Kipp C. Kruse

Largemouth bass (Micropterus salmoides) learn to avoid feeding on toad (Bufo) tadpoles

Abstract Largemouth bass ( Micropterus salmoides ), when starved for 1 day, almost totally abstain from eating toad ( Bufo americanus and Bufo woodhousei ) tadpoles. However, there is a positive relationship between bass hunger levels and the acceptability of Bufo larvae as food items. With experience, there is a decrease in the number of toad tadpoles engulfed (taken into the mouth), and that actually consumed by the bass, while the number of larvae expelled (spat out) increases. Bass strongly prefer Hyla crucifer tadpoles to B. americanus larvae, and learn to distinguish between the two species. These results agree with the hypothesis that the schooling behaviour found in B. americanus tadpoles functions, at least in part, as a deterrent to predation.

Effects of environmental and social conditions on homosexual pairing in the Japanese beetle (Popillia japonica Newman)

Homosexual pairing between males occurs under natural conditions in a wide variety of taxa, including many insect species, but few studies have investigated how environmental and social conditions affect same-sex pairing in insects. We investigated factors affecting homosexual pairing in male Japanese beetles (Popillia japonica Newman) in the field and in the laboratory. Specifically, we investigated how time of day, sex ratio, beetle density, and temperature affected the likelihood of homosexual pairing. In the field, male–male pairs constituted 1–6% of the pairs we collected. Homosexual pairs were more common in the afternoon than in the morning and the evening. Sex ratio, density, and temperature were all related to the likelihood of finding a homosexual pair, but the relationships were not linear. In the laboratory, higher male densities and relatively male-biased sex ratios were associated with an increase in the frequency of homosexual pairs. Homosexual pairs were more frequent at relatively low and relatively high temperatures. Males that mounted other males tended to be smaller than the males that they mounted. In addition, compared to males that were not homosexually paired, there was some indication that the mounting males were smaller, and the mounted males larger, than the unpaired males. Our data suggest that homosexual pairs are a result of males mistaking other males for females, and we hypothesize that the environmental and social factors cause changes in homosexual pairing through their effects on the frequency of pair formation and pair duration.

Male backspace availability in the giant waterbug (Belostoma flumineum Say)

SummaryFollowing bouts of courtship and copulation, female giant waterbugs (Belostoma flumineum Say) deposit eggs on the backs of their mates. Throughout a 6– to 12-day brooding period, males display several behaviors that are vital to egg-nymph survival. Consequently, females depend on male post-copulatory behaviors for successful reproduction and the possibility exists for male backspace availability to limit female reproduction in this species. I studied seasonal trends and factors that affect male backspace availability in populations of B. flumineum in east-central Illinois (USA). Early in the spring/summer, giant waterbug populations are relatively small and a large majority (188/205=91.7%) of the males are egg-laden; males experimentally added to the population during this period quickly became encumbered. In contrast, later in the summer after young-of-the-year emerge as adults, the waterbug population density increases dramatically and fewer (670/1274\2= 52.6%) of the males are encumbered (egg-laden). Of the males that are egg-laden both early and late in the season, significantly more are completely encumbered (i.e., 100% of the dorsum egg covered) early in the spring. The adult sex ratio is generally not biased and the number of eggs/pad that completely covers a male approximates a full ovarian complement. Therefore, these factors probably do not cause male backspace to become limited. The primary factor that appears to limit male backspace availability is the ability of females to synthesize a second partial clutch in a short time, often within 1 to 4 days. Females are capable of ovipositing partial clutches on 12 males within 30 days, whereas male brooding period is temperature dependent and ranges from 6 to 13 days. Newly emerged males are capable of breeding significantly sooner than can newly emerged females, thereby creating ample oviposition substrate for the females in the population after young-of-the-year adults appear. The evolution of sex-role reversal is not well understood; however it should not evolve in waterbugs unless male backspace limits female reproduction. Such a situation appears to exist in B. flumineum early in the season but not later in the summer.

Male Preference for Large Females and Female Reproductive Condition in the Japanese Beetle, Popillia japonica Newman (Coleoptera: Scarabaeidae)

Abstract In the field, paired (in copula) female Japanese beetles (Popillia japonica Newman) tend to be larger than unpaired females. In this study, we investigated whether this size pattern could be explained by a male preference for large females, and whether larger females tended to have more and/or larger eggs than smaller females. In a laboratory study, both small and large males, when given a choice of a large and small female, tended to choose the large female. We dissected field-caught paired and unpaired females, measured their body size, and counted and measured their eggs. Larger females tended to have more and larger eggs than smaller females, indicating that males may benefit from choosing larger females due to the egg characteristics of these larger females. Paired females in the field were consistently larger than single females; paired females also had more and larger eggs, even when body width was statistically controlled. Thus, although body width of the female correlates with her fecundity, males may either use cues in addition to a females body width to determine her immediate fecundity or fecund females may be more available for or less resistant to male mating attempts.

