Pachyiulus hungaricus (Karsch, 1881)

Jovanović, Zvezdana, Pavković-Lučić, Sofija, Ilić, Bojan, Vujić, Vukica, Dudić, Boris, Makarov, Slobodan, Lučić, Luka & Tomić, Vladimir, 2017, Mating behaviour and its relationship with morphological features in the millipede Pachyiulus hungaricus (Karsch, 1881) (Myriapoda, Diplopoda, Julida), Turkish Journal of Zoology 41 (6), pp. 1010-1023 : 1013-1014

3.1. Mating behaviour of P. hungaricus

3.1.1. Mating arena

In the mating arena test, 47% of all contacts resulted in copulation, while 53% of contacts ended with no copulation. Descriptive statistics of the observed behavioural traits (ML, DC, CC, ECC, ECWC, and CWC) are shown in Table 1.

Our direct observations of mating behaviour indicated that a pattern of daily dynamics is present in the number of copulations. To be specific, the most matings were observed between 1000 and 1200 (21 copulations) and between 1400 and 1600 (10 copulations). Watching millipedes in the arena, we observed that males are the more active sex during the search for mates, while females are the “choosier” sex. We also recorded successive behavioural sequences that may define precopulatory behaviour in this species. The male actively taps the dorsal side of the female with its antennae and moves forward to the head. After reaching the female`s head, the male bends the anterior part of its body below the female, and, if the female is receptive and ready to mate, it will uncoil the anterior part of its body and expose the gonopores. The male then ejects its gonopods, and the next step in achieving copulation is extrusion of the female’s vulvae. During copulation, animals can be found in a parallel position ( Figure 3A ) or the male can coil around the female ( Figures 3B and 3C ).

3.1.2. Female and male choice tests

In the female choice test, 66% of all contacts resulted in copulation (78% of them were achieved with the previous partner and 22% with a new partner). In the male choice test, 54% of all contacts resulted in copulation (64% of them were with the previous partner and 36% with a new partner) ( Figure 4 ). The results also indicated that individuals of both sexes achieved significantly more copulation with the previous partner than with a new partner. In the female choice test, 68% of all observed contacts (with and without copulation) were with the previous partner, while 32% were with a new partner. In the male choice test, 53% of all contacts (with and without copulation) were with the previous partner, while 47% were with a new partner. A higher percentage of all contacts ending in copulation was observed in both the female (71% vs. 29%) and the male (69% vs. 31%) choice tests ( Table 2).

Additionally, the highest percentage of copulations was detected after 1 contact in both the female and the male choice tests (63% and 77%, respectively), while smaller percentages of copulations was recorded after 2 or 3 contacts (19% and 9%, respectively). The smallest number of copulations (5%) was noted after 5 contacts in the male choice test.

Results of the Χ 2 test indicated that the previous partners achieved significantly more copulations than the new partners in both tests (male choice test: Χ 2 = 7.44, df = 1, P <0.01; female choice test: Χ 2 = 30.86, df = 1, P <0.01). The number of all contacts (with and without copulation) with the previous and new partners was significantly different only in the female choice test (Χ 2 = 13.58, df = 1, P <0.01).

We also tested differences of behavioural traits in millipedes that mated twice (in the mating arena and in the female or male choice test). No significant differences of DC and ECC were found between 2 successive copulations, but CC was significantly different ( Table 3).

3.2. Morphological variation in P. hungaricus

3.2.1. Analyses of linear measurements

No significant differences in mean values of 6 morphological traits were observed between mated and nonmated females, or between mated and nonmated males ( Table 4).

3.2.2. Analyses of leg and gonopod size and shape variation

The centroid size of legs did not differ between mated and nonmated individuals (F 3,150 = 1.614, P = 0.1885). The first principal component (PC1) explained 81.03% of the morphological variability of walking legs ( Figure 5 ). Canonical variate analysis revealed that leg shape in males significantly varied as a function of mating status (P = 0.0174), but that was not the case in females (P = 0.7503). The first axis in CVA explained 82.26% of shape variation of the analysed trait ( Figure 6 ). The degree of grid deformation, size of the vectors, and their direction illustrate the pattern of leg shape variation in mated and nonmated individuals ( Figures 5 and 6 ).

We found no significant difference in CS of gonopods between mated and nonmated males (F 1,32 = 0.06, P = 0.8043). The pattern of gonopod shape variation in mated and nonmated males is illustrated by deformation grids with vectors ( Figures 7 and 8 ). The shape of the observed gonopod part did not significantly differ between the 2 groups of males (P = 0.0662) ( Figure 8 ).

Figures

Figure 3. Positions of mating pairs of P.hungaricus. A – male and female in parallel position; B, C – male coiling around female (photo:B. Ilić).

Figure 4. Percentage of copulations achieved with the previous partner (white bars) and with a new partner (black bars) in female and male choice tests.

Figure 5. Principal component analysis (PCA) of leg shape in relation with mating status (rectangle: black – nonmated females, grey – mated females; circle: black – nonmated males, grey – mated males).

Figure 6. Canonical variate analysis (CVA) of leg shape in relation with mating status (rectangle: black – nonmated females, grey – mated females; circle: black – nonmated males, grey – mated males).

Figure 7. Principal component analysis (PCA) of gonopod shape in relation with mating status (circle: black – nonmated males; grey – mated males).

Figure 8. Canonical variate analysis (CVA) of gonopod shape in relation with mating status (circle: black – nonmated males; grey – mated males).