Maternal investment in offspring

Elephants show an extended period of maternal care and direct investment of energy through nursing of calves (fig. 4.2). The average suckling duration of 2-4 minutes per hour observed in elephants younger than 1 year is one of the highest rates reported among ungulates. Even though calves have developed the motor skills needed for feeding on plants, they continue to derive significant amounts of nutrition from their mothers until 3 years or older. The maternal contribution beyond 2 years is not needed for survival, but could be crucial for maintaining growth rates, body condition, and ultimate reproductive ability of offspring. Therefore, it can be expected that elephant mothers would regulate their investment in children in a manner that would maximize their own long-term reproductive success.

In a polygynous species with marked sexual dimorphism, such as in elephants, the variance in reproductive success is much greater among males than among females (chapter 3). Mothers should thus preferentially invest in offspring of the sex that brings them the highest genetic returns. While these ideas had been in the literature for some years, Robert Trivers and D. E. Willard in 1973 presented a theoretical framework for the evolution of parental ability to vary investment in male and female offspring. In a polygynous, sexually dimorphic mammal, it pays to invest preferentially in male offspring when

Elephant With Curled Trunk

Figure 4.2

An elephant calf suckling. Among African elephants at Amboseli, Kenya, the mothers bias investment toward male calves.

Figure 4.2

An elephant calf suckling. Among African elephants at Amboseli, Kenya, the mothers bias investment toward male calves.

a mother has ample resources (when she is in good body condition, for instance). The logic here is that a well-provisioned son would grow large, achieve dominance over other competing males in the population, and enjoy a higher than average reproductive success. On the other hand, a weak mother is better off investing in a daughter with an opportunity anyhow to reproduce rather than produce a weak son with a low chance of successful mating. The Trivers-Willard hypothesis, as originally formulated, related largely to parental ability in adjusting the sex ratio of offspring, although it did state that, for species with prolonged maternal investment, this should be extended to the total investment in offspring. Studies on a variety of mammal species have so far been equivocal in support for this hypothesis.

The Amboseli study of Phyllis Lee and Cynthia Moss provides the only detailed observations of elephants on this subject (fig. 4.3). Lee and Moss observed 110 male calves and 125 female calves aged 0-54 months. From about 3 months, male calves attempted to suckle from their mothers more frequently than did female calves. The male calves were also more successful at suckling. When averaged across all ages, male calves suckled once every 37 minutes, while female calves did so every 50 minutes. The duration of each bout of suckling did not vary significantly between the sexes (86 seconds for males and 89 seconds for females). However, when the frequency and the duration of suckling bouts were combined into a single measure of milk intake, it was clear

0-3 4-6 7-9 10-12 13-16 18-24 25-30 31-36 37-42 43-48 49-54

Age (months)

Figure 4.3

The mean estimated milk intake (duration of suckling times number of successful suckling bouts per hour) for male and female elephant calves of various age classes at Amboseli. (From Lee and Moss 1986. Reproduced with the permission of Springer-Verlag GmbH & Co., K.G.)

that male calves spent more time on the nipple than did female calves. This pattern was consistent across all ages from birth to 3 years. While it was the calf that generally initiated suckling, either the calf or the mother terminated suckling. Maternal termination of suckling by male calves became pronounced after 3 years.

Weaning itself was a gradual process. There was no specific age at which suckling suddenly terminated; in a small proportion (15%) of cases, an older sibling continued sporadic suckling along with the younger sibling. The duration of maternal suckling in a calf is obviously important in determining onset of estrus in the mother and interbirth intervals, an important consideration in ultimate reproductive success. The Amboseli data showed that investment in a male calf resulted in interbirth intervals that were longer by 2 months (wet years) to 5 months (dry years) compared to having a female calf previously. There seems to be a certain cost, in direct demographic terms, that goes along with the higher level of investment made in male calves. Obviously, there is a tradeoff between investing more in a son in the expectation of increasing his reproductive success and incurring a higher short-term demographic cost through longer interbirth interval.

Lee and Moss (1986) aptly asked whether elephant mothers may "be investing more in males than females in order for them to grow faster, or alternatively because they grow faster" (p. 360, italics in original) Data from Amboseli indicated that males grew faster than females on average from birth onward, and the dimorphism was clear by 5 years, by which time they had been weaned (in a study of growth rates in Asian elephants born in captivity, I found no difference between male and female calves in growth until 2 years, after which differences surfaced; see appendix 2). Because of their faster growth rates, male calves could be demanding and obtain more milk from their mothers.

The observation that elephant mothers bias postnatal investment toward sons is consistent with the Trivers-Willard hypothesis, although by itself, it is not necessarily substantial evidence for the same. At the population level, mothers may have to invest more in sons than in daughters only because sons grow faster. The crucial question is whether individual mothers in good (higher-than-average) body condition are preferentially investing in sons such that these sons would enjoy greater than average reproductive success (or that mothers in poor condition do the opposite). This has to be integrated over various stages, taking into consideration prenatal mechanisms (adjustments in sex ratios of offspring), postnatal investment (bias in suckling), demographic cost-benefit (changes in inter-birth interval) considerations, and offspring reproductive potential. In a long-lived species, these conditions would vary over the life span, as with age-related body condition, interannual variation in climate and food availability, population densities, and so on. This integration is possible only when a population is studied over the long term, as at Amboseli, although pieces of the jigsaw puzzle may be assessed elsewhere. Among 261 elephants born in captivity in southern India, I found that mothers in the 2040-year age group produced more sons than daughters, while no bias was noticed for other ages. Middle-aged mothers in their reproductive prime can also be expected to be relatively in good condition, thus lending support to the Trivers-Willard model.

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