Does reproductive state affect a lizard's behavior toward predator chemical cues?

We examined how reproductive state affected shifts in behavior of adult female Podarcis sicula toward chemical cues from a natural snake predator (Coronella austriaca). The oviparous P. sicula lizards do not experience a major physical burden during reproduction, but gravid females substantially increase duration of basking. Therefore, gravid lizards are likely to experience a greater risk of mortality because they are exposed to predators for longer periods. Both gravid and non-gravid females shifted patterns of locomotion when confronted with snake chemical cues, but the change was notably larger when females were non-gravid. When non-gravid, lizards responded to predator scent by increasing the number of stand-ups and starts, while such a response was not observed when females were gravid. By contrast, gravid lizards clearly reduced the time spent basking in the presence of predator scent, whereas no change in basking behavior was observed when females were non-gravid. Thus, females exhibit differential behavioral responses to predator scents that is dependent on reproductive state.     

Distribution of Anoplostoma viviparum (Nematoda) in Southampton Water sediments

The juveniles, non-gravid females, gravid females and males of the free-living marine nematode Anoplostoma viviparum Bastian showed markedly different distributions within layers of mud from Southampton Water sampled during the early spring of 1975. Of the total population, 92% of gravid females occurred in the anoxic layer, 73% of juveniles occurred in the flocculant layer, non-gravid females and males were found in approximately equal numbers in both the flocculant and oxidised layers but none in the anoxic layer. A tentative activity cycle based on these distributions is put forward for consideration, taking into account the water movement within the estuary.     

Flight initiation distance decreases during social activity in lizards (<Emphasis Type="Italic">Sceloporus virgatus</Emphasis>)

Flight initiation distance, the predator–prey distance when escape begins, is predicted by escape theory to decrease if fleeing entails loss of benefits. Shortening of flight initiation distance during social interactions is known only in males and only in a few species. In a previous study, male, but not female, Sceloporus virgatus lizards had shorter flight initiation distance when interacting with tethered conspecifics. Females in that study were not gravid or close to ovulating. I predicted that flight initiation distance would be shorter in gravid females that perform sidle-hopping displays to reject courtship than in lone females. I tested this prediction and examined effects of social interactions by males with free-ranging conspecifics to ensure that previous findings were not artifacts of tethering and experimental introduction of conspecifics. Flight initiation distance was shorter in females when interacting with males than when alone; it was also shorter in males interacting with either sex. Thus, when beneficial for reproductive reasons, social interaction affects flight initiation distance in females, but at other times, it does not. Lesser shortening of flight initiation distance in females than males may be a consequence of greater social benefit to males and protection of reproductive investment by females.     

Loss of mating opportunities influences refuge use in the Iberian rock lizard,<Emphasis Type="Italic"> Lacerta monticola</Emphasis>

Because time spent in refuge may be costly if prey lose opportunities to forage, fight, or mate, prey allow predators to approach closer before beginning to flee when opportunity costs are high. Because the same opportunity costs may apply to refuge use as to escape, prey should make similar trade-offs between risk of emerging and cost of remaining in refuge. In the Iberian rock lizard, Lacerta monticola, we studied the effects of sex, reproductive season, speed of predator approach, and potential loss of mating opportunities on time spent in refuge following simulated predatory attacks. Lizards of both sexes adjusted refuge use to the level of risk by spending more time in refuge when approached rapidly than slowly. Females remained in refuge for equal times in the mating and postreproductive seasons, but males emerged sooner during the mating season, suggesting adjustment to a cost of lost opportunity to search for mates during the mating season. When a tethered female was nearby, males emerged from refuge earlier than if no female was present, indicating a trade-off between risk and mating opportunity. Approach speed affected emergence time when females were absent, but not when a female was present. Approach speed did not affect the probability that, after emerging, a male would return to court the female. For males that courted females intensely (bit them) before entering refuge, approach speed did not affect latency to emerge, but males that courted less intensely emerged sooner if approached slowly than rapidly. These findings show that males adjust the length of time spent in refuge to both risk of predation and reproductive cost of refuge use.Communicated by A. Mathis     

Locomotor impairment and defense in gravid lizards (Eumeces laticeps): behavioral shift in activity may offset costs of reproduction in an active forager

