3.6 The numbers game … or the struggle for existence
In the majority of The Life of Mammals TV sequences there is relatively little evidence of any struggle for existence, apart from the occasional predator/prey interaction. Even then you are offered the comforting reassurance that 'four out of five chases end with the prey escaping'. So you could be forgiven for thinking that most mammals survive to a ripe old age, or at least until, like the topi, they have fulfilled their reproductive potential. Not so! Four out of five chases may end with the prey escaping, but that doesn't mean four out of five animals escape being eaten over a longer period of time.
But there is another, often more potent, threat to survival. One of the special characteristics of rodents that we drew attention to at the beginning of this course is their prolific breeding prowess. The calculation for the Norwegian rat in Section 1 demonstrated that the world would long since have been totally overwhelmed by rats if their numbers went unchecked! This prodigious ability to breed means that there is direct competition. Rats compete with other rats, mice compete with mice. A glimpse of the problem was revealed in the 'Chisellers' programme when we were shown a plague of mice.
Watch the TV programme from 40.20-41.42 and jot down any information on aspects of the breeding of the common house mouse. Compare what you learn of their behaviour with what DA describes towards the end of LoM Chapter 3 about population explosions in lemmings.
In cereal-growing areas in Australia, mice feeding on spilt grain in the field can invade grain stores, where the numbers proliferate rapidly, reflecting the enormous breeding capacity of these rodents. The economic consequences of such population explosions are clearly severe. The population explosions of the Scandinavian lemmings [p.85] occur under more natural conditions but have a less clear cause. They result in increased dispersion of the species, though no such benefit is as obvious from house mouse population explosions.
In the TV programme we are told that females of the common house mouse can breed when they are five weeks old, and monthly thereafter. The litter size is between four and six. How does this compare with the Norwegian rat mentioned in Section 1?
Within 20 weeks of being born, the female Norwegian rat can have produced 56 offspring. The common house mouse can have had four litters in this time but, as it only has up to six young in a litter, it can (at most) produce 24 young after 20 weeks and so lags behind the Norwegian rat with its 56 young.
These vast numbers would not seem to be a good measure of success because so many of these creatures die. However, although I have already flagged the rodent's 'record-breaking productivity' [p. 84] as a feature that contributes to the rodent success story, I need to justify this claim that high productivity is an effective measure of success.
Large populations can be sustained if food is available for all, but the mice in the programme 'disappeared' once they had stripped the cornfields and the barns. As with the lemmings described in LoM p. 85, the population crashes - another example of the 'struggle for existence', as Darwin described it. Famine exerts a strong selection pressure on the rodents - few survive. We surmise that a mouse which survives, does so not by chance but because it possessed some variant of a character that was 'profitable to itself' (Darwin's words). For example, perhaps it could run further and faster than others to find food, or could survive on food that others found unpalatable, or had a better sense of smell and found food that others overlooked, or had sharper teeth and could break open nuts that others could not open. Whatever the detail of the character that confers advantage, the benefit of high productivity followed by a strong selection pressure is that the advantageous character can very rapidly increase in frequency within the population. Darwin saw that this mechanism suggests how new species could arise - a process called speciation. Indeed, it was this aspect that inspired the full title of the book On the Origin of Species by means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life. One measure of rodent success mentioned in Section 1 is the number of species. So, by this logic, the other criterion for success - productivity - could be responsible for the group's ability to have diversified and formed so many different species. When DA says at the end of LoM 'rodents are the most adaptable of mammals' you can now appreciate that he is not referring to an individual's behaviour but to the range of adaptations that are seen across the many rodent species.
My hope is that LoM and this series of units will give you an appetite to read beyond the materials supplied here. There is a wealth of high-quality popular books on evolution and natural selection, many of which describe Darwin's life and work. One of the best is Almost Like a Whale by Steve Jones, described as an 'update' of Darwin's Origin of Species. Reading it will give you a clear sense of why natural selection is regarded as such a fundamental biological concept. Another is What Evolution Is: From Theory to Fact by Ernst Mayr. Both books are available as paperbacks.