A particular problem for herbivores is that they have to spend so much of their time just eating. It does depend exactly what it is that they eat; if their diet consists largely of something highly nutritious, such as fruit, it may not be much of an issue. At the opposite extreme, browsers have a particularly tough time of it, because grass isn't especially nutritious, and takes a lot of time to digest. And you can't scan the horizon for predators while your head is down on the ground. Furthermore, while many herbivores spend much less time sleeping than carnivores do (consider how much time a cat spends kipping), they do have to rest at some point.
These problems become particularly acute during the winter. The food, such as it is, is even less nutritious than usual, and the carnivores are going to be especially hungry as well, which might lead them to take on prey they wouldn't otherwise. But it's more than that, because, for many large herbivores in the temperate regions of the world, winter is also the time for mating. That ensures that it's spring by the later stages of pregnancy, and when the young are still being suckled, and the mother needs the most food. But it does mean that, just when food is short, and predators are about, both females and males have other things on their mind than keeping watch.
This is partly why so many large herbivores live in herds. There is safety in numbers, and only a few animals need to be on watch at any given time, so long as they alert their kin quickly when they spot something dangerous. But even so, the essential conflict remains: how best to juggle your waking hours between eating, watching, and breeding?
There have been a number of studies of how different animals respond to this issue. Unsurprisingly, the answer depends, not just on such things as herd size, gender, season, and so on, but on which animal we're talking about - there is no one rule that fits everything. For example, during the mating season, male elk are far too busy to bother watching for predators as they normally would. But male impalas are the exact opposite, actually spending more time on guard during the mating season - although they're probably more worried by the thought of other males than they are by the possibility of lions.
Przewalski's gazelle (Procapra przewalskii) is one of the more endangered species of large, land-dwelling, mammal. Most gazelles are found on the fertile plains of east Africa, but these, although they are true gazelles, are part of a widespread Asian group. Once found wandering across the high plateaus of central Asia, they now live only around the shores of Qinghai Lake in northern China, amidst a landscape of cold steppe grassland and semi-desert. With probably less than two thousand left alive, their herds are now so small that, during the summer, they often wander together with the closely related Tibetan gazelles for protection.
In December, as the winter starts to set in, males compete with one another, clashing with their horns to establish dominance. The 'winners' gather a herd of females, which they lead about across the steppes, keeping them close by to determine when they are ready to mate. The 'losers' mope about on their own, keeping to the fringes and hoping for a second chance before the season is over. The region is so remote that they don't need to worry too much about human hunters - although encroaching agriculture is limiting their remaining habitat - but wolves are another matter.
Chunlin Li, of the Chinese Academy of Sciences, and co-workers, recently published a study of the behaviour of these rare and endangered animals to see how they coped with the demands of the rut and of keeping an eye on the surrounding scenery. The answer, it seems, depends on whether we're talking about males or females.
Females spent more time vigilant - that is, with their heads raised and peering around at the surroundings - during the winter rut than they did during the summer. However, they also spent more time eating, presumably because of the difficulty of finding high-quality grasses at that time of year. In order to spend more time on both activities, they sacrificed their rest, spending far less time snoozing during the day than they otherwise would - although we don't know what they were up to at night. Presumably, not taking the time to rest must take some toll on them, or they wouldn't do it so much when the opportunity arose during the summer. It's likely that that's largely due to the need to keep moving in search of food, but it also seems that they feel it's extra important to keep an eye on what's going on during the rut.
They might be more concerned by wolves at such times, but it's at least as likely that their primary concern is the males. The dominant males can get quite aggressive, with sexual harassment being particularly common. Indeed, there are certainly some related animals in which the males can seriously injure females during the rut, and if that's also the case here (which has not, so far as I can tell, been confirmed), the females would have good reason to stay alert, if only for members of their own species.
Dominant males, on the other hand, are, if anything, less vigilant during the rut than at other times. They too, go without rest, but it's not so they can feed more, but because they're devoting all their energy to herding the females, showing off their sexual prowess with visual displays and scent marking, and, of course, to actually mating. After all, they have a lot of females to mate with, so it all takes up a lot of their time. When you add in all the time spent on displaying and so forth, it's clear that the males are far more sexually obsessed than the females are, spending hours trying to look impressive, while the females just carry on grazing. Since they are, after all, keeping a reasonably close eye on what the females are doing, they can probably afford to let them do all the scanning for danger... assuming they even care, since there was also some evidence that they were too busy displaying to bother running away when a threat really did appear.
The males wandering about on their own were a different matter; they were constantly alert, perhaps because they no longer had the protection of any kind of herd. Females were, as expected, less vigilant individually the larger the group they were in, so a lone male might well have more to worry about when he has no one else to protect him. He might also, for that matter, be on the look out for any opportunity for mating that presents itself - and, every now and then, one did get a chance.
All of which means that Przewalski's gazelles act more like elk than impalas. That's probably because of their mating system, with the unlucky males being (mostly) excluded early on in the season, while male impalas are always trying their luck, giving the dominant males more to worry about.