Summary Female Eumeces laticeps experience a substantial decrease in running speed (ca. 25%) and an even greater loss of endurance (slightly over 50%) while gravid. Because some widely foraging lizards, including E. laticeps, rely primarily on running to escape predators, the decreases in speed and stamina may contribute to an increased risk of predation. However, observations suggest that gravid females become less active or conspicuous on the surface. Ambush foraging lizards rely relatively more on crypsis associated with immobility to avoid predation and thus can have greater average relative clutch mass (RCM) than active foragers. Behavioral compensation for locomotor impairment by becoming less active or conspicuous may allow some species the advantages inherent in both high relative clutch mass when gravid and the increased energetic profitability of active foraging when not gravid. As females gain weight during the breeding season, they may forage actively until the risk due to increasing locomotor impairment becomes too great and then change defensive strategy to greater reliance on crypsis. Without such a shift, widely foraging squamate reptiles may be less able than ambush foragers to exploit life-historical strategies demanding high current investment in reproduction.     

Experimental manipulation of mate choice by male katydids: the effect of female encounter rate

Summary This study tests the general prediction that discrimination among potential mates increases with the availability of potential mates. Specifically, we conducted two experiments that examined mate choice by male zaprochiline katydids in relation to their prior encounter rate with females. The probability of mate acceptance or rejection was measured for males given either frequent or no contact with females in the laboratory (experiment 1) and males taken directly from natural areas of either high or low female abundance (experiment 2). In both experiments, males with low female encounter rates were more likely to mate than males with high female encounter rates. In both cases, the decreased mating probability of males in the high encounter treatment resulted from their tendency to reject lighter (and less fecund) females. Despite the presumed advantage to males of selecting heavier females, field data indicate that, unlike females, males do not aggregate in rich food patches. Possible explanations for this finding are discussed. Offprint requests to: T.E. Shelly at the present address     

Sex,food and conflicts: nutrition dependent nuptial feeding and pre-mating struggles in scorpionflies

Female and male reproductive interests often differ. In species in which matings are accompanied by a transfer of resources valuable for both participants, such as nuptial prey gifts, conflicts may readily occur. Scorpionflies may use alternative mating tactics. One is to offer a prey item (dead arthropod) to females in exchange for mating. This prey gift tactic includes a conflict because a male must decide on whether to offer the gift rather than to fight the female and consume the gift. The outcome may depend on the nutritional status of both males and females. Males may be more willing to give if they themselves are satiated and the condition of the females may influence the payoff from the males’ investment. Similarly, females may be more willing to accept food gifts if they are in poor nutritional condition. In this study of the scorpionfly Panorpa cognata, I experimentally manipulated the feeding history of both males and females. I observed the outcome of the direct interactions that followed when males that were holding prey were approached by females. I found that well-fed males offered the food gift sooner than males in poor nutritional condition that fed extensively on the food item before offering. Female condition had no significant influence on whether prey items were offered by males or accepted by females. I also found that well-fed males rarely searched for prey to pursue the prey gift tactic in courtship. Thus, the prey tactic does not seem to be the males’ first option.     

Biology of the deep-sea octopus Bathypolypus sponsalis (Cephalopoda: Octopodidae) from the western Mediterranean Sea

Several aspects of the biology of Bathypolypus sponsalis were studied from 297 individuals (115 males, 180 females and 2 indeterminates) caught in a depth range of 200–800 m depth in the western Mediterranean Sea. The paper presents data on sizes (length-weight relationships, size-frequency distributions) and reproduction (sex ratio, maturation, condition), and also analyses of the diet of B. sponsalis from samples taken throughout the year. Length-weight relationships showed that females are heavier than males at the same mantle length. Although mature individuals were found all year round, the maximum number occurred in spring and summer. Sexual maturation data revealed that males mature at smaller sizes than females. The gonadosomatic index increased with maturity in both sexes; the increase was gradual in males, but abrupt in females. The digestive gland index was used as a condition index and showed a differential behaviour with maturity; it increased gradually in females, but decreased in males. Like other octopus species, B. sponsalis appears to be an opportunistic predator, feeding on a great variety of preys. Stomach content analysis yielded a total of 19 different prey items belonging to four major groups (Crustacea, Mollusca, Ophiuroidea and Osteichthya). The first three groups were the more frequent preys, since crustaceans, molluscs and ophiuroids appeared in 76%, 49% and 30% of the stomachs, respectively. Decapoda Reptantia (among crustaceans) and cephalopods and bivalves (among molluscs) constituted the more abundant prey items. While the Decapoda Reptantia group was significantly more abundant in stomach contents of females, gastropods were taken more frequently by males. These differences in diet could reveal females as a more active predators than males. Received: 5 March 2000 / Accepted: 7 November 2000     

Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay,New Jersey,USA

Adult horseshoe crabs Limulus polyphemus (L.) feed on a wide variety of infaunal and epifaunal invertebrates during their spring spawning migration in Delaware Bay, New Jersey, USA. Comparison of the gut contents with estimates of available prey showed that the most abundant potential prey item, the bivalve Gemma gemma, was avoided. The thinner shelled but comparatively scarce clam Mulinia lateralis was a preferred prey item. In the laboratory, crabs fed on G. gemma when it was the only available item but not when M. lateralis or soft-shell clams, Mya arenaria, were offered in conjunction. Large M. lateralis (>10mm) were preferred to small M. lateralis; there was no discrimination between M. lateralis and M. arenaria of the same size. Male and female horseshoe crabs had similar gut contents and laboratory feeding preferences, despite the fact that females are larger than males. Crabs spawning later in the summer contained more food than did crabs collected at the peak of spawning activity.     

Different escape tactics of two vole species affect the success of the hunting predator, the least weasel

In the ongoing evolutionary arms race between predators and their prey, successful escape from the predator leads to the evolution of improved escape tactics in prey, but also predators become more effective in following and attacking the prey. Antipredatory behavior of prey is considered to be the strongest towards their most dangerous predators. However, prey species can differ both in vulnerability and efficiency of escape to a shared predator. We studied escape reactions of two vole species, the bank vole (Myodes glareolus) and the field vole (Microtus agrestis), under a simulated predation risk of the least weasel (Mustela nivalis nivalis). We conducted a laboratory experiment where a vole was given a possibility to escape from a weasel by fleeing to a horizontal tunnel or climbing the tree. Subsequently to the vole escape decision, we released a weasel to the same tunnel system to test how the weasel succeeded in following the vole. Weasel presence changed the behavior of voles as especially bank voles escaped by climbing. Instead, the majority of field voles fled into the ground-layer tunnel. The different escape tactics of the voles affected the success of the weasel, because climbing voles were less often successfully followed. We suggest that the difference in escape tactics has evolved as an adaptation to different habitats; meadow-exploiting field voles using ground-level escape while bank voles living in three-dimensional forest habitat frequently use arboreal escape tactics. This is likely to lead to different habitat-dependent vulnerabilities to predation in Microtus and Myodes vole species.     

Patterns of selective predation by juvenile,benthivorous fish on estuarine macrofauna

Benthic feeding on macrofauna was studied in juveniles of the sparids Lithognathus lithognathus and Rhabdosargus holubi in the upper reaches of the Gamtoos Estuary, South Africa. Fish and benthic macrofauna were sampled simultaneously, and the selection of invertebrate prey assessed. Both fish species strongly selected for corophioid amphipods and consumed other benthic taxa in low numbers. R. holubi also exploited aquatic autotrophs, while L. lithognathus had a narrow prey-spectrum, feeding almost exclusively on the tube-dwelling amphipod Grandidierella lignorum. G. lignorum was the most abundant prey species, both in the benthos and the fish's diet. This species also showed prominent behavioural differences between the sexes; males were markedly more active on the sediment surface than females, who rarely left their tubes during the day. Males switched from an infaunal to epifaunal microhabitat in search of receptive females, concurrently increasing their exposure to fish predators. Consequently, L. lithognathus selected significantly more males than female amphipods, causing a marked bias towards females in the sex ratio and age-structure of the amphipod population. Juvenile amphipods were less preyed upon, presumably as a result of lower prey-detection or capture efficiency by the predators. Accepting current notions about predation as an important structuring element for benthic communities, our data also stress the prominence of size-and sex-selective predation in structuring individual prey populations.     

Shark-inflicted injury frequencies, escape ability, and habitat use of green and loggerhead turtles

Interactions between large marine predators and their prey are difficult to observe and little is known about the risk of predation faced by sea turtles. The frequency of predator-inflicted injuries, however, has afforded insights into the predation risk faced by many taxa. We measured the frequency of shark-inflicted injuries on green (Chelonia mydas) and loggerhead (Caretta caretta) sea turtles in Shark Bay, Western Australia with a view to determining differences between species and sex-classes in the risk of predation from tiger sharks (Galeocerdo cuvier). Furthermore, we investigated how escape ability and habitat use might influence the probability of turtles being injured by sharks. Shark-inflicted injuries were more frequent on loggerhead than on green turtles, and most frequent on adult male loggerhead turtles. Species effects could not be attributed to differences in habitat use, since green turtles were found in habitats favored by tiger sharks more often than were loggerhead turtles. Green turtles, however, were faster and maneuvered better than loggerhead turtles, suggesting that escape ability is a factor in interspecific differences in injury frequency. The sex-class difference in injury frequency of loggerhead turtles suggests that males face greater predation risk than females and may take more risks. For green turtles, the lack of a sex difference in injury frequency might be due to greater escape ability lowering overall predation risk or to no differences between sexes in the benefits of risk-taking.     

