We are all familiar with animals of many species that developed interesting mechanisms to avoid being eaten. This includes, for example, animals that look like plant parts:
and animals that merge with the background:
There are also animals that look like other, unpalatable or dangerous, animals, in order to push predators away:
But we rarely think that plants also use this sort of mechanisms to avoid being eaten. There are, however, some recorded cases of similar behaviors in plants. One case is that of the plant Corydalis benecincta, whose leaves commonly have the brownish color of the surrounding rocks:
Recently, a study on plants of the genus Amorphophallus found another interesting case of mimicry. This genus, which includes the famous titan arum, usually develops a single large leaf that in some species can attain the size of a small tree or shrub. Such a gigantic leaf seems to be a perfect meal for some herbivores but, to avoid them, many species of this genus developed a series of marks along the petiole of their leaf that look like lichens or cyanobacteria.
With this mimicry, the petioles, which are quite tender, end up looking like a hard and old trunk that does not look that interesting as a meal for most herbivores. The lichen marks are so well represented that they can even be associated with real lichen genera. For example, the marks seen on the figures B and C above look like lichens of the genus Cryptothecia.
How and why this marks evolved across Amorphophallus species is still not well understood. Despite the hypothesis that they help the plant mimic a tree trunk, some species with small leaves also have those marks, while some with large leaves do not have any marks or have them in simpler patterns. The titan arum Amorphophallus titanum is a good example of the latter:
For a long time, plants were regarded as less dynamic organisms than animals, but in recent years our knowledge about them is increasing and showing that they are actually very versatile creatures that developed similar creative and complex strategies.
Claudel C, Lev-Yadun S, Hetterscheid W, & Schultz M 2019. Mimicry of lichens and cyanobacteria on tree-sized Amorphophallus petioles results in their masquerade as inedible tree trunks. Bot J Linn Soc 190: 192–214.
Sexual cannibalism is the act of eating a sexual partner right before, during or right after copulation. Despite being a considerably rare behavior, its occurrence is very popular among the general public.
When sexual cannibalism occurs, it usually consists of the female eating the male. Two popular cases are those of mantises and of spiders, especially the black widow. This phenomenon, at least among black widows, is much rarer than most people think.
Although sometimes sexual cannibalism occurs because one of the partners mistakes the other for food, in many species it is a evolutionary selected strategy that assures that the female will eat enough for the offspring to develop properly. It may look horrible from our human point of view, especially if we think from the perspective of the male, but we must remember that passing your genes to the next generation is the main purpose of most organisms and, if the male succeeded in fertilizing the female’s eggs, his life has served his purpose and he can die happily.
Sexual cannibalism is, of course, almost exclusively observed among predators, which is kind of obvious. And, as I said above, is commonly performed by the female. One group that is famous for its female-empowered species is the insect order Hymenoptera, which includes bees, ants, wasps, sawflies, among others. Since many hymenopterans have some degree of sociality, in which societies are composed almost exclusively of females, and males are generated only for the purpose of reproduction, it is curious that sexual cannibalism has never been recorded in this group… until now.
A recently published study examined the mating behavior of a small parasitoid wasp, Gonatopus chilensis. This species belongs to the family Dryinidae, of which all species lay their eggs on insects of the suborder Auchenorrhyncha, which includes cicadas, leaf hoppers, plant hoppers, among others. The larvae, after hatching from the egg, feed on the hosts. Adult females of dryinid wasps are also voracious predators and feed on the same species on which they fed as larvae.
After copulation, females of G. chilensis were often observed trying to capture the males in the same way they capture their prey. However, in only one occasion the female was successful in capturing the male and ate its gaster (the large round portion that forms most of the abdomen in wasps). Since only one instance of cannibalism was observed, it may be a rare phenomenon in this species, but since several attempts to capture the male were seen, it seems that eating the male is an interesting idea for the females.
This is the first known case of sexual cannibalism in hymenopterans and, therefore, an important record that increased the number of groups in which this behavior is known to occur.
A recently published paper describes a new species of insect of the order Zoraptera from two specimens found in mid-cretaceous amber from northern Myanmar.
