Long incubation's may defend birds against parasites

Some tropical birds have longer egg incubation times than their temperate cousins, even though their habitat is teeming with egg-eating predators. The reason for this has long been a mystery, but a recent study has applied new methods to confirm the evidence for the old idea that a longer development period leads to a stronger, more efficient immune system.

The University of Missouri-St Louis’s Robert Ricklefs first studied this relationship in the early 1990s, using data from the microscopic examination of avian blood samples to look for the presence of parasites, primarily those that cause malaria. He found that the longer a species’ incubation period, the lower its prevalence of parasites. However, Ricklefs remained concerned that especially low parasite loads could have been missed during microscope examination, affecting prevalence estimates.

Ricklefs's newly published study offers evidence that the answer lies in the birds’ immune systems, which may grow stronger and become more able to defend against blood parasites thanks to this longer period of development.

Advances in DNA sequencing helped find this new solution. Ricklefs and his colleagues collected blood samples from birds in the eastern United States and several Neotropical countries, and checked for the presence of parasite DNA. They then tabulated how many individuals from various bird families were sampled at each site and how many were infected with Haemoproteus or Plasmodium parasites. About 22 per cent of individual birds in both temperate and tropical regions had parasite DNA in their blood. While incubation times vary little among temperate species, there are differences among tropical birds, in which parasite prevalence was significantly lower in species with longer incubation times.

These results confirm those of the old blood smear analysis. While there is still no direct evidence for the hypothesis that a longer incubation time promotes a stronger immune system, this correlation provides a strong indication that this could be the solution to the mystery of why the embryos of some tropical birds take so long to develop.

“My interest in blood parasites was stimulated by a former graduate student, Victor Apanius, primarily in the context of community ecology. However, I had been working on the diversification of avian life histories, particularly embryo and chick growth rates, and I couldn't help but notice a connection between the two,” said Ricklefs. “I wasn’t surprised that the new results confirm the old ones so well, really, because the two techniques estimate the same attribute. However, more detailed studies of the avian immune response and how variation in host defence is related to development certainly are warranted.”

San Francisco State University’s Ravinder Sehgal, an expert on avian blood parasites, commented: “This paper is a nice follow up to the 1992 study that showed an inverse relationship between parasite prevalence and egg incubation period. It will be now be important to test the work [experimentally] to study the parasitology and explore the trade-offs between embryo growth rate and immune function.”