Group histories and offspring sex ratios in ringtailed lemurs

Statistically significant departures from 50:50 birth sex ratios have been reported for a variety of mammals. General explanations for these deviations are usually based on differences  in the net benefits of producing sons versus daughters, depending on attributes of individual mothers or their populations.

There are four separate hypothesis to understand this, the first being the Trivers-Willard (T-W). It focuses on male intrasexual competition. The effects are anticipated when offspring of a given sex in superior physical condition have greater reproductive potential than offspringof the other sex in a similarly augmented condition, or females can influence the physical condition of offspring at maturity. A second hypothesis, local resource competition (LRC), applies to situations in which one sex is philopatric and so faces competition mainly from philopatric individuals. When LRC increases, females are predicted to produce a greater number of the dispersing sex to mitigate local competition. This LRC effect is

an attribute of the population or group and so is not expected to vary among individuals. The third hypothesis also involves competition among members of the philopatric sex. the relative advantages of producing sons versus daughters vary among mothers within groups, and so we call this hypothesis LRC-individual. Under LRC-individual, only high-ranking females bias production toward the philopatric sex because they can best support these offspring, usually daughters, against harassment from likesexed individuals. A fourth hypothesis concerns delayed benefits that may arise from producing the philopatric sex. This local resource enhancement hypothesis (LRE) proposes that when one sex effectively “repays” costs of reproduction by providing benefits to the mother, this sex should be overproduced under appropriate conditions.

These four hypotheses were tested on Lemur catta, at Duke University. Intra- and intergroup competition is intense for female ringtailed lemurs, suggesting that factors associated with both LRC and LRE are important. However, maternal agonistic intervention is irregular and matrilineal rank inheritance unreliable in ringtailed lemurs, predicting that LRC effects will be observed predominantly at the population level. Competition patterns vary across wild populations of ringtailed lemurs, and so the factors that influence sex ratios may also vary. Male intrasexual competition is intense, and a male’s physical condition should aid him in this competition. If adjustment of offspring sex ratios is an adaptive element of female life histories, individual females are expected to respond facultatively to varying circumstances, perhaps as part of an adaptive norm of reaction.

Two social groups (Lc1 and Lc2) ranged semi-freely inlarge forested natural habitat enclosures. Both
indoor and outdoor groups have experienced a range of demographic conditions, particularly with respect to the composition of the groups and their stability. Males have transferred freely between the two groups and females have defended stable territorial boundaries without obstruction. Evicted females have been removed from the enclosures by Duke University. Animals in the natural habitat enclosures foraged extensively on the natural vegetation while also receiving daily provisions. The following information was obtained from the records for each birth: infant sex, mother’s location, her age, and the number of mature females and mature daughters present in the group at the likely time of conception. Ringtailed lemurs are highly seasonal in their reproduction, with conceptions occurring in the fall mating season and births the following spring.

In the LRC-population results, species with male dispersal predicts a polutaion leve male bias in the secondary sex ratio. Even though the ratio was slightly biased toward males, it was not statistically different from an equal sex ratio. LRC-population also predicts that mothers overproduce sons when groups are large, as males eventually disperse and will not compete for limited resources. Females in large natural habitat enclosure groups tended to overproduce sons. For the T-W results, mothers

that lose an infant early in the birth season are predicted to produce sons in the next birth season, as they are expected to have more energy available for investing in this next infant. The percentage of twins was slightly higher among females that were recorded as having lost an infant. The sex ratio of infants born to these females, however, was not significantly different from equality with a binomial test. As for the LRE, top-ranking females overproduced daughters when resources were maximal provides indirect support for
LRE modified by LRC. LRE makes two more direct predictions for ringtailed lemurs.

Female with offspring.

     Pereria, M. and Nunn, C. (1999), Group histories and offspring sex ratios in ringtailed lemurs ( Lemur catta ). Behavioral Ecology and Sociology, vol 48.1.