Why haven’t hedgehogs evolved not to have late litters by now?


Evolution is a blind, impartial process that cannot predict the future and, contrary to common misconception, does not work ‘for the good of the species’ – a species either adapts or dies out. Evolution, and natural selection, the mechanism by which it operates, requires genetic variation that offers an individual some selective advantage - a higher survival rate, increased attractiveness to the opposite sex - over 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 “work with”. In short, evolution works with whatever genes the individual in question has – if a variation doesn’t exist, evolution cannot create it. Thus, we cannot expect something will evolve in a particular way that seems adaptive to us.

So, the fact that the hedgehog breeding season extends until late summer suggests that no mutation/variation has yet arisen in the population to truncate it. Interestingly, some adaptation does appear to have occurred in that there is a small pool of data suggesting that late-born hedgehogs put on weight more rapidly than those born earlier in the year.

The details

Every autumn, very small hedgehogs start appearing in parks and gardens across the country. The current advice from the British Hedgehog Preservation Society is that a hedgehog needs to weigh at least 700g (1.5 lbs) if it is to survive hibernation, and these little ones are too small to put on sufficient weight in time. The reason? They’re born too late in the year and there simply isn’t enough time before winter to lay down the fat reserves necessary to get them through to spring. Without human intervention, most won’t see the New Year.

Hoglets with their mother. Many females produce litters late in the summer, leaving the youngsters insufficient time to fatten up enough to survive hibernation. - Credit: Krzysztof Kozłowski

The regular appearance of so many of these ‘autumn orphans’ has prompted some to ask why hedgehogs haven’t evolved not to have such late litters. After all, if many of these late-born hoglets are destined to die during winter, and producing such late litters puts a strain on their mother to replenish her fat stores before hibernation, sticking to only one litter born early in the summer seems like an obviously beneficial evolutionary step. In his The New Hedgehog Book, mammologist 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, 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, although there are actually two peaks in breeding in the UK, generally speaking they’re sexually active for most of the time they’re out of hibernation. 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 cross the 700g threshold. Given 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 have come across the term “evolution”, so I don’t plan to go into detail about what it is and how it works. There are, however, a couple of fundamental points that we need to understand in order to answer the question of why we shouldn’t expect something to evolve in a particular way.

Hoglets suckling from their mother. - Credit: Hedgehog Bottom Rescue

Simply put, evolution is the process by which populations of organisms change over time, though successive generations. 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 the production of sperm and eggs (meiosis) and as DNA is repaired. This meiotic recombination means that offspring have a different combination of genes from either parent. Mutations, by contrast, are the random errors that occur during the copying of genetic material that happens during meiosis and as cells are replaced or repaired as we grow (mitosis), 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 you’re unlikely to 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; a bit like a game of Chinese whispers. Our cells are subject to the same potential ‘copy errors’ during meiosis and mitosis, and errors can be created by environmental factors, such as radiation. Most animals have a pretty faithful replication system, thanks to some nifty built-in error correction (like having a spell-checker or proof-reader when copying your article) and this means that mutations are usually rare; but errors do still creep in and they serve as important evolutionary “fodder”.

As mutations happen and genes are mixed up, new combinations can be beneficial (positive) or detrimental (negative) to their host, or may have no impact at all (we call these ‘neutral’ or ‘silent’ mutations). If a gene or gene complex is positive it makes its host more likely to survive and reproduce with it than without it. Negative combinations make the host less likely to survive and reproduce, while a silent mutation has no discernible impact either way. It’s not tough to picture how a positive change that gives you an ‘edge’ in a particular environment makes it more likely that you’ll find a mate and reproduce, and thus pass the trait on to your offspring. This differential survival of gene complexes (genotypes), is known as natural selection. So, in short, changes to genes happen and these changes either promote or hamper the host’s life; if they make the host ‘fitter’ they get passed along the family line, while the species tends to face extinction if they reduce survival. The critical point here, though, is that these mutations aren’t planned: they happen at random and we cannot predict when a mutation will crop up or what it will do. This brings us to a couple of important concepts that are often misunderstood:

