Does Vitamin E Supplementation Enhance Growth Benefits of Breeding Helpers at No Oxidative Costs?

Passive and active parental food allocation in a songbird

Parent-offspring conflict over food allocation can be modeled using two theoretical frameworks: passive (scramble competition) and active choice (signaling) resolution models. However, differentiating between these models empirically can be challenging. One possibility involves investigating details of decision-making by feeding parents. Different nestling traits, related to competitive prowess or signaling cryptic condition, may interact additively or non-additively as predictors of parental feeding responses. To explore this, we experimentally created even-sized, small broods of pied flycatchers and manipulated nestling cryptic quality, independently of size, by vitamin E supplementation. We explored how interactions between nestling cryptic condition, size, signals, and spatial location predicted food allocation and prey-testing by parents. Parents created the potential for spatial scramble competition between nestlings by feeding from and to a narrow range of nest locations. Heavier supplemented nestlings grew faster and were more likely to access profitable nest locations. However, the most profitable locations were not more contested, and nestling turnover did not vary in relation to spatial predictability or food supply. Postural begging was only predicted by nestling hunger and body mass, but parents did not favor heavier nestlings. This suggests that size-mediated and spatial competition in experimental broods was mild. Pied flycatcher fathers allocated food in response to nestling position and begging order, while mothers seemingly followed an active choice mechanism involving assessment of more complex traits, including postural intensity interacting with order, position, and treatment, and perhaps other stimuli when performing prey-testings. Differences in time constraints may underlie sex differences in food allocation rules.

Heat shock proteins and antioxidants as mechanisms of response to ivermectin in the dung beetle Euoniticellus intermedius

Ivermectin is the most common antiparasitic drug used in livestock in many regions of the world. Its residues are excreted in dung, threatening non-target fauna such as dung beetles, fundamental for cleaning dung in pastures. However, it is unclear which are the physiological mechanisms used by dung beetles to cope with ivermectin. Here we evaluated experimentally the physiological responses of the dung beetle Euoniticellus intermedius to ivermectin-induced stress. We measured metabolic rates, heat shock protein 70 (Hsp70) expression, antioxidant capacity, and oxidative damage in lipids in both males and females exposed to a sublethal dose. Compared to control beetles, ivermectin-treated males and females had increased metabolic rates. Moreover, ivermectin-treated females increased their expression of Hsp70 whereas males increased their antioxidant capacity. No changes in the levels of oxidative damage to lipids were detected for either sex, suggesting a process of hormesis, such that exposure to a moderate concentration of ivermectin could stimulate the action of a protective mechanism against oxidative stress, that differs between sexes. However, it does not exclude the possibility that damage to other biomolecules might have occurred. Sexual differences in physiological responses can be interpreted as the result of hormonal differences or life-history trade-offs that favor different mechanisms in females and males. Hsps and antioxidants are involved in the physiological response of beetles to ivermectin and may be key in providing resistance to this contaminant in target and non-target species, including dung beetles.