr~hO1Og~(Eberhard 1985). The ex~la- genitalia are "locks" that allow access to only the appropri- ate male "keys" (Shapiro and Porter 1989). An alternative hypothesis proposed by Eberhard (1985) is that male geni- Received February 10, 1995. Accepted June 29, 1995. talic elaborations result from sexual selection by female H.C. Proctor,' R.L. Baker, and D.T. Gwynne. choice; males whose genitalia provide greater stimulation to Department of Zoology, Erindale College, University of the female during copulation have a greater chance of trans- Toronto, Mississauga, ON L5L 1C6, Canada. ferring an ejaculate. There is good evidence from a wide Author to whom all correspondence should be sent at range of terrestrial that males "internally court" the following address: Department of Biology, Queen's females during sperm transfer (Eberhard 1994). University, Kingston, ON K7L 3N6, Canada Eberhard (1985) broadly defines male genitalia as any (email: proctorh@biology .queensu.ca). part or product of a male that touches females in a sexual

Can. J. Zool. 73: 2010-2020 (1995). Printed in Canada 1 ImprimC au Canada Proctor et al.

context, such as intromittant organs, claspers, or spermato- male does not have a prolonged interaction with one female, phores. In one test of the lock-and-key hypothesis, Eberhard regardless of whether he requires previous contact with examined the prediction that spermatophores of species females for spermatophore production (nonpairing) . We whose males and females do not contact each other during therefore classified a species as pairing if the male had close sperm transfer should be more complex than those of species behavioural interactions with a particular female during sper- with male-female contact. The rationale is that complexity matophore transfer and as nonpairing if there were no such is required in noncontact species because females have no interactions. As well, we subdivided pairing species into two species-recognition cues other than those provided by sper- groups depending on whether the male physically brings the matophore morphology; in contact species, spermatophore female and spermatophore in contact (manipulative) or complexity is less important because of cues in the appear- simply indicates the location of the spermatophore to the ance or behaviour of the male. In his test, Eberhard divided female and allows her to move over it of her own accord spermatophores from 64 taxa into four categories of morpho- (nonmanipulative) . logical complexity, and differentiated between contact and noncontact taxa on the basis of whether males touch females Choosing species for the test before depositing spermatophores. Instead of finding a trend A lack of statistical independence may arise if one uses towards high spermatophore complexity in noncontact taxa, speciose but monotypic taxa in comparative tests (Harvey as predicted by the lock-and-key hypothesis, Eberhard found and Page1 1991). In Eberhard's (1985) analysis, for example, a positive association between high complexity and male- 14 of the 17 contact taxa assigned to the highest level of female contact. He interpreted this as support for his female- spermatophore complexity belong either to a single super- choice hypothesis, arguing that in contact species a male has of pseudoscorpions or to three closely related orders the opportunity to physically bring a marginally receptive of pedipalp (Uropygi, Amblypygi, Schizomida) ; female and his spermatophore together so that stimuli from because of the close phylogenetic relationships between many complex spermatophore elaborations can bring the female of these taxa the true sample size should be lower than 14. into a more receptive state. In noncontact species, a female Ideally, one should use independent evolutions of a character is likely to approach a spermatophore only if she is already in a comparative test (Ridley 1989). We were unable to do fully receptive, and spermatophore elaborations designed to this with spermatophore complexity because complexity is a "convince" her would therefore be unnecessary. continuous character and we would have had to arbitrarily Our paper provides a detailed analysis of complexity of delimit categories which, because of our knowledge of the spermatophores in light of the female-choice hypothe- hypothesis being tested and of species' pairing behaviour, sis. Unlike Eberhard, we use a comparative approach that could have led to subconscious bias in the analysis. Instead controls for phylogenetic effects (Brooks and McLennan we tried to maximize independence of data points by con- 1991; Harvey and Page1 199 1). Our approach further differs sidering each change of mating behaviour in a clade from from Eberhard's in that we use a larger data set involving a pairing to nonpairing (or vice versa) to be an independent For personal use only. greater diversity of species, provide statistical analysis, and evolutionary opportunity for modification of spermatophore control for subconscious bias in ranking the complexity of complexity. When available, we used phylogenies to recog- spermatophores . nize independent evolutionary changes in behaviour, and combined species into groups monotypic for pairing or non- pairing behaviour (Fig. 1). We did this by sequentially com- Methods bining sister-groups that showed only one type of mating Categorization of species behaviour until we encountered an example of the opposite Eberhard considered only those taxa with substrate-mediated behaviour (Fig. la), or until a sister-group was encountered spermatophore transfer in his study. As well as adding to the that contained taxa with both behaviours (Fig. 1b). Phylogenies number of species that use this sort of transfer, we further for myriapods and hexapods were from Boudreaux (1979); extended the list by including taxa that release planktonic for orders, the majority opinion of Weygoldt and spermatophores or place preformed spermatophores on or Paulus (1 979), Shultz (1990), and E.L. Smith (personal com- into females. Thus, we are not simply reanalyzing Eberhard's munication); for pseudoscorpions, Harvey (1992); and for data, but testing his hypothesis with a broader range of major taxa, Lindquist (1976) and Norton et al. (1993). organisms. To avoid problems of interference from other For other groups we used as a proxy for phylo- Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 genitalic cues, we excluded species that insert spermato- geny (water , Cook 1974; all other taxa, Parker 1982). phores into females through an intromittant organ, or that After sorting species into groups monotypic for pairing or form spermatophores inside the female's genital tract. nonpairing, we randomly chose a species from each mono- Although division of taxa into contact and noncontact typic group and used its spermatophore in the test. Although species (Eberhard 1985) is suitable for testing the lock-and- our method may still result in some taxa being overrepre- key hypothesis, it is not the most appropriate division for sented, lack of independence should be greatly reduced rela- testing female choice. Because Eberhard's argument for tive to using species as independent points. female choice rests on the ability of males to elicit a sexual response in females by bringing them in contact with sper- Ranking complexity of spermatophores matophores, a more appropriate division is between species Subjective categorization of qualitative characters is common in which the male interacts closely with a particular female in comparative tests of evolutionary hypotheses (e. g ., phylo- during sperm transfer (pairing) and species in which the genetic increase in complexity of sperrnatophores, sperm Can. J. Zool. Vol. 73, 1995

