Strategic adjustment of copulatory plug size in a nematode

Copulatory plugs(CP)are substances produced during copulation that block the genital openings of the female.In several species of Nematoda,males produce CP that are thought to impede female remating and thus sperm competition.The relatively large size of the CP in several nematodes,and its evolutionary loss in self-fertilizing populations of Caenorhabditis elegans,suggests that CP are costly to produce.If CP production is costly,the application of basic concepts of strategic ejaculation theory suggests a modulated allocation of CP in response to sperm competition risk.This hypothesis led us to predict that males perceiving a higher risk of sperm competition will produce larger CP.We tested these ideas with the entomopathogenic,gonochoristic nematode Rhabditis regina.Our first experiment provides evidence suggesting that production of CP is costly,because the size of CP is negatively affected by stressful conditions(high population density,small male adult size,and suboptimal food type).The results of our second experiment support the prediction that males adjust the size of CP to sperm competition risk:the average size of CP increased as the number of males competing for one female increased.Overall,our study supports the idea that in R.regina the production of CP is costly for males and that the size of the CP produced is influenced by sperm competition risk.

The movement of ＂false antennae＂ in butterflies with ＂false head＂ wing patterns

In many butterfly species of the family Lycaenidae, the morphology and color pattern of the hind wings, together with certain behaviors, suggests the presence of a false head （FH） at the posterior end of the perching individual. This FH is consi- dered an adaptation to escape from visually oriented predators. A frequent component of the FH are the tails that presumably resemble the antennae, and the typical hind wings back-and-forth movement along the sagittal plane （HWM） performed while perching apparently move the tails in a way that mimics antennal movement. By exposing 33 individuals from 18 species of Lycaenidae to a stuffed insectivorous bird, we tested two alternative hypotheses regarding HWM. The first hypothesis proposes that, when the butterfly is observed at close range, the HWM distorts the shape of the false head thus reducing its deceiving effect and, therefore, selection will favor butterflies that stop moving their wings when a predator is close by; the second hypothesis says that an increase in the frequency of HWM improves its deflective effect when the butterfly confronts a predator at close range. Our results tend to support the second hypothesis because half of the butterflies started to move their hind wings or increased the rate of HWM when exposed to the stuffed bird; however a substantial proportion of butterflies （30%） stopped moving their hind wings or decreased the rate of HWM as expected from the first hypothesis. Our observations also showed that there is great variation in the rates of HWM, and demonstrated the existence of alternative ways of producing ＂vivid＂ movement of the hind wing tails （the ＂false antennae＂） in the absence of HWM [Current Zoology 61 （4）： 758-764, 2015].