Antlers and Ageing

Ageing is an inevitable fact of life. Without it, populations would rapidly expand to the point that insufficient resources existed to maintain them, unless we also do away with reproduction. And, if we do that, then the creature in question will never be very numerous, and will be wiped out by the first accident, natural disaster, or change in climate conditions to come along. This is something that has been the case since well before mammals existed, even if the nature and pace of ageing might be different for, say, an oak tree or a coral colony, or conceptually vague, as in a mycelial network.

When it comes to reproductive senescence, however, there is a difference in the way this affects male and female mammals. Females are born with a finite supply of eggs, although, in practice, this is far more than they will need, so they don't cease to be fertile simply because they run out. What actually triggers the menopause in humans is complex, even assuming no confounding health conditions, but the number of remaining egg follicles falling below a required level and thereby lowering the production of certain hormones is thought to be key.

Cheating Little Vixens

The majority of mammal species are either polygynous, where one dominant male mates with multiple females, or promiscuous, where both sexes have multiple partners. However, while monogamy may be less common, it isn't exactly rare, either, with it having evolved several times in widely separated mammalian groups. In some cases, this is what we would term "facultative" monogamy, where animals (often large predators) are sufficiently widely spaced that it's simply difficult for a male to find multiple partners, or, if he can, they live sufficiently far apart that he can't plausibly defend more than one of them from his rivals. In others, monogamy is an essential part of the breeding process, typically because the young are too much effort for one parent to raise alone.

Of Pregnancy and Progesterone

Female mammals, like those of many other creatures, are not permanently fertile. Instead, they go through regular cycles, ovulating at intervals under the control of hormonal signals produced by the pituitary gland and the ovaries. In humans, this manifests as the menstrual cycle, but this is a relatively unusual feature of our species.

Whether or not other mammals menstruate may depend on your exact definition of the term. Chimpanzees certainly do (and, indeed, rarely experience menopause), and it's present to a variable extent in other apes and Old World monkeys. In New World monkeys it's microscopic and it's completely absent in lemurs. At least some bats menstruate, as do sengis (elephant shrews) and, so far as we know, just one species of rodent.

Boys or Girls?

Generally speaking, a newborn mammal is equally likely to be male or female. The sex ratio in the resulting population may not always be a perfect 50/50 if one sex has a shorter life expectancy than the other, but it's still going to be pretty close. There is a sound reason for this, and it's called Fisher's Principle, after geneticist and mathematician Ronald Fisher, who promoted it in the 1930s (although he probably wasn't the first to have thought of it).

The argument runs like this. Let's say that a particular species produces more females than males. Then males will have more mating opportunities than females, and will, on average, have more offspring. If a mutation then arises in a given individual that makes her more likely to give birth to sons, she will tend to have more grandchildren, many of whom will carry that mutation. Since they will also have an advantage, the mutation will spread through the population... until males are more common, at which point it's preferable to have more female offspring, and so on. 

Love on the Mountain Tops

Caprines - members of the goat subfamily - are amongst the mammals most adapted to harsh environments, with the majority of species adapted to living in the cold, barren, and precipitous slopes of mountains. There are some exceptions; sheep (which are taxonomically a subtype of "goat") originally evolved to live in barren rocky hills rather than on true mountains, while some of the East Asian species inhabit forested slopes. 

There are, as with many animal groups, more species of caprine than one might at first think, and I covered them all individually about ten years ago. Looking through that series, it should be possible to appreciate that the group is also varied, not only inhabiting a range of environments but also living varied lifestyles, from those that are near-solitary to those that prefer large herds. This is also reflected in their mating habits which, are as one might expect, related to the size of the community in which they live. One would also expect that the habitat would have some effect on how the animals choose to live, and, in turn, on that mating behaviour.

