Tag Archives: #prey

Unusual Prey: Spiders Eating Snakes (Biology)

There are spiders that eat snakes. Observations of snake-eating spiders have been reported around the world. Two researchers from Basel and the US consolidated and analyzed over 300 reports of this unusual predation strategy.

Spiders are primarily insectivores, but they occasionally expand their menu by catching and eating small snakes. PD Dr. Martin Nyffeler, arachnologist at the University of Basel, and American herpetologist Professor Whitfield Gibbons of the University of Georgia, USA, got to the bottom of this phenomenon in a meta-analysis. Their findings from a study of 319 occurrences of this unusual feeding behavior recently appeared in the American Journal of Arachnology.

It turns out that spiders eat snakes on every continent except Antarctica. Eighty percent of the incidents studied were observed in the US and Australia. In Europe, on the other hand, this spider feeding behavior has been observed extremely rarely (less than 1 percent of all reported incidents) and is limited to the consumption of tiny, non-venomous snakes of the blind snake family (Typhlopidae) by small web-building spiders.

Black widows are particularly successful

Incidents of snake predation by spiders have never been reported from Switzerland. A possible explanation is that Switzerland’s native colubrids and vipers are too big and heavy even when freshly hatched for Swiss spiders to subdue them.

Juvenile Eastern garter snake, Thamnophis sirtalis, trapped in a brown widow web (Latrodectus geometricus) observed in Douglas, Georgia, USA. (Photo: Julia Safer)

The data analysis also showed that spiders from 11 different families are able to catch and eat snakes. «That so many different groups of spiders sometimes eat snakes is a completely novel finding,» Nyffeler emphasizes.

Black widows of the family Theridiidae were the successful snake hunters in about half of all observed incidents. Their potent venom contains a toxin that specifically targets vertebrate nervous systems. These spiders build webs composed of extremely tough silk, allowing them to capture larger prey animals like lizards, frogs, mice, birds and snakes.

Big catch 

Another new finding from the meta-analysis: spiders can subdue snakes from seven different families. They can outfight snakes 10 to 30 times their size.

The largest snakes caught by spiders are up to one meter in length, the smallest only about six centimeters. According to the statistical analysis done by the two researchers, the average length of captured snakes was 26 centimeters. Most of the snakes caught were very young, freshly hatched animals. That some spiders are able to subdue oversized prey is attributable to their highly potent neurotoxins and strong, tough webs.

Possible insights into the effect of spider venom

Many spider species that occasionally kill and eat snakes have venom that can also be lethal to humans. That means the venom of various spider species has a similar effect on the nervous systems of snakes and humans. For this reason, observations of vertebrate-eating spiders can also be important for neurobiology, as they allow conclusions to be drawn about the mechanisms by which spider neurotoxins affect vertebrate nervous systems.

Brown widow spider feeding on a Brahminy blindsnake in a garden house in Zaachila, Oaxaca, Mexico. (Photo: Matias Martinez)

«While the effect of black widow venom on snake nervous systems is already well researched, this kind of knowledge is largely lacking for other groups of spiders. A great deal more research is therefore needed to find out what components of venoms that specifically target vertebrate nervous systems are responsible for allowing spiders to paralyze and kill much larger snakes with a venomous bite,» says Martin Nyffeler.

The captured snakes are anything but helpless themselves: about 30 percent are venomous. In the US and South America, spiders sometimes kill highly venomous rattlesnakes and coral snakes. In Australia, brown snakes – which belong to the same family as cobras – often fall prey to redback spiders (Australian black widows). Martin Nyffeler says, «These brown snakes are among the most venomous snakes in the world and it’s really fascinating to see that they lose fights with spiders.»

Storage of energy reserves

When a spider catches a snake, it will often spend hours or days feasting on such a large prey. Spiders have an irregular feeding pattern. When a lot of food is available, they eat in excess, only to go hungry for long periods again afterward. They store excess food as energy reserves in their body and use it to tide them over longer periods of starvation.