But the most impressive thing about this new pre-historic species, named Zorotypus pusillus, is the fact that the fossil contains a male and a female that apparently died while they were mating. This is concluded because the two individuals are very close to each other and the male has an elongate structure coming out of his abdomen, which is probably the aedeagus or intromittent organ, a penis-like organ found in most zorapterans and used to deliver sperm into the female.
The order Zoraptera contains a very small number of species, currently 44 extant ones and 14 fossils. They are very small, live in groups and look like tiny termites, although they are not closely related to them. Most extant species mate with the male introducing its aedeagus into the female to deliver sperm, but at least one species, Zorotypus impolitus, does not copulate. In this species, the male deposits microscopic spermatophores on the abdomen of the female.
The discovery of the preserved mating behavior in this species from the cretaceous period indicates that the mating behavior seen in most extant species was already used by species living 99 million years ago. The origin of zorapterans is not well known yet, but this and other fossil species indicate that they exist at least since the beginning of the cretaceous.
Chen X, Su G (2019) A new species of Zorotypus (Insecta, Zoraptera, Zorotypidae) and the earliest known suspicious mating behavior of Zorapterans from the mid-cretaceous amber of northern Myanmar. Journal of Zoological Systematics and Evolutionary Research. doi: 10.1111/jzs.12283
Hagfish are primitive chordates that make up the class Myxini. They are marine animals that live at the bottom of the sea and feed mainly on polychaete worms that they pull out of the substrate. However, they are also scavengers and have a peculiar behavior in which they perforate the body of dead fish and enter it, eating the dead animal from inside out.
Morphologically, hagfish are characterized by the presence of a cartilaginous skull, like vertebrates, but lack a vertebral column, keeping the notochord, the dorsal cartilage-like structure of chordates, during their whole lives. Due to this lack of vertebrae, the hagfish were classified outside of the vertebrates, but united to them due to the presence of the skull. Thus, Myxini was seen as the sister-group of Vertebrata and both together formed the clade Craniata.
Among the vertebrates, most extant groups have a jaw that evolved from modified gill arches, making up the clade Gnathostomata. The only animals with a vertebral column that lack jaws are the lampreys (Petromyzontiformes) and, although this lack of jaws is shared with hagfish, it is not usually seen as a synapomorphy uniting these groups. In hagfish, the jawless mouth have lateral keratin plates with tooth-like structures that act somewhat like the true jaws of Gnathostomata, but working from the sides and not from above and below. In lampreys, on the other hand, the mouth is circular and have keratin tooth-like structures arranged circularly.
There are a lot of morphological features that unite lampreys to vertebrates and separate them from hagfish, the main one being the already mentioned vertebrate column. Likewise, lampreys and jawed vertebrates have dorsal fins while hagfish lack them. Lampreys also have lensed eyes in common with jawed vertebrates, while hagfish have simple eyesposts without lenses or even associated muscles.
Some of the traits shared between hagfish and lampreys, just as the lack of jaws, are usually seen as a primitive state that changed in jawed vertebrates, or have clearly evolved independently. For example, both hagfish and lampreys have only a single nostril, while jawed vertebrates have two, but this is likely a primitive character. Adult hagfish and lampreys have also a single gonad, but this appears in hagfish by an atrophy of the left gonad, so that only the right one develops, while in lampreys the left and right gonads fuse into a single organ.
Therefore, morphologically, it seems logical to consider hagfish as a sister group of vertebrates, which include lampreys and jawed vertebrates. It is also important to mention that there are more groups of jawless vertebrates that are currently extinct, such as the class Osteostraci, one of several fossil groups traditionally called ostracoderms. Although lacking a jaw as well, these vertebrates had paired fins just like jawed vertebrates. Thus, the phylogenetic organization of these major groups based on morphology would be as shown in the figure below:
However, in the last decades, the use of molecular phylogenetics has challenged this view by grouping hagfish and lampreys into a monophyletic clade that is sister-group of jawed vertebrates. But how could this be possible? Such a relationship would imply that the primitive state of hagfish is the result of secondary loss.