Newborn hoglets, about a day old, illustrating the first coat of short, white spines. - Credit: Caroline Gould
  1. The raw materials for evolution (i.e. mutations and recombination) are randomly generated, but this does not mean evolution and natural selection are random processes. That would be akin to saying if you take the ingredients for making a cake out of the cupboard in a random order you must use them in a random order. The mixing and baking part of the process isn’t random, just the collection of the ingredients. All this means is that you can’t say exactly when, or even if, you’ll be able to make the cake until you’ve found all the ingredients you need.
  2. Evolution is just a process – it has no goal, no ‘master plan’, and cannot predict the future.
  3. Natural selection does not work ‘for the good of’ a population or species. It operates at the level of the individuals, because some survive and reproduce more effectively than others. A side effect may be that a species is better adapted to a given environment, but evolution is just a process and it cannot aim to do anything.

A better adapted hedgehog?

There can be little debate that, for hedgehogs, having late litters can be detrimental, even fatal, to both the mother and the offspring – both may struggle to lay down enough fat in time for the winter, making it more likely they will die. In an ideal world, hedgehogs would evolve not to breed any later than, say, early August to allow themselves and their young plenty of time to fatten up before winter. In our non-ideal world, however, such a shift would require two main criteria to be met:

  1. A hedgehog somewhere must be born with an ‘updated’ genetic blueprint (genome) that instructs their body to switch off the mating drive after a certain time in the year.
  2. Having this ‘new and improved’ blueprint shouldn’t compromise the hedgehog in other ways. This drive to breed early might reduce the likelihood of dying from starvation during winter, but it may not do much good if all your spines fall out and you get eaten in spring because you have no protection. Equally, if this ‘early breeder’ trait also happens to cause a disfigurement, or excessive odour, members of the opposite sex may give you a wide berth and the trait will never make it to future generations. This may sound extreme, and these examples probably are, but we have plenty of evidence now that a change to a single gene can elicit more than one effect on the host (we’ll call it pleiotropy). In cats, for example, a gene that causes them to develop white fur and blue eyes, both desirable traits among cat owners, also affects their hearing such that about 40% of cats with white fur and blue eyes are deaf.

With the foregoing in place, we have something on which natural selection can work, but before we can see how this might spread in the population we need to understand how a ‘late breeding’ gene has persisted for so long. The reason is that winter is a key force driving selection, because if a hedgehog is going to die it’s most likely to happen during hibernation.

European hedgehog in a UK garden. - Credit: Dave Webb

In the wild, the hedgehog breeding season starts around May. Once the first litter has left the nest, the female can breed again to produce her second brood in September or October. Hedgehogs rarely breed during their first year and those born in the first litter usually have plenty of time to gain weight for hibernation. So, by the time these guys reach their second winter, they’ll already have bred, which means they’ll pass on their ‘late breeder’ gene(s) before they undergo the physiological stresses of hibernation again. Those born late in the year may not survive to breed, and the mother may also die during that winter, but by this stage at least one litter carrying the ‘late breeder’ gene(s) have probably reached 700g and are tucked up in their hibernaculum. Nonetheless, if a truncated breeding season makes it more likely the hedgehog will survive hibernation and thus potentially live to breed in the following season, all other factors being equal, we could expect the incidence of these ‘early’ genes to increase in the population at the expense of the late ones.

Additional complications, such as the unpredictability of Britain’s climate making some winters milder than others, are also likely to influence the ultimate outcome. It might also be argued that the excellent work of carers who over-winter and release underweight hedgehogs could reduce any selective advantage were a mutation to occur.

We can see that, hypothetically at least, the introduction of an “early breeder” gene into a population could lead to a decrease in late-born litters. Why don’t we see this, then? Quite simply, it appears that there is no such mutation currently at large in the population. Remember, natural selection and evolution cannot create genetic diversity; they can only work with what they find. If the mutation doesn’t exist (or is silent) there is nothing to select for. Indeed, if we consider that evolution is essentially a ‘game’, with the goal of each organism being to stay in it (i.e. live to pass on your genes) rather than folding (i.e. dying before passing on your genes), hedgehogs might be following a simple strategy. Hedgehogs probably have a gene/complex that simply instructs them to breed as often as possible, be that early or late in the season; this appears to be supported by reports of three litters in some hedgehog populations in New Zealand.

