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.

By a strict definition, however, the vast majority of mammal species never experience menopause. This is for the simple reason that they don't menstruate in the first place; the uterine lining is simply resorbed back into the body rather than being shed. Nonetheless, so far as we know, all female mammals do eventually cease to be fertile in that they eventually stop ovulating. Technically, that's "oopause" not "menoapause", but the practical effect is much the same. 

In the wild, few animals reach that sort of age, although there are some exceptions - most notably among cetaceans. 

We know all this because there has been a fair amount of research into the loss of reproductive ability in elderly female mammals. Even if they don't reach full oopause before other symptoms of ageing lead to death by predators or disease, one review has found that at least two-thirds of mammal species experience some form of female reproductive decline in the wild.

Whether the same is true of males has been much less studied. That's largely because it's much harder to assess the paternity of a newborn mammal than it is the identity of its mother, although the absence of a hard dividing line doesn't help. Sperm, after all, is not limited in quantity, with more being produced all the time, whether it's needed or not and male mammals remain theoretically capable of producing offspring throughout their lives.

But not necessarily with equal likelihood.

There are several ways that a male mammal might become less likely to father children as he ages. This could be something inherent in the reproductive biology, such as a drop in sperm count or a change in the quality of the fluid the sperm are swimming in. Or, in the case of mammals where each female has multiple sexual partners, if one of them simply produces a smaller volume of sperm than his competitors, that would disadvantage him.

But another possibility is what we might, in the technical language, refer to as "precopulatory factors". That is to say, there could be something that makes an older male less likely to get the chance to mate in the first place. We know, for example, that this happens in some bird species. The singing voice of great tits (Parus major) declines with age, which has obvious implications for a species that relies on birdsong to find mates. Similarly, the black spots on the back of male kestrels (Falco tinnunculus) that make them look sexy to females fade with age in the same way that human hair goes grey.

Many of the same processes could be present in mammals, which can use multiple different ways to signal sexual prowess and the ability to father healthy children. Indeed, we have evidence that elderly male mice produce a smaller amount of sex pheromones than their more virile counterparts. However, it's a lot easier to look for obvious physical signs that might make a male less attractive. For instance, if there is strong physical competition for partners among rival males, any general age-related decline might become increasingly problematic.

One of the most visibly obvious signs of sexual attractiveness in male mammals comes from the presence of horns or antlers. Only one species of deer has antlers in the female (the reindeer), and horns are less common, and typically smaller where they are present, in members of the cattle/goat/antelope family. Looking specifically at deer, we know, for example, that roe deer with bigger antlers father more children, and this is true of at least some other species, too. 

That could be because larger antlers make the male more effective at fighting off rivals, and there's likely at least some truth in that. But it has been shown that female white-tailed deer find males with bigger antlers more sexually attractive. This is probably for good reason; males with larger antlers have diverted more of their resources to developing large and pointless ornaments, indicating a high degree of physical fitness... and, at least in red deer, males with big antlers really do produce higher quality sperm. 

The thing about antlers, as opposed to horns, is that they are regrown each year. Grow large horns and... well, you have large horns; they aren't going anywhere. Grow large antlers, though, and you have to grow them all over again the following year. Sure, antlers get larger and more complex as the stag grows, but it's plausible that it becomes an increasing struggle to grow new ones when it starts becoming truly elderly. So, in deer, unlike goats or antelopes, the head ornamentation really could shrink with age.

But does it, and, if so, how much difference does it make?

Fallow deer (Dama dama) are among the species with the largest antlers relative to their body size, constituting up to 2.8% of their total body weight - almost double what you expect by looking at other deer. Furthermore, while they may not quite be in the same league as seals, fallow deer also have an unusually large variation in body size between males and females, even by the standards of deer, presumably because their males also happen to be amongst the most competitive.

A study published earlier this year looked at a herd of around 600 fallow deer living in Phoenix Park, Dublin. This is a 700-hectare (1,750 acre) walled park that has been home to deer since the 1660s. Obviously, these are not fully wild deer, but it's precisely this that allows us to see what happens as they age in an at least partially natural environment, with fear of significant predation. Young fawns are sometimes taken by foxes and traffic collisions do occur from time to time, but, otherwise, aside from any disease or old age, the only mortality risk is the annual culls required to stop the population outstripping the natural resources - and these are split evenly among age and sex groups so as not to disturb the overall population structure.

The study was carried out over a six-year period, with cameras and range-finders being used to photograph the bucks and estimate the size of their antlers. An obvious measurement here is the overall length, measured once the antlers are fully grown and shorn of their velvet. In other deer, such as the red or white-tailed species, you would probably also want to count the number of tines, which increase as the antlers grow in size and weight. For fallow deer, however, that's not so useful, because their antlers are, like those of moose, palmate - that is to say, once full-grown, they are broad sheets with a variable number of projections along the rim. So, in this case, it made more sense to measure the width of the flaring part of the antlers, and combine that with the length to get an overall score relating to their size.

According to the results of this study, antlers grow steadily between the time they reach full development at four years, and around six or seven years. After that, they do indeed start to shrink again, declining at about the same rate that they grew. Even in this herd, very few individuals reach the age of eleven, but the few that do can apparently no longer manage to grow antlers the size that they had when they were four. Both the length and palm-width of the antlers decrease at more or less the same rate.

This decline in antler size after seven years corresponds with the age that previous studies have shown bucks start to become less successful at mating, being pushed out by younger, fitter rivals. Significantly, perhaps, it's a couple of years earlier than they start dying of old age - although they can live for at least three years after that. 

This doesn't absolutely prove that antler size is the only factor here, since there are other reasons that does might prefer seven-year-old to nine-year-old bucks. If nothing else, the study didn't look at body size, and this might decline a little as well, albeit probably in line with the antlers, and there could be other factors involved, such as scent or stamina that would be harder to measure.

But nonetheless, it does seem that older fallow deer bucks are less able to produce large antlers every year, and that this does have an effect on their chances of securing mates even before age starts to catch up with them in other respects. Age comes to us all in the end, and it may only be humans that try to fight against it.

[Photo by Garry Knight, from Wikimedia Commons.]