Science Lesson

An explanation of my project is in order for those of you who aren’t familiar with the science behind why I am halfway around the world.

I am studying maternal investment decisions in Thomson’s gazelle, or “tommies”. That is, I am interested in how mother gazelle decide how much to care for their offspring. Some of you likely expect that mothers are naturally caring and pre-programmed to sacrifice anything and everything for the well-being of their progeny. A warm fuzzy thought, but not so in the animal kingdom (or in humans, for that matter! For more on this, see Sarah Blaffer Hrdy’s extremely interesting and accessible book, Mother Nature).

A mother’s evolutionary “goal” is to make the largest possible contribution to the gene pool. This is best accomplished by producing a large number of offspring who are highly reproductively successful themselves. Mothers are limited in their ability do to this in two ways. First, time is limiting: mothers only have from the time they reach sexual maturity until the time they die (or reach menopause, in the case of humans and some few other species) to produce offspring. Each offspring produced is also time-consuming: the female must first become pregnant (a quick step in the tommy’s case thanks to ever-vigilant and eager males), then the fetus must gestate (about five and a half months), and finally the female’s body must become receptive to pregnancy again (a relatively short two weeks or so in tommies, much longer in other species that experience lactational amenorrhea). Thus, a female tommy, who will reach maturity sometime between 9 and 12 months of age and, if she is lucky, will live to a maximum of 12 years in the wild, can produce at the very most twenty-two young (tommies give birth only to singletons, so we do not find super-productive Octo-moms).

So we might expect every tommy mother to produce a fawn every six months and invest identically in each fawn such that at the end of her 12 years she has produced the maximum twenty-two offspring. However, a mother is limited in her capacity to invest. According to parental investment theory, investing in one offspring decreases the mother’s (or father’s) ability to invest in other offspring. This decrease in investment ability can come to pass in two main ways: the mother can either suffer a survival cost (i.e. she dies before she can reproduce again) or a fecundity cost (she is alive, but not fertile). For example, a mother investing heavily in her young may attempt to defend it against an attacking predator. This benefits the offspring by increasing its chances of survival, but inflicts a survival cost on the mother by increasing her chances of being killed by the predator. A mother may also invest in her offspring by keeping it in an area of low predator density, as humpback whales, some caribou populations, and many seals do. These areas are often also lacking or completely devoid of food for the mother. Again the investment increases the offspring’s chance of survival, but this time the mother’s fecundity may suffer as a result of her reduced forage intake; she may have to fatten up before reproducing again.

Given that raising young is costly, mothers should be choosy about which young they invest in and how much they invest. So how do they decide? There are many variables that can influence a mother’s level of investment. She may invest more in stronger or older offspring as those are the most likely to make it to adulthood. Or she may invest in weaker or younger offspring because those stand to benefit more from higher levels of investment. She may invest more in one sex that is likely to have higher reproductive success. She may invest more when environmental conditions are favorable such that the cost of investment is lower. There are many predicted patterns of investment, few of which have been demonstrated conclusively or seem to hold true across all species. In my project, I will be examining four possible drivers of maternal investment decisions: predation risk, maternal forage intake, offspring age, and maternal condition.

Ungulate species employ one of two primary maternal care strategies. Horses, camels, rhinos, wildebeest, and caribou (among others) exhibit the “following” strategy wherein the offspring stays with its mother continuously from the time it can walk until it is independent. Tommies, along with all other gazelle, most other antelope and many cervids (deer) exhibit the “hiding” strategy. Hiding is a bit more complex than following. After the young is born and gains its feet, it selects a hiding spot in nearby vegetation. It remains crouched and motionless in this hiding spot for hours until its mother retrieves it, feeds and grooms it, and leads it to another hiding area where it hides again. This process repeats for the duration of the hiding phase, which can last anywhere from one day (in impala) to four months (in bushbuck). For tommies, hiding is intensive for the first two weeks of life and then tapers off until, by two months of age, the fawn no longer hides at all.

While field scientists have often marveled at the infant’s ability to stay more or less motionless for hours on end, waiting patiently for its mother to return, I am more interested in the mother’s side of the equation. It appears that hider mothers have the opportunity to greatly limit the amount of investment required of them and thus raise relatively cheap offspring. Compared to follower mothers, who must constantly monitor their young, be highly vigilant for predators that may target their vulnerable offspring, and endure their infant’s disruptive play, begging and general harassment, hider mothers seem to get off easy. They park their young for hours at a time during which they are free to tend to their own needs (i.e. maintain themselves in order to enable future reproduction). However, rather than leave to seek the safety of groups or lush feeding habitats, mother tommies often hang around near their hiding young, seemingly incurring costs that may reduce their ability to reproduce in the future. The point of this “hovering” as I have come to call it seems to be to enable the mother to intervene in the event of a predator attack. Tommy moms are known to vigorously attack jackals and birds of prey that disturb their young and even try to distract cheetahs and hyenas that come across them.

My primary goals are to determine just how costly hovering is to mothers and to identify drivers of hovering behavior. Do mothers hover only when it is cheap for them in terms of predation risk and feeding costs? Only when the environment is particularly dangerous for their offspring? Some combination of both? To this end I will be spending many hours watching tommy mothers and recording their behavior. I will also record the behavior of non-lactating females. These non-mothers will serve as a baseline against which to compare maternal behavior: they have no offspring to worry about, so their behavior represents the optimal behavior pattern for a female investing completely in self-maintenance. Deviations from this pattern on the part of mothers represent potential costs of maternal investment. In addition to collecting behavioral data, I will be setting up camera traps on the plains on which I am watching the tommies. These camera traps will hopefully capture images of predators that use the plains. Relative densities of predator species will help to contextualize mothers’ behavior: if there are many small predators that threaten tommy young but not adults, one might expect mothers to be more willing to hover (since the predation risk to themselves will be low, but the benefit to their young will be high). However, if the plain is commonly used by cheetahs, which are highly dangerous to adult gazelle, I would expect mothers to behave so as to reduce their own predation risk, perhaps by reducing their investment in their young. The last main component of the project is habitat analysis. I will return to the locations where I observe mothers and young and measure grass height, greenness and leafiness. I will also take vegetation samples from which I will measure various nutritional components. This will help me to determine if mothers are incurring a foraging cost while hovering by confining themselves to suboptimal foraging habitats.

So that’s roughly the project. I will try to go into more detail on particular components as my field season progresses!