In many polygynous species, males do not participate in brood care duties. Offspring frequently have long maternal dependence periods during which they receive protection and food from their mother. When a mother has more than one offspring and food is scarce, she has to decide whether to try to raise the whole brood or ensure the survival of at least part of her progeny.
The problem is complicated when the sex of these offspring comes into play. Since these species are sexually size-dimorphic, with males larger than females, male offspring need to grow faster, demanding more maternal care than their sisters. Life-long studies on some species have shown that maternal care can be crucial for the future reproductive success of well-nourished young males, with no similar effect in young females. This is because dominant adult males will mate with multiple females and leave few or even no matings to low-condition competitors, whereas most adult females will generally raise at least some offspring over their lives. When resources are limited, a mother with two offspring of different sex faces the dilemma of concentrating into raising a good son that might give her many grandchildren, securing the survival of her less-demanding daughter, or trying to raise both.
In 1973, Robert Trivers and Dan Willard proposed that in some mammals the decision of these mothers would depend on their own condition. Mothers in good condition, or those breeding in years with high food abundance, would take advantage of their better chances and tend to invest more in sons, whereas mothers in poor condition or in years with limited resources would tend to raise more daughters. This hypothesis raised great interest, although the numerous empirical studies done in the following decades have not yet clarified whether in birds and mammals parents can really control the sex of their progeny. There might be other mechanisms to determine which sex survives in each case that do not imply maternal manipulation.
We investigated this problem in the great bustard, the species showing the highest sexual size dimorphism among birds, with adult males more than doubling the weight of adult females. We observed the mother-chick interactions in families of two chicks of different sex and compared them with those observed in broods of two sons or two daughters, and in broods of one chick. Great bustard chicks have a long maternal dependence period of 6 to 18 months, during which they mostly feed by themselves but receive additional feedings from the mother.
In previous studies, we had shown that great bustards belong to that group of species where an early maternal care had a strong positive influence on the future breeding success of sons but not of daughters. So we could in principle expect mothers of two-chick broods, which are found mostly in good years, would provide more attention to their sons than to their daughters.
What we found is that in mixed-sex broods, male chicks indeed received twice the number of feedings from the mother than their sisters. However, mothers never refused to feed her daughters. What happened was that male chicks stayed closer to the mother twice as long as their sisters, who were prevented to access their mother probably simply by the presence of their begging brother. Therefore, when the mother took a prey in her beak and offered it to both offspring, it was nearly always the son who approached earlier to be fed.
In mothers with same-sex broods, both chicks spent the same time close to the mother, and the maternal feeding frequency to each of them did not differ. Finally, female chicks in single-chick broods received significantly more maternal feedings than female chicks in mixed-sex broods. Based on this evidence, we concluded that in mixed-sex broods it was the presence of a brother what determined the reduced feeding rate to the female chick.
The higher food required by the faster growth of male chicks compared to their sisters is the main reason for their more intensive begging. But as we say above, we never saw any signs of maternal preference to feed either son or daughter. The mother just offered food and gave it to the chick approaching first, in the hope that this was the chick that was hungrier at that moment. The behavior we observed does not need to invoke manipulation by the mother. Our results support the alternative, more parsimonious explanation that the crucial factor was the differential competitive ability of male and female offspring to manipulate the amount of food or care provided by the mother.
In other birds and mammals, where offspring share the same nest or den, larger siblings sometimes attack or even kill smaller siblings. An extreme case of sibling competition has been described in the Soay sheep, where female lambs with a male twin show a reduced birth weight and lower lifetime breeding success compared to those with a female twin, and there was no such effect in male-male co-twins.
In great bustards, the negative consequences for female chicks sharing maternal care with a brother are such that sisters have more chances to die by starvation than their brothers and female chicks without brothers. Competition among siblings is a well-known fact in many species, but a systematic disadvantage for the female sex is a grotesque biological consequence of a too-large difference in size between both sexes.
These findings are described in the article entitled Sibling competition and not maternal allocation drives differential offspring feeding in a sexually size-dimorphic bird, recently published in the journal Animal Behaviour. This work was conducted by Juan C. Alonso and Enrique Martín from the Museo Nacional de Ciencias Naturales, Manuel B. Morales from the Universidad Autónoma de Madrid, and Javier A. Alonso from the Universidad Complutense.