Fig. 1. Two examples of how we used cladograms to (Berlyne 1971). Because both hypotheses about the male determine which taxa to include in groups monotypic for genitalia are concerned with complexity as a transmitter of mating behaviour (P, pairing; N, nonpairing). (a) Taxa 1 -3 information (the lock-and-key hypothesis concerns species are a group monotypic for pairing; taxa 4 and 5 are a group identity and Eberhard's female-choice scenario concerns monotypic for nonpairing. (b) Taxa 1 and 3 are monotypic male quality), human assessment of spermatophore complexity for nonpairing; taxon 2 is monotypic for pairing. is likely to measure characteristics relevant to the hypoth- eses. Because relative size of objects has been shown to p P P N N affect judgement of complexity (Farley and Weinstock 1980), we redrew spermatophores to be similar in size and line thick- ness. As well, because we were interested only in morpho- logical elaborations that could be tactually sensed by female , we did not include pigmentation or internal struc- tures (e.g., sperm cells) in the drawings. For the polychaete Kinbergonuphis simoni and the mites virgulata, tricuspis, and Eriophyes sheldoni, we drew sper- matophores from photographs. For the millipede Glomeris transalpina and the collembolans Tullbergia quadrispina, Heterosminthurus bilineatus, and Sminthurus viridis, we drew spermatophores from published verbal descriptions. Complex structures are difficult to reconstruct from written descriptions; however, difficulty is minimized when struc- tures are very simple (A. M . Shapiro , personal communica- tion). The spermatophores we drew from verbal descriptions were simple (spheres or unelaborated stalks and spheres) and, for the collembolans , differed from spermatophores already illustrated only in the proportions of the stalk and sperm droplet. Spermatophores of the mites Unionicola sp. 4 and papillator were drawn as integrated groups, since they are deposited as such and a female would not encounter individual sperrnatophores. If a human subject ranked two or more spermatophores as being equally com- plex, we assigned those spermatophores the same average rank (e.g., if the first three tied for being the least complex,

