ON THE EDGE OF EXTINCTION - A - THE TIGER (Panthera tigris)

[peter]

ON THE SIZE OF CAPTIVE AMUR TIGERS - III

III.1 - Introduction

In this post, a very recent study, 'Body measurement parameters and body weights of sub-adult and adult Amur tigers (Panthera tigris altaica)', will be discussed. It was published in the Chinese Journal of Wildlife, 2021, 42(4), pp. 965-972. 

The authors, Cui (Y), Liu (D), Mu (G), Gong (M), Xu (H), Li (X), Yang (S), Fan (X), Liu (Y) and Jiang (G), collected detailed information about the weight and 8 body parameters (chest, waist and neck circumference, shoulder and hip height, width of the front and hind palm and total length) of 45 subadult (22♂, 23♀) and 74 adult (40♂, 34♀) captive Amur tigers (n=119). 

Their goal was " ... to explore the growth and development of (captive) Amur tigers ... and to provide a scientific reference for the artificial breeding and protection of the wild Amur tiger ... " (from the abstract). 

The model they developed suggests that " ... shoulder height is an important parameter for assessing the weight and length of the Amur tiger ... " (abstract). 

Again, it was 'Betty' who found the study, sent me the link and answered a few questions (many thanks!). The study is in Chinese, but the abstract and the tables have an English translation. 

I'm not quite sure, but I think all tigers measured were from the Siberian Tiger Park, Harbin. They are a bit larger than those in Hengdaohezi (discussed in the previous post).

III.2 - Scans

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

*This image is copyright of its original author

III.3 - Results

a - Male tigers

As I'm unable to read Chinese, definitions are unclear. Combining Table 2 and Figure 3, I concluded, at least for now, that 2-3 year old males were considered as subadult, whereas tigers of 4 and older were considered as young adults. It's quite likely I'm wrong. Better remember that when you read the post. I'll ask 'Betty' to help out. 

a1 - Length

In contrast to the study discussed in the previous post, male tigers in the Siberian Tiger Park in Harbin did not reach their greatest length before the age of 4, but just before they reached 6 years of age. Adult males averaged 304,3 cm in total length measured 'between pegs' (Table 2). After 6, however, male tigers gradually lost length with age. The number of observations in the category 6-9 years of age, however, is very limited (see Figure 3). 

a2 - Weight

Like in the study discussed in the second post of this series, subadult males peak when they reach 4 years of age (at about 210 kg, see Figure 3). Young adults, in contrast, quickly lost weight. The difference between the Hengdaohezi and the Harbin Amur tigers is Harbin males reached a new peak after reaching 6 years of age. Between 6-7 years of age, they average about 230 kg. One of the males reached 295 kg (Figure 3). After 7, males gradually lose weight. 

a3 - Chest circumference

Subadult males peak at about 4 years of age, when they average well over 150 cm (Figure 3). Between 4 and 5 years of age, however, they lose about 25 cm (...). Mature males (6 years and over) average about 135 cm (Figure 3). The higher average of adult males in Table 2 (141,8 cm), most probably, is a result of the decision to consider tigers of 4 years of age as adults. After age 7, males slowly lose size. 

a4 - Neck circumference

Table 2 says subadult males average 80,3 cm and adults 90 cm. Figure 3, however, strongly suggests the opposite is true (...). Tigers up to 4 years of age average over 90 cm, whereas 5-year old males average just over 80 cm. As males get older, the neck circumference again increases. It's the exception to the general rule that captive male Amur tigers (at least those in the Siberian Tiger Park) lose size after reaching 6-7 years of age.  

a5 - Shoulder height

Table 2 says adult males average 112,2 cm at the shoulder. Using reliable information in other studies and books, I think it's very unlikely the actual standing height at the shoulder of adult males really is over 112 cm. According to 'Betty', the protocol is clear, but then not quite. It could depend on the expertise of those measuring the tiger, we think. Like 'Betty', I think the 'height at the shoulder' was measured when the animals were sedated. For this reason, the results are unreliable.  

