Vertebrates depend on an efficient transport system to supply their bodies with vital nutrients and oxygen. The vertebrate heart is highly versatile and adjusts its performance when a body's demands change in response to factors such as temperature, digestive state, or behavioural responses. In the longer term, other factors influence the heart's workload, such as sexual maturation and reproduction. While cardiac output can be changed short-term by altering heart rate or the amount of blood ejected per heart beat, the only way to change output long-term is to change the heart's size. To investigate what factors influence long-term changes in the heart, Filippo Garofalo and his colleagues from University of Calabria, Italy, investigated the influence of sex and season on heart morphology and performance in the green frog Rana esculenta.
Over the course of eight years, the team captured 696 green frogs at different times of year. To see how heart mass differed between the sexes and seasons, they carefully removed the animals' hearts and weighed them. Heart mass increased with body mass in both sexes, however males had bigger hearts for a given body size, even though they are smaller than females. This suggested that males are more active.
Next, to estimate how much blood each heart could pump in a particular time, the team inserted tiny tubes into the veins and arteries of whole hearts and artificially pumped fluid into them. They used pulse pressure – the difference in blood pressure between when the heart is filling with blood and when it is contracting – as a measure of a heart's performance. They found that heavier hearts pumped more fluid and had a higher maximum pulse pressure in both males and females. They also found that female hearts pumped relatively more fluid than male hearts, meaning that they pumped more blood for their size.
The team found out why females' hearts pumped more blood when they investigated which part of the heart was responsible for the increase in overall heart mass. They separated out and weighed each heart's ventricle,which pumps blood to the body, finding that females' ventricles were heavier than males' ventricles, even though they had relatively smaller hearts. The larger ventricles caused the better than expected heart performance in females.
Comparing ventricle size in frogs captured at different times of year, they found that females' ventricles were larger still during the crucial parts of reproductive cycle: spring, when mating occurs; and winter, when animals recover and prepare for the next breeding season. This suggested that there was an extra strain on females at these times of year. Also, they found that as males became sexually mature their ventricles increased in mass, which raises pulse pressure. Wondering what morphological changes occurred in heavier hearts, the team found that larger ventricles had more heart muscle tissue, which would account for their higher pulse pressure.
The authors conclude that when frogs are very active during the breeding season, both sexes tune up their cardiovascular system to cope with the increased metabolic demands: females have to produce a whole clutch of good quality eggs and choose a mate; males use up a lot of energy vocalising to attract females. At these times, females' ventricles are relatively larger,meaning that they can maintain a higher blood flow than the males, provide more oxygen to their tissues, and cope with the enhanced work load caused by their bigger body size and more intense breeding behaviour.