For personal use only. each was given the rank of (1 + 2 + 3)/3 = 2, and the next more complex one the rank of 4). Humans take several factors into account when judging morphological complexity (Berlyne 197 1). We used computer- aided image analysis to determine the importance of two of these factors: the ratio of the perimeter to the surface area, transfer and storage structures in decapod crustaceans, Bauer and the surface area alone. The perimeter 1 surface area ratio 1986; song complexity and plumage dimorphism in birds, (PIS) is commonly used as a measure of lake complexity Hamilton and Zuk 1982, Hoglund 1989; penis complexity and ("shoreline development, " Lind 1974). The ideal simplest conspicuousness of ovulatory signs in primates, Dixson 1987; figure is a circle; departures from perfect circularity result in SillCn-Tullberg and Moller 1993). Because there is the danger higher PIS values. Because the relationship between P and S of subconscious bias if the purpose of such categorization is changes with the absolute size of an object, the ratios must known, several authors have used naive human subjects as be normalized for comparison. We calculated the percent judges (e.g., plumage brightness in birds, Read 1987; Read difference between actual perimeter of the spermatophore and Weary 1992; brightness in fish, Ward 1988; showiness drawings and the ideal P value of a circle of the same surface

Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 in lizards, Lefcort and Blaustein 1991). To avoid subcon- area: difference = [(actual P - ideal P)lideal PI 100. We scious bias in ranking spermatophore complexity, we pre- determined P and S by scanning the images of the ranked sented drawings of spermatophores to 30 human subjects spermatophores with an Abaton Scan 300 color scanner and who were unaware of the hypothesis being tested and asked using the National Institutes of Health (NIH) Image 1.4.9 them to use their own concept of complexity to rank the image analysis program (NIH, Freeware). We then com- objects from least to most complex. Complexity is very diffi- pared the computer rankings with those of the human sub- cult to define using a single mathematical measure (Maddox jects by means of Spearman's rank correlations. 1990; Lewin 1994), but humans use a multifactorial approach when judging complexity (Berlyne 1971). Complex objects Statistical analyses are those that show greater asymmetry, irregularity, hetero- Unless otherwise mentioned, tests were performed with the geneity of elements, amount of material, and number of STATISTIX I1 software package (NH Analytical Software, angles (Day 1967; Berlyne 1971). All of these measures Roseville, Minn.). All measures of central tendency are reflect higher information content in the more complex object given as means + SE. Proctor et al.

Table 1. Friedman's nonparametric ANOVA for spermatophores of manipulative (M), nonmanipulative (NM), and nonpairing (NP) species using individual human subjects as blocks (Sokal and Rohlf 198 1, p. 446).

(A) Behavioura

NM NP M (n = 6 species) (n = 23 species) (n = 11 species)

Rank sum 85 56 39

(B) A posteriori multiple comparision (Zar 1984, p. 230)

Comparison Difference 4 P

NM vs. NP 29 4.. 10 <0.05 NM vs. M 46 6.50 <0.001 NP vs. M 17 2.40 >0.1 ns

Note: ns, not significant. "Microspio mecznikowianus and Halacarellus basteri were not included because of insufficient information about male behaviour.