The Sex Lives of Female Jaguars

There are many ways of classifying mating systems in animals, but one of the most basic uses four main types. In polygynous species, the male mates with as many females as he can get away with, driving away or out-competing any potential rivals. This ensures he can sire as many children as possible, while the female gains the advantage of a strong father for her offspring. This pattern is perhaps seen most strongly in deer and seals, but it's also seen, for example, in lions and gorillas. 

Monogamy is somewhat less common. Sometimes, it happens only because the species is sufficiently widespread that any given male is unlikely to find more than one receptive female during the breeding season, but it can also occur by choice, typically where raising young is a sufficiently arduous task that the father has to stay around after the birth to help. This is commonly associated with birds, but many mammals also form pair bonds for raising young. These include species of gibbon and small antelope that, in paternity tests, have shown essentially 100% loyalty to their mates. The prairie vole is well-studied in this regard, with the formation of the pair bond through prolonged and repeated mating having been linked to, among other things, the "cuddle hormone" oxytocin.

Fruit Bats of Madagascar

While most attention tends to focus on large charismatic animals such as rhinos and tigers, it will probably come as no surprise to discover that many species of bat are endangered. Here in Britain, all bats are legally protected by the Wildlife and Countryside Act 1981, making it an offence to disturb their roots. That's perhaps an unusually robust example, but other protections and conservation efforts exist across the world.

On the other hand, it is true that the majority of bat species are not especially threatened, at least on a worldwide scale - although things may be different locally. Bearing in mind that around one in six bat species are so recently identified and so little studied that we simply don't know how common they are, only around another one in six are rare enough to be listed as "threatened" by the International Union for the Conservation of Nature. Still, that's not exactly a small proportion, and since there are somewhere around 1,400 named species of bat, it's not small in absolute terms, either.

Tha bats are a highly diverse group, representing no fewer than 21 taxonomic families, none of which are likely as familiar to the layman as terms such as "cat family" or "deer family". Some of these contain very few species, representing oddities that don't quite fit into any of the main subgroups, but there are still five families with over a hundred species each. Of these, the one that contains the highest proportion of threatened species is the fruit bat or "flying fox" family, the Pteropodidae.

I Would Swim Five Thousand Miles...

Breeding can be an energetically costly business, whether that's the effort put into finding and attracting a mate, or that required to raise young. The latter is a particularly important factor in mammals, which can't simply lay eggs somewhere where there's plenty of food and hope that the hatchlings do well for themselves as, say, an insect might be able to. Therefore, we might well expect that mammals will feed more during the breeding season, to compensate for all that extra energy they will be using. 

For males, there can be a downside, in that all the time you are spending finding and eating food is time not spend wooing and mating with females. Thus, in animals such as deer, we may find that males actually eat less during the mating season than they do at other times because their mind is far too much on other things. But females, given the needs of both pregnancy and lactation, ought to be different.

Naked Aggression

Evolutionarily speaking, it is in the interests of animals to have as many offspring as they can, so long as those offspring will themselves reach reproductive age. However the means for a male to do this may be different from those best employed by females. This is especially true in mammals, because the female has to put considerable resource into pregnancy, lactation, and general care of the young until they are old enough to fend for themselves. The male, however, doesn't have to do any of this, and, in many species, his role in matters may be completed as soon as he's finished mating.

This leads to the concept of sexual selection, where the evolutionary demands on the male may be different than those on the female, leading to different behaviours and appearances. There are often said to be two main driving forces behind such sexual selection. The first is the need for males to compete with one another for access to females, resulting in larger, more heavily built males, often with natural weapons of some kind, such as horns or tusks. The second is driven by the females, when they choose a mate based on some particular feature, leading the males to emphasise that feature in response. (This is, perhaps, most obvious in birds).

All the World's Deer: The Red Deer Species Complex

Red deer

When the first list of scientific animal names was drawn up in 1758, all six known species of deer were placed in the genus Cervus - which is, of course, simply the Latin word for "deer". Over the following decades, as taxonomy became more refined, all but one of those species were moved to other genera, and when the deer family, Cervidae, was named in 1810, it had roughly the same meaning that the genus had had so many years before. As its name implies, though, Cervus was selected as the type genus of the family - the one that defines the archetypal group against which all others are compared. Although newly described species had been added since, the one species that was so obviously deer-like it had never been moved anywhere else thus became, in a sense, the defining species of its family.

