Polyandry enhances offspring survival in an infanticidal species

The adaptive significance of polyandry is an intensely debated subject in sexual selection. For species with male infanticidal behaviour, it has been hypothesized that polyandry evolved as female counterstrategy to offspring loss: by mating with multiple males, females may conceal paternity and so prevent males from killing putative offspring. Here we present, to our knowledge, the first empirical test of this hypothesis in a combined laboratory and field study, and show that multiple mating seems to reduce the risk of infanticide in female bank voles Myodes glareolus . Our findings thus indicate that females of species with non-resource based mating systems, in which males provide nothing but sperm, but commit infanticide, can gain non-genetic fitness benefits from polyandry.

Associations with multiple male groups increase the energy expenditure of humpback whale (Megaptera novaeangliae) female and calf pairs on the breeding grounds

In mammalian mating systems, where operational sex ratios are male skewed and males must compete for access to females, increased levels of male attention may amount to harassment of females and their offspring. To evaluate how male associations affect the behaviour of humpback whale (Megaptera novaeangliae) female and calf pairs in breeding regions, we compiled time budgets and monitored calf breathing regimes for females with calves in a range of associations, with (N = 71) and without (N = 19) males, in Hawaiian waters. We found that, while associations with a single male did not significantly change the behaviour of female–calf pairs, associations with multiple males led to increases in the time spent traveling (median increase 35%; p < 0.001) and decreases in time spent at rest (median decrease 29%; p < 0.001). Additionally, calves spent less time at the surface (median decrease from 10% to 0; p < 0.001) and the frequency of intermittent breaths between dives increased (median increase from 16 to 22%; p = 0.006). We show that these behavioural changes would require increased energy expenditure, which could impact calf fitness, and we speculate that the association between a female–calf pair and single male escort comprises a female counterstrategy that offsets male harassment, consistent with Mesnick's (1997) bodyguard hypothesis.

Parent-Offspring Conflict in Budgerigars

Despite widespread theoretical interest in genetic conflict between parents and offspring, there is little empirical evidence that it exists in nature. Theoretical models suggest two outcomes of conflict not predicted by alternate theories: (1) offspring that control the allocation of parental investment might show escalated demand behaviours (e.g., begging) and demand more resources than they could efficiently use and (2) parents might evolve behavioural counterstrategies which prevented offspring from obtaining extra resources, but which were more costly than a "laissez-faire" parental strategy allowing offspring control. These predictions were tested in budgerigars, (Melopsittacus undulatus) in large flight cages at Davis, California. Budgerigar clutches hatch extremely asynchronously, yet all nestlings grew at similar rates and fledged at similar sizes and ages. This independence of hatch order and performance seemed due primarily to the mother budgerigar's allofeeding strategy: females allofed offspring mainly on the basis of size, and only secondarily attended to begging rate. Offspring of a given age and size were treated the same by their mothers regardless of hatch order, and offspring undersized for their age were fed as if they were younger. In contrast, male budgerigars attended to offspring begging rates. Males tended to initiate feeding bouts when offspring begged, and to allofeed vigorous beggers more often. Variance in male allofeeding behaviour allowed comparisons of size-matched families in which females performed nearly all of the allofeeds to nestlings (= female-fed families) with families in which males and females both allofed nestlings (= male-aided families). The parent controlled the allocation of food in female-fed families, while offspring had greater control over food allocation in male-fed families. As was predicted by conflict theory, the female counterstrategy was effective but potentially costly: the food delivery rate of females was only half as fast as males'. Conversely, offspring control resulted in an escalation of beg rate for the same degree of need (as measured by size and growth), and male-aided offspring obtained nearly three times more regurgitations than female-fed nestlings, yet grew at comparable rates and fledged at comparable sizes and ages. Hence, offspring demanded and obtained more food than they could effectively use. Other avian parents also seem to use effective but potentially costly counterstrategies, and other avian offspring may demand more food than they require. Even if initial hatch asynchronies functioned in brood reduction, the parental strategies described here would allow parents to retain control over the timing and conditions for offspring loss.