Results Thus, our subjects' decisions were not affected by differences in spermatophore size in the drawings, a comple~elyartificial Mating behaviour and spermatophore morphology were com- construct, but were influenced by more biologically relevant piled for 201 After controlling for phylogeny, there specie^.^ characteristics. were 19 pairing and 23 nonpairing groups. We used a random- A potentially stronger test of Eberhard's female-choice number generator to select one species from each of these hypothesis involves comparison of complexity rankings among groups (N = 42 species). manipulative (M) , nonmanipulative (NM) , and nonpairing Thirty subjects ranked the spermatophores of these species, (NP) species. The female-choice argument predicts a decline taking, on average, 17.1 f 1.2 min to rank the illustrations in complexity with less opportunity for male manipulation (range 8-34 min). Fourteen subjects knew the drawings (M > NM > NP); conversely, the lock-and-key hypothesis represented spermatophores, while the other 16 did not; no predicts a decline in spermatophore complexity with greater subject knew the purpose of our test or whether a given sper- contact between the sexes (NP > NM > M). We tested For personal use only. matophore was produced by a pairing or a nonpairing species. these predictions using Friedman's nonparametric ANOVA Figure 2 shows spermatophores arranged from lowest to with blocks (= individual subjects) and found that neither highest complexity based on mean ranks calculated from all predicted the of rankings. Instead, spermatophores of 30 subjects. The grand means for pairing and nonpairing nonmanipulative species were significantly more complex f species were not significantly different (pairing: 21.8 2.6; than those of the other two groups, and those of manipulative f nonpairing: 21.2 2.0; t = 0.15, two-tailedp = 0.88). To species had the lowest rank sum (NM > NP > M) (Table 1). control for variation between subjects in their concept of Data from this last analysis indicated that among pairing complexity we calculated the mean rank for spermatophores species, spermatophores placed on a substrate were ranked from pairing and from nonpairing taxa for each subject, then more highly (T = 26.3 f 2.5, N = 10) than those placed compared the 30 pairs of means in a t test; there was no sig- on or in a female (T = 13.1 f 4.2, N = 7) regardless of nificant difference (t = 0.92, p = 0.366) (for other methods whether courtship was manipulative or nonmanipulative. All of controlling for interrater differences see Tinsley and nonrnanipulative pairing species deposit on substrates; the Weiss 1975). To test whether ranking was affected by the higher complexity of nonmanipulative spermatophores as knowledge that illustrations represented spermatophores, we revealed in the Friedman's test (above) may be due to this. used t tests to compare rankings assigned by the two groups Comparing complexity across a broad taxonomic scale Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 of human subjects. There was no evidence that subjects who is often difficult (the "apples and oranges" problem; see knew the drawings were spermatophores ranked them differ- Read and Weary 1992). Because some of our subjects com- ently from those who were ignorant of this, either for sper- mented on the difficulty of ranking spermatophores that matophores of pairing species (t = 0.03, p = 0.978) or for were extremely different in morphology, we looked at pairs those of nonpairing species (t = -0.03, p = 0.976). of species from sister-groups with opposite monotypic pair- Comparison between computer rankings of complexity ing behaviour (e.g . , groups 1 and 2 in Fig. lb). We reasoned and those of humans indicated that departure from circularity that for these species, changes in spermatophore morphology was more important in human decisions than the relative size would be more recent than changes between species from of the spermatophore image (S measured in pixils2) (Fig. 3). more phylogenetically distant groups (e.g ., groups 2 and 3 A complete set of tabular data may be purchased from the in Fig. lb) and that differences in spermatophore morphol- Depository of Unpublished Data, Document Delivery, CISTI, ogy might therefore be less extreme. Ten pairs of species met National Research Council Canada, Ottawa, ON KIA OS2, these requirements (Table 2). For each pair we determined Canada. whether the human subject had ranked the spermatophore 201 4 Can. J. Zool. Vol. 73, 1995

Fig. 2. Spermatophores ranked according to increasing complexity. Mean rank, whether the spermatophore was produced by a pairing (P) or nonpairing (N) species, and standard error (in parentheses) are listed beneath each spermatophore. Species are listed from left to right within each row (a-h). (a) Glomeris transalpina (class Diplopoda, order Glomerida) (Haacker 1969), Tullbergia quadrispina (order Collembola, family Onychiuridae) (Mayer 1957), Microhedyle lactea (class Gastropoda, order Opisthobranchia) (Swedmark 1964), Petaloconchus monteryensis (class Gastropoda, order Mesogastropoda) (Hadfield and Hopper 1980), Arachnocephalus vestitus (class Insecta, order Orthoptera) (Boldyrev 1914), and Peripatus corradoi (phylum Onychophora) (Schaller 1979). (b) Campodea remyi (order Diplura) (Schaller 1954), Heterosminthurus bilineatus (order Collembola, family Sminthuridae) (Bretfeld 1970), Phoronis vancouverensis (phylum Phoronida) (Zimmer 1967), Scheloribates laevigitus (order , suborder ) (Woodring and Cook 1962), Lamellisabella coronata (phylum Pogonophora) (Southward 1975), and Tracheobdella punctata (phylum Annelida, class Hirudinea) (Schaller 1979). (c) Sminthurus viridis (order Collembola, family Sminthuridae) (Mayer 1957), Cheiridium museorum (order Pseudoscorpiones, family Cheiridiidae) (Weygoldt 1966b), Scutigerella immaculata (class Symphyla) (Schaller 1971, 1979), Nanorchestes amphibius (order Acariformes, suborder , cohort ) (Schuster and Schuster 1977), Haemogamasus hirsutus (order , suborder ) (Schaller 1979), and Arrenurus globator (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Bottger 1962). (d) Ambystoma annulatum (class Amphibia, order Urodela) (Spotila and Beumer 1970), Microspio mecznikowianus (phylum Annelida, class Polychaeta) (Rice 1978), Spio jilicomis (phylum Annelida, class Polychaeta) (Rice 1978), Saxidromus delamerei (order Acariformes, suborder Prostigmata, cohort Anystina) (Coineau 1976), and Pseudotriton ruber (class Amphibia, order Urodela) (Organ and Lowenthal 1963). For personal use only. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 Proctor et al. 201 5

