Indiscriminate oophagy by ant larvae: an explanation for brood serial organization?

Summary Last instar larvae ofLasius niger under standard laboratory conditions and abundant food supply feed on conspecific eggs and, if forced to a choice, show a significant statistical preference for trophic versus fertilized eggs (approx. 60% of the trials observed). On the other hand, they are unable to discriminate between kin and non-kin eggs, both fertilized and trophic. Fertilized eggs killed by freezing and trophic eggs handled in the same way are also selected in a random manner.Last instar larvae ofMessor semirufus regularly failed to discriminate in a significant way between trophic and fertilized and between kin and non-kin eggs.Substantial experimental evidence confirms that larvae of both these species — as already suggested in the literature for other ants-rely on trophic eggs as an essential component of their diet in order to be able to develop. Additionally,L. niger last instar larvae appear to be able to attack and pierce the egg chorion with their own mandibles, while this capacity is at least very reduced or very rare inM. semirufus. First and second instar larvae of both species never succeeded in piercing the egg membrane alone.These findings imply that the right (i.e. trophic) eggs should be presented to the larvae by the workers (i.e. worker discrimination should be assumed) and, at least in the case of youngLasius larvae, and probably for all stages ofMessor larvae, the egg membrane must be pierced by the workers in order to allow the larvae to feed.The following evolutionary sequence is suggested to explain the origin of trophic eggs in ants: 1) larval oophagy (obligatory at least for the species founding new colonies in an independent claustral manner), 2) generalized facultative or obligatory larval oophagy for larvae of all colonial stages, 3) production of trophic eggs by the queen(s) and/or workers in order to avoid cannibalism of nestmates.Lack of discrimination or weak discrimination capacity between trophic and fertilized eggs and the consequent larval cannibalism in form of oophagy, as demonstrated in this paper, is suggested to explain age segregation among eggs and larvae from workers, a widespread phenomenon in ants which must have been selected to avoid the oophagy of viable eggs.     

Sibling cannibalism in the ladybird beetleHarmonia axyridis: Fitness consequences for mother and offspring

The adaptive significance of sibling cannibalism was analyzed in the ladybird beetle Harmonia axyridis at two prey densities. Possible costs and benefits were considered from three points of view: the mother, the cannibal and the victim. Cannibals ate both infertile and fertile eggs, thereby increasing in body length and survival rate with the intensity of sibling cannibalism. The cannibalistic trait was clearly beneficial to the cannibal when aphid density was low. However, it was not always beneficial when aphid density was high and the victims were full siblings. The altruistic behavior of being a victim was beneficial only when the victim was cannibalized by full siblings at low aphid density. The mother attained almost equal fitness at low aphid density, regardless of the intensity of sibling cannibalism. This suggests that sibling cannibalism is not maladaptive for the mother. At high aphid density, however, mother's fitness decreased with the intensity of sibling cannibalism, indicating that sibling cannibalism is maladaptive for the mother when larval food availability is high.     

A test of alternative hypotheses for kin recognition in cannibalistic tiger salamanders

The function of kin recognition is controversial. We investigatedtheadaptive significance of kin discrimination in cannibalistictiger salamanderlarvae, Ambystoma tigrinum. Previous laboratoryexperiments show thatcannibals preferentially consume lessrelated individuals. We hypothesizedthat this example of kinrecognition (1) is a laboratory artifact, (2) is aby-productof sibship-specific variation in escape responses, becausecannibalsfrom families with rapid responses may be more likely to cannibalizeslowlyescaping non-kin, (3) is an epiphenomenon of species recognition,(4)functions in disease avoidance, because kin may be moreinfectious thannon-kin, or (5) is favored by kin selection.We evaluated these fivehypotheses by using laboratory and fieldexperiments to test specificpredictions made by each hypothesis.We rejected hypotheses 1-4 above because(1) kin recognitionwas expressed in the wild, (2) escape responses did notreliablypredict whether a cannibal would ingest kin or non-kin, (3)kinrecognition was not most pronounced in populations wheretiger salamandersco-occur with other species of salamanders,and (4) non-kin prey were morelikely than kin to transmit pathogensto cannibals. However, we establishedthat the necessary conditionfor kin selection, Hamilton's rule, was met.Thus, our resultsimplicate kin selection as the overriding reason thatcannibalistictiger salamanders discriminate kin.     

