Wednesday, June 22, 2005

'Boiled' Eggs & Dinosaur Sex Ratios

Image from HERE.

In the discussion of what killed off the dinosaurs at the end of the Cretaceous you’ll often read that the asteroid impact coincidental with the Cretaceous-Tertiary boundary caused extreme environmental changes. Scientists know that the sex ratio of some incubating reptile eggs can be strongly effected by temperature with more males hatching at low temperatures and more females hatching at higher temperatures. If the average temperature at the end of the Cretaceous changed enough to effect the sex ratio of enough different types of dinosaurs over a long enough period of time, the resulting imbalance in the number of males to females born could have contributed to their extinction.

In the paper below, Ann Göth and David T. Booth show for the first time that the incubating eggs of some living birds show a similar response to temperature. Since extinct dinosaurs are phylogenetically bracketed by reptiles and living birds it stands to reason that dinosaurs could have been susceptible to the same problem.

Temperature-dependent sex ratio in a bird. Ann Göth and David T. Booth. Biology Letters 1: 31-33. DOI: 10.1098/rsbl.2004.0244

Abstract: To our knowledge, there is, so far, no evidence that incubation temperature can affect sex ratios in birds, although this is common in reptiles. Here, we show that incubation temperature does affect sex ratios in megapodes, which are exceptional among birds because they use environmental heat sources for incubation. In the Australian brush-turkey Alectura lathami, a mound-building megapode, more males hatch at low incubation temperatures and more females hatch at high temperatures, whereas the proportion is 1:1 at the average temperature found in natural mounds. Chicks from lower temperatures weigh less, which probably affects offspring survival, but are not smaller. Megapodes possess heteromorphic sex chromosomes like other birds, which eliminates temperature-dependent sex determination, as described for reptiles, as the mechanism behind the skewed sex ratios at high and low temperatures. Instead, our data suggest a sex-biased temperature-sensitive embryo mortality because mortality was greater at the lower and higher temperatures, and minimal at the middle temperature where the sex ratio was 1:1.