The adaptive value of venom (the measure of its useful-
ness once delivered) is one of the key criteria for ‘being
venomous’, and is probably the least contentious. For
example, we humans have proteins in our oral secretions
(saliva) which can also be found in venoms. We also have
specialised glands that produce these secretions.
However, the said secretions do not really offer us any
adaptive value once out of our mouths. Therefore, we
are not venomous.It is not yet understood precisely how, genetically,
venoms were recruited as a biochemical weapon.
However, DNA replication is an imperfect process, and
mutations provide the perfect platform for the
emergence of new, beneficial traits. Current research
indicates that most snake venom toxins are essentially
derived from normal body proteins - digestive pancreatic
enzymes, for example - which, through some kind of
genetic hiccup, were duplicated, expressed, and secreted
by cells in oral glands, and were subsequently exapted
and weaponised: delivered from one organism into the
body of another, in which they exert a toxic effect.Over time, selection favoured mutated forms of these
molecules which induced their effects with greater
power, speed, and/or efficiency. Specialised deliverysystems were developed in parallel, such as venom
glands coupled with fangs or stingers. Research points
toward a similar mechanism of recruitment in some
other lineages. For example, investigations are underway
to demonstrate the homology between the proteins
secreted by the skin’s regulatory mucosal glands and
those of the venom glands adjoining the spines in some
species of catfish. This coupled ‘venom system’ is
considered by many scientists to be a key aspect of
differentiating between animals which incidentally
introduce their oral secretions into the body of another
organism and those that actively do so.Ok, we are getting somewhere. So, to meet the criteria of
being venomous, an animal must have specialised glands
or cells which produce a toxic secretion that is adminis-
tered into the body of the victim, via a wound, through
the use of a specialised delivery mechanism? Well, sort
of. Cue caveat number one. Heard of the slow loris
(Nycticebus spp.)? This genus is considered by many to
contain the world’s only venomous primates. There are
currently four described species, all of which are found in
Southeast Asia. These nocturnal primates have brachial
glands on their inner elbow that excrete a strong-
smelling substance when they feel threatened. The slow
loris licks this exudate, and the enzymes in its saliva break
down and activate the toxins. The loris may bite
defensively, thereby introducing the toxins to the animal
that is the source of the threat.However, neither the secretion nor the licking occurs the
moment that the threat is perceived, but instead followLeft: the Bengal Slow
Loris (Nycticebus
bengalensis). There
has been at least one
fatality from a slow
loris bite. Image by
Hoang Mai Thach.
Below right: the
venom of the blue-
ringed octopus
(Hapalochlaena spp.)
contains tetrodotoxin,
a potent neurotoxin
that is also lethal if
ingested. Image by
kaschibo.‘most snake venom toxins snake venom toxinsare derived from normal body normal bodyproteinsproteins—digestive enzymes forexample’many minutes later. Furthermore, the loris’s teeth,
though sharp and effective at penetrating flesh, are fairly
typical for mammals which share its omnivorous diet,
and thus are probably not specialised for the purpose of
envenomation. And while there is at least one reported
fatality from a slow loris bite, the responsible toxins are
essentially allergens, the same type as those expressed
by cats, and therefore exert limited toxic effect unless
the victim has an anaphylactic reaction. To further
complicate matters, the loris will also spread the exudate
throughout its fur as a poisonous deterrent to ectopara-
sites and predators.So, toxic secretion via specialised glands? Strictly, yes.
Administered through the infliction of a wound? Partly,
yes. Via a specialised delivery mechanism? Maybe.
Venomous? Probably. Examples like these have led
researchers - toxinologists, in particular – to broaden the
definition of venom slightly by omitting the requisite for
a specialised delivery system. When evaluating cases
such as these, it is important to remember that evolution
is a continuum. It is probably easier than you think to find
examples of extant species which exist at stages all along
the continuum from non-venomous to venomous, and
perhaps the slow loris is one of them. So, at what point
on this continuum do we actually class something as
venomous? Good question. And how, then, does thecomposition of the toxic secretion come into play here?
What actually is venom?A complex cocktail.
Venom is, very simply speaking, a complex mixture of
bioactive proteins and peptides. While there are also non- proteinaceous molecules in venoms, little is known
about their specific function and it is thought that most
of them perform some kind of housekeeping role.
However, as this is not necessarily always the case, to be
a bit more accurate we generally refer to the bioactive
constituents of venom as toxins: molecules which can
induce a toxic effect.
Molecular size and structure often differ categorically
between a venom toxin and a poison toxin. Biological
poisons are typically very small, organic molecules - that
is, they are not usually proteinaceous - and their toxic
effects can be experienced following ingestion, absorp-
tion, or inhalation of the poisonous substance. Venom
toxins are usually comparatively large proteins whose
activity relies on their structural integrity, which exposes
active sites on the molecule. However, this also makes
them fairly unstable. Any significant changes to structure- by heat, for example - results in a loss of toxin activity.
This is called denaturation, and explains why the first aid
treatment for a stingray sting is to immerse the affected
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