Fig. 2 (continued). (e) undulata (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Proctor 1992b), Atractides nodipalpis (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Proctor 1992b), Kinbergonuphis simoni (phylum Annelida, class Polychaeta) (Hsieh and Simon 1990), Bdellodes virgulata (order Acariformes, suborder Prostigmata, cohort Eupodina) (Wallace and Mahon 1976), and Euscorpius italicus (order Scorpiones) (Angermann 1955). (f) Unionicola tricuspis (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Hevers 1978), Podura aquatica (order Collembola, family . Poduridae) (Schliwa and Schaller 1963), Anystis baccarum (order Acariformes, suborder Prostigmata, cohort Anystina) (Schuster and Schuster 1966), Abrolophus rubipes (order Acariformes, suborder Prostigmata, cohort Parasitengona) (W itte 1975), and Scolopendra cingulata (class Chilopoda, order Scolopendromorpha) (Klingel 1960). (g) Neumunia distincta (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Proctor 1992b), Stylopauropus pedunculatus (class Pauropoda) (Schaller 197I), Eriophyes sheldoni (order Acariformes, suborder Prostigmata, cohort Eleutherengona) (Sternlicht and Griffiths 1974), and Dactylochelifer latreillei (order Pseudoscorpiones, family Cheliferidae) (Weygoldt 1966b). (h) Nicoletiella jaquemurti (order Acariformes, suborder Prostigmata, cohort Labidostommatina) (Vistorin 1978), Labidocera aestiva (subphylum Crustacea, order Calanoida) (Blades and Youngbluth 1988), Unionicola sp. 4 (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Proctor 1992b), Neumunia papillator (order Acariformes, suborder Prostigmata, cohort Parasitengona) (Proctor 1991, 1992b), and Halacarellus basteri (order Acariformes, suborder Prostigmata, cohort Eupodina) (Kirchner 1967). For personal use only. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 Can. J. Zool. Vol. 73, 1995

Table 2. Paired comparison between closely related pairing and nonpairing species showing positive and negative scores for the 30 human subjects and X2 goodness-of-fit tests for each pair of species.

Pairing species Nonpairing species Positivea Negativea Corrected X2 p

Microspio mecnikowianus Spio filicornis Microhedyle lactea Petalochonchus monteryensisb Podura aquaticab Tullbergia quadrispina Heterosminthurus bilineatus Sminthurus viridisb Dactylochelifer latreilleib Cheiridium museorum Halacarellus basterib Bdellodes virgulata Saxidromus delamerei Anystis baccarumb Unionicola tricuspis Unionicola sp. 4b Neumania papillato@ Neumania distincta Pseudotriton ruber Ambystoma annulatum

- - - - "Zero scores are not included. bSpecies that received significantly more positive scores.

