Snake Predation Strategies – Part 2: Venom and constriction
Last week, in our Predation strategies - Part 1: Bodies and behaviours blog, we discussed the links between a snake’s morphology (body form and structure) and its predation strategy. This week, we are taking a closer look at prey subjugation strategies - that is, the strategies snakes use to overpower their prey.
Strategies for survival
Over previous blogs, we have introduced the concept of animals being specialised to their environment (adapted) and discussed a few of the adaptations that help snakes survive, particularly their predation strategies. Their vomeronasal organ helps them to pick up a scent trail and their ectothermy and low metabolic rate allow them to go for long periods of time between meals. Many ambush-hunting snakes have heat-sensing pits, and as a rule-of-thumb most tend to have a thick and powerful body, which helps them to strike quickly or overpower their prey. In contrast, active hunters tend to have longer, thinner bodies with greater agility, which complement their prey capture strategy.
The dwarf crown snake (Cacophis kreffti) is a small venomous elapid that hunts lizards. Image: Scott Eipper CC2.
When making these generalisations about the links between snake morphology (body form) and predation strategy, it is important to remember that there are many exceptions to the rule. You may recall from last week that an animal’s adaptations increase its chance of survival, which in turn increases its chance of evolutionary success – its fitness. However, fitness is influenced by all aspects of an animal’s ecology – its interactions with other living things, the conditions of the environment, and its role within that environment; its niche. A snake’s adaptations will therefore reflect the variables of its niche. If we consider microhabitat, for example, snakes that sit hidden in the leaflitter for weeks on end will have very different morphology and colouration than snakes that live amongst the vines and branches of trees – even if they are both ambush predators, such as the two snakes pictured in the top row below. As all these factors play a role in shaping snakes’ adaptations there is a diverse mix of morphologies and behaviours, including some agile snakes that ambush their prey as well as some stout snakes that actively hunt (bottom row).
Top left: Copperhead (Agkistrodon contortrix), image Judy Gallagher, CC2; top right: Large-eyed pit viper (Trimeresurus macrops), image Rushen, CC2; bottom left: Amazon tree boa (Corallus hortulanus), image Charles (Chuck) Peterson, CC2; bottom right: Shield-nosed cobra (Aspidelaps scutatus), Mokele, CC2.
Subjugation strategies
In previous blogs, we have mostly focussed on adaptations in the context of locating prey. So, what adaptations and behaviours do snakes have to actually catch and kill their prey? Before we get into it, it is worth noting that successful predation is determined by prey capture – not necessarily prey death. A snake is probably not too bothered if its meal still has a heartbeat, so long as that heartbeat is not powering a defensive attack or a bid for freedom. It just so happens that causing rapid death is often an outcome of meeting these criteria. For this reason, we prefer to use a term such as “subjugating” (bringing under control) or “immobilising” as opposed to “killing” when we talk about a snake catching and overpowering its prey.
When it comes to the act of subjugating prey, we find two dominant strategies utilised by snakes: venom and constriction. However, there is an interesting spread of these two strategies among active and ambush predators. Venom, in particular, is widespread amongst the so-called “advanced” snakes (Colubroides - the clade that contains some 85% of all snake species). But it’s not as simple as saying that some snakes use venom while others constrict. We can find examples all along the continua of the “venomous to non-venomous” and “constrictor to non-constrictor” axes as well as the points of intersection between the two. There are ambushing non-venomous constrictors (such as pythons and boas), ambushing venomous non-constrictors (such as most vipers), active venomous non-constrictors (such as many elapids), active non-venomous constrictors (some colubrids), and even active venomous constrictors (other colubrids). And, of course, there are lots of in-betweens!
When venom takes on constriction. The snake whose head is touching the sand is a reticulated python (Malayopython reticulatus), which uses constriction to subjugate prey. The other snake is a venomous king cobra (Ophiophagus hannah), infamous for feeding almost exclusively on other snakes. The king cobra probably attempted to eat the python, but it looks like its venom did not act quickly enough to mitigate the python's defensive attack. In this case, these two snakes were evenly matched - and it didn't end well for either of them. Photo: Reptile hunter.
Constriction
The pythons and boas are most famous for this strategy, and they are also non-venomous ambush predators. These snakes usually follow the large head and strong, thick body plan that is typical of ambush predating snakes. Their bulk means they are not particularly fast or agile, but when they encounter a prey animal after a period of lying in wait, they will strike and swiftly immobilise the animal by coiling around it. Pythons and boas also typically have long bodies as they need plenty of length to wrap around and immobilise their target. Their large heads and numerous teeth help them to secure the animal in the first crucial moments of contact – they bite and don’t let go. In fact, to help them do this, pythons and boas have two rows of hook-like teeth on their upper jaw. These snakes will sustain their constriction until their prey’s circulation is arrested. The adaptive value of their solid heads and bodies is therefore built upon having the strength to physically overpower their prey. This, along with the size and length of their bodies, also means that they are able to consume very large animals. Such meals can keep them going for months before they must feed again. Pythons also hold the record for the most extreme metabolic shifts pre-and post-meal (see this blog for a recap on metabolism), and this is strongly linked to their predation strategies and body size.
