All monitors reproduce by laying eggs and usually viable eggs are produced only within two
months of mating. Observations in captivity (Card 1993, Eidenmuller 1993) suggest that in
some cases sperm storage of up to three months is possible. In many species mating and
egglaying occurs only during a few months of the year but in some tropical species the
breeding season is extended and may continue throughout the year. Very few studies of freeliving
monitors have been able to gather much infonnation about reproduction but a great
deal of data is being accumulated for captives (see Hom & Visser 1990, 1991 for reviews).
Plior to mating the female must expend a great deal of energy in the production of eggs. The
male uses a comparatively minuscule amount of energy making spenn. For this reason
females are usually less active than males, grow at slower rates and achieve smaller sizes.
The number and size of the eggs produced varies enormously between species. Some produce
clutches of eggs that weigh almost half of their body weight. The eggs take up so much space
in the body cavity that the female may be unable to feed whilst heavily gravid. Clutch size and
relative mass vary greatly between species. Some large monitors lay comparatively light
clutches whilst some small species produce what appears to be an impossibly huge clutch.
The most prolific species are the African white-throated, Nile, and Bosc's monitors, all of
which lay large numbers of eggs which hatch quickly into small offspring, each with very
little chance of survival to adulthood. Other large monitors produce small clutches of eggs
that may require longer incubation and hatch into much larger offspring. Even among dwarf
monitors clutch size varies by several magnitudes. Females may accumulate the energy used
for egg production as fat whilst food is abundant and make the eggs during periods of
relative inactivity. In other species egg production does not appear to rely on large fat
Whether a monitor can make a full clutch of eggs depends on the amount of energy she can
afford to invest in them. Observations in captivity have shown that many monitor liza
Courtship in monitors
capable of producing more than one clutch of eggs per year if they have unlimited food
resources. In most cases two clutches of eggs are produced in quick succession but some
monitors may produce a clutch of eggs every three months. Some can produce their own
body weight of eggs in a year, but it is doubtful that free-living individuals ever have life so
good. The methods currently used to investigate the reproductive biology of monitor lizards
in the wild require that the animals be killed and so there is no evidence that multiple
clutching occurs in the wild, but it certainly occurs wherever suitable food and climate exist.
Having produced a clutch of eggs at great expense, the female needs some good quality
sperm to fertilise them. She indicates her sexual receptiveness by releasing scent signals
which may be detected by a large number of males. But the males, literally bursting with
sperm, first seek out other males and engage in the bizarre contests of strength which seem
to detennine their likelihood of achieving successful copulation. The amount of energy
female monitor lizards must expend to produce young with a good chance of survival forces
them to live relatively sedentary lives during at least part of the year. It is clearly in her
interests to have them fathered by the most desirable male available. From her point of view a
desirable mate is a big, strong one. Monitor lizards, as we have seen, are very well equipped
for killing other animals and often practise cannibalism. Therefore in their social interactions
with other monitors they must adopt very different behaviours than those employed in the
quest for food . The ritual combat practised by monitor lizards permits a hierarchy to be
established without continual fighting. Similarly courtship in monitor lizards, although less
ritualised, reduces the chances of serious injury or death when investigating possible mates.
Detailed accounts of courtship and mating can be found in Auffenberg (198Ia&b, 1988),
Moehn (1984), Carter (1990), and Green & King (1993). In some species the female tends to
be the dominant partner and only males strong enough to completely immobilise her are able
to achieve copulation. In some species the act of love is performed very gently, but in others
it is accompanied by a great deal of biting and scratching. Some accounts of mating could be
interpreted as rape. Actual copulation is very short in some species and protracted in others,
with mating occurring over several days.
Male monitors are endowed with two penis-like structures which allow them to mate on
either side of the female. They may use them alternately or prefer one to the exclusion of the
other. The surfaces of the hemipenes are amazingly convoluted and they vary so much
between species that they are used as a taxonomic tool (Branch 1982; Bohme 1988). Females
also have twinned appendages that are partly eversible and may represent glands which
produce sexual pheromones.
Most records of counship and copulation in monitor lizards come from captivity, where
animals have less opportunity to escape from each other and serious injuries are not
uncommon. Nevertheless mating is a dangerous activity and Auffenberg (1981) has
suggested that pair bonding may occur between some male and female Komodo dragons.