Still, a spider often eats only a small part of a dead snake. Scavengers (ants, wasps, flies, molds) consume what remains.

Original publication

Martin Nyffeler and J. Whitfield Gibbons (2021).
Spiders (Arachnida: Araneae) feeding on snakes (Reptilia: Squamata).
Journal of Arachnology 49, (1-27), doi: 10.1636/JoA-S-20-050

Featured image: Scarlet snake entrapped and killed in a black widow web in the corner of the front porch of a house in Gulf Breeze, Florida, USA. (Photo: Trisha Haas)


Provided by University of Basel

Neurons In A Visual Brain Area Of Zebrafish Are Arranged As A Map For Catching Prey (Neuroscience)

Spotting, pursuing and catching prey – for many animals this is an essential task for survival. Scientists at the Max Planck Institute of Neurobiology now show in zebrafish that the localization of neurons in the midbrain is adapted to a successful hunting sequence.

Visually responsive neurons in zebrafish are arranged in a map that serves to efficiently catch prey. ©MPI of Neurobiology / Förster

Far away, in the periphery of its visual field, a tiny zebrafish larva detects a small dot moving sideways. Is it prey or is it a threat, for instance, a distant predator sneaking up on it? Within the shortest possible time, the fish decides that it must be potential prey. The larva turns toward the object, approaches it, until it is right in front, and snaps shut – one of its daily hunting routines is successfully finished.

What might sound straightforward, is actually a highly complex process. Many different visual stimuli are detected simultaneously, transferred from the eye to the brain, and further processed. Interestingly, the stimuli don’t reach the brain at random locations: every position on the retina is transmitted to a very specific location in the tectum of the midbrain, the processing hub for visual stimuli. However, apart from that, there is not much knowledge of how the neurons are wired and organized, or which signals they specifically react to.

Dominique Förster and a team from Herwig Baier’s laboratory analyzed how retinal ganglion cells transfer visual information from the eye to the tectum and how this input is further processed. To do so, zebrafish larvae were presented in a virtual reality area with different visual stimuli, ranging from small and big prey-like objects to approaching threats similar to predatory fish. Using a special microscopy technique, the researchers not only analyzed the activity of hundreds of neurons in parallel, but also the location of their cellular projections.

Response to optical stimuli

Analyzing this pool of data showed that retinal ganglion cells as well as neurons in the tectum respond to optical stimuli in a highly specialized manner: While some cells are activated by small objects, others react to bigger objects, or even threats. Some cells are interested in the direction of motion, others only in whether the environment gets darker or brighter. Interestingly, the special-purpose neurons do not distribute randomly in the tectum. Retinal ganglion cells reacting to threats send their projections into deep tectal layers, where they meet the receiving processes, or dendrites, of tectal neurons. In contrast, cells responding to prey-like objects make synaptic connections in more superficial layers. This specialization of different tectal layers most likely allows zebrafish to quickly distinguish between prey and threat – an essential skill for survival.

The researchers then discovered that the prey-specific cells of the upper layers are arranged in a way that is advantageous for the hunting sequence: prey usually appears first at a distance in the peripheral visual field. Such images are represented in the back part of the tectum. Strikingly, this region of the tectum is enriched with cells reacting to small moving objects. When the fish turns toward the prey and approaches it, the prey’s image moves to the front part of the tectum and gets bigger, until it appears directly in front of the fish. There it is held steady by the fish’s own movement, until the prey can be captured by a vigorous strike. Neurons tuned to visual input with these characteristics, that is, large and steady, sit in the frontal region of the tectum – exactly where this kind of information reaches the brain.

