After laying a clutch of eggs, there's only one thing on a chicken's mind;keeping her brood warm until they hatch. Safe in the egg warmed by mum, chick embryos have no need for thermoregulation. But once liberated from their insulated incubators, young chicks are at the mercy of the elements. They must be able to maintain their metabolic rate and body temperature in a cold world,or face certain death. Just how a creature's complex thermoregulatory system develops fascinates Warren Burggren and his student Juli Black. Although chick embryos are essentially cold blooded, depending on their mother for warmth,the team suspected that the bird's thermoregulatory system must be primed and ready to go before hatching. Keen to know how the system develops, the team decided to test how chilly and warm chick embryos' thermoregulatory systems develop, finding that a cold start in life compromises the chick's ability to thermoregulate by cutting their oxygen carrying capacity.
Burggren explains that most gestating creatures try out postnatal reflexes before they are born `its a bit like a dress rehearsal for opening night on Broadway' he says. Knowing that human foetuses practice breathing in the womb,he wondered whether incubating chick embryos could test out their thermoregulatory systems while safe inside their eggs? But a self-warming chick can't make enough heat to warm an entire egg, so Burggren and Black decided to monitor the egg's metabolic rate, to see whether the tiny occupant was trying to maintain its body temperature. Transferring eggs from a cosy 38°C to a chilly 35°C, Black recorded the cold eggs' metabolic rates and found that that most of the older chicks maintained their metabolic rate as the temperature fell, and in some cases, even increased it! Burggren admits he was pleased to see that the chicks were trying to thermoregulate.
But what happened to chick embryos that were raised at 35°C? Would the cold conditions affect the embryo's development and change their ability to thermoregulate? Black incubated eggs at the lower temperature, and monitored the chicks. Sure enough the chick's physical development lagged behind their warmer cousins, slowing in proportion to the 3°C fall in temperature. Monitoring the chick embryo's ability to thermoregulate, the team realised that when they compared cold and warm chicks, the youngsters reared in the warm successfully maintained their metabolic rate, while cold reared chicks couldn't (p. 1543). The cold incubation had altered the course of the cold chick's thermoregulatory development.
Puzzled by the change in the cold chick's physiology, Burggren and Black decided to track down what might have changed to impair the chicks'thermoregulatory capacity. Knowing that a chick faced with a drop in environmental temperature must maintain its oxygen consumption rate to stay warm, and that adequate blood oxygen transport is a key feature of maintaining oxygen supply, the team decided to see if the cold chick's blood was different to that in the warm reared youngsters.
Although extracting tiny blood samples from the developing chick embryos was tricky, Black soon became adept at manipulating volumes as small as 8μl as she tested both groups of chicks' oxygen carrying capacity. Testing a whole suite of blood characteristics, she found that not only were the cold chicks' hematocrit and haemoglobin levels lower, but the haemoglobin's oxygen affinity was also significantly reduced. It appeared that incubation in the cold had affected the embryos' haematological development, in turn compromising their ability to carry oxygen and maintain their metabolic rate as the temperature falls (p. 1553).
Although Burggren points out that there are probably a whole host of other physiological regulators that need to interact during successful thermoregulation, blood oxygen carrying capacity is clearly one significant factor in an animal's ability to stay warm.