There are established differences between the lifespans of males and females in many species of mammals and birds.

Evolution, of course plays a significant role. Sexual selection and genetic factors, as well as the parental care role, have an impact.

One key finding in a recent study was found by comparing mammals and birds.

1176 species were in the study (we like a large sample size!) and it concluded:

• Female mammals live 13% longer than males on average
• Male birds live 5% longer than females on average

The answer lies in the sex chromosomes. In mammals, females have two X chromosomes but the males are heterogametic and have XY chromosomes. When carrying out a genetic cross to investigate sex-linkage (AQA & OCR), you will see that females rarely succumb to sex-linked genetic diseases or traits. This is because they have two X chromosomes so have two copies of the genes located on them. If one gene were to be mutated so that the protein they code for is non-functional, then they can be protected from symptoms because they still have one gene that is fully functional. Whereas males, being heterogametic, only have one X chromosome, and they have no second copy of some sex chromosome genes on their Y chromosome. So, if their only copy is mutated, then they will suffer the symptoms. So having a pair of non-homologous chromosomes has some risk. You will see on pedigrees that the sufferers are males - the sons of usually symptomless carrier mothers (a common type of exam question).

What is really interesting is that in birds the situation is reversed because it is the males that have the same chromosomes and it is the females who are the heterogametic sex and more vulnerable to sex-linked diseases. Exam questions (AQA & OCR) have to test your AO2 and AO3 skills – and looking at the unfamiliar example of sex-linkage in birds has been a feature in the past. You apply what you know about sex-linkage in the familiar mammal’s scenario but adapt to swapping the chromosomes for birds and can make predictions about the outcome.

Another interesting factor involved is that of sexual selection. When competition for a mate is higher – lifespan takes a hit. You can spot this in birds with bright colours or large body size and indeed body weapons! They have to compete hard to get a mate and this can shorten their lifespan. They follow polygamous relationships (have many mates) so the competition is always ongoing. The downsize of such bright colours is they attract predators too! When the parent relationship is monogamous (one partner), the pressure of competition is much lower and the males live longer. Another contributory factor is the parental role. The sex that invests in raising their young lives longer. From a genetic point of view, the investment ensures the parent’s genes are passed on to the next generation and then the one after too.

Analysing zoo data helped to reveal the effect of the environment on this variation (OCR and AQA). Animals in zoos don’t have the same pressures in the wild – they get fed and are protected from predators and disease. This definitely reduces the difference in lifespans between males and females, showing that the genetic component was still having an effect.

Competition, predation, adaptation, variation, causes of variation and sex-linked inheritance, wrapped up in a context of natural selection during evolution are topics central to A-Level Biology understanding.

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