from the pairing species as having equal (0), higher ( +), or in which associations between characters are adaptively main- lower (-) complexity than that from the nonpairing species. tained in closely related species (in this case, high complexity We added the number of plusses and minuses for each subject and pairing). However, because there is as yet no way to and performed a Wilcoxon signed-rank test on the 30 pairs discriminate between adaptive maintenance and phylogenetic of values. We found that subjects tended to rank the sper- inertia without performing experiments for each species, we matophore from the pairing species higher than that of the had to choose independent events for statistical tests. nonpairing species (two-tailed p = 0.0026). To identify which pairs of spermatophores showed the strongest deviation Human visual assessment of morphological complexity from the predicted 1: 1 distribution of plusses and minuses we Recently, criticism has been leveled against using humans to performed a X2 goodness-of-fit test for each of the 10 pairs. judge colour patterns viewed by birds (Bennett et al. 1994), Two pairs were not significantly different from a 1: 1 model, because the spectrum visible to birds includes wavelengths four had the nonpairing spermatophore ranked higher, and invisible to us (ultraviolet; Bennett and Cuthill 1994). Bird four had the pairing spermatophore ranked higher (Table 2). feathers often have ultraviolet patterns that would not be This suggests that the significant results from the Wilcoxon included in human assessment of, for example, relative showi- test stemmed from the greater number of plusses awarded to ness (Burkhardt 1989). Similarly, human impressions of For personal use only. spermatophores from pairing species on the four occasions complexity may not agree with those of other species (see that they were ranked as more complex than were awarded Read and Weary (1990) re song complexity in birds). It is not to spermatophores from nonpairing species when they were clear how the morphological complexity of spermatophores ranked as more complex (Table 2). as assessed by human vision in both Eberhard (1985) and this study relates to what is actually sensed by females when they Discussion contact spermatophores. There have been very few observa- Spermatophore complexity and pairing behaviour tions of the parts of a spermatophore that are touched by the We found little support for the association between pairing female genitalia. In some cases only a small portion is func- and morphological complexity of spermatophores predicted tional in stimulating the female; for example, although the by Eberhard's female-choice hypothesis. The only favoura- spermatophore stalk of the uropygid arachnid Typopeltis ble evidence from our study came from a comparison of crucifer Pocock is quite elaborate, only the tips of the sperm spermatophores from 10 pairs of closely related pairing and packets are grasped by the female's genital operculum nonpairing species in which spermatophores from pairing (Weygoldt 1978). Also, even if observations of sperm-uptake species were ranked as being, on average, more complex; behaviour are made, one cannot be certain that parts of a spermatophore which touch the female provide the type of

Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 however, this pattern was not consistent when each pair is tested separately (Table 2). stimulus predicted by the female-choice hypothesis. On the Although our results do not support the female-choice other hand, parts of a spermatophore may stimulate female hypothesis, it would be premature to dismiss it as an expla- receptors in locations other than the genitalia. Clearly, as nation for variation in spermatophore complexity because Eberhard (1985, p. 185) suggests, neurobehavioural studies (i) the conservative nature of our phylogenetically controlled are necessary to determine whether and how much females tests may render the predicted relationship undetectable, and are stimulated by the various genitalic elaborations of males. (ii) there may be an imperfect correlation between realized Human visual assessment may also fail to reflect sper- spermatophore complexity and human visual assessment. We matophore complexity because complexity can be chemical discuss these issues below. as well as morphological. Recognition of conspecific sper- matophores is likely accomplished through pheromones, The conservative nature of phylogenetically controlled tests particularly in species with no male-female contact dur- A problem inherent in phylogenetic controls is the possibility ing sperm transfer. There is behavioural evidence for of being too rigidly conservative and thereby rejecting cases spermatophore-associated pheromones in polychaetes (Rice Proctor et al.

