Blair R. Costelloe Behavioral Ecologist

Thomson's Gazelles

Maternal and antipredator behavior of Thomson’s gazelles

Parents with dependent offspring face interesting challenges in optimizing their behavior to maximize their reproductive success. They must meet all the same requirements as non-parents – getting enough to eat, avoiding predation – while also protecting and providing for one or more vulnerable, needy offspring. Thomson’s gazelles are a great system in which to study maternal decision-making, behavior, and trade-offs for several reasons: Cheetahs feeding on an adult male Thomson’s gazelle Cheetahs feeding on an adult male Thomson’s gazelle

Maternal tactics for preventing predation of newborn fawns

The time immediately after birth, known as the postpartum period, is a very risky time for ungulate infants. They are small, weak, and unable to run and are thus easily killed by predators. Once a gazelle is born, the fawn and its mother enter a race against the clock to get the fawn hidden before a predator detects and kills the newborn. For this study, I observed eleven mother-fawn pairs during the postpartum period and noted the habitat (short or tall grass) and social context (in a group or alone) in which parturition occurred. I measured how long it took for the fawn to move away from the birth site and begin hiding or be killed by a predator, and recorded any disturbances to the mother-fawn pair by other non-predator animals.

In my study, 6 of the 11 fawns were encountered by predators within an hour of birth and all but one of these were killed. I found that there are two effective tactics mothers can use to improve their fawn’s probability of surviving the treacherous postpartum period: they can either leave their social group and give birth alone in a tall grass habitat OR they can remain in their social group and give birth in their normally-preferred short grass habitat. The first strategy reduces the duration of the postpartum period (that is, fawns are able to stand, walk, and ultimately hide sooner) because the mother-fawn pair suffers fewer disturbances. However, this strategy increases the detectability of the fawn because a lone female in tall grass is a pretty sure sign to predators that there is a fawn nearby. The second strategy reduces the detectability of the fawn (because the mother and fawn are less conspicuous when they are within a group of other gazelle), but increases the duration of the postpartum period (because other gazelle are attracted and displace the mother from the fawn, which means it takes longer for the fawn to develop the ability to stand). Fawns whose mothers use one of these two strategies had about a 60% chance of surviving until hiding. In comparison, the females that did not use one of these strategies (that is, they gave birth alone in short grass or in a group in tall grass) all lost their fawns within the first hour of life.

In this course of this study, I observed several instances of warthogs attacking and/or killing newborn gazelle (click here for a video), a behavior that had not been previously documented. The window of opportunity for warthog predation seems to be pretty limited: to kill the gazelle, the warthog first has to knock it over and hold it down with a hoof. Once they are an hour or two old, gazelles are able to recover from being knocked over fairly quickly, and thus avoid being killed. Warthogs were very numerous at Ol Pejeta while I was conducting this research, so the encounter rate between warthogs and gazelle neonates may have been unusually high, which might explain why I observed this behavior but previous researchers did not. Interestingly, tommy mothers do not exhibit antipredator behavior in response to warthogs near their young, providing further evidence that this is a rare or emerging predator behavior.

The video below shows a fawn’s postpartum period, starting approximately 20 minutes after birth, during which the fawn is learning to stand, walk and nurse. The fawn’s mother gave birth within a herd so the mother-fawn pair suffer some disturbances by conspecifics. However, this video has a happy ending as the fawn survives to begin hiding.

Relevant publications:

Roberts, B.A. and Rubenstein, D.I. 2014. Maternal strategies for mitigating neonate predation risk during the postpartum period in Thomson’s gazelle. Behaviour 151: 1229-1248. PDF

Roberts, B.A. 2014. The trials of motherhood: maternal behavior patterns and antipredator tactics in Thomson’s gazelle (Gazella thomsonii), a hiding ungulate. PhD thesis, Princeton University. PDF

Roberts, B.A. 2012. An attack by a warthog (Phacochoerus africanus) on a newborn Thomson’s gazelle (Gazella thomsonii). African Journal of Ecology 50: 507-508. PDF

Maternal vigilance behavior relative to fawn activity

Vigilance behavior is an important means by which gazelles reduce their predation risk. More vigilant animals are more likely to detect predators while they are still far away, giving the prey a better chance of successfully escaping. However, for many animals vigilance comes at a cost. For gazelle and other grazing ungulates, there is a trade-off between vigilance and grazing: when the animal’s head is down in the grass, it doesn’t have a very good view of its surroundings. But if a gazelle keeps its head up all the time, it will starve. So gazelle and other prey animals have to decide when to be vigilant and when to focus more on foraging.

The Risk Allocation Hypothesis (formulated by Steven Lima & Peter Bednekoff in a 1999 paper) proposes that animals should be more vigilant when risk is high and less vigilant when risk is low. But for animals to follow this strategy, they have to have some idea of when they are at relatively high or low risk. Their ability to achieve the optimal vigilance strategy will depend on their ability to accurately assess their current risk level. We know that animals tend to increase their vigilance when they are in risky situations, for example when they are in treacherous habitats or when there is evidence that predators may be nearby. They also respond with extreme vigilance (and escape behavior) when they actually detect predators. But how do animals know when they AREN’T at risk? It is usually hard to tell because predators have lots of adaptations (like camouflage and stealthy behavior) that help them avoid detection.