Aggregation during oviposition and predation risk in Sympetrum vulgatum L. (Odonata: Libellulidae)

Summary Tandem pairs of the libellulid dragonfly Sympetrum vulgatum always start oviposition with contact guarding directly over the surface of shallow water, where they are exposed to green frogs (Rana esculenta). Tandems which approached locations with other pairs already present started oviposition nearby regardless of whether or not predators were actually present. With predators present attack rates during arrival were lower on tandem pairs in groups than on pairs that oviposited alone. During oviposition the attack rate on groups was similar to that on solitary pairs, but predation risk to individual tandem pairs was lower due to dilution effects. Predation risk during tandem oviposition was similar for both sexes, but females had a higher risk of falling prey to frogs during post-tandem oviposition than males. If tandems were attacked by frogs, females left the site after tandem oviposition despite the male hovering above her, and the frequency of non-contact guarded post-tandem ovipositions was reduced.     

Vulnerability of the copepod Acartia tonsa to predation by the scyphomedusa Chrysaora quinquecirrha : effect of prey size and behavior

Although scyphomedusae have received increased attention in recent years as important predators in coastal and estuarine environments, the factors affecting zooplankton prey vulnerability to these jellyfish remain poorly understood. Current models predicting feeding patterns of cruising entangling predators, such as Chrysaora quinquecirrha (Desor, 1948), fail to account for the selection of fast-escaping prey such as copepods. Nevertheless, our analysis of gastric contents of field-collected medusae showed that this scyphomedusa fed selectively on the calanoid copepod Acartia tonsa (Dana, 1846) and preferentially ingested adult over copepodite stages. We measured feeding rates in a planktonkreisel while simultaneously videotaping predator–prey interactions. C. quinquecirrha consumed adult A. tonsa ten times faster than copepodites. Differences in prey behavior, in the form of predator–prey encounter rates or post-encounter escape responses, could not account for the elevated feeding rates on adults. Prey size, however, had a dramatic impact on the vulnerability of copepods. In experiments using heat-killed prey, feeding rates on adults (1.5 times longer than copepodites) were 11 times higher than on copepodites. In comparison, medusae ingested heat-killed prey at only two to three times the rate of live prey. These results suggest that during scyphomedusan–copepod interactions, prey escape ability is important, but ultimately small size is a more effective refuge from predation. Received: 26 September 1997 / Accepted: 20 May 1998     

Dominance and competition in an herbivorous lizard

Summary A study of social organization in an herbivorous lizard Ctenosaura hemilopha investigated the role of dominance in group member behavior. Attention focused on a very populous (16 ind.) colony to examine causes and effects of crowding with respect to competitive factors. Lizards were colonial, with a top-rank male, one or more adult females, and various subadults and juveniles. Top-rank males defended harems, with colonies female-biased (1:4 in focal colony). Dominance hierarchy was observed, with females more aggressive than males, except for the top-rank male. Group awareness facilitated adaptive responses to threatening dominants and predators. Strong correlations existed among individual size, rank, and aggression. Top-rank male aggressiveness was partly explained by harem defense. Food resource competition, which causes aggression in female insectivorous lizards, did not explain female C. hemilopha behavior. Response to predators, predator fecal pellet analysis, and tail break frequencies implicate crevice escape sites for predator avoidance as a prime controller of social and population structure in these lizards.     

Morphology,fluid motion and predation by the scyphomedusa Aurelia aurita

Although medusan predators play demonstrably important roles in a variety of marine ecosystems, the mechanics of prey capture and, hence, prey selection, have remained poorly defined. A review of the literature describing the commonly studied medusa Aurelia aurita (Linnaeus 1758) reveals no distinct patterns of prey selectivity and suggests that A. aurita is a generalist and feeds unselectively upon available zooplankton. We examined the mechanics of prey capture by A. aurita using video methods to record body and fluid motions. Medusae were collected between February and June in 1990 and 1991 from Woods Hole, Massachusetts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita create fluid motions during swimming which entrain prey and bring them into contact with tentacles. We suggest that this mechanism dominates prey selection by A. aurita. In this case, we predict that medusae of a specific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected prey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusac because flow field velocity is a function of bell diameter. On the basis of prey escape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton such as barnacle nauplii and some slow swimming hydromedusae, while faster copepods will be negatively selected.     

