Abstract

This work sprouted as a sequel of investigation on the problem of rat infestation in Mauritius. The work was on Rattus norvegicus as it was more easily obtained than Rattus rattus. Most rats available were either found crushed, mutilated, decomposed or with missing parts. Their body weights could not then be accurately obtained and investigations were then necessary to estimate their body weights. The body weights, body lengths, girths and tail lengths of dead rats were measured. Relationships were established between the weight of the body and the remaining parameters. Regression between the body weight and the other respective parameters were as follows: (a) girth was 0.7424 with equation y=1.0658x - 0.1993 (b) body length was 0.7887 with equation y=1.0401x - 0.064, (c) tail length was 0.8387 with equation y=0.8798x + 0.2737, (d) body length plus girth was 0.78 with equation y=0.365girth + 0.723 body length, (e) tail length plus girth was 0.81 with equation y=0.403 girth+0.625tail length and (f) total body length (body length plus tail length) with equation y=0.4994x + 0.3049., where y was the body weight and x represented the respective parameters as detailed in the text. The total body length was the most positively correlated with the body weight followed by the latter's relationship with the other length of the tail. Most of the equations could be used to estimate body weights of Rattus norvegicus with around 80 to 90% accuracy.

Key words: rat, Rattus norvegicus, body weight, body length, girth, tail length.

Introduction

The first settlers in the island of Mauritius were the Dutch who occupied the island in 1598. They abandoned the country for the first time in 1658 (Moree, 2000), the main reasons being cyclones and damages by rats. The second and last occupation was from 1664 to 1710. This contributed to further introduction of rats in the country. Unfortunately, more rats were accidentally brought in during the French and British periods until recently. These rats multiplied over centuries and are presently found everywhere both in towns and villages causing considerable damages.

There are presently two species of rats namely Rattus norvegicus and the Rattus rattus (Felix, 2003). They live in close association with man's activities. They have adapted, both genetically and biologically, to man's modified environment and are difficult to control (Howard, 1994). Extensive damages have been noted in reports from several newspapers. These animals damage crops, livestock products, stored foods and infrastructure such as electric cables and telephone lines (Thumiah, 2002). They are also responsible, directly or indirectly for well-known disease problems such as the plague and the hanta viral disease.

The objectives of the work were primarily to locate rats in the centres of towns and take measurements in order to establish relationships, if any, between the weight of the body and the other chosen parameters. This would provide information to estimate the body weight of these animals. The body weight would indicate the amount of food consumed and would somehow quantify the extent of damage in chosen regions. The extent of damages would also be associated with the total population and the growth rates of rats, both of which are presently difficult to estimate since there is implication of a long term and complex exercise.

Materials and methods

In Mauritius, the control measures were in progress only in towns under the aegis of the municipal councils and the Ministry of the environment. The work was given by tenders and contracts to private companies. Different control measures were used to control the rat population in the island. Figure 1 shows the percentages of rats obtained by using bait blocks, bait pellets, glue traps and live traps. Of these, the former two produced the best results. Thus, most of the rats obtained were from bait pellets, which was the method of choice used by the contractors. Baits were effective and easy to use.

Only the species Rattus norvegicus was obtained for measurements since Rattus norvegicus was more easily available than Rattus rattus. It was a more aggressive species and thus more likely to get trapped as it dominated in most regions (Khodabocus, 2002). It dominated in most areas and was more liable to be trapped during the competition for food with that of Rattus rattus. Measurements of parameters on sites presented several problems. Stray cats, dogs, mongooses and even crows, ate the dead bodies or major parts thereof. Thus, most of the rats obtained were found to be crushed, mutilated, decomposed or with missing parts. Bodies in states of decomposition were not considered for measurements. The dead rats were collected from five town centres of the island.

The use of chemicals in the baits caused the animals to bleed from most of their orifices (William, 1975). Hence, observations taken were from rats available within a few hours. This led to the availability of a reduced number of rats. Forty-three specimens were found suitable for the work.

Placing the dead animal directly on a commercial balance, the body weight was obtained. The girth was considered as the circumference of the chest, that is that part of the body just behind the fore limbs. A non-elastic cotton piece of string was placed around the girth and the length of the string was obtained by placing it on a measuring tape. The body length recorded started from the tip of the nose to the end of the trunk that is at the starting point of the tail as noted by touching the bones. The same string was used similarly to obtain the length of the tail from the start of the tail to its tip. When taking the values of the parameters parallax error was avoided and the body of the animal was made to lie on a flat level (board/concrete floor).

The data was converted into logarithms to base ten in order to decrease the extent of variation of coordinates on graphs produced and to have a clearer line of fit. Regression analyses were carried out using the Excel and Minitab softwares. Graphs of correlations, product moment correlation coefficients, respective equations and regression results were produced. Each product moment correlation coefficient was tested at 5% significance level to find out differences in correlations. This was done by the use of the table of critical values for product moment correlation coefficients. The parameters chosen were weights of the rats, their girths, body lengths and the lengths of the tails.

