Reproduction

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 reserves.

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 rds are 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).

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.