So, rather than being specifically early or late breeders, they are merely continuous breeders because, from an evolutionary perspective, the possibility of some of your genes getting into the next generation is a better bet than the certainty of none making it through had you stopped earlier in the year. Spray and pray, if you like. Even if this late litter is your second, the hoglets still represent a chance of more of your genes getting into the next generation – it’s like not putting all your eggs in one basket (or all your genes in one litter). Ultimately, even if only one hoglet from this late litter survives, it’s still one more to hopefully pass on part of your genes to subsequent generations than if you hadn’t tried at all.

The road to extinction

A recent estimate by the Peoples Trust for Endangered Species suggests that Britain’s hedgehog population has declined by some 300,000 animals since the turn of the century and some conservation bodies have raised concerns that the species might be extinct in the UK by 2025 unless drastic action is taken. Late litters probably don’t help, but it’s impossible to draw conclusions at the present time – we simply do not have sufficient data on the population.

Many people feed hedgehogs visiting their garden and this may be a genuine lifesaver for those born late in the year. - Credit: Colin Shaw

Perhaps the biggest question regarding the impact of late breeding on the hedgehog population is how they managed before humans were around to care for the autumn orphans? After all, it seems likely that hedgehogs have always been programmed to breed continuously. In a fascinating paper to the journal Lutra during 2009, former University of Hull biologist Toni Bunnell provided an  interesting insight into the survival of late litters. Between 1998 and 2006, Bunnell collected data on 119 hedgehogs brought into her Yorkshire-based hedgehog sanctuary, and established that:

mean growth rate for each month that hedgehogs arrived at the sanctuary was lowest in July and highest in September”.

In other words, hedgehogs born in late litters put on weight faster than those from early litters. Granted, the difference between early and late litters was small, 1.68g (about one-seventeenth of an ounce) per day, but it was statistically significant and at the end of a week that’s a difference of almost 12g (just under half-an-oz.). Bunnell conceded that we don’t know how this happens, but suggested that changes in ambient light and temperature associated with autumn may alter the hoglet’s physiology and stimulate its appetite, much as is seen in some bear species. Bunnell concluded that:

These findings dispel previous suggestions that all young hedgehogs born late in the year are automatically doomed to die due to a failure to achieve a satisfactory weight which would allow them to survive hibernation.”

That said, an adaptation that allows late-born hoglets to put on weight faster doesn’t seem to stop many hundreds being taken into care each year; nor does it appear to be arresting the apparent decline in numbers. It seems that the life strategies that have kept hedgehogs around for the last 15 million years may no longer be sufficient to compensate for the high mortality imposed by our modern world. Thus, it may be that late breeding has only become a problem in the last 80 to 100 years, which may be insufficient time for a beneficial mutation to become widespread – although not always the case, evolution is typically a slow process.

As ruthless as it sounds, in a changing environment, the alternative to adaptation is extinction. It would not be a surprise to find that there have been autumn orphans for as long as there have been hedgehogs and, although it provides little comfort, there is currently no evidence that the relatively recent decline in hog populations is a result of late breeding. Indeed, most agree that problems such as strimmers, habitat destruction, insecticides and molluscicides are probably more important.

Autumnal hedgehog. - Credit: Mark Houghton

So, in conclusion, where a beneficial mutation occurs, natural selection can see it propagate through a population and potentially through a species. The genetic variation on which natural selection (and thus evolution) operates, however, arises by chance and so, although we can predict the sorts of useful adaptations we’d expect to see, we cannot expect that they will occur. Evolution has no foresight or creative capacities; it does not work ‘for the good of the species’, it cannot plan, and it cannot create variation where none exists.