When a mother bird lays her eggs, she settles down on top to keep them warm; but not reptile mums. They usually abandon their eggs after burying them in a nest. `We know from experimental work that embryos are exquisitely sensitive to external conditions in the nest,' explains Rick Shine from the University of Sydney; the incubation length can vary enormously depending on the temperature. But it wasn't clear exactly how temperature affects developing embryos. `We couldn't measure what was happening inside the egg, we had to treat it as a “black box”,' explains Shine. But all that changed when Shine's postdoc, Rajkumar (Raju) Radder, stumbled across a piece of equipment called `Buddy' on the internet. Buddy measures the tiny heartbeat of developing chicks in the egg. Shine and Radder realised that it may also work on reptile eggs, and if it did, they could begin to find out more about what was going on inside (p. 1302).
Wondering whether the amount of work that an embryo does in the egg determines incubation duration, Shine, Radder and Wei-Guo Du collected freshly laid three-lined skink eggs from fields near Canberra, and took them back to the lab where they could measure the youngsters' heart rates during incubation at temperatures ranging from 20 to 35°C. The skinks took anything from 30 days to hatch at 30°C, up to 60 days at a chilly 22°C, so temperature certainly affected the duration, but how many heartbeats did the babies take?Measuring the embryos' heart rates, Radder found that they varied from 60 beats min–1 at 20°C up to over 120 beats min–1 at 33°C. The warmer lizards' hearts were racing and only took 5×106 heartbeats before hatching, while the cold lizards' hearts beat more slowly, and they took 6×106heartbeats over their 2 month gestation.
But how would other reptile embryos fare? Travelling to Zhejiang, eastern China, with the Buddy, Du repeated the experiments with Bo Sun, from Hangzhou Normal University, on another lizard species, Takydromus septentrionalis, and a turtle, Pelodiscus sinensis. Again they found that the coolest lizards used more heartbeats to develop fully, but the turtle seemed to take many more: 6.72×106 heartbeats compared with the lizard's 4.79×106 heartbeats at 30°C.
Shine suspects that one of the reasons for the difference is that lizard mums allow their young to begin developing in utero before laying their eggs, while turtle mums lay their eggs directly after fertilisation. So lizard embryos are already partially developed before their eggs are laid. When the team took this into account the heartbeat numbers began to look more similar.
Shine and Du also knew that the embryos allocate some of their energy to simply surviving, so they wondered how many heartbeats were allotted by the reptiles to growth and development? Shine and Du subtracted the number of heartbeats required for survival from the total number of heartbeats and found that the numbers were also similar, with the Australian and Chinese lizards using 4.04×106 and 3.98×106 heartbeats to grow, while the turtle used 5.56×106.
So although the nest's temperature has a dramatic effect on the number of heartbeats that a developing embryo takes, with colder embryos putting more energy and heartbeats into staying alive than warmer embryos, it seems that lizards and turtles use a similar number of heartbeats to grow and develop,and only hatch once each of those heartbeats has been taken.
Tragically, Raju Radder did not live to see this project's completion. He died unexpectedly in May 2008 at the age of 34 from a massive heart attack while home for his sister's wedding in India. `Raju was a superbly gifted young biologist; he was intelligent, creative, hard working and a joy to collaborate with,' remembers Shine. He adds, `It is truly bittersweet to see papers coming out with Raju's name on the list of authors.'
