Odour-Mediated Interactions Between an Apex Reptilian Predator and its Mammalian Prey

An important but understudied modality for eavesdropping between predators and prey is olfaction, especially between non-mammalian vertebrate predators and their prey. Here we test three olfactory eavesdropping predictions involving an apex reptilian predator, the sand goanna Varanus gouldii, and several species of its small mammalian prey in arid central Australia: 1) small mammals will recognise and avoid the odour of V. gouldii; 2) V. gouldii will be attracted to the odour of small mammals, especially of species that maximise its energetic returns; and 3) small mammals will be less mobile and will show higher burrow fidelity where V. gouldii is absent compared with where it is present. As expected, we found that small mammals recognised and avoided faecal odour of this goanna, feeding less intensively at food patches where the odour of V. gouldii was present than at patches with no odour or a pungency control odour. Varanus gouldii also was attracted to the odour of small mammals in artificial burrows, and dug more frequently at burrows containing the odour of species that were energetically profitable than at those of species likely to yield diminished returns. Our third prediction received mixed support. Rates of movement of three species of small mammals were no different where V. gouldii was present or absent, but burrow fidelity in two of these species increased as expected where V. gouldii had been removed. We conclude that olfaction plays a key role in the dynamic interaction between V. gouldii and its mammalian prey, with the interactants using olfaction to balance their respective costs of foraging and reducing predation risk. We speculate that the risk of predation from this apex reptilian predator drives the highly unusual burrow-shifting behaviour that characterises many of Australia's small desert mammals.

Odour-Mediated Interactions Between an Apex Reptilian Predator and its Mammalian Prey

An important but understudied modality for eavesdropping between predators and prey is olfaction, especially between non-mammalian vertebrate predators and their prey. Here we test three olfactory eavesdropping predictions involving an apex reptilian predator, the sand goanna Varanus gouldii, and several species of its small mammalian prey in arid central Australia. Firstly, as expected, we found that small mammals recognised and avoided the faecal odour of sand goannas, feeding less intensively at food patches where the odour of V. gouldii was present than at patches with no odour or a pungency control odour. Secondly, V. gouldii was attracted to the odour of small mammals in artificial burrows, and dug more frequently at burrows containing the odour of species that were energetically profitable than at those of species likely to yield diminished returns. Our third prediction received mixed support. Rates of movement of three species of small mammals were no different where V. gouldii was present or absent, but burrow fidelity in two of these species increased as expected in an area where V. gouldii had been removed. We conclude that olfaction plays a key role in the dynamic interaction between V. gouldii and its mammalian prey, with the interactants using olfaction to balance their respective costs of foraging and reducing predation risk. We speculate that the risk of predation from this apex reptilian predator drives the marked and highly unusual burrow-shifting behaviour that characterises many of Australia's small desert mammals.

Australian funnel-web spiders evolved human-lethal δ-hexatoxins for defense against vertebrate predators

Australian funnel-web spiders are infamous for causing human fatalities, which are induced by venom peptides known as δ-hexatoxins (δ-HXTXs). Humans and other primates did not feature in the prey or predator spectrum during evolution of these spiders, and consequently the primate lethality of δ-HXTXs remains enigmatic. Funnel-web envenomations are mostly inflicted by male spiders that wander from their burrow in search of females during the mating season, which suggests a role for δ-HXTXs in self-defense since male spiders rarely feed during this period. Although 35 species of Australian funnel-web spiders have been described, only nine δ-HXTXs from four species have been characterized, resulting in a lack of understanding of the ecological roles and molecular evolution of δ-HXTXs. Here, by profiling venom-gland transcriptomes of 10 funnel-web species, we report 22 δ-HXTXs. Phylogenetic and evolutionary assessments reveal a remarkable sequence conservation of δ-HXTXs despite their deep evolutionary origin within funnel-web spiders, consistent with a defensive role. We demonstrate that δ-HXTX-Ar1a, the lethal toxin from the Sydney funnel-web spider Atrax robustus, induces pain in mice by inhibiting inactivation of voltage-gated sodium (NaV) channels involved in nociceptive signaling. δ-HXTX-Ar1a also inhibited inactivation of cockroach NaV channels and was insecticidal to sheep blowflies. Considering their algogenic effects in mice, potent insecticidal effects, and high levels of sequence conservation, we propose that the δ-HXTXs were repurposed from an initial insecticidal predatory function to a role in defending against nonhuman vertebrate predators by male spiders, with their lethal effects on humans being an unfortunate evolutionary coincidence.

