Age-determination of the tsessebe, plus labelling by body mass, using photos in iNaturalist
What, exactly, does a given species of ruminant look like at a given age? Can we estimate the body mass of infants and juveniles in photographs, based on objective criteria?
The tsessebe (Damaliscus lunatus lunatus, https://www.inaturalist.org/observations?taxon_id=132770) provides an unusual opportunity, because
- its growth and reproduction show a definite relationship to the calendar, and
- the subspecies is well-covered by photos in iNaturalist.
Child et al. (1972, https://www.degruyter.com/document/doi/10.1515/mamm.1972.36.3.342/html) 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 (https://journals.co.za/doi/pdf/10.10520/AJA03794369_3447) states 'horns became externally visible at five weeks'.
Based on the above information, I have annotated photos of the tsessebe in iNaturalist.
BOTSWANA
According to Child et al. (1972, 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.
November
The following show infants < 1 month old, with body mass <15 kg:
https://www.inaturalist.org/observations/100694354
https://www.inaturalist.org/observations/66701641
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, and body mass is little more than 20 kg:
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
July
The following show juveniles about 8 months old, when the dark markings are not yet fully-developed, particularly on the face. Body mass is probably <60kg, which is about half of maternal body mass:
https://www.inaturalist.org/observations/143915475
https://www.inaturalist.org/observations/57781574
https://www.inaturalist.org/observations/38168446
https://www.inaturalist.org/observations/4131
https://www.inaturalist.org/observations/125828092
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/11383030
https://www.inaturalist.org/observations/33810120
https://www.inaturalist.org/observations/33769054
https://www.inaturalist.org/observations/138261267
October
The following shows juveniles about 11 months old. Body mas is probably >70 kg:
https://www.inaturalist.org/observations/69009855
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
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 (South Africa) and Zimbabwe, the tsessebe gives birth in October.
October
Newborn
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
February
4 months old
https://www.inaturalist.org/observations/87448750
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, the tsessebe gives birth in September.
December
3.5 months old
https://www.inaturalist.org/observations/6899966November
September
1 year old
https://www.inaturalist.org/observations/138408064
November
14 months old
https://www.inaturalist.org/observations/36316646
DISCUSSION
The following photo-pair nicely show the difference between 11 months old and 23 months old:
https://www.inaturalist.org/observations/99518233
https://www.inaturalist.org/observations/16954883