Second Litters

Q: Every year hundreds, if not thousands, of young hedgehogs are taken into care -- or die in the wild -- because they’re born too late in the season to fatten up in time for winter. Shouldn’t they have evolved not to have this second litter by now?

Short Answer: Evolution is a blind, impassive process that cannot predict the future and, contrary to some misconception, does not work ‘for the good of the species’ – a species either adapts or dies out. Evolution, and the mechanism by which it operates (natural selection), requires genetic variation that offers an individual some selective advantage (a higher survival rate, or increased attractiveness to the opposite sex, than those carrying the alternative). The genetic variation that is required for evolution to operate occurs randomly (through mutation and recombination) and without it natural selection has nothing to select for. In short, evolution works with the genes the individual in question has got – if the variation doesn’t exist evolution cannot create it and thus we cannot expect something to evolve in a particular way. Thus, the lack of a truncated breeding season in hedgehogs suggests that there is no such mutation/variation at large in the population and hence no selective pressure away from late litters. Interestingly, some adaptation does appear to have occurred in this species and late-born animals appear to put on weight more rapidly than those born earlier in the season.

The Details: The sheer number of small hedgehogs that show up in our parks and gardens each year, and which are too small and born too late to put on sufficient weight to survive hibernation, has prompted some to question the lack of evolution in this species towards what would be a rather obviously beneficial evolutionary step. In his The New Hedgehog Book, mammalogist Pat Morris sums up hedgehog evolution succinctly, writing:

“The first hedgehogs probably appeared over 15 million years ago, long before sabre-toothed tigers, woolly rhinos and other modern upstarts. Those creatures are now extinct, but the hedgehog is with us still. It’s as though the Mark 1 hedgehog was sufficiently well adapted to its way of life that nothing better has yet evolved to replace it.”

Many species have a definite breeding season that maximizes the chance that their offspring will be born at an appropriate time of the year (i.e. sufficiently early to gain suitable condition in time for winter). Hedgehogs do not seem to be genetically programmed with such a ‘cut-off point’ and are sexually active for most of the time they are ‘awake’ (with two peaks in breeding in the UK). The result is that many litters are born late in the year -- generally during October and November, although some may be born as late as December -- and are consequently weaned too late to get to the 700g (1.5 lbs) threshold recommended for surviving hibernation. So, given that late-born litters are unlikely to survive without human intervention, and that hedgehogs have been around since the Miocene, surely this should have been more than enough time for evolution to ‘step in’ and prevent the adults going to all the trouble of having such late litters?

Evolution 101

Most of us with even a basic grounding in science have come across the term “evolution”, so I don’t plan to go into great detail about what evolution is and how it works, but there are a few fundamental points that we need to understand in order to answer the question of why we shouldn’t expect something to evolve.

Simply put, evolution is the change that occurs to a species over successive generations (arguably the term applies to anything that has specific ‘versions’, not just biological organisms). These changes principally come about via two processes: genetic recombination and mutation. Recombination is the cutting, splicing and general randomized ‘mixing up’ of genes that happens during meiosis (sperm and egg production) and as DNA is repaired. The recombination that occurs during meiosis means that offspring have a different combination of genes to either parent. Mutations, by contrast, are the random errors that occur during the copying of genetic material that happens during meiosis and mitosis (growth and repair) as well as under the influence of environmental factors (e.g. radiation, chemical exposure etc.). A good, and widely-used, analogy for random mutation is that of copying a document. Imagine that you chose to re-type one of the articles from this website (or a chapter of your favourite book); even if you’re a monastic scribe it’s unlikely that you’d be able to re-type the whole thing without making a single mistake – if you then made a copy of that copy the error count would rise. Many animals have a pretty faithful replication system (thanks to a kind of built-in error correction – like having a spell-checker when copying your article) such that mutations are comparatively rare, but errors do still creep in and they serve as important evolutionary ‘fodder’.