In spite of that, the information is interesting. Assuming they measured the height at the shoulder in the same way as hunters did in the recent past (referring to British India in particular), one could conclude the difference between this method and the actual standing height at the shoulder in adult male tigers of large subspecies (P. tigris tigris and P. tigris altaica) could be about 4 inches, perhaps a tad less. 

The difference between both methods in adult male Amur tigers could be about 10%. In this study, adult males averaged 112,2 cm., whereas it's known standing male tigers average, depending on the location, 100-103 cm. The method used by hunters suggested adult male Indian tigers averaged 3.3-3.5 (99,1-104,1 cm). If my assumption is correct, it would mean adult male Indian tigers average 89-94 cm at the shoulder while standing. Exceptional individuals (referring to both subspecies) could be about 4 inches taller (100-105 cm in Indian tigers and 110-115 cm in Amur tigers).   

a6 - Heel width

Table 3 says the average width of the palm of the front foot in captive adult male Amur tigers ranges between 9,6-10,0 cm. Apart from the table published in 2005 (and discussed in the posts I referred to in the first post of this series), there's not much known about the average 'heel width' of wild adult males. Biologists, however, agree the minimum heel width in wild males is about 10,0 cm. 

According to A. Batalov, male tiger 'Ochkarik' has a heel width of about 10,0 cm. This tiger ranges between 160-180 kg in weight. Another male in that district, 'Obor', larger than 'Ochkarik', has a heel width of 12,0 cm. In large wild male Amur tigers, the heel width can exceed 13,0 cm (up to 13,5-14,0 cm).

Back to the Harbin tigers. At age 6, adult males average about 230 kg, just over 10 feet in total length measured 'between pegs' and 9,99 cm in heel width (referring to the palm width of the front paw). My guess is the heel width of a wild adult male of that size would be over 12,0 cm (referring to averages). Assuming the relationship between heel width and weight would hold for all males, one could conclude the difference in heel width between wild adult males and captive males is 20-25%. A significant difference. 

b - Tigresses

Like in subadult males, subadult females peak in many respects before they reach adulthood. The difference with subadult males is peaks in females are less pronounced. Furthermore, most peaks are reached well before they reach 4 years of age. 

In contrast to adult males, adult females continue to grow after they reach 5 years of age. The growth also is more prolonged. At age 8, females are still growing in most respects (see Figure 4). The only exception is the heel width. After 7, females gradually lose width.

The averages for adult females in Table 2, if anything, seem to be quite misleading. At age 8, for example, females do not average 135,4 kg and 270,4 cm in total length. Figure 4 says females of that age average just over 150 kg and about 275 cm (...). 

c - A few remarks 

Although the results seem to point in a different direction, there isn't much to choose between the results of this study and the results of the study discussed in my previous post (referring to the dissertation of Weilin). There are different reasons.  

The first reason is table 2 is misleading to a degree. This is a result of the uneven spread of observations. Most tigers used for the study ranged between 2-6 years of age. The number of tigers of 6 years and older, however, is very limited (see Figure 3 and Figure 4). 

It could be that subadult and young adult male tigers, as is suggested in both studies, really are a bit larger than mature male tigers, but there's not enough information about mature male tigers (referring to both studies) to get to sound conclusions.    

Another reason is the decision to distinguish between 2 groups only: subadults and adults. This decision is not supported by the measurements (see Figures 3 and 4). These indicate there are 3 stages during growth. For this reason, one has to distinguish between subadults (2-3 for females and 2-4 years for males), young adults (4-5 for males and 3-4 for females) and mature adults (males of 6 years and older and females of 5 years and older). 

I'm not saying the results of this study are unreliable, but the lack of data about mature tigers has an effect on the conclusions. For example. The 4 mature females used in this study were quite large individuals. The logical conclusion is mature females are larger than young adults. That conclusion, however, is based on a very small sample. In mature males, it was the other way round. Again, the conclusion (mature males are smaller than young adults) is based on a very small sample. 

The information I found (referring to the table I'll post in a few days) suggests mature male Amur tigers, like the females in this study, continue to grow, both in length and weight, after reaching 6 years of age.  

Anyhow. It is clear there is a difference between the tigers in Hengdaohezi (see the previous post) and those in Harbin. The Hengdaohezi tigers (referring to males only) seem to be decidedly shorter and not as heavy as those in Harbin. Remarkable, as both belong to Panthera tigris altaica. One would like to read a bit more about the possible reasons.  