A Test for Male Parental Care in a Fundulid, the Bluefin Killifish, Lucania goodei

Under field conditions, breeding male bluefin killifish, Lucania goodei, have been observed aggressively defending territories from other breeding males, non-breeding females, and minnows (mainly Notropis harperi). We performed an aquarium experiment to test whether male aggression serves to protect newly deposited eggs from predation. We allowed a male and a female to spawn in a yarn mop, removed the female, and exposed the eggs to one of four treatments (spawning male present, two minnows present, spawning mal+two minnows present, no adult fish present). Mops were censused daily for seven days. Egg predation rates were highest in the male+minnows and male only treatments. Egg predation rates in the male+minnows treatment did not differ from the predicted predation rate (sum of male only and minnows only treatments). Hence, there is no evidence for male parental care in L. goodei. In addition, we compared the egg predation rates (filial cannibalism) between males of 3 different color morphs and found no evidence for differential egg cannibalism.     

Does cooperation mean kinship between spatially discrete ant nests?

Eusociality is one of the most complex forms of social organization, characterized by cooperative and reproductive units termed colonies. Altruistic behavior of workers within colonies is explained by inclusive fitness, with indirect fitness benefits accrued by helping kin. Members of a social insect colony are expected to be more closely related to one another than they are to other conspecifics. In many social insects, the colony can extend to multiple socially connected but spatially separate nests (polydomy). Social connections, such as trails between nests, promote cooperation and resource exchange, and we predict that workers from socially connected nests will have higher internest relatedness than those from socially unconnected, and noncooperating, nests. We measure social connections, resource exchange, and internest genetic relatedness in the polydomous wood ant Formica lugubris to test whether (1) socially connected but spatially separate nests cooperate, and (2) high internest relatedness is the underlying driver of this cooperation. Our results show that socially connected nests exhibit movement of workers and resources, which suggests they do cooperate, whereas unconnected nests do not. However, we find no difference in internest genetic relatedness between socially connected and unconnected nest pairs, both show high kinship. Our results suggest that neighboring pairs of connected nests show a social and cooperative distinction, but no genetic distinction. We hypothesize that the loss of a social connection may initiate ecological divergence within colonies. Genetic divergence between neighboring nests may build up only later, as a consequence rather than a cause of colony separation.     

Kinship Influences Cannibalism in the Wolf Spider, Pardosa milvina

Recent studies have called into question the role of Wright's coefficient of relatedness (r) in the interactions among relatives. Kin selection theory predicts a positive relationship between relatedness and frequency of altruistic acts, but a number of researchers have reported the opposite relationship. I used a lycosid spider (Pardosa milvina) to test the hypothesis that genetic relatedness would affect the propensity of a cannibalistic species to prey on genetic relatives. I considered lack of predation to be a form of altruism where the predator incurs a cost (loss of a meal) that benefits potential prey. Specifically, I questioned whether direct genetic offspring would be avoided as prey items and whether the sex or reproductive condition of a cannibalistic predator would affect the likelihood of predation on conspecific juveniles. As predicted by kin selection theory, spiderling mothers ate significantly fewer of their own offspring than they did of nonkin spiderlings of the same age. Adult virgin female and adult male spiders ate significantly more spiders than females that had recently carried spiderlings. Females with egg sacs consumed significantly fewer spiderlings than did virgin female spiders. These findings support Hamilton's rule and suggest that, in some systems, genetic relatedness plays a strong role in governing altruistic behavior toward relatives.