The Sounds of Mother and Calf

While for humans, speech is our primary means of communication, for many mammals scent-marking is more important. This is not to say, however, that audible signals are irrelevant. This can be especially true for social mammals (such as ourselves), which often have a need for something more immediate and complex than the sort of long-lasting "keep away" or "I'm ready to mate" signals that scent marks can provide. Communication through sound can help keep a herd together, signal aggression, provide instant warnings, and so on. It's key to many primates, for instance, because most of them live up in the trees where it may be difficult to keep sight of all troop members when they are out foraging.

It's also important for many hoofed herd animals and, at least in the wild, few North American mammals are more sociable than the bison (Bison bison). While herds are no longer as vast as they were 200 years ago, recovery plans for the species are underway, and, in many cases, may rely on some degree of fencing or other containment at least for the time being. Understanding bison behaviour, including communication, could help with that, making it easier to assess how comfortable the animals are feeling - and, perhaps, the likelihood of a 750 kg (1,600 lb) bull deciding it's had quite enough of that fence and heading off somewhere it doesn't realise is less safe.

This is certainly true of domestic cows, where studies of animal welfare have often found bovine communication helpful in evaluating not just physical pain, but emotional wellbeing. These agricultural studies cover a wide range of sounds and contexts, but in the case of wild bison, most communication studies to date have focussed on the loud bellows of bulls. The volume of such bellows is a good indicator of how large and physically fit the bull is, although this is more a means of boasting about prowess to intimidate rivals than it is a means of enticing females to mate.

It doesn't take much, however, to realise that it isn't only adult male bison that make sounds and that it would also be useful to know how females and young are communicating. In this respect, we can note that for any mammal, communication between mother and child is crucial for starting out in life. A mother, for example, needs to know where her child is, to call it to her when needed, and, in a herd, to identify which of many young animals it might be. 

Among herd animals, the great majority of studies on mother-infant vocal interactions have been conducted, as we might expect, on domestic animals - cows, sheep, goats, and horses. There is, however, at least one on deer, and, as of last month, we can now add bison to that list.

This new study was based on observations of a herd of around 2,000 adult bison and their offspring living in a 40,000 hectare (155 square mile) private ranch in Montana. Within this area, the animals can wander about almost freely, are not provided with food that they don't graze for themselves, receive no assistance with calving... plus the ranch does have both wolves and bears living wild on it among other animals. So, while this doesn't count as genuinely wild habitat and the bison are occasionally herded between pastures, it's certainly a lot more natural than most domestic cattle farms.

The sounds made by bison cows to communicate with their calves are short grunts, lasting just 0.3 seconds on average. This is distinct, not only from the louder and longer bellows of the demonstrating male bison, but also from the longer, more harmonic, "moo" sounds of domestic cattle. The calves' responses were even shorter and of higher pitch than those of adult females, a trend seen generally in young animals and tied to their smaller size.

Judging by the contexts within which the mother called to her calf, and the response that then followed, the most common reason - 39% of all calls - was to tell the calf to follow her. Consisting of a short burst of urgent grunts, these are basically a "come along" signal used when the herd is about to move somewhere else and ensure that the calf is paying attention and doesn't get left behind. Since bison herds are inherently mobile, grazing for a few hours at a time in a particular spot and then moving to a new one at least once a day, and often more frequently, this is obviously going to be important.

Once they are moving, however, especially if the herd is moving quickly, mother and calf are usually silent, with only occasional grunts - possibly of encouragement. Given that the adult males aren't displaying either, the authors of the study note that a stampeding herd of bison is a lot quieter than you'd expect unless their hooves are thundering on hard ground.

Bison mothers try to stay close to their calves as far as possible, keeping a close watch on them. But it is in the nature of a large bison herd that sometimes the calf will be obscured by other individuals, and temporarily move out of sight. This is usually because the calf wants to interact with another of its own age, something that will be important to developing the social skills that any herd animal needs as it grows up. 

Thus, the second most common call type, at around 21%, involves the mother checking up on where her calf has got to. When the mother notices that she can't immediately see her calf, she produces short grunts at intervals. Assuming that it responds within the first five calls - as they usually do - the mother looks towards where the sound came from and then, assuming she is satisfied, calms down and returns to happily grazing. It's worth noting, of course, that this requires that both she and the calf be able to identify each other by sound alone, although visual appearance and scent surely also play a part in recognition when the calf is nearby.

Another common call is an invitation to the calf to suckle, while sometimes the mother just wants her calf to come over quickly and stop playing about elsewhere. In both cases, the calf tends to respond to these calls quickly, recognising their importance.

Two rarer types of call are of particular interest. First is the "imprinting call", which the mother gives shortly after birth. With her calf still lying on the ground and not yet strong enough to stand, she walks slowly around it repeatedly giving out soft, quiet grunts over a period of up to half an hour. The newborn calf doesn't visibly respond to these, but they are probably a key part of the recognition process by which it will later identify its mother's voice. We see something similar in sheep, and even in much more distantly related mammals such as dolphins, and one can certainly understand why it would be important.

The remaining type of call is the rarest, heard just four times during the two-month study. This is what happens when the calf doesn't respond to a regular "contact call" and the mother realises that it is genuinely missing. At this point, as you might imagine, she panics, running around screaming loudly over and over again, occasionally rearing up to get a better view of her surroundings. Short, quiet grunts are clearly not going to cut it at this point.

One can, perhaps, take comfort in the fact that, given how rare this type of call is, baby bison must be pretty good at staying close to their mother.

[Photo by Doreen Van Ryswyk of the US Fish & Wildlife Service, in the public domain.]