The Tyranny of the Bruce Effect

Monogamy is a relatively rare phenomenon among mammal species. It tends to occur where it requires considerable investment to raise children, whether that be in terms of supplying food over a long period, or careful protection from external threats. If you can spread that effort between both parents, the child is more likely to survive and the species to be perpetuated.

That's common in birds, perhaps in part because they have to physically fetch and deliver food rather than producing milk with the calories from their own diet, not to mention the effort of incubating eggs. But, in mammals, while it certainly occurs in some species (as do the other two options) the most common pattern is polygyny. 

The Sexy Face-Masks of Lekking Bats

The mating behaviour of mammals is, unsurprisingly, highly varied across different groups and species. We would hardly expect the behaviour of dolphins to resemble that of reindeer, for example, or hedgehogs to resemble cheetahs. Reproductive information is one of the key biological factors we tend to look at when describing a species, even if it isn't quite as easy to evaluate as, say, diet or habitat. (Or anatomy, of course, which doesn't even necessarily require the animal to be alive). Look at almost any write-up of a mammal species in an encyclopedia, and there'll be something on reproduction - assuming we know it.

But, for a great many mammal species, we don't. This may be because it's rare, or difficult to observe in the wild, or perhaps that it's a newly discovered species that we can reasonably assume isn't that different from close relatives we already knew about. But, at least when it comes to reproductive behaviour, one of the biggest gaps in our knowledge concerns the bats.

Why Males Are More Muscular

There's a general rule in the animal kingdom that females tend to be larger than males. This is likely because of the extra demands of producing and laying eggs, or of spawning by whatever other method they use. Clearly, it's not a rule that holds among mammals where, while there are some exceptions (especially among bats), it's usually the male that's the larger and more physically imposing sex.

I discussed this a few months ago, in the context of seals, where this size difference is especially noticeable. But it's true in other mammal groups, too, including the primates. In general, the reasons for this are much the same among primates as they are among other mammals; males compete with one another for access to mates, and the most successful ones have more mating opportunities, and hence more children, as a result. Genetic inheritance then carries the trait of "large males" down to future generations, amplifying it until other constraints get in the way.

Musth and the Older Elephant

Male Indian elephant, probably practising his social distancing
rule of "stay at least 1.2 km apart."

As with many other group-living mammals, male and female elephants not only spend much of their lives apart, but also live in quite different sorts of society. Females live in small family groups, dominated by the eldest among them and consisting of her children and other close relatives. These groups, in turn, form larger aggregations known as 'clans', the membership of which can change over time, with individual families joining or leaving as circumstances dictate. The precise details vary between the three different species of living elephant, but these basic rules seem to be universal.

Males, on the other hand, leave the family unit of their birth as soon as they reach puberty. From then on, they spend much of their lives alone, without the benefit of the matriarch guiding their sisters. . From time to time, they may meet up with other males, or even join female groupings, but these are always short-term arrangements. This creates a situation where solitary males regularly travel about, hoping to encounter different female groupings as they do so; similar behaviour is seen among mammals as diverse as giraffes, polar bears, and killer whales.

Primate Penis Bones

At a certain level, the skeletons of all mammals follow a broadly similar pattern. Most of the bones that we see in the human skeleton are also found in the majority of mammals, and often in the same numbers. Famously, for example, a giraffe has exactly as many bones in the neck (seven) as humans do - they're just rather longer. Well, there's a reason giraffes can't bend their necks like swans.

