Photos of the tsessebe in iNaturalist, labelled for body mass and age since birth

Ruminants grow remarkably rapidly. This limits our ability to estimate the size and age of individuals from photographs.

The tsessebe (Damaliscus lunatus lunatus, https://www.inaturalist.org/observations?taxon_id=132770) is a useful example to begin with, because

• its body size is intermediate, in the range of ruminant species from bambis to megaherbivores,
• its growth and reproduction show a particularly precise relationship to the calendar,
• the subspecies is well-covered by date-labelled photos in iNaturalist,
• the sexual dimorphism is not enough to blur the overall patterns, in either body size or the size and shape of the horns, and
• the relevant criteria have been established in three published studies (Child et al. 1972, https://www.degruyter.com/document/doi/10.1515/mamm.1972.36.3.342/html and Huntley 1979, https://journals.co.za/doi/pdf/10.10520/AJA03794369_3447 and https://journals.co.za/doi/abs/10.10520/AJA03794369_3447 and Anthony and Lightfoot 1984, https://journals.co.za/doi/pdf/10.10520/AJA03794369_3138).

Child et al. (1972) found the following mean body masses for the tsessebe:

Mature males 140 kg

Mature females 127 kg

Adult males 3 years old 135 kg

Adult females 3 years old 115 kg

Adolescent males 2 years old 105-110 kg

Adolescent females 2 years old 95 kg

Juvenile males 1 year old 82 kg

Juvenile females 1 year old 68 kg

Juvenile males 6 months old 55 kg

Juvenile females 6 months old 45 kg

Newborns 10-11 kg

What this amounts to is a progression of body mass of 10 kg-50 kg-75 kg-100 kg-125 kg-130 kg, at the ages of 0 years-0.5 years-1 year-2 years-3 years-6 years.

Huntley (1979) states 'horns became externally visible at five weeks'.

Based on the above information, I have annotated photos of the tsessebe in iNaturalist, as follows.

BOTSWANA

According to Child et al. 1972 (page 366 in https://www.degruyter.com/document/doi/10.1515/mamm.1972.36.3.342/html), the tsessebe in northern Botswana breeds extremely seasonally, giving birth in November.

The results are as follows, beginning with the birth-month of November, and categorising observations by subsequent months. Throughout this Post, I have ignored the sex of the individuals described.

November

The following shows newborns, of body mass about 10 kg. The withers of the infants are hardly as high as the maternal belly (shown even more clearly for the closely-related topi, Damaliscus jimela, in https://www.alamy.com/topi-damaliscus-lunatus-female-with-new-born-calf-massai-mara-kenya-image61099252.html?imageid=524FB694-797E-4AF9-969C-995F7380E022&p=191343&pn=3&searchId=aa055d5f893ca3a60402a79b4144734b&searchtype=0):
https://www.inaturalist.org/observations/100694354

The following show infants <1 month old, with body mass probably <15 kg:
https://www.inaturalist.org/observations/66701641

The following shows an individual infant ostensibly only 1 month old, but with body mass already at least 20 kg:
https://www.inaturalist.org/observations/122347068

Late December/early January

The following show infants about 1.5 month old, when the horn-tips have just appeared. Body mass is about 20 kg, having doubled since birth. The withers of the infants now reach at least the height of the maternal knee:
https://www.inaturalist.org/observations/146520990
https://www.inaturalist.org/observations/146636730
https://www.inaturalist.org/observations/146528727

February

According to the stated date, the following shows an infant individual about 3 month old. This seems questionable in view of the small size of the horns. However, this individual was possibly born at the end of the birth-season, say in early December, in which case the age might be 2-2.5 months:
https://www.inaturalist.org/observations/147820164

March

The following show juveniles about 4.5 months old, when the horns are easily visible and the ground-colour of the juvenile figure is still noticeably paler than that of adults. Body mass is probably about 40 kg, which is a quarter of maternal body mass:
https://www.inaturalist.org/observations/37787394
https://www.inaturalist.org/observations/10004638

Late April

The following show juveniles about 6 months old:
https://www.inaturalist.org/observations/171576691

Late June-early July

https://www.inaturalist.org/observations/173664182
https://www.inaturalist.org/observations/125828092

Late July

The following show juveniles about 8 months old. The dark markings are far from fully-developed, particularly on the face. Body mass is probably <60kg, which is about half of maternal body mass. The withers of these juveniles have already reached the height of the maternal sacrum:
https://www.inaturalist.org/observations/143915475
https://www.inaturalist.org/observations/57781574
https://www.inaturalist.org/observations/38168446
https://www.inaturalist.org/observations/4131

August

The following show juveniles about 9 months old:
https://www.inaturalist.org/observations/138887828
https://www.inaturalist.org/observations/41206964
https://www.inaturalist.org/observations/41206958

September

The following show juveniles about 10 months old. Body mass is probably <70 kg:
https://www.inaturalist.org/observations/189516047
https://www.inaturalist.org/observations/11383030
https://www.inaturalist.org/observations/33810120
https://www.inaturalist.org/observations/33769054
https://www.inaturalist.org/observations/138261267

