9. FEEDING AND GROWTH OF JUVENILE CROCODILES

9.1 Feeding

As crocodiles grow, needless to say, they need more food although as a percentage of their own body weight the amount they eat gets less as Table 1 shows.

Table 1. Food Requirements of Young Crocodiles. Based on feeding records of C. porosus and C. novaeguineae in Papua New Guinea.

Commercially the aim is to grow crocodiles to marketable size as quickly and cheaply as possible. Management must therefore encourage feeding through good husbandry and then provide food which promotes good growth. Unfortunately the nutrition of crocodiles is still poorly understood but, as far as is known, they can not make use of vegetable-based proteins. Commercial rearing is therefore dependent upon a reliable supply of animal protein - a commodity often in short supply for human consumption.

A further constraint is the fact that reptiles can so easily do without food. They can not be starved into eating what they do not like. Attempts to use dried and reconstituted or salted preparations have failed because the crocodiles simply won't eat. This means that where animal protein is only seasonally plentiful (often the case with fish) freezer storage is essential. In Papua New Guinea, long-term rearing of crocodiles in remote villages was not generally successful, partly for this reason (Bolton, 1980. Bolton & Laufa, 1982).

In the wild most crocodile types eat a wide variety of food. The usual pattern is for invertebrates to predominate in the diet of the very young. As they grow crocodiles eat more vertebrates, mainly fish but including whatever land animals they are able to catch and kill in or near the water. In captivity crocodiles have been grown to commercial size on a number of diets but usually with far less variety than they would have in the wild. Crocodiles in captivity commonly grow twice as fast as their wild counterparts but their faster growth in captivity is almost certainly the result of having more to eat rather than being given a better diet. Until more is known about crocodile nutrition the best plan is to offer crocodiles as much food as they will eat and to provide as much variety in the diet as possible.

As with hatchlings food should be prepared in a fly-screened enclosure and chopped to a size that can be swallowed. Food must be fresh and should not be re-frozen after being thawed.

Over a period of a week or a month a pen of crocodiles will eat about the same amount whether they are fed every day or every other day. Less frequent feeding means less work but if this is not important then it is probably best to offer food daily or on the basis of a 5 day working week. This way there will probably be less squabbling over food.

The only way to ensure that crocodiles have as much as they want is to increase the ration until there is a little left uneaten. Experienced managers can estimate the amount very accurately and make allowances for such factors as the weekend fast, recent disturbance in the pens or a change in the weather. Laying down food in the late afternoon and removing uneaten scraps early the following morning is a satisfactory system.

For planning purposes the amount of food required can be roughly estimated from Table 1 and the growth rate shown in Fig. 9. It can be seen that in 4 years the average crocodile will measure about 2 metres long and weigh about 37 kg. During this time it will have consumed about 260 kg of food. On this basis a farm of 1000 crocodiles, with 250 animals in each age class from year I to year 4 will need about 1.25 tonnes of food each week.

This is based on the rearing of C. porosus in the hot tropics. Growth rate and food consumption will be less in the case of smaller species and cooler climates and will vary with diet.

Mugger (C. palustris) in India are reported to need comparable quantities of food to reach a length of 2m as can be calculated from Table 2.

Table 2. Food Requirements of C. palustris in India. (From De Vos, 1982).

Probably, when crocodiles are young and growing most rapidly, they have a similar capacity for utilising their food in body building. Detailed comparisons can only be made under carefully controlled conditions. In alligators it has been found that during the first 33 months 49.5% of food consumed (dry weight) was converted into body mass in animals fed on fish (McNease & Joanen,1981).

Many factors, beside food intake, are known to influence growth (see section 5.7) but the subject is complex and some very basic work has still to be done. The subject is brief ly reviewed under the headings below.

9.2 Factors Affecting Growth

9.2.1 Diet

The growth shown by C. porosus in Fig. 9 was achieved on a diet of whole, chopped trash fish from the sea with no supplements. Feeding was on the basis of a 5 day week. Very good growth rates have been recorded with other crocodiles on a diet of whole fish. In environmental chambers alligators were fed finely ground fish on five days a week for the first year then they were fed chopped fish and fed on only 3 days of the week. After 19 months they averaged 106 cm total length and 4.02 kg body weight. After 33 months they averaged 160 cm and 19.4.kg with 10% of the alligators measuring more than 180 cm (McNease & Joanen, 1981).

The fish used in this alligator study was obtained from trawl nets and was dominated in occurrence by the Atlantic croaker (Micropogon undulatus). An analysis showed the diet to be deficient in vitamins and a vitamin premix was added to the fish at a maximum rate of 1% by weight. In Papua New Guinea a vitamin and mineral premix (as sold for poultry) was tested for 50 days but had no effect on the growth rate of juvenile crocodiles fed on mixed sea fish.

Fresh fish can be confidently recommended as a diet on which to rear crocodiles to commercial skin size (it may not be an adequate diet for breeding animals) but the fish must be whole so that liver, entrails and bone are included.

Red meat may produce even faster growth. In the alligator study mentioned above, animals fed on the carcases of nutria I/ were 3% longer and 20% heavier than fish-fed alligators. The meat of game animals has proved successful for rearing Nile crocodiles in southern Africa but comparative data are not available because of differences in temperature and other factors.

1/ Coypu (Myocastor coypus) a large rodent killed for its fur.

Fig 9. Growth Curve for C.porosus The curve shows average rate of growth and is based on records from over 600 animals at Moitaka Farm, Port Moresby, Papua New Guinea.

