Indeterminate growth in Ardeotis

(writing in progress)
 
One of the interesting features of the biology of the large bustards in the genus Ardeotis is ‘indeterminate growth’.
 
The ‘typical’ pattern for birds is to grow rapidly to adult size (or even exceeding adult size if temporary deposits of fat are considered). Associated with this is a pattern in which the juveniles look quite different from the sexually mature even at similar body size.
 
An odd feature of bustards, at least in the genus (Ardeotis) of the largest bustards, is a pattern in which growth is extended beyond sexual maturity, associated with a pattern in which the juveniles look like miniature adults.
 
I have been familiar with Ardeotis for most of my life, but it is only recently that I have noticed how odd the juveniles look, in their appearance as ‘miniature adults’.
 
I will seek photos showing this pattern for the three spp. other than A. kori (shown below). However, I think the pattern applies to all four spp.
 
At the same time, it is interesting that the male continues to grow for years after full sexual maturity, a pattern which I find to be odd for birds. It is this indeterminate growth, particularly in the male, which helps to explain why fully mature males can sometimes weigh 2.5-fold adult females. However, what is seldom explicitly pointed out is that the proportions remains similar, which means that the fully mature males are ‘adult males writ large’ – as opposed to the proportions changing with growth.
 
If one sees a photo lacking clues to scale (which is usually the case), it is difficult to tell whether a given individual of Ardeotis is juvenile, adult, or old – even though the range of body masses here may be from a mere 2 kg to a whopping 10 kg. It is only when the photo captures adult together with juvenile (as shown below) or male together with female, that the strange pattern of isometric scaling becomes obvious.
 
Ardeotis kori struthiunculus, presumably adult female with juvenile:

https://www.alamy.com/kori-bustard-choriotis-kori-adult-and-a-chick-image66738758.html

https://africawild-forum.com/viewtopic.php?t=8081&start=430

https://www.inaturalist.org/guide_taxa/1101272

(writing in progress)

Publicado el julio 8, 2022 11:31 TARDE por milewski