The social environment affects mate guarding behavior in Japanese beetles, Popillia japonica

Abstract The effect of the social environment on post-copulatory mate guarding duration in Japanese beetles, (Popillia japonica Newman), was examined in a laboratory experiment. The mate guarding durations of beetles in different sex ratios and densities were observed for 10 hours. Guarding duration was longer when females were larger, suggesting the presence of ‘cryptic’ male mate choice. Densities, but not sex ratio, affected the duration of guarding bouts, with males guarding for longer at higher densities. This result implies that males increase their guarding duration under conditions in which their female may be likely to be encountered by other males. The lack of a sex ratio effect on the duration of guarding bouts is consistent with other studies on this species that indicate males have difficulty distinguishing females from males. Consequently, because the sex ratio on food plants is typically male-biased, a paired male may react just to density, treating surrounding individuals as if they were mostly males. The total amount of time males spent guarding was lower at lower densities and at male-biased sex ratios; this suggests that after ceasing to guard one female, males were less able to find a subsequent mate under these conditions.

Optimal foraging by predaceous diving beetle larvae on toad tadpoles

SummaryThe feeding behavior of predaceous beetle larvae (Dytiscus fasciventris) on American toad (Bufo americanus) tadpoles was investigated in the laboratory. The rate of food extraction from an individual tadpole decreases as the prey item is being depleted. Beetles hand fed single tadpoles were shown to have a significantly longer mean handling time than beetles capturing similar size tadpoles from constant prey densities of 5 or 20 tadpoles/1. Handling times were not correlated with mean intercatch intervals but were found to decrease throughout a 10-tadpole catch sequence at all prey densities. Beetles allocated less time per unit weight to large prey items than to smaller tadpoles. However, the marginal ingestion rate (amount consumed/unit of handling time) did not differ with prey size throughout a 10-tadpole catch sequence. These results support the rate expectation hypothesis (Charnov 1976) as an explanation for time allocation to patches.

Factors affecting the allocation of paternal care in waterbugs (Belostoma flumineum Say)

SummaryMale giant waterbugs (Belostoma flumineum Say) brood eggs oviposited on their dorsa by conspecific females. Laboratory observations indicate that viable egg pads are occasionally discarded before hatching. Theory predicts that such behavior should occur only if the costs incurred by brooding exceed the benefits of hatching the egg pad. We studied the effects of egg pad size, time invested in brooding, and egg viability upon the continuation of paternal care in the giant waterbug. We found that smaller egg pads are less likely to hatch than larger ones, and males appear to be less likely to discard egg pads as temporal investment increases. However, the inviability of eggs did not appear to affect the probability of an egg pad being discarded. Males of this species appear to have evolved a decision-making process involving the continuation of paternal care.

Physical Contests for Females in the Japanese Beetle, Popillia japonica

Abstract We conducted field observations of physical competition for mates, in which a single male attempts to usurp a female from another male, in male Japanese beetles, Popillia japonica Newman (Coleoptera; Scarabaeidae). Physical contests for mates were relatively rare, but when they occurred the challenger male was able to successfully takeover females by dislodging the previously paired resident male in only 18% of contests, suggesting that a substantial prior residency advantage exists in this species. Challenger males that were successful in takeover attempts were significantly larger than the resident male. In contrast, no size pattern was found between intruding males and residents in unsuccessful takeover attempts. The frequency of contests for existing pairs was examined throughout the day. Pair frequency was greatest in early morning and in the evening but contest frequency was highest during the middle of the day. Contest frequency was negatively related to beetle density but not related to temperature. Overall, physical contests for mates appear to be an important part of the mating behavior in Japanese beetles. The frequency of the contests relates to the time of day and social conditions and contest outcome is related to prior residency and the size of the intruding male relative to the paired male.

Male-Male Mounting and the Unreliability of Body Size as a Character for Mate Choice in Male Japanese Beetles (＂Popillia Japonica＂ Newman)

BackgroundSame-sex pairing is common in many animal species. In many insects, same-sex pairing is often thought to be a result of poor sexual discrimination (i.e., a mistake), but few detailed studies of the mechanisms underlying the mistaken pairing have been conducted. Previous studies have found that in the field, a small proportion of Japanese beetle (Popillia japonica) mating pairs consist of two males instead of a male and a female. In the current study, we investigated the relationship between body size, the tendency to mount other males, and the duration of these mounts, in laboratory experiments on male Japanese beetles.ResultsIn the first experiment, we observed male-male mounting in all-male groups in which each male had been uniquely marked. Males of all sizes were likely to mount other males and extend their aedeagus (copulatory organ), but the mounts were longer, and aedeagus extension was more likely to occur, if the mounted beetle (in the ‘female’ position) was larger than the mounting beetle (in the ‘male’ position). In the second experiment, we observed male-female behavior in mixed-sex groups. Females did not immediately copulate with males that had mounted them. If copulation did occur, males tended to remain on the back of females for an extended period of time. Males that mounted other males in mixed-sex groups tended to mate subsequently with a female and then stay with her.ConclusionsWe propose that the minimal physical difference between the sexes, in combination with benefits to the males of rapidly attempting to pair with any available female, explains the tendency for males to mount other males. Extended mounts may occur because larger individuals are more likely to be female and because of selection on males to persist in a copulation attempt when females do not immediately copulate with a male.