Effect of predator cue on escape and oviposition behaviour of freshwater snail

When suitability of sites for parent is incompatible with that for their eggs, parent–offspring conflict occurs — the parent must choose between the optimal oviposition site for eggs and that suitable for itself. We examined effects of predator cue on parental habitat use and oviposition site in Physa acuta. We assessed desiccation risk in eggs and the relationship between water depth and predation risk. Predator cues induced the elevation of parental habitat away from the bottom but had no effect on the oviposition site. Oviposition site choice is not by-product of escape response. Eggs that were exposed to desiccation failed to hatch, indicating high desiccation risk. Eggs laid at deeper levels were at higher risk of stripping from oviposition substrates by crayfishes. Although the middle-water level would be suitable oviposition sites, the parents selected an oviposition site in the upper level. There are other benefits and costs of oviposition site selection.

Trait evolution under multiple selection pressures: Prey responses to predictable and unpredictable variation

When a strong selection pressure, such as predation risk, varies widely in space and time, how should prey respond? When risk varies predictably, prey are hypothesized to respond in a risk-sensitive manner. It is less clear how prey should respond when risk varies unpredictably.Additionally, prey response may also depend on how predation risk interacts with other selection pressures. Our understanding of the complex action of multiple and variable selection pressures on prey traits is still comparatively poor. Here, we examine how predictable and unpredictable aspects of predation risk act together with another important selection pressure to influence prey behaviour in the rock pool breeding mosquito, Aedes vexans. Through the selection of sites for oviposition, female mosquitoes can influence the predation risk faced by their offspring. We tested how females select oviposition sites, when encountering pools that vary in larval predation risk and desiccation risk. We comprehensively quantified spatial and temporal variation in predation risk by measuring densities of predatory dragonfly nymphs in rock pools of different sizes, along the mosquito breeding season. We also measured hydroperiod length. We next conducted manipulative experiments in rock pools and measured female oviposition responses to variation in predation and desiccation risks. Predation risk varied widely in space and time. Desiccation risk only appeared important for the small pools. Ovipositing females appeared to respond to these multiple aspects of variation in selection pressures. Females seemed to respond to predictable variation by avoiding large pools that permanently harboured predators in natural settings. Female responses were more variable to medium-sized pools with naturally stochastic predator densities, highlighting the role of unpredictability in predation risk in shaping behaviour. Females did not clearly prefer small pools that were naturally devoid of predators but carried high desiccation risk, suggesting that they balance multiple risks – predation versus desiccation – when choosing oviposition sites. Our study suggests that wild populations may commonly experience multiple and variable selection pressures that can favour seemingly puzzling trait variation. We highlight the need to quantify variation in selection pressures and investigate how such variation, especially the unpredictable aspects, shapes prey traits.

Oviposition site choice under conflicting risks demonstrates that aquatic predators drive terrestrial egg-laying

Laying eggs out of water was crucial to the transition to land and has evolved repeatedly in multiple animal phyla. However, testing hypotheses about this transition has been difficult because extant species only breed in one environment. The pantless treefrog, Dendropsophus ebraccatus , makes such tests possible because they lay both aquatic and arboreal eggs. Here, we test the oviposition site choices of D. ebraccatus under conflicting risks of arboreal egg desiccation and aquatic egg predation, thereby estimating the relative importance of each selective agent on reproduction. We also measured discrimination between habitats with and without predators and development of naturally laid aquatic and arboreal eggs. Aquatic embryos in nature developed faster than arboreal embryos, implying no cost to aquatic egg laying. In choice tests, D. ebraccatus avoided habitats with fish, showing that they can detect aquatic egg predators. Most importantly, D. ebraccatus laid most eggs in the water when faced with only desiccation risk, but switched to laying eggs arboreally when desiccation risk and aquatic predators were both present. This provides the first experimental evidence to our knowledge that aquatic predation risk influences non-aquatic oviposition and strongly supports the hypothesis that it was a driver of the evolution of terrestrial reproduction.

Variable reproductive strategies of an African savanna frog, Phrynomantis microps (Amphibia, Anura, Microhylidae)

West African savannas are habitats with unpredictable rainfall. Species with varying life-history traits may be more successful in these environments than species with fixed traits. We studied the reproduction strategies of the microhylid frog Phrynomantis microps in different savanna types, a humid savanna in Ivory Coast and a drier one in Benin. We recorded 5437 clutches in eight ponds in the humid savanna during five consecutive rainy seasons. A further 694 clutches were investigated in 10 ponds in Benin in one rainy season. For each clutch, we recorded egg numbers, deposition time, location within the pond and rainfall. Precipitation was important in triggering reproduction. However, the amount of rainfall needed differed in relation to rainy season length and total annual rainfall. Especially in years and regions with a late rainy season a threshold of minimum precipitation was needed to initiate spawning, indicating a trade-off between the need to reproduce and the survival probabilities of offspring in ponds with a high desiccation risk. Egg numbers per clutch further differed between pond sizes, breeding season length, as well as to time within the rainy season. Potential explanations for these differences, in particular desiccation and predation risks are discussed, but need further experimental support.

Does desiccation risk drive the distribution of juvenile cane toads (Bufo marinus) in tropical Australia?

Immediately after their transition from aquatic to terrestrial life, juveniles of many anuran species are restricted to the margins of natal ponds. Understanding the factors determining the duration of that pondside aggregation has direct management ramifications in the case of the invasive cane toad (Bufo marinus) in tropical Australia. Previous work suggests that dispersal confers biotic advantages (reduced risk of cannibalism, enhanced feeding opportunities) to juvenile toads, but desiccation risk constrains these small animals to the moist margins of the pond. If so, juvenile dispersal should be sensitive to fluctuating hydric conditions on a diel and seasonal cycle. We tested this prediction with field observations (monitoring of dispersal to and from the pond) and field experiments (manipulating hydric regimes). Our results support a dynamic model of juvenile distribution, with a primary role for temporal variations in desiccation risk as the primary factor driving dispersal. During the dry season, strong diel cycles in desiccation risk generate a ‘tidal’ flow of juveniles, dispersing out in the moist morning but retreating to the pond margins at midday. Dispersal rates were enhanced by artificial watering during the dry season, and by the onset of the wet season.

The reproduction of the Apennine yellow-bellied toad (Bombina pachypus) in central Italy

Amphibians are declining worldwide and many species are threatened for unknown reasons. In fact, information on ecology of several species are not available due to the lack of studies. The Apennine yellow-bellied toad (Bombina pachypus) is an Italian endemic species in progressive decline in many areas, often for unclear reasons. A two-year study of 116 temporary ponds in a grazing area of the Majella National Park (central Italy) from 2001 to 2002 revealed that: (a) Apennine yellow-bellied toads reproduced in small ponds characterized by high desiccation risk; (b) breeding activity occurred for a prolonged period (at least from May to the beginning of August), during which females spawned clutches of very few eggs in several, successive events; (c) the reproductive success was very low especially because of high egg mortality; and (d) the main cause of mortality was the desiccation of ponds. Consequently, the characteristics of breeding areas (i.e., small, ephemeral ponds) are the most important feature influencing the reproduction. The safeguard of temporary ponds is crucial for the conservation of this amphibian and could preserve also the other species using these pools.