Compensating for the costs of polygyny in hen harriers Circus cyaneus

In polygynous species, the adults are faced with a dilemma during chick rearing. Males must decide how to distribute food between their females and food allocation patterns are often highly unequal. In turn, the females that receive less food from males have to decide how much time to invest in additional hunting. If they spend more time hunting, then they leave their young exposed to weather and predators. However, if they stay at the nest, they increase the risk of their chicks starving. One way that birds may compensate for reduced provisioning is by increasing the size of prey caught. We tested this hypothesis by comparing prey deliveries to nests of hen harriers, Circus cyaneus, with females of different breeding status. As expected, male harriers delivered less food items to the nests of polygynous females, and especially their secondary, or β females. However, both sexes were able to compensate by delivering larger items and there was no difference in the overall mass of food delivered to nests. Moreover, females spent a similar amount of time at the nest, irrespective of status, and there were no overall differences in breeding success. Our results show that polygynous female harriers can compensate for the costs of polygyny, but we suggest that their ability to do so will vary according to the abundance of both large prey and predators.     

Sex- and performance-based escape behaviour in an Asian agamid lizard,Phrynocephalus vlangalii

In lizards, males are predicted to sprint faster and run for longer than females by virtue of higher testosterone levels and differences in morphology. Consequently, escape behaviour is also predicted to be associated with sex and locomotor performance, yet these links have rarely been explored. Here, we tested whether escape behaviour is associated with locomotor performance in the toad-headed agama, Phrynocephalus vlangalii, and whether it is sex-dependent. This species is also characterized by elaborate tail displays, which we examined as a potential pursuit-deterrent signal. Tail waves were performed by a very small proportion (2/58, 3 %) of individuals during predatory trials, suggesting that tail signalling functions exclusively in a social context. To understand the relationships between sex, escape behaviour and performance, we first measured escape behaviour (flight initiation distance, flight distance—measured differently compared to previous studies of lizard escape behaviour, and refuge use) in the field before measuring maximal sprint speed and endurance on the same individuals in the laboratory. Flight initiation distance did not differ between the sexes and was unrelated to performance capacity (maximal endurance and sprint speed) but was positively related to body size with larger individuals fleeing earlier. Males fled farther than females, but flight distance was also unrelated to either endurance or sprint speed. Interestingly, faster females were less likely to enter a refuge than slower females, whereas sprint speed and the probability of taking refuge were unrelated for males. Our results suggest that when males and females are not obviously sexually dimorphic, they are more likely to overlap in escape tactics.     

Tail loss reduces mating success in the Iberian rock-lizard,Lacerta monticola

Summary Many lizards autotomize their tails to escape when grasped by a predator. It is hypothesized that tail loss causes a reduction in social status, thereby potentially lowering their reproductive success. We experimentally induced tail loss in Lacerta monticola in a semi-natural enclosure, and show that tail loss reduced social status and mating access in males. Tailless males increased body mass more rapidly than tailed dominant males, probably due to lower aggression costs. Also, tailless females were courted less and copulated less than tailed females, supporting the hypothesis that tail loss decreases reproduction potential.Correspondence to: J. Martin     

The special role of male Cebus monkeys in predation avoidance and its effect on group composition

Summary Vigilance behavior, predator detection abilities, and responses to real and model predators were studied in two species of capuchin monkeys (Cebus albifrons and C. apella) in a Peruvian lowland rain forest. Adult males were more vigilant than adult females in both species, mainly because the males spent less time feeding and foraging and partly because they were at the periphery more often than the females. The increased vigilance of adult males is reflected in their superior performance in the detection of (model) predators. Adult and subadult males were also far more likely to approach and mob real and model predators. Adults that were outside the center of the group increased foraging activities but cut back an feeding, much of which was done in exposed tree crowns. Current theory suggests that primate groups are multi-male when a single male is unable to defend sexual access to the group of females. In these small capuchin groups, which are multimale, the enhanced safety of females and young provided by extra adult males furnishes a more plausible explanation. A comparison of the two capuchins with the ecologically similar Southeast Asian Macaca fascicularis suggests that the high predation risk outside the group may also have caused the unusual male career profile in capuchins, which have a long tenure of dominants and a very long potential lifespan. Further predictions of this hypothesis are developed.