Fig. 3. Correlations between human ranking of trations will remain. Researchers can minimize this potential spermatophore complexity with computer measurements of by choosing illustrations produced by the most skilled and two aspects of complexity: deviation from a circular outline reliable artists. (a) and area of spermatophore drawing measured in pixils on the computer screen (6).Because the spherical spermatophores The predicted association between pairing and of Glomeris transalpina and Tullbergia quadrispina were spermatophore complexity: an alternative hypothesis morphologically identical and were given the same A problem with the female-choice hypothesis is that the pre- complexity rank (2.4), we considered them to be a single dicted positive association between pairing and spermato- data point. phore complexity is not exclusive to this scenario. Females of a pairing species probably approach spermatophores from a predictable direction (e.g., a female may walk behind a male as he deposits spermatophores). A likely result of this behaviour is that spermatophores of pairing species will be morphologically specialized on the side first encountered by the female. This predicted asymmetry would be interpreted as higher complexity by humans because of the way we perceive asymmetrial objects (Berlyne 197 1). Conversely, spermatophores of nonpairing species are likely to be sym- metrical in order to accommodate females approaching from many different directions, and would therefore be scored as less complex by humans. Associations between predictabil- ity of female approach and spermatophore asymmetry have been recorded. In the nonpairing pseudoscorpion Serianus carolinensis the male builds a web that funnels females towards a spermatophore that is strongly asymmetrical in comparison with those of related species that do not build guiding webs (Weygoldt 1966~).Arnold (1976) also noted greater asym- metry in the spermatophores of salamander species in which females approach from a predictable direction. An analagous pattern occurs in insect-pollinated flowers in which asym- metry seems to be an adaptation in inclined flowers to probing by insects from a particular side (Robertson 1888). How- ever, despite this evidence our study failed to find an associa- For personal use only. tion between pairing and complexity. This may indicate that asymmetry is not always greater in the spermatophores of pairing species, or that the importance of other morphologi- cal features outweighed that of asymmetry in human ranking of spermatophore complexity.

Spermatophore complexity and deposition site Among pairing species, spermatophores of nonmanipulative species were ranked as more complex than those of manipu- lative ones (Table 1). This may be related to the fact that nonmanipulative species deposit spermatophores on the sub- strate more often than do manipulative species, and that sub- jects considered substrate-deposited spermatophores to be more complex than those placed directly on the female. Higher ranking may be due in part to the presence of a supporting Can. J. Zool. Downloaded from www.nrcresearchpress.com by Guangzhou Jinan University on 06/06/13 Spermatophore complexity rank stalk or web in most substrate-deposited spermatophores that increases perceived complexity. As well, substrate-deposited 1980) and in a large number of terrestrial and aquatic arthro- spermatophores may require more complex protuberances to pods (Proctor 1992~). help with the insertion of sperm. This is less important Finally, there is an inherent problem in using two- in spermatophores that are positioned directly by the male. dimensional figures to illustrate three-dimensional objects, In silverfish, pseudoscorpions, and scorpions (Thys 1989; be they spermatophores, birds, or primate genitalia. The Weygoldt 1975), substrate-deposited spermatophores have illustrator controls the image that ,the viewer judges, and if explosive or extrusive mechanisms to force sperm into the he or she has neglected or embellished particular aspects of female .that may require morphological elaborations to oper- complexity or brightness, rankings based on these illustra- ate. Humans perceive complexity to be higher not only when tions may be wrong. However, as the ideal alternative of asymmetry is increased (see above) but also when structures presenting judges with the real objects is almost always impos- have greater amounts of material, a greater number of inde- sible to achieve, the problem of potentially misleading illus- pendent units, and more heterogeneous elements (Berlyne Can. J. Zool. Vol. 73, 1995