Changes in positive associations among vertebrate predators at South Georgia during winter

We studied positive associations among seabirds and marine mammals at South Georgia on research cruises during the Austral winters of 1985, 1991 and 1993 and found statistically significant differences. We collected data on abundance and distribution, providing a critical reference for sub-Antarctic conservation in anticipation of future environmental changes. We found significant changes in the abundance of 29% of species surveyed and a consequent change in species diversity. We postulate that the resulting altered community composition may have previously unanticipated population effects on the component species, due to changes in positive interactions among species which use each other as cues to the presence of prey. We found a near threefold reduction in spatial overlap among vertebrate predators, associated with warming sea temperatures. As the strength and opportunity for positive associations decreases in the future, feeding success may be negatively impacted. In this way, environmental changes may disproportionately impact predator abundances and such changes are likely already underway, as Southern Ocean temperatures have increased substantially since our surveys. Of course the changes we describe are not solely due to changing sea temperature or any other single cause—many factors are important and we do not claim to have removed these from consideration. Rather, we report previously undocumented changes in positive associations among species, and argue these changes may continue into the future, given near-certain continued increases in climate-related changes.

The effect of insect cyanoglucosides on predation by domestic chicks

Cyanogenic insects release cyanide and other byproducts that are thought to make them unpalatable to would be predators. In fact, aposematic Heliconius butterflies and Zygaena moths are rejected by vertebrate predators. Nevertheless, there have been few studies testing the effect of cyanoglucosides on vertebrate predation. Here I report tests undertook with captive domestic chicks in order to evaluate the effect of two chemically diverse cyanoglucosides, linamarin and prunasin. In insects as well as plants, cyanoglucosides are stored in vacuoles and are enzymatically broken down when the tissue is disrupted as in the case of a predator attack. Linamarin is an aliphatic that releases cyanide and acetone upon breakdown, while prunasin is an aromatic cyanoglucoside that releases cyanide and benzaldehyde. Using concentrations that mimicked the average concentration of a Heliconius butterfly, supplemented by test with higher concentrations, I ran a series of trials with naïve chicks by offering prey laced with chemicals. I categorized prey acceptance and compared the behavior of the predators towards test and control larvae. Prey coated with cyanide and benzaldehyde were significantly rejected by the birds, while acetone did not elicit avoidance behavior. Intact cyanoglucosides apparently were not detected by the predators, presumably because of fast ingestion time or lack of enzymes to breakdown cyanoglucosides. The rejection of cyanide laced prey confirm the protective nature of cyanoglucosides against a vertebrate predator. Additionally, the rejection of the pungent but not toxic benzaldehyde suggests that some species that store aromatic cyanoglucosides could be detected via smell as well by taste. These results provide support for cyanoglucosides as defensive chemicals of aposematic lepidopterans and related arthropods.

Consumption of the eggs, hatchlings, and tadpoles of Green Frogs (Lithobates clamitans) by native and non-native predators

Predation by native and non-native predators on the eggs, embryos, and early stage tadpoles can affect the recruitment of offspring into a population. We examined the effects of native (Little Brown Mudbugs, Cambarus thomai; overwintered Rana tadpoles; Common Green Darner, Anax junius, larvae) and non-native (Western Mosquitofish, Gambusia affinis) potential predators on the eggs, hatchlings, and early tadpoles of the Green Frog (Lithobates clamitans). The predators had no effect on survivorship or hatching of L. clamitans eggs. However, tadpole survivorship was significantly reduced by dragonfly larvae and crayfish, but not G. affinis or the overwintered ranid tadpoles. Our observation that invertebrates consumed Green Frog tadpoles while vertebrates did not is consistent with palatability contributing to the tadpoles’ susceptibility to different predators. Our results therefore suggest Green Frog tadpoles, but not eggs or embryos, from some populations may be subject to differential predation by invertebrate and vertebrate predators.