The diamond python (Morelia spilota) has the large head and thick, strong body that is typical of constrictors. Image Doug Beckers CC2.
However, there are many other smaller species of snake that constrict their prey too, including some active predators and some venomous species. When it comes to active-hunting constrictors, the importance of an agile body appears to somewhat outweigh the need for sheer physical strength to overpower large prey, as snakes that fit this category are typically long and slender with small heads (though there are of course exceptions, such as the large, strong, and agile rat snakes). However, active-hunting constriction is common among species that also have mild venom, such as many colubrids (members of the family Colubridae). These snakes have specialised fangs at the rear of their mouth. However, these fangs aren’t especially efficient at injecting venom, which cannot really be said is their primary means of prey subjugation. However, as their means of physical subjugation is less powerful than that of a typical ambush constrictor, it may be that the venom weakens their prey enough to help overcome the disadvantages associated with the trade-off between agility and strength (a more in-depth discussion on this topic can be found here).
The brown tree snake (Boiga irregularis) uses both venom and constriction during prey-handling - but the contribution of its venom to subjugating prey is contentious. Image teejaybee CC2.
Venom
Venom is a molecular adaptation, so we can’t really see it properly. However, we can look at the behaviour and morphology of a venomous snake to see the adaptations it possesses that are associated with being venomous.
Venomous ambush-hunting snakes, such as most vipers (members of the family Viperidae), possess the thick bodies associated with ambush predation. However, as they subjugate their prey in an entirely different manner to their constricting counterparts – through the use of chemical weaponry rather than physical strength – we see many differences between venomous and non-venomous ambush species. As they do not physically immobilise their prey, i.e. via constriction, many vipers will bite-and-release in order to reduce the risk of injury to themselves if the animal retaliates. Vipers possess a highly sophisticated venom system in the form of long, hollow fangs that are coupled with large venom glands and powerful compressor muscles. This enables them to deliver a large yield of toxic venom through a high-pressure venom delivery system deep into their prey’s tissue. The short, thick bodies of these snakes generate the power behind their explosive strike, which they can execute in around one tenth of a second. The acceleration they experience when striking this quickly – around 2.5 metres per second – is twice as great as the maximum acceleration experienced by a fighter-jet pilot. They then rely heavily on their forked tongue and vomeronasal gland to locate the animal, which may have travelled a fair distance before it succumbed to the venom’s effects. There is also evidence to suggest that components of their own venom act as a chemical tag to help them do this.
There are, however, many arboreal vipers that deviate a little from these general trends. They typically have longer and thinner bodies, a reflection of the need to better navigate through and camouflage within their arboreal habitat. They also often hold on to their prey after biting, presumably as it's pretty difficult to follow the scent trail of an animal that falls out of a tree!
Venomous hunters. Left: A terrestrial ambush predator, the bushmaster (Lachesis muta) (image Thomas Stromber CC2); Right: a mostly terrestrial active predator, the Indian cobra (Naja naja) (image Chandan Singh CC2).
When we look at the adaptations of highly venomous active predators, we can see that they share a similarity of body morphology and behaviour with their active hunting non-venomous or mildly venomous counterparts. This tells us that having an agile body is very important for an active predation strategy. Though some colubrids are mildly venomous, when it comes to active predation strategies utilising venom, the elapids (members of the family Elapidae) are the masters. This is their primary means of prey subjugation, and their venom is typically very powerful.
That being said, many elapids will also coil around their prey while waiting for the effects of their venom to set in. The role that constriction plays in prey subjugation is a little unclear in these cases, but by observing predatory and prey-handling behaviour we can make some hypotheses. For example, it may be a means of immobilising the animal so that it cannot injure the snake. However, the fangs of many elapids are relatively short in comparison to those of a typical viper. Fang morphologies will influence their ability to penetrate different species’ skins. We often see a trend whereby bite-and-release is employed by elapids when subjugating prey that have the potential to cause serious injury (such as biting rodents), while constriction is utilised when predating upon prey animals whose skin is harder to penetrate (such as scaly skinks). In these snakes, constriction may therefore facilitate envenoming by enabling the snake to effectively pierce its prey’s protective scales.
There is still a lot we don’t know about snakes, their use of venom, or even how it works to subjugate prey. But next time we will discuss some of the things we do know (or think we know!) - so thanks for reading, and stay tuned for the next instalment!
- Bianca