Evidence from fossils could help clarify this issue, but most fossils that have been associated with hagfish have not good enough morphological characters preserved to assess their correct phylogenetic position. Recently, however, a well preserved hagfish fossil from the Cretaceous helped to elucidate part of the hagfish phylogeny. The divergence between lampreys and hagfish, considering previous knowledge, was usually put around the early Cambrian period, just after the beginning of the divergence of most animal phyla, but with data of the new fossil, it is pushed to a more recent point in time, around the Early Silurian, more than 130 million years after. This new fossil, named Tethymyxine tapirostrum, clearly lacks a skeleton or dorsal fins as seen in lampreys and jawed vertebrates, but has several characters shared with extant hagfish.
At least two synapomorphies can be found uniting hagfish and lampreys and separating them from jawed vertebrates. The first one are the teeth, which in these two groups are composed of keratin plates. The second one is the organization of the myomeres, the series of muscles arranged along the body of chordates in a somewhat segmented fashion, that in both hagfish and lampreys begin right around the eyes.
Considering the evidence from molecular data, the new fossil that makes it likely that hagfish and lampreys diverged more recently if they form a monophyletic group, and the likely true synapomorphies uniting these two jawless vertebrate groups, it seems that hagfish and lampreys are indeed sister-groups, forming a clade called Cyclostomata and sister-group of the jawed vertebrates Gnathostomata. If this is really the case, then the apparently more primitive features of hagfish are in fact the result of secondary losses and its ancestor likely had a more vertebrate look, with a vertebral column, dorsal fins and lensed eyes.
But let’s keep watching. Things may change again in the future as new data become available.
Miyashita T, Coates MI, Farrar R, Larson P, Manning PL, Wogelius RA, Edwards NP, Anné J, Bergmann U, Palmer AR, Currie PJ (2019) Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. PNAS116(6): 2146–2151. doi: 10.1073/pnas.1814794116
Oisi Y, Ota KG, Kuraku S, Fujimoto S, Kuratani S (2012) Craniofacial development of hagfishes and the evolution of vertebrates. Nature 493: 175–180. doi: 10.1038/nature11794
Parental care, here defined as any behavior in which an animal takes care of its young, is a widespread practice in the animal kingdom, having evolved repeatedly in many taxa. It is not difficult to see, considering natural selection, why parental care is an adaptative trait. It enhances the chance of one’s offspring to survive and thus carry one’s genes to the next generation.
On the other hand, the related behavior known as alloparental care is not that easy to explain in every instance of its occurrence.
While parental care means caring for your own offspring, alloparental care means caring for the offspring of another individual. If you spend time and resources in taking care of an animal that is not your direct descendant, you must have a good reason to do it, a reason that somehow benefits you. Or you may just be too dumb.
Most animals reject or even kill the offspring of other individuals of the same species. A classic example is a male lion that kills the cubs that he knows are not his. He does that because he sees no advantage in allowing the offspring of another male to survive.
An extreme example of caring for juveniles that are not your direct offspring is found in social insects such as bees and ants. Worker ants usually do not reproduce but they raise their siblings as if they were their own children. In this case, it is more advantageous to make siblings than to make children because of the peculiar reproductive system of hymenopterans. I will not enter in details but, basically, ants share 100% of their father’s DNA and 50% of their mother’s DNA, so that two sibling ants have 75% of their genes in common, while the relationship between a female ant and her female offspring is of only 50%.
However, alloparental care is found in many other animals, especially in mammals. Although not having 75% of similarity between siblings as in ants, many mammals and other animals help their mothers and/or fathers to raise their siblings. This has less direct advantages but they are still there. After all, your siblings (if they are of the same mother AND father) share 50% of your DNA, the same amount that you share with your children. But alloparental care may also happen with more distantly related relatives, such as grandchildren and half-siblings, which share only 25% of their DNA with you. This is not a problem, though, because if you are unable to have your own kids at that time, it is better to help raise those juveniles that share some DNA with you than to do nothing because 25% of your genes is still better than nothing.
A recently published paper reports the first observation of alloparental care in the field in the cichlid fish Neolamprologus savoryi. The team observed a male fish helping take care of the eggs of another male that was found to be his father, although the mother of the eggs was not his mother. The male helper was small and probably sexually immature, so that, as said above, helping his half-siblings, which have 25% of his genes, survive is better than doing nothing.