This does not imply any sort of monogamous relationship (as incorrectly stated by Selbert
(1994), but rather that acquainted animals mate with each other more than with other
individuals and thus reduce the danger of sustaining injuries from an incompatible liaison. In
bonded pairs mating is accompanied by much less preliminary courtship than in unacquainted
animals and some courtship behaviour may occur between the pair at times of year other than
the mating season. The extent to which pair bonding exists in other monitor species is
unknown, but reports of captive breeding indicate that it may be widespread.
Female monitors have limited ability to store sperm and so eggs are always laid within a few
weeks of mating. The eggs are covered with a flexible, leathery shell (Zwineberg (1972)
fancifully portrayed monitors emerging from chicken eggs) which are very prone to
desiccation in dry conditions. Therefore they must be deposited in a nest which provides the
heat and humidity necessary for them to develop into fully-formed lizards. Monitor lizard
nests are very carefully concealed and there are very few records of nests discovered in the
wild (Tsselarius & Menshikov 1995 provide a detailed exception). In all species studied
female monitors reach the peak of their activity as they begin to search for suitable nesting
sites. They may have to fight against other females to gain access to good areas. Nests tend
to be situated in an elevated position, presumably to reduce the danger of flooding. Despite
many claims in the literature no monitor lizard eggs have been found in tree hollows. Many
species deposit their eggs in chambers at the ends of burrows of varying depths which are
always refilled to some extent. A number of test, or decoy holes may be dug nearby. Females
may actively defend their nests for a few days after egg laying, but this behaviour does not
persist for long before the weak and hungry lizard goes in search of food .
For many species termite mounds provide the perfect incubator. The female simply locates an
active mound, tears a hole in it and deposits her eggs. For reasons not understood the
termites do not destroy the eggs but quickly repair the damage done by the lizard, thus
sealing the eggs inside. Active termite mounds are maintained at almost constant
temperatures and are much more humid than the surrounding air. Furthermore the nest is
well protected from predators and as long as the mound is not abandoned by the termites the
eggs are safe. Surprisingly many young monitors born in termite mounds do not feed on their
guardians (Carter & King. pers.comm.).
The construction of some termite mounds allows the newly hatched lizards to escape via
ventilation shafts that lead to the outside. but in other mounds they may be entombed. lacking
the strength to break through the tough outer walls. The question of how these lizards escape
from the mound has not been solved. Several times it has been suggested that the mother
returns to the nest when incubation is completed and digs out the youngsters (Cogger 1967;
Boonratana 1988: Ehmann et al
1991). Many people remain unconvinced that the monitor
lizard is capable of remembering her nesting site and returning there at the correct time and
unfortunately the results of field work investigating this phenomenon are not available
(Carter 1989: Boylan 1995). A television documentary which purported to show the release
of hatchlings from a termite mound by their mother used broad artistic license (Marven
1990). My own opinion is that a complete lack of parental care in monitor lizards would be
surprising, especially in view of their apparently analogous evolution with snakes, many of
whom protect their eggs throughout incubation.
As well as utilising active termitaria many monitor lizards deposit eggs in mounds that are no
longer inhabited by termites. An important distinction can be drawn between these
behaviours. Although any monitor can lay its eggs in an abandoned mound the ability to use
active mound may depend on special (probably chemical) adaptations that prevent the
termites from recognising the eggs as foreign and destroying them. If this is the case it would
suggest that the lizards are only able to make use of the mounds of certain species of termite.
At present there is not enough data to confirm or refute this.
Speed of incubation is determined largely by temperature and in the wild some eggs may take
almost a year to hatch. Incubation times in captivity exist for most species but virtually none
exist for eggs laid in the wild and the only published data on the conditions experienced in
natural nests is provided by Ehmann et al
1991. Clearly it is beneficial if the young lizards
emerge at a time of year when their is plenty of food available, but this is not always the case.
The eggs of the desert monitor, for example. may hatch shortly before the onset of winter
and the youngsters remain together in the nest until the following spring. In some species the
development of the embryos may be completely halted during very cold weather and in
others development may be completed months before the eggs hatch. Most hatch (and all
emerge) from the nest at a time when insect abundance is high. They lead very secretive lives,
remaining mainly in shady. humid microhabitats and feeding voraciously. Mortality is very
high in the first year of life and the small amount of data available suggests that many eggs
may fail to hatch at all (Ehmann et al
1991; Phillips & Packard 1994; Tsellarius & Cherlin 1995).