Of course, when we get into detail, there are many exceptions to this. For instance, the default pattern for the paws of mammals is that they all have four digits with three bones each, and one with just two bones (the thumb and big toe in humans). But, obviously, this isn't true of all mammals. For instance, dogs have no big toe on their hind feet, and, while they do have a full set of ankle bones, the metatarsal that would normally connect to the big toe isn't there, either. There are rather more dramatic alterations in, say, horses and two-toed sloths, let alone dolphins.

Male Chauvinist Seals

There are four basic mating systems that can be seen in mammals, or, indeed, any other creature that possesses two clearly defined sexes. In monogamous systems, a male and female pair mate with one another and then typically remain together to raise their young. This tends to occur wherever raising young is an energy-intensive task that requires the full-time attention of two adults to work.

In polygynous systems, one male mates with multiple females, to maximise the number of offspring he can sire. In the polyandrous system, it's the other way round, with a single female mating with multiple males (some mole rats do this, but it's rare in mammals). The final option is a promiscuous system, where both sexes have multiple partners.

A Hole of Your Own

Many animals dig burrows for shelter, whether from the weather or from predators. In some cases these are complex or extensive burrow systems, such as we find with rabbits or gophers, and some animals, such as moles, try not to leave their burrows at all, adapting to a subterranean life. Most are much simpler than this, a basic hole in the ground in which the animal can rest securely at night - or during the day, as the case may be.

At the opposite extreme to the specialised diggers, however, are those animals that don't dig burrows at all, but still find it useful to seek shelter in this manner. These are creatures that will either use natural cavities or take over an abandoned burrow originally dug by something else. If they can't find one, it's usually not a disaster, although it may make life a little uncomfortable. But that's not necessarily true when it comes to time to breed.

Lots of Little Deer Mice

If a sexually reproducing animal is to preserve its species without population loss, each pair of parents has to produce, on average, two offspring that will themselves live long enough to reproduce. This is just basic arithmetic, but there are at least three different ways that an animal can achieve this result and which one is used varies from species to species.

One approach is to maximise the chances of each of your offspring surviving. This is called the K-selection strategy, and results in the population being as close to the maximum capacity of the local habitat as possible. (The 'K' stands for 'capacity'... in German). Such animals don't need to reproduce very often, or produce very many offspring when they do, but they have to invest a lot of resources in their survival. They tend to be relatively large animals, with few predators... elephants, primates, and whales are all good examples among mammals. Humans are a particularly extreme example, given how long it takes us to raise our children, and, as with other strong K-selectors, twins are rare in our species.

Breeding Cats in Captivity

The Sumatran tiger is a critically endangered subspecies

Although the actual number may be higher, depending on the status of certain subspecies, there are 38 widely recognised species of wild cat. Of these, five are formally considered to be "endangered species", and a further thirteen are "threatened", but not sufficiently so to fall into the riskier bracket. Taken together, that's almost half of the total species, and it ignores the significant number of species that are threatened in parts of their natural range, but survive well enough somewhere else.

The primary reasons for this are habitat loss and poaching, and virtually every species of cat - even those, such as the puma/cougar/mountain lion, that aren't threatened overall - has a declining population. Fortunately, the same thing that makes wild cats attractive to poachers - their undeniable charisma and beauty - also inspires conservation efforts of the sort that, say, the Ethiopian amphibious rat (Nilopegamys plumbeus) could only dream of.

Small British Mammals: Field Mice

Wood mouse

While the common house mouse is likely one of the most familiar of all animals living wild in Britain, it is only one of four species of mouse native to the islands. (France and Germany, for comparison, have six each, although they aren't the same six). Of the other three, the closest relatives of the house mouse are the two species of field mouse, which are also found widely on continental Europe.

Field mice have benefited less from the presence of humans than house mice, since they tend to avoid human dwellings, and there are rather a lot of those in Europe. Given their common name, however, one might suppose that they have at least benefited from the spread of agricultural land. This, however, only true of one of the British species. This is the wood mouse (Apodemus sylvaticus), sometimes also known as the "long-tailed field mouse". In Britain, when the term "field mouse" is used without qualification, it typically means this one.