October

The following show juveniles about 11 months old. Body mass is probably >70 kg:
https://www.inaturalist.org/observations/69009855
https://www.inaturalist.org/observations/192022183

November

The following shows a juvenile individual about 12 months old. Body mass is about 75 kg:
https://www.inaturalist.org/observations/17087

Late December/early January

The following shows a juvenile individual about 13.5 months old. Body mass is probably <80 kg. The dark markings, particularly on the face, remain incomplete, and seem not to have changed over the previous 6 months:
https://www.inaturalist.org/observations/10639034

March

16 months old:
https://www.inaturalist.org/observations/21793037

16.5 months old:
https://www.inaturalist.org/observations/10006014

May

18 months old:
https://www.inaturalist.org/observations/61938247

June

19 months old:
https://www.inaturalist.org/observations/139307973
https://www.inaturalist.org/observations/139301317

November

24 months old. Body mass is about 100 kg:
https://www.inaturalist.org/observations/2588518

December

25 months old:
https://www.inaturalist.org/observations/15469

KRUGER NATIONAL PARK AND ZIMBABWE

In Kruger National Park (https://en.wikipedia.org/wiki/Kruger_National_Park) and Zimbabwe, the tsessebe gives birth in October (Child et al. 1972).

Accordingly, the results are as follows.

October

Newborns:
https://www.inaturalist.org/observations/32595531
https://www.inaturalist.org/observations/9323974

December

1.5 months old:
https://www.inaturalist.org/observations/107290193

2 months old:
https://www.inaturalist.org/observations/36703962

January

3 months old
second photo in https://www.inaturalist.org/observations/184603378

February

4 months old:
https://www.inaturalist.org/observations/87448750

The following is stated to be in February. However, this individual infant is certainly <2 months old, suggesting that either the date is incorrect or the population in Hwange National Park does not conform to the seasonality given by Child et al. (1972):
https://www.inaturalist.org/observations/38602291

March

5 months old:
https://www.inaturalist.org/observations/151731918
https://www.inaturalist.org/observations/144870681

April

6 months old:
https://www.inaturalist.org/observations/75355938
https://www.inaturalist.org/observations/12119329

June

8 months old:
https://www.inaturalist.org/observations/13368268

July

9 months old:
https://www.inaturalist.org/observations/99261092
https://www.inaturalist.org/observations/20887477

August

10 months old:
https://www.inaturalist.org/observations/107534355

September

11 months old:
https://www.inaturalist.org/observations/99518233

October

1 year old:
https://www.inaturalist.org/observations/143044598
https://www.inaturalist.org/observations/44272021

November

13 months old:
https://www.inaturalist.org/observations/37087281
https://www.inaturalist.org/observations/7733155

December

13.5 months old:
https://www.inaturalist.org/observations/107290193

January

15 months old:
https://www.inaturalist.org/observations/9412515

February

16 months old:
https://www.inaturalist.org/observations/105321502
https://www.inaturalist.org/observations/35886830
https://www.inaturalist.org/observations/34375710

September

23 months old:
https://www.inaturalist.org/observations/16954883

November

25 months old:
https://www.inaturalist.org/observations/7733155

CAPRIVI STRIP

In the Caprivi Strip of Namibia (https://en.wikipedia.org/wiki/Caprivi_Strip), the tsessebe gives birth in September (Child et al. 1972).

Accordingly, the results are as follows.

The following shows an infant individual about 1 month old. Either the stated date is incorrect, or the population no longer conforms to the seasonality given in Child et al. (1972):

https://www.inaturalist.org/observations/68999662

September

1 year old:
https://www.inaturalist.org/observations/138408064

November

14 months old:
https://www.inaturalist.org/observations/36316646

DISCUSSION

At about 6 months old, the horns reach the length of the ear pinnae, and the body reaches 50 kg.

The progression from 10 kg through 50 kg to 100 kg is summarised, approximately, by the following three photos:

10 kg
https://www.inaturalist.org/observations/9323974

50 kg
https://www.inaturalist.org/observations/57781574

100 kg
https://www.inaturalist.org/observations/2588518

The following photo-pair shows the difference in appearance between 11 months old and 23 months old:
https://www.inaturalist.org/observations/99518233
https://www.inaturalist.org/observations/16954883

These series show that the colouration of the tsessebe does not merely change continuously from infancy to maturity. Instead, there are three stages, within each of which body size covers a considerable range.

The infantile colouration, which is plain fawn with countershading, is unexplained by crypsis, because infants of the tsessebe do not hide even when newborn.

The juvenile colouration, in which the dark markings of adults have nominally appeared but lack intensity for more than a year, is also unexplained. Most precocial is the darkness on the anterior surface of the forelegs. The delay of the darkening on the face until adulthood suggests a social clue rather than a feature adaptive in the context of anti-predation.

Posted on Απρίλιος 01, 2023 0919 ΠΜ by milewski