The cost of living in the animal kingdom is a phenomenon which has led to evolutionary changes distinguishing species’ reaction to predators and prey. Venom has revealed itself as a chancy investment, being a large energy expenditure resulting in compromise over millions of years. Not to be confused with poison, venom is a poisonous liquid that is produced and can be put into another animal's body by biting or stinging. Nevertheless, venomous reptiles thrive, some using venom for predation and others for defence. The distribution of venomous animals favour the Class Reptilia over Mammalia or Aves for significant reasons - rivalry between the efficacy of physical versus chemical means of survival. Some of the few venomous non-reptilians have been explored, revealing their competitive advantage and reasons why being venomous has not spread to more species.

Metabolic trade offs and energy efficiency

Producing and storing venom involves huge metabolic trade offs; it is therefore compromised through other adaptations, often being more energy efficient. For example, carnivorous birds (like the falcon) have sharp claws, muscular legs and spear-shaped beaks which help them hunt and defend themselves. Hypothetically, if a falcon began producing venom, it may be easier to subdue their prey, however, the huge energy requirement in producing venom would deflate the available energy for flight and the agile movements required when hunting prey. These kinds of adaptations are seen across the majority of mammals and birds, who have mainly adapted physical forms of defence/predation compared to chemicals. Utilising and preserving claws and teeth are an alternate, less energetically expensive method of predation and defence in mammals and birds.

Instead, ‘Toxic Birds’ are poisonous rather than venomous. These species obtain toxins from the plants or insects they feed on, and do not produce their own toxins. This implies that their toxicity is not produced by extra glands, instead obtained via their food. Not only are Toxic Birds gaining the necessary nutrients required, but also toxins to support their predation and defence (suggesting a cross-functional diet). Toxic birds therefore have no extra expenditure to be poisonous, implying why a small fraction of birds who have a highly unique diet can profit from this advantage.

Being endotherms, mammals and birds have the added energy cost of thermoregulation. This is to ensure that an optimum temperature is maintained for enzymes to catalyse chemical reactions. This added fee reduces the available resources in mammals and birds, hence an even greater emphasis on energy efficiency is stressed. In contrast, all reptiles are ectotherms, par one species of lizard, the Argentine black and white Tegu; pythons, although endotherms, have the unique ability to shiver to generate their own heat. Furthermore, ectotherms depend on external sources for energy rather than metabolism, which is why the majority of reptiles live in deserts or tropical environments - the common denominator being higher ambient temperature. Ectotherms need far less energy to function, suggesting that the remaining discretionary energy may have evolved to be invested into the maintenance of venom.

One tends to see that some venomous animals lack other means of predation or defence, such as a deficiency of limbs (notably arms, legs or wings). This raises the need for a defence mechanism for those who generally can’t retreat from predators. Venom therefore serves as an excellent defence mechanism; the cobra’s distinct ability to spit venom in their predator’s eyes results in instant pain and temporary or permanent blindness. Birds on the other hand may fly away from land predators, exemplifying the unneeded nature of venom as a defence mechanism. This further supports the idea of a metabolic trade off, as bird’s wings are used in defence and predation, showing its efficiency, so maintaining venom would be superfluous.

Some snakes have the ability to consume prey which seem impossible due to their size difference. A (venomous) black mamba is an example of this, devouring baby pigs, large mice, bats and squirrels in one go. This is due to adaptations of its skull having extremely flexible bones and skin, the quadrate bone which adds an extra turning point to the lower jaw and recurved teeth which help snakes to eat large prey whole. The greater energy source obtained by consuming these larger prey replenishes the taxing metabolic cost of creating the venom used to subdue it. This is an example of efficient usage of energy which helps reptiles, like snakes, to survive. Many constrictor snakes have evolved to not produce venom but also consume larger prey; these snakes, including boas and some pythons, are far larger and can defeat prey by wrapping themselves around them (constriction), which smaller snakes (eg. the black mamba) can not (hence they rely on venom).

Venomous mammals in Mammalian taxonomic Orders:

Eulipotyphla

In the order Eulipotyphla, five of over four hundred species are confirmed to be venomous, the greatest number of venomous mammals out of the four mammalian order. Their reasons for this metabolic compromise mimic that of some reptiles (the most venomous being solenodons and shrews). Due to the high basal metabolic rate (BMR) of eulipotyphlans more energy is required to sustain their metabolism, therefore more prey (or larger prey) are needed to be digested. Venom plays a crucial role in this mechanism, as it doesn’t kill the prey, instead paralysing it. This keeps the prey conscious, resulting in the animal’s nutrients being preserved at the time of consumption. More significantly, this means eulipotyphlans can hoard paralysed prey and reduce the frequency of exiting into danger; less frequent departure from their habitats reduces the chances of predators (particularly large birds) discovering their secure habitat and waiting in ambush, or simply preying on eulipotyphlans while they are also hunting. An example being Blarina brevicauda who paralyses insects to store for up to 3 days. Kowalski et al. (2018) observed a behaviour where shrews ate smaller prey immediately and stored larger prey. This links to Optimal Foraging Theory (OFT) where immediate consumption or storage is assessed by the shrew, in an attempt to minimise foraging time. Here it is observable that taking fewer predation trips and utilising venom to paralyse larger prey is more energy efficient than taking multiple trips, suggesting energy profitability.