A few other staple diets have been tested and found to be inferior to fish. High protein animal feeds manufactured as fish or dog food are useless if they are based on vegetable protein and prohibitively expensive (except in small quantities for hatchlings) if they are not.

Butchers' raw mince was tested in Papua New Guinea but crocodiles would eat little of it - apparently because of its sticky consistency. It could be used if mixed, a little as a time, with other food. It could be nutritious although the sample tested was very fatty.

Poultry offal is the staple diet of tens of thousands of crocodiles in Papua New Guinea and northern Australia. It produces satisfactory growth rates but was found to be inferior to fish in a 100 day trial. Over this period fish fed crocodiles showed a 112% increase in their average weight while those fed on poultry offal increased their mean weight by only 48%. The crocodiles in both groups averaged about 800 gms to start with (Bolton & others, 1981). The offal consisted of the heads, feet and intestines of poultry mixed together and chopped as necessary. For routine feeding of poultry offal a very coarse mincing machine has proved successful.

With poultry offal as a basic diet there is obviously scope for experiment with supplements and the addition of other ingredients. What proportion of fish or red meat, for example, would produce a significant improvement in growth? In a commercial situation costs and benefits would also have to be carefully monitored.

9.2.2 Size

As Fig. 9 shows, the rate of growth slows down as crocodiles get bigger. The middle part of the growth curve is fairly straight, that is to say the decreasing growth rate results in a fairly constant gain in size and weight - in this case about 4.6 cm per month during the first two years.

Growth rates expressed in this way can be misleading if the details are not given. For example, when a crocodile of 30 cm gains 5 cm in a month and another of 130 cm gains 5 cm in the same time the smaller animal grows by 16.6% but the larger one only grows by 3.8%.

9.2.3 Species

The rate of growth slows down very noticeably as crocodiles approach maturity. Presumably the species which mature at a smaller size will show this slowing of growth proportionally earlier.

There is some evidence however that different species grow at different rates in captivity even during their first and second years. In Papua New Guinea the freshwater species was, on average less than three quarters the size of C. porosus at one and two years old. Both types were kept under identical conditions. It made no significant difference to growth rates whether the two species were kept separately or together. In a two-month trial porosus groups increased their weight by more than 40% while the C. novaeguineae groups, matched for size, increased by around 30%

In India reported growth rates for mugger and gharial suggest that under good management both species, at least during the first two years, grow almost as fast as C. porosus in Papua New Guinea. De Vos ( 1982 ) mentions that 50 mugger hatchlings had an average increase in length of 4.25 cm per month over a period of 24 months. Ghariai, which are about 37 cm long at hatchling, reached 1.2 m after 28 months - a growth of about 4.6 cm/month. Various published figures for American alligators roared in heated enclosures indicate growth rates of 4-4.5 cm/month during the first 2 years.

Published growth rates for Nile crocodiles in captivity are from southern Africa where cold winter months have reduced or prevented feeding. As would be expected, the rates are relatively slow. At Victoria Falls a sample of 50 crocodiles grew by 33 cm/yr their first 27 months (Blake & Loveridge, 1975). Since These crocodiles would have grown little or not at all during the winter the monthly average (2.75 cm) is nor very meaningful. Wild crocodiles grew only about half as fast.

9.2.4 Sex

In Papua New Guinea male crocodiles of both species grew significantly faster then females in a trial where the starting weight was 4.8 kg and growth was measured by weight gain over 100 days. From numerous growth records it was calculated that male C. porosus averaged 3 yrs 11 months at maximum commercial skin size (about 2 m total length live animal) while females averaged 4 yrs 4 months. This represents about 11% faster growth in males (Bolton & others, 1981).

At larger sizes the sex differential will be greater because growth in females becomes very slow indeed as they approach maximum size. With wild alligators in Louisiana Chabreck and Joanen(1979) found that after 3 yrs of age males grew almost 20% faster than females and were growing 62% faster by the age of 10. At 20 years old the differential was almost 200%. A sample of 49 captive alligators in their seventh year showed males to average 2.37 m in length and females to average 2.06 m. Rearing conditions had been identical and the difference (15%) could be attributed to sex.

9.2.5 Individual Variations

In captivity, at least, the different rates of growth shown by crocodiles are difficult to explain by any of the factors mentioned so far. Sometimes crocodiles just grow much more slowly, or rapidly, than the average. This is true even of hatchlings from the same clutch of eggs which are reared under identical conditions. Within a nest incubation temperature can vary from top to bottom but it seems unlikely that this could account for the extreme variation that sometimes occurs in hatchling growth. At the age of six and a half months, for example, the heaviest individual in a batch of 27 C.porosus weighed three times as much as the lightest one, and was 70% heavier than the average. The sexes were not known but exceptionally rapid growth can occur in either sex and can override the tendencies normally attributed to sex and sometimes species. In Papua New Guinea, for example, over a period of 414 days a saltwater male grew from 10 to 16 kg while a freshwater male grew from 12.5 to 26 kg and saltwater female grew from 12.5 to 28 kg. All three animals were apparently healthy and received the same attention.

A female saltwater crocodile in Papua New Guinea increased its length by an average of 4.82 cm/mth in its first two years and Bustard (unpublished,report) mentions that an exceptional group of mugger (which hatch at about 30 cm) reached 1.5 m in 2 years. the equivalent of 5 cm/month. Exceptional growth rates have also been recorded in Nile crocodiles (Blake & Loveridge, 1975).

The extent to which growth rate is influenced genetically is not known but if rapid growth is heritable then the possibilities for selective breeding are obvious and exciting.