Results and discussions

Body weight and girth

It was found that there was a positive correlation at 5% significance between body weights and girths, with a product moment correlation coefficient of 0.7424. From Figure 2 it was observed that when the values of the parameters increased there was an increase in the variation between them. This demonstrated that some animals with higher body weights might be having a smaller girth. Generally, girths of rats increased with increasing body weights. It is natural that when rats grow, their bodies develop and increase in weight together with girth but may be at different rates for different individuals (Howard, 1994).

Figure 2. Relationship between body weight and girth of rats

Body weight and body length

Body weight and body length were positively correlated with a coefficient of regression of 0.7887. There was a positive relationship between the two parameters at 5% level of significance. It was found from Figure 2 that there was an increase in variation between the two parameters when their values increased. This meant that some animals might be having higher body weights but a smaller body length or fat animals might be short and heavy (Niethammer, 1981, Greaves, 1989). Howard, 1994, has pointed out that in general, rats of higher body weights tended to have longer bodies with some exceptions.

Body weight and body length

There was a positive relationship between body weight and tail length with a product moment correlation coefficient of 0.8387. The value of RČ was tested it found that there was a positive correlation. From Figure 3 it was noted that as values of the parameters increased there was an increase in variation between them. This showed that a rat of higher body weight might be having a shorter tail or vice versa, which might happen in some cases. Howard, 1994, pointed out that in general, a heavier rat tended to have a longer tail.

Body weight with body length plus girth

The RČ value was 0.775 and the equation produced was body weight (g) = 0.365 girth (mm) + 0.723 body length (mm). There was a positive relationship since the product moment correlation coefficient was 0.78. A little more than 78% of the variation in body weight was explained by the relationship of the combination of the two parameters. At 5% significance level, there was a positive correlation between body weight and the two parameters. Also, there have been some unusual observations by getting values with high percentages of residuals. This was due to the fact that some animals might not be having the same proportional increase in body lengths and girths (Jordan. C.1998).

Body weight with tail length plus girth

The product moment correlation coefficient RČ was 0.811 and the equation produced for the two combined parameters was body weight (g) = 0.403 girth (mm) + 0.625 tail length (mm). There was a positive relationship. It was found that there was a positive correlation between the two sets of values at 5% significance level. Some values had high residual percentages. This was because there were some individuals, which had large differences with increase in these parameters (Greaves, 1989).

Body weight with body length plus tail length

There was a good relationship between the weight of the body and the total length of the rat as measured from the tip of nose to the end of tail. The product moment correlation was 0.8971. The total length of the rat was more positively correlated with that of body weights as the coefficient of regression was highest. This correlation was the best obtained after the calculations of values for all parameters and their combinations.

All the analyses made at 5% significance level showed that there were satisfactory correlations between the body weight and their respective parameters. Higher relationships were obtained for the total body length and the tail length. These might be due to variations within parameters. These results confirm the findings of Niethammer, 1981. There were indications that the total body length and that of the tail might be having a better link with the body weight than the rest of the parameters. By using tail length, body weight can be predicted more accurately. Tail length could be measured although the bodies of rats have started to decompose or crushed which is an advantage compared to the other three parameters. There were differences in the extent of easiness of measuring tail length as compared to the other two parameters.

The body weight of the animal gives an idea about the extent of growth and amount of food consumed. The latter can be used to extrapolate the extent of damage caused as a consequence of its activities.

According to Howard, 1994 and Greaves, 1984, the body length, tail length, and weight of Rattus norvegicus were 260mm, 187mm and 400g respectively. But those found in Mauritius had smaller weights, longer tails and shorter bodies. This may be due to the type of environment in which they were found. Adaptation to environments develops some body changes. Those which live in burrows might be having longer tails to be more sensitive and dominant ones might have heavier weights to be more competitive. These observations were also found in the work of Greaves, 1994. The type of food consumed was important since the rats that lived near residential areas might not be getting more food protein and therefore might be having longer bodies with lighter weights.

To determine the equations for predicting body weight it were assumed that all the data of the parameters obtained were normally distributed and when one parameter equalled zero all other parameters would also be nil and there should not be a linear increase within the parameters. This was because parameters of animals were being dealt with and therefore could contain some extreme values. Also, there could not be linear increases as in the case of parameters for plants or physical components. There should be a point where stabilization of growth is reached that is when the rats come to an age nearing 2 years (Howard, 1994; Wetzel, 1983)

Conclusions

The analyses revealed that body weight had the best relationship with the total body length and tail length. The respective equations were namely, Log y = 0.4994 Log x + 0.3049 and Log y=0.8798Log x + 0.2737 could be used to estimate body weights of rats. Girths, body length and their respective combinations could also be used.

The use of appropriate equations would depend on the quality of the rats obtained. For example when the trunk is found mutilated only the length of the tail could be used; when the body is intact the total length as measured from the tip of nose to the end of tail could be convenient.

Acknowledgements

The authors are grateful to the officers of all the town councils for their collaboration. They also thank Mr. Jimmy Felix, Director of Sanitact Pest Control department of Rentokil Initial.

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