Beremendia fissidens were studied by Cuenca-Besócs (2007), which was an early ancestor of the shrew (from early pliocene), and depicted that venom evolved for hunting large prey due to B. fissidens’s large envenomation apparatus and powerful bite. These advantageous characteristics may have meant B. fissidens, and their descendants, over-hunted larger vertebrate prey (to extinction), which caused a shift towards hunting insects and smaller prey - mirroring what is mainly hunted by their descendants today. The less reliable food source of larger prey (and larger prey being dangerous to hunt) implies the unprofitable appearance of venom in eulipotyphlans, causing an evolving shift against its dependance - hence Eulipotyphla mammals are highly uncommon to be venomous. 1% of all eulipotyphlans being venomous today may imply that a stable balance has arose between larger vertebrate and smaller prey hunting by shrews and solenodons. This links to the ideas of the past over-predation concept Dufton (1992), which supports the contemporary diet of shrews and solenodons consisting of primarily insects but also vertebrate as a secondary source (compared to vertebrate being a primary source for ancient shrews/solenodons). This suggests that only certain mammal species can profit off venom production, as they have particularly high BMR.

Additionally, the tooth and claw concept implies that eulipotyphlans do not need venom because of their advanced physical features. Venom never acts with immediate effect whereas physical mechanisms do, demonstrating a window of danger for the venomous eulipotyphlans between giving a venomous bite and reaping its rewards. Shrews require an almost continuous food supply (high BMR means shrews need to eat every few hours) which may suggest why some species of shrew have lost their venomous traits - if venom is produced but no prey is caught, valuable energy is lost. Venomous shrews (Blarina and Neomys) have shallow, open grooves on their lower incisors which form a concave trough for venom admission; non-venomous shrews (Crocidura russula and Sorex araneus) also possess a similar shallow groove, however have evolved to no longer produce venom - suggesting why so few eulipotyphlans are venomous.

Monotremata

One of the five total species in the order Monotremata is venomous - the male platypus (Ornithorhynchus anatinus). Male platypuses’ envenomation apparatus, called the crural system, consists of a spur on each hindlimb connected to modified sweat glands called crural venom glands. Although also having spurs, female platypuses are non-venomous perhaps due to the extra energy expenditure paid in gestation and not competing for mates; the extra mass of carrying eggs, creating nutrients for the zygote. are all expenditures reducing the energy balance available for the female platypus. The male platypus uses venom for intraspecific competition to deter other males from potential mates. Platypuses only use venom in their breeding season, occurring in the winter (August to October). This therefore justifies their expense as affordable as it is a fee paid in a small portion of the year. This contrasts with venomous reptiles who use venom as a defence/predatory mechanism implying its regular (year round) use.

Chiroptera

The second largest order of mammalia species, Chiroptera, contains three venomous mammals out of over 1400 species (Diaemus youngi, Desmodus rotundus and Diphylla ecaudata - three species of vampire bats). Venom within these hematophagous bat species allows for longer feeding times, and does not kill or paralyse their prey as discussed previously. Vampire bats have no adipose stores, therefore they can not store energy in the form of fat. As a result of this, a constant food supply is needed which explains why vampire bats can die of starvation after two days without food. All three venomous species contain plasminogen activators (the key domains being K2 and F) which help vampire bats by preventing clotting during feeding, allowing blood to flow smoothly. A sufficient meal is obtained to compromise for their lack of adipose, and undisturbed feed for up to 30 minutes. Most bat species can rely on their adipose tissue for insulation, however vampire bats lack this; consequently, a reliance on food and blood supply is needed to regulate their body temperature. In addition, Vampire bats are the only sanguivores amongst over 1300 bat species, therefore venom is only advantageous to them. If other bats adopted venom it would unnecessarily increase their chances of lethal rabies infections (occurring to 10% of vampire bats).

Conclusion

The keystone reason why there are so few venomous mammals and birds in comparison to reptiles is due to the cost of living resulting in energy being budgeted to where it has evolved to be most profitable. Most venomous reptiles sacrifice other features seen commonly in mammals to afford to use and replenish venom production. The choice of predation and defence mechanisms have been carefully selected over millions of years through evolution, and venom steered towards reptilia due to their favourable habitat and physical characteristics. Being venomous is not necessarily advantageous over being non-venomous, as both thrive in their respective predatory and defence operations. Previously, venom may have been so due to its ability to overcome larger prey, however due to over-predation across millions of years, a balance has emerged taming venom reliance.