In this study, we tested whether mother gazelles followed the predictions of the Risk Allocation Hypotheses. We reason that mothers have to manage periods of relatively high and low risk as a result of their fawns’ activity. When a fawn is hiding it is relatively safe, and when it is active it is exposed to predators and therefore at high risk. We already showed that mothers were more vigilant during fawn active periods than hiding periods in our 2015 paper. In this more recent study, we tested two further predictions:

Our main findings are summarized in the figure below. The left side (A) shows that maternal vigilance rates (the pink and green lines) increase as the mother prepares to retrieve the fawn. Maternal vigilance increases during this period, and are especially high in the hour prior to retrieval. The right side of the figure (B) shows maternal vigilance rates after the fawn has gone back to hiding. Vigilance is very low at the start of this period, but then gradually increases as time goes on.

Both of our predictions were confirmed, suggesting that mothers can effectively tailor their vigilance behavior to match current levels of predation risk. The idea that mothers adjust their vigilance behavior in response to conditions of low risk is particularly interesting. This behavior, known as predator sampling, has only been reported in a few species of birds and fish based on experimental studies.

Relevant publications:

Roberts, B.A. and Rubenstein, D.I. 2018. Temporal structuring of vigilance behaviour by female Thomson’s gazelles with hidden fawns. Animal Behaviour 145: 87–97. PDF

Roberts, B.A. 2014. The trials of motherhood: maternal behavior patterns and antipredator tactics in Thomson’s gazelle (Eudorcas thomsonii), a hiding ungulate. PhD thesis, Princeton University. PDF

Behavioral changes during the transition out of the hiding phase

In this study, I compared the behavior of young fawns (less than one month old) and their mothers to that of older fawns (one to two months old) and their mothers. While young fawns are fully engaged in the hiding strategy, older fawns are in the midst of a transition out of hiding. They spend less time hidden and emerge from hiding more frequently, often without waiting for their mothers to retrieve them. These behavioral changes on the part of the infant are expected to affect the behavior of the mother. Young fawn in hiding Young fawn in hiding

At the onset of the hiding phase, mothers in hiding species commonly undergo a series of behavioral changes. In some species, mothers exhibit altered habitat preferences as they seek out environments that offer greater protection to their vulnerable offspring. Some females leave their social groups and spend more time alone. Generally, mothers tend to be more vigilant than non-mothers, spending more time scanning the environment to detect predators that may threaten their young. These behavioral changes can inflict costs on mothers: habitats that conceal hiding infants can also conceal stalking predators; prey animals tend to be at greater risk of predation when they are alone compared to when they are in groups; and vigilance behavior takes time away from foraging. As infants shift away from hiding behavior, we might expect these maternal behavioral patterns to shift as well. Protective environments may be less important to offspring survival as infants develop their escape abilities, so we expect to see mothers returning to their typical habitats. More frequent emergences allow mothers relocate their infants more frequently, enabling them to track group movements more effectively and spend more time in groups. Finally, older fawns are thought to be at greater risk of predation because they spend less time in hiding. We would expect mothers to increase their vigilance during the transition out of hiding to mitigate this increased risk.

I observed 40 mother-fawn pairs and noted their behavior, habitat type (tall or short grass), and grouping behavior. I confirmed that fawns in my population undergo the same behavioral transition described in previous studies of Thomson’s gazelles: older fawns tend to stand up more frequently than younger fawns and are more likely to emerge on their own rather than waiting for their mothers. Furthermore, I found that when fawns wait for their mothers to retrieve them, they are protected to some extent by maternal vigilance. Meanwhile, fawns that emerge on their own forfeit that protection. In other words, when a mother is preparing to retrieve her fawn, she becomes super vigilant to make sure that there are no predators nearby that might attack the fawn once it is exposed. When the fawn gets up without the mother initiating the retrieval, she cannot protect her fawn in this way. Older fawns are therefore at greater risk of predation than younger fawns.

Contrary to our predictions, we found no differences in habitat use between mothers of young and old fawns. Both types of mothers spent the majority of their time in short grass habitats, which is the habitat type normally preferred by adult Thomson’s gazelles. However, we did find that mothers of transitioning fawns spent less time alone and more time in large groups than did mothers of young fawns. Adult gazelle are safer when they are in groups compared to when they are alone, and risk of predation tends to decrease as group size increases. We argue that the more frequent emergences of older fawns enable their mothers to adopt these safer grouping tendencies. Whereas young fawns hide for several hours at a time, older fawns only stay hidden for a half hour or so. Thus mothers with young fawns are spatially tethered to the same area for hours, whereas mothers with older fawns can relocate their offspring more frequently and thus follow groups of other gazelle as they move across the landscape.

All mothers, regardless of fawn age, are more vigilant when their fawns are active than when their fawns are hidden. When their fawns are hidden, mothers of old fawns are significantly less vigilant than mothers of young fawns. This is contrary to our expectations: old fawns are prone to emerging from hiding unexpectedly, and we would expect mothers to be more vigilant in order to monitor their behavior and keep an eye out for predators in order to reduce the risk of fawn predation. However, this is not the case. We hypothesize that mothers rely on the vigilance of the group, rather than their own vigilance, for fawn protection. Thus, for Thomson’s gazelle mothers, the transition out of hiding appears to be a time during which the costs of motherhood subside rather than increase despite heightened fawn risk.

Relevant publications:

Roberts, B.A. and Rubenstein, D.I. 2015. Coping with transition: fawn risk and maternal behavioral changes at the end of the hiding phase. _Animal Behaviour _109: 217-225. PDF

Roberts, B.A. 2014. The trials of motherhood: maternal behavior patterns and antipredator tactics in Thomson’s gazelle (Gazella thomsonii), a hiding ungulate. PhD thesis, Princeton University. PDF