A really hard thing to explain is why some animals accept to take care of the offspring of unrelated individuals, in which there is no clear adaptative advantage. Such a situation was recently discovered to occur with the common earwig Forficula auricularia. Females that had their egg clutches replaced with the eggs of an unrelated female took care of them as if they were their own. No advantage of any kind can be extracted from this behavior, so the most likely explanation is simply the lack of adaptative pressure to reject unrelated eggs. It is likely that, under natural conditions, a female earwig never encounters the eggs of another female. Thus, there was never a scenario in which the capacity to recognize one’s own eggs (and differentiate them from others) could evolve. Natural selection needs opportunities to act.
But sex is usually much more complex and how it occurs is usually shaped by environmental conditions, especially by the presence of competitors. In dioecious species, males usually compete for the females but is there a similar behavior applied to hermaphrodites?
According to the sex allocation theory, hermaphrodite organisms have to choose how much they invest in the male versus the female function. If you produce more eggs, thus preferring the female side, you end up producing less sperm and vice versa. So what should hermaphrodites do?
One way to try to take the best of this situation is allocating resources to the female or the male role according to what will give you more advantages in the current scenario. This would be determined mainly by the number, or the density, of individuals in the population.
When there are a lot of individuals, there is a lot of sperm, and being able to fertilize the eggs becomes more difficult. Thus, hermaphrodites would increase their investment in the male function to have a better chance against the sperm of the others. In other words, it is better to be a male when there are too many guys around you.
On the other hand, when finding a mate is rare, there is little sperm competition, so focusing on being a female is more advantageous. After all, the little sperm you produce is enough to fertilize the eggs of the few other individuals around there.
Most studies looking for evidence of the sex allocation theory found conflicting results. In many organisms, only one of the sexual functions changes according to population density, with either the number of eggs or the amount of sperm remaining the same and sometimes both functions are enhanced at the same time, going against the idea of a trade-off that the sex allocation theory predicts.
The problem may be simply a matter of how to look at things. Most studies focused on gamete production only, but sex is much more than that. One important part that has been neglected is sexual behavior. In order to test whether behavioral investment may show sex allocation differences, a recent study compared the investment of the hermaphrodite polychaete worm Ophryotrocha diadema in a female-related and a male-related behavior. According to their hypothesis, a low density of organisms would increase parental care, a female-related behavior, while a high density of organisms would increase motility in order to find a mate, a male-related behavior.
And their hypothesis proved to be correct! Worms kept in pairs, i.e., with few mating opportunities due to the low density of individuals, moved less but took more care of the eggs. On the other hand, worms kept in groups, i.e., with more mating opportunities, moved more and did not take so much care of their eggs.
More than being nice evidence for the sex allocation theory, this study highlights the need to look beyond gamete production to assess sex allocation not only in hermaphrodites but in all organisms.
Sexual selection is a frequent subject of my posts here but they are usually focused on how females and males behave regarding each other. However, there is a third element that results from their interactions: the children.
Females tend to select the best males to be the father of their children because they are interested in having a healthy and strong offspring with better chances of surviving. But what happens when a female has no choice but to mate with a low-quality male? Will she take care of their children the same way?
A recent study conducted with the Honduran red point cichlid, Amatitlania siquia, investigated this question. This fish species is native from Central America and, as usually between cichlids, a female and a male form a bond and take care of their eggs and young together.
The researchers placed a female in an aquarium with transparent walls in which she was able to visually analyze two males, one placed in a chamber to the left and another in a chamber to the right. One of the males was larger than the other, both being larger than the female. After 48 hours, the female was placed randomly with either the larger or the smaller male for them to mate.
The results indicate that females produce similar egg clutches and take care of the eggs in equal amounts when mated with either larger or smaller males. However, after the eggs hatch and the larvae develop to the fry stage, the female spends more time caring for them if their father is the larger one.
They don’t seem very excited to waste their time with low-quality children. Afterall, they may meet that handsome big fish again in the future.