1971); spermatophores placed on a substrate seem more Engineering Scholarship to H .C .P. and NSERC operating likely to show extreme development of these characteristics grants to R.L.B. and D.T.G. than those placed on the female. References Species-specific morphology in spermatophores of nonpairing species Angermann, H. 1955. Indirekte Spermatophorenubertragung bei Eberhard's female-choice hypothesis predicts that spermato- Euscorpius italicus (Hbst.) (Scorpiones, Chactidae). Naturwis- phores of nonpairing groups will not show species-specific senschaften, 42: 303. differences in morphology. This holds for some taxa, including Arnold, S .J . 1976. Sexual behaviour , sexual interference and oribatid mites and collembolans, which have morphologically sexual defense in the salamanders Ambystomu muculatum, Ambystomu tigrinum, and Plethodon jordani. Z. Tierpsychol. uniform sperrnatophores. However, there are other nonpair- 42: 247-300. ing taxa that do show interspecific differences in spermato- Bauer, R.T. 1986. Phylogenetic trends in sperm transfer and phore structure (e.g., bdellid mites, Wallace and Mahon storage complexity in decapod crustaceans. J. Crustacean Biol. 1972, 1976; some water mites, Proctor 1992b). Eberhardian 6: 313-325. female choice could select for complex species-specific mor- Bennett, A.T.D., and Cuthill, I.C. 1994. Ultraviolet vision in phology in nonpairing species if females tactually sample a birds: what is its function? Vision Res. 34: 1471- 1478. number of spermatophores before deciding to take up sperm; Bennett, A.T.D., Cuthill, I.C., and Norris, K.J. 1994. Sexual however, to our knowledge such behaviour has never been selection and the mismeasure of color. Am. Nat. 144: 848 -860. observed. Another, less direct form of "female choice" Berlyne, D.E. 197 1. Aesthetics and psychobiology. Meredith seems to be a more probable cause of species-specific mor- Publishing Corp., New York. Blades, P.I., and Youngbluth, M.J. 1988. Morphological, physio- phology. Females of different species are likely to differ in logical, and behavioural aspects of mating in calanoid . size, shape, or microhabitat. If one broadly defines female In Proceedings of the 3rd International Conference on Copepoda. preference as "any trait that biases the probability that females Edited by G.A. Boxshall and H.K. Shminke. Kluwer Academic mate with different kinds of [conspecific] males, and does Publishers Group, Dordecht, the Netherlands. pp. 39-51. not necessarily involve any cognitive process" (Kirkpatrick Boldyrev, B.T. 1914. Contributions a 17Ctudede la structure des and Ryan 1991), then any slight morphological or behav- spermatophores et des particularites de la copulation chez ioural variation between females of different species can Locustodea et Gryllodea. Horae Soc. Entomol. Ross. 41: 1-245. result in selection for different structural modifications in Bottger, K. 1962. Zur Biologie und Ethologie der einheimischen spermatophores . For example, interspecific size differences Wassermilben Arrenurus (Megaluracarus) globator (Mull.), in females may affect stalk height, differences in genital 1776 Piona nodata nodata (Miill.), 1776 und Eylais indun- dibulifera meridionalis (Thon), 1899 (Hydrachnellae, ) . aperture shape may affect sperm capsule morphology, and Zool. Jahrb. Abt. Syst. Oekol. Geogr. Tiere, 89: 501 -584. differences in habitat may affect stalk structure if those habi- Boudreaux, H .B. 1979. phylogeny with special refer- tats exert different environmental stresses on spermato- ence to insects. John Wiley and Sons, New York. For personal use only. phores. For example, some stalk modifications in water mite Bretfeld, G. 1970. Grundzuge des Paarungsverhalten europaischer spermatophores appear to enhance spermatophore stability in Bourletiellini (Collembola, Srninthuridae) und daraus abgeleitete situations of hydrodynamic stress (Proctor 1992b). In less taxonomisch-nomenklatorische Folgerungen. Z. Zool. Syst . stressful situations such modifictions may be lost. For exam- Evolutionsforsch. 8: 259 -273. ple, the spermatophore of Unionicola intermedia, a copulat- Brooks, D.R., and McLennan, D. H. 1991. Phylogeny, ecology and ing water mite, has simpler stalk and capsule morphology behaviour. University of Chicago Press, Chicago. than those of its nonpairing congeners (Hevers 1978); this is Burkhardt, D. 1989. UV vision: a bird7s eye view of feathers. J. Comp. Physiol. A, 164: 787-796. probably due to relaxed selection on structures that prevent Coineau, Y. 1976. Les pariades sexuelles des Saxidrominae Coineau stalk breakage and loss of sperm in nonpairing species. As 1974 (Acariens Prostigmates, Adamystidae). Acarologia, 18: Eberhard (1985, p. 154) noted, other copulating water mites 234 -240. also have spermatophores with very simple external mor- Cook, D.R. 1974. Water mite genera and subgenera. Mem. Am. phology. Clearly, detailed studies of how spermatophores Entomol. Inst. (Gainesville) No. 2 1. function in both their biotic and their abiotic environments Day, H .I. 1967. Evaluations of subjective complexity, pleasingness are needed if we are to fully understand the evolutionary and interestingness for a series of random polygons varying in pressures that shape them. complexity. Percept. Psychophys. 2: 28 1- 286.

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