This study shows that the arrangement and connectivity of neurons in the tectal brain map is adapted to the demands of hunting. Specialized cells localize to brain regions where their function is best suited for an efficient catch. By using the hunting behavior of zebrafish as an example, the researchers were able to demonstrate the impact of natural selection on the layout of relevant brain regions. These results remind us that the way animals (including us) perceive the world is shaped by evolution. The way the brain is wired has worked best to ensure survival in the past.

References: Dominique Förster Thomas O Helmbrecht, Duncan S Mearns, Linda Jordan, Nouwar Mokayes, Herwig Baier et al., “Retinotectal circuitry of larval zebrafish is adapted to detection and pursuit of prey”, Neuroscience, 2020 DOI: 10.7554/eLife.58596 http://dx.doi.org/10.7554/eLife.58596

Provided by Max Planck Gesellschaft

Snake Island Is Teeming With Nothing But Outrageously Venomous Snake (Amazing Places)

Some names really say it all. You shouldn’t have to do much research to decide if you’d like to go to “Gumdrop Mountain.” Likewise, you wouldn’t expect to have to warn people to stay away from “Snake Island.” But if you manage to find your way there, you’ll find a place where the most toxic bite rules.

©gettyimages

According to some estimates, there’s about one snake per square meter on Ilha da Queimada Grande (as it’s properly called in Portuguese) and up to 4,000 of those are deadly golden lancehead vipers. Legends say that pirates brought the snakes themselves in order to protect the treasure they’d hidden on the island, but in reality, the snakes have been there for thousands of years.

In fact, the snakes of Snake Island have been there since before it was an island. 11,000 years ago, rising water levels turned a peninsula into an island, leaving the snakes stranded to evolve on their own. With no land predators to worry about, the snakes had it made. There was just one problem: They didn’t have any land prey to eat, either. There were birds, and that fact shaped the evolution of the golden lancehead over the next few thousand years. Evolving to dine on their feathered neighbors gave the snakes one of the most toxic bites in the animal kingdom.

The birds that land on Snake Island aren’t giants, and they aren’t particularly resistant to poison. So why did these snakes need to develop one of the strongest venoms on the planet? The answer is speed. While most venomous snakes can sink their teeth into their prey and then wait for the toxin to take effect, the golden lancehead’s meals will fly back to the mainland if given half a chance. That’s why their venom has to work so quickly — the fact that it’s literally strong enough to melt human flesh is an unintended side effect.

©gettyimages

You might not expect a place that’s literally crawling with deadly snakes to require a warning, but the government of Brazil has found it necessary to do so. It’s not just for the people’s sakes either — the golden lancehead is listed as critically endangered, and this is the only place in the world that it lives. That’s why it’s illegal for people to visit the island, with the exception of the herpetologists that study the wildlife from their on-site labs and the Brazilian navy’s annual trip to keep the island’s lighthouse in order.

You don’t have to tell us twice. 

How Venus Flytraps Catch Their Prey? (Botany)

A 2015 study has found that Venus flytraps recognize, ensnare, and digest their prey by “counting” the number of times the insect brushes against its sensory hairs. A fly that brushes its leg against the plant’s hair only once could live another day. But a second brush is crucial: it prompts the flytrap to send water into its leaves, which can snap shut over the fly in a tenth of a second. As the trapped insect continues to struggle against the hairs, the plant continues to receive electrical impulses from the contact. The fifth impulse is the next important one, and it prompts the fly trap to produce digestive enzymes.


References: Jennifer Böhm, Sönke Scherzer, Elzbieta Krol, Ines Kreuzer, Katharina von Meyer, Christian Lorey, Thomas D. Mueller, Lana Shabala, Isabel Monte, Roberto Solano, Khaled A.S. Al-Rasheid, Heinz Rennenberg, Sergey Shabala, Erwin Neher, and Rainer Hedrich (2016). The Venus Flytrap Dionaea muscipula Counts Prey-Induced Action Potentials to Induce Sodium Uptake. Curr. Biol. 26. Link: https://www.cell.com/current-biology/fulltext/S0960-9822(15)01501-8