III.4 - Conclusions

The study discussed in this post is of interest for zoos and those working with captive Amur tigers, because it offers a bit of insight in the growth and development of captive Amur tigers up to about 6 years of age. 

Subadult females (at least those in the Siberian Tiger Park in Harbin) quickly develop up to age 3. Between age 3-5, their body changes. They no longer grow and lose quite a bit of weight. After reaching about 5 years of age, they start growing again. At age 8, they're still growing in most respects (see Figure 4). At that age, they averaged over 150 kg (331 pounds), 275 cm (just over 9.0) in total length measured 'between pegs' and 122 cm (about 4.0) in chest circumference. Three females exceeded 285 cm (9.4) in total length measured 'between pegs', whereas five reached or exceeded 160 kg (364 pounds). 

Subadult males quickly develop until they reach 4 years of age. Between 4-6, they lose quite a bit of weight. After reaching 6 years of age, they start growing again. The period of growth, however, is not as extended as in females. After they reach 7 years of age, they gradually lose length and weight. The only exception is the circumference of the neck. At age 9, males average 195-200 kg (430-442 pounds), about 295 cm (9.8) in total length measured 'between pegs' and 135 cm (4.5) in chest circumference. At their peak, when they're 6-7 years of age, adult males average about 230 kg (508-509 pounds) and 307-308 cm (10.1) in total length measured 'between pegs'. Exceptional individuals reach 320-325 cm (10.6-10.8) in total length measured 'between pegs' (see Fig. 3). Only one of the males exceeded 250 kg. He was 295 kg (651 pounds).   

Table 2 suggests the averages for adults are different. This is a result of the decision to consider young adults as 'adult'. The conclusion I got to, however, is far from solid as well. The reason is only very few mature tigers were included in the study. 

The study doesn't offer a reason for the difference in development between mature males and females. It is quite likely the conclusion regarding the different development of mature males and females I got to will prove to be incorrect. The reason, again, is the very small sample (referring to tigers of 6 years and older). 

In the abstract, the authors said " ... shoulder height is an important parameter for assessing the weight and length of the Amur tiger ... ". The problem is this parameter was measured in an unclear way. The height at the shoulder has to be measured when tigers are standing. A female reaching 95 cm (just over 3.1) measured in this way is very tall. Same for a male reaching 110 cm (just over 3.7). 

The study has reliable information about the size and development of subadult and young adult Amur tigers. If you want to know a bit more about the size and development of mature tigers (6 years and older), however, you have to look elsewhere.  

III.5 - About the study

I read the study more than once. Although interesting all the way, I still feel some parts were a result of rushwork. Here's some of the reasons. 

One is the character of Table 2. The measurements, as was stated before, strongly suggest one has to distinguish between subadults, young adults and mature adults. The authors no doubt got to that conclusion as well. In spite of that, they decided to distinguish between subadult and adults only. The only logical outcome of this decision can be a confusing, somewhat misleading, table.

Two is they apparently didn't notice the discrepancies between Table 2 and Figures 3 and 4. Subadult males, as Table 2 suggests, didn't average 170,8 kg. At least, not if 3-year old males would have been included. They weren't, because this department (3-year old males), apart from 1 male, was completely empty. This means only 2-year old males were weighed. It also means the average of subadult males is incomplete, and therefore, inaccurate. Same for subadult females.  

Three is the lack of information about mature tigers (6 years and older).  

Four is they, based on the model developed, conclude the height at the shoulder could be the best indicator of the size of tigers. This although the method used to measure the height is unclear (referring to the protocol) as well as inadequate (it's bound to produce unreliable results).  

Five is the abstract is a somewhat shallow summary of some of the findings. The wealth of data (see Figures 3 and 4) offers the authors a lot of room for a decent analysis.  

The amount of information collected, however, is impressive. Apart from the dissertation of Weilin discussed in the previous post in this series, it's one of the very few about he growth and development of subadult and young adult Amur tigers born and raised in captivity. Figures 3 and 4 in particular offer plenty of opportunities to get to a few conclusions. Reply