Out on the grassy plains of Kenya’s Masai Mara National Reserve, a group of six female topi antelope (Damaliscus lunatus) walk across the savanna. It is the time of the annual rut – a one and a half month period in which most males control small patches of land and try to attract adult females which, for one day, are in estrus. The small group walks by one of the lone males, but just as they reach the edge of his territory he snorts an alarm. It means that somewhere, out ahead of them, a predator waits to pounce on any topi foolish enough to blunder towards it, and so the females stay close. What the females don’t know, however, is that the alarm call was a lie.
In many natural history documentaries, mating among mammals is presented as a relatively straightforward affair. Males compete with each other for access to females – be it through the control of a territory, dominance in a social group, or some other route – with the biggest and most impressive males coming out on top. Not surprisingly, things are not as orderly in nature as they are on television, and this goes for the topi as for any other animal. Males lucky enough to hold down a mating territory are not content to simply let females choose who they are going to mate with. As Jakob Bro-Jorgensen and Wiline Pangle demonstrate in a new American Naturalist paper, deceitful males can enhance their chances of siring part of the next generation.
Detecting deception among non-human animals has been notoriously difficult. How can you tell if an animal is telling a lie? In the case of topi antelope, specifically, it would appear that males are emitting false alarm snorts in order to retain females in their territories, but what if these “false alarms” are really just mistakes which just happen to have a beneficial effect for those males?
In order to test these competing hypotheses, Bro-Jorgensen and Pangle observed 73 female topi (53 in estrus and 20 not in estrus) over several rut seasons between 2005 and 2009 for a total of 274 hours. The scientists recorded the salient details of how the females moved among male-controlled territories as well as the occurrence of alarm calls by males which were determined to be “true” or “false” on the basis of whether predators were actually present (which was possible to determine given the open habitat in which the topi live). Once they observed the occurrence of both true and false alarms in the field, they recorded these calls (along with a grunt, which they used as a control) and played them back a total of 20 times to 60 grazing female topi to see whether females could detect the difference between them. Furthermore, the researchers approached male topi on foot in order to see if they emitted alarm snorts. If so, this might mean that the snorts are more about communicating to a predator – “I see you. You have lost the element of surprise” – than to other topi.
Deception in action – A) A male topi (background) emits a false alarm snort to keep a female (foreground) from moving out of his territory. B) Now that the female has been prevented from moving away, the male shifts his attention to her as seen by the position of his ears and his gaze. C) The male mates with the female shortly thereafter. In a more dramatic video example included in the supplementary information, a male alarm snorts several times before running over to a female and attempting to mate with her. From Bro-Jorgensen and Pangle, 2010.
When Bro-Jorgensen and Pangle looked at the acoustic breakdown of the alarm calls they could not find any difference between true and false alarms – the calls could only be distinguished by the context in which the males emitted them. Likewise, the playback experiments also failed to find a difference between true and false alarm snorts. Females reacted the same way to both, often standing vigilant before walking away since males typically snort from a place between females and the danger, meaning the females would – under normal circumstances – be headed back into the male’s territory. These observations were consistent with the natural behavior of these animals. Consistent with the female-retention hypothesis, the scientists observed that males emitted false snorts most often when a female in estrus was nearby. When females began to move away, the male would look in the direction she was going, with his ears alert, and snort.
Rates of false alarms versus true alarms emitted by male topi, accounting for the context in which they were made. The number of false alarms in the presence of estrus females greatly exceeds the number of true alarms. From From Bro-Jorgensen and Pangle, 2010.
Sex, of course, was the reason why the males were raising false alarms, and this strategy appeared to work well for them. On average, males who emitted false alarms mated two-to-three times with estrus females during the time those females remained in their territory, and in a few cases – 10% – males only succeeded in mating after snorting. Likewise, since a female topi is only in estrus for one day, the longer a male can keep a female in his territory, the better – every minute she is in one male’s territory is a minute that she’s not mating with another male. Even if a male does not mate very many times with a female that is in his territory, delaying her departure can still confer reproductive benefits.
Interestingly, the results of the “threat” experiment (in which researchers approached male topi standing alone) showed that true alarm snorts might have more to do with letting predators know that they have been spotted than communicating to other topi. Even though no other topi were nearby, the males still snorted when approached. If the primary purpose of the snorts was to warn other topi, then the male would have just drawn attention to himself for no good reason, and so it seems more plausible that alarm snorts are actually “deterrent snorts” meant to discourage predators which rely on stalking. Hence, since true alarms were elicited by lone males and false alarms were closely tied to the presence of females in estrus, it was unlikely that the male topi were making mistakes (i.e. thinking that they were warning females about a predator in the grass). They were using the alarm calls as a manipulative tool to gain more mating opportunities, and males which did so enjoyed enhanced mating success. As the authors state, males gain significant reproductive advantages from false alarm calls, but females cannot afford to ignore alarm calls – as there is no difference between false and true alarms, ignoring any alarm call may mean that they are walking right into the path of a predator.
Just what the male topis are thinking when they emit the false alarms is beyond our ability to discern, but the results of the observations are strong – male topis use alarm calls to manipulate females. Since there is no way to tell the difference between true and false alarms, and the cost to females for ignoring the alarms is potentially high, it is easy for the male topi to use the snorts as dishonest signals. In fact, alarm calls in general are probably susceptible to becoming used as manipulative signals for much the same reasons – perhaps deceit is more widespread among animals than has previously been appreciated.
[Photo of topi on an anthill (top) by Jakob Bro‚ÄêJ√∏rgensen.]
Bro‚ÄêJ√∏rgensen, J., & Pangle, W. (2010). Male Topi Antelopes Alarm Snort Deceptively to Retain Females for Mating The American Naturalist, 176 (1) DOI: 10.1086/653078