zoology biology benno meyer rochow science blog calcium bones

Long Live the Females

In praise of the tougher (not “weaker”) sex

Even if there are some exceptions like hamsters, for instance, there can be no doubt that in most mammalian animal species the female is the longer lived of the two sexes. Why should female longevity be favoured by Nature and what is the basis for this phenomenon?

A host of reasons has been advanced to explain this statistical fact. In virtually all species the male gametes (the spermatozoa) vastly outnumber female gametes (the egg cells). Consequently, one male suffices to fertilize eggs from a whole lot of females. Also, the females of most species tend to have a greater direct input into the brooding and brood-care business than the males. In short, males appear to be the more expendable sex and perhaps even a drain on a species’ resources if food and shelter should become scarce. In a way, because of a female’s limited capacity to spawn offspring and her frequently greater long term contribution toward bringing up the young, she, herself, represents a “limited resource” for the males – and they have to compete for it. They struggle to obtain access to the female “resource” and in this process of competition males expand energy in preparing and defending nest sites, establishing and guarding territory and endangering themselves through fighting and disregard of protective cover and health issues.

The mortality of males of most species is undeniably higher, but additional reasons may operate. Males in most species of animals are, of course, males because they possess one x and one y chromosome; females, usually, have two x chromosomes. Any genetically-based abnormality or disease located on an x-chromosome can affect the male more easily, because if his single x-chromosome is a “disease carrier”, then there is no second “healthy” x-chromosome to compensate. In females there is, and immediately the genetic disease “haemophilia”, carried on one x-chromosome and only affecting males comes to mind. Several Royal dynasties in the 19th and early 20 th century were affected by it and the haemophilia of the only son (the Tsarevich Alexei) of the last Russian tsar Nikolai II. and the influence Rasputin had at court were some of the reasons that led to the Bolshevik revolution and the end of the Russian monarchy. That this chromosomal difference between males and females could, indeed, be an important factor, is suggested by the fact that in the few birds that have been examined for longevity, the males appear to be the longer lived: in birds males are the homogametic sex with two equal chromosomes and it’s the females that are the ones who possess two different chromosomes , i.e., one x and one y chromosome.

The question that a student upon this revelation asked me was this: does it follow from this then that in birds males are females and females are males? It’s a matter of definition how you define what a male and what a female is. There is, of course, also the big “why” question: why are sex-chromosomes and sex determination in birds (and incidentally also in some insects like butterflies and moths) the opposite to that from mammals? I am unable to answer this “why” question. Have you got a convincing answer?

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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