Researchers at Stellenbosch University think they are starting to understand why ostrich chicks vary so greatly in size, with some chicks up to three times larger than their peers. The research could also provide answers to the high chick mortality rate within the first three months.
With ostrich chick mortalities of between 10 and 50%, one of the greatest challenges facing ostrich farmers is raising chicks that will survive beyond the first three months.
Now researchers from the Department of Botany and Zoology at Stellenbosch University (SU) have found that the bright white feathers of the male ostrich may be one of the best indicators of faster-growing and healthier chicks.
According to Prof Michael Cherry, a specialist in bird and animal behaviour at SU, surprisingly little is known about the mating system of ostriches, even though it is an iconic bird of the open savannas: "They are the largest living birds on earth and at the same time also the largest of the flightless birds, called the ratites. They are not only dimorphic (meaning the sexes differ, in this case in colour), but also promiscuous (males and females have multiple partners)."
What evolutionary biologists do know, however, is that the bright plumage of male birds of some species appears to indicate a stronger immune system. Prof Cherry explains: "Having bright plumage comes at a cost, so only the fittest males can afford to have both a strong immune system and bright feathers. At the same time females might prefer bright males because they are better able to resist disease and pass on to their offspring copies of genes for resistance."
Proving the genetic link
Back in 2005, Prof Cherry and a colleague realised the breeding system used by the ostrich industry provided the ideal experimental setup to test whether this assumption also held true for ostriches. In most instances the ostrich eggs are collected shortly after being laid. The eggs are then incubated for six weeks under controlled conditions. After hatching, the chicks are sold off to different growers.
Dr Maud Bonato, who conducted the research as part of her doctoral studies, was then given access to a research flock of 38 South African black ostriches (Struthio camelus) on a research farm in Oudtshoorn, managed by the Western Cape Department of Agriculture.
The birds were divided into two camps, with a male to female ratio of 7 to 11 and 8 to 11. During 2005 all the parents and their chicks were genotyped for parentage analysis. This means the DNA were extracted from blood samples taken from the parents and their offspring, and then subjected to a number of tests and analysis, including growth rates and immune responses. The researchers also took colour intensity measurements of each male, specifically traits such as the bill, neck, shins and black and white feathers.
In the first part of her study Dr Bonato investigated whether ostriches, like many other bird species, prefer to mate with males showing the most elaborate ornamentation.
"Current understanding is that females should invest more in reproduction when mated to an attractive male than when mated to a less attractive male. Maternal investment, particularly nutrients and energy, can profoundly influence the development of embryos and survival of hatchlings. In other words, larger eggs produce larger offspring with a higher chance of survival and faster growth," she explains.
The results, which indicated that ostrich females did indeed lay heavier eggs when mated with males with brighter coloured feathers, were published in the journal Animal Behaviour in 2009.
"We found that the colour of the neck, white and black body feathers, and the brightness of black feathers, predicted egg mass. These traits are exposed during the male courtship display, so we suggest that these visual cues influence the degree of maternal investment in eggs through their influence on female perception of mate quality," she writes in the article.
Dr Bonato then set out to establish whether there is a significant correlation between chick growth and immune defence, and the levels of immune defence in their genetic parents. She also wanted to establish whether specific male traits such as colouration could be used to predict the growth rate and immune responses of their offspring.
Published in the journal Behavioral Ecology Sociobiology this month, the research results showed that parents' immune responses predicted both chick immune responses and growth rates. Furthermore, the colouration of the father's white feathers was found to predict offspring immune response to typical avian diseases such as diphtheria, while the colouration of both the father's white feathers and bill predicted offspring growth rate.
In other words, an ostrich chick with a healthy immune system can invest more energy into growing faster. Compare this with an ostrich chick with a weak immune system, where all the energy is going towards fighting bacteria and other diseases, Dr Bonato explains.
Advice to ostrich farmers
Dr Bonato, who is now with the Department of Animal Sciences at Stellenbosch University, advises ostrich farmers to utilise the ostrich's natural mating behaviour to their advantage: "This means providing ostriches with enough space and mates to be able to express their full natural behaviour."
She also cautions against plucking feathers during or close to the breeding season: "After being plucked a male may not be able to show his full quality to the females. As a result the female might not mate or invest as much as she would have to an unplucked male," she concludes.
Dr Bonato is currently continuing her research into ostrich mating behaviour at the research farm in Oudsthoorn.
Photo above: During courtship, the bright white feathers of the male ostrich are displayed in kantling behaviour to attract the attention of the best female, but together they also produce healthier and faster-growing chicks. In a recent study, researchers from Stellenbosch University showed a direct correlation between the white colour of male ostrich feathers and the growth rate and immune response of their offspring. Photos: Maud Bonato
Media enquiries
Prof Michael Cherry
Tel: +27 72 6875845
E-mail: mic@sun.ac.za
Dr Maud Bonato
Tel: +27 44 27 26 077
E-mail: mbonato@sun.ac.za
Released by
Wiida Fourie-Basson, Media: Faculty of Science, Stellenbosch University, 021 808-2684, science@sun.ac.za, 071 099 5721
