Tag Archives: animal behavior

Male resistance: when females disappear and hermaphrodites don’t like you

by Piter Kehoma Boll

During the evolution of life, sex was certainly a great innovation. It allows organisms to reproduce while mixing their genes with that of another individual. Although it usually makes your offspring to have only half of your genes, which does not seem to be as great as an offspring that carries you as a whole into the next generation, there are certainly advantages in mixing. The most evident advantage is that your genes can combine with other versions and, as a result, produce a better team of genes than the one that you had. Even though each of your children carries only half of you, that half is more likely to survive than a child that carries you as a whole. In other words, sex gives the possibility for a population of genes (those that make up an individual) to get rid of some of the less efficient ones and replace them with better copies.

As you know, most sexual organisms make such a recombination by fusing two sexual cells, the gametes, and those are usually of two different kinds: a small one (the male) and a large one (the female).

In some species, each individual can only produce either male or female gametes, therefore being either a male organism or a female organism. In such species, sexual reproduction requires a male to mate with a female. This is the pattern found, for example, in most vertebrates and arthropods.


A female (large) and a male (small) of the tick Ixodes ricinus mating. Image by Jana Bulantová.*

In other species, each individual can produce both male and female gametes, therefore being called a hermaphrodite. The advantage of such a system is that hermaphrodites can mate with any individual of their species, sometimes even with themselves! One of the main problems with hermaphroditism is when you decide to play only one role, which may lead to conflict during sex.


Two snails Helix pomatia (hermaphrodites) making love. Photo by Wikimedia user Jangle1969.**

Now what evolved first? Dioecious species (those having male and female individuals) or hermaphrodites (allso called monoecious species)? It’s hard to tell, but we can be sure that during evolution many lineages switched from one system to the other and back. And the coolest part is that such switches still happen today.

You may know that most flowering plants are hermaphrodites. Flowers usually have both male and female organs, although they are rarely able to fertilize themselves (self-fertilization). Among plants, the cases of dioecious species seem to be mainly due to some mutation that ended up partially sterilizing an individual. For example, a mutation could appear that makes the plant unable to produce male organs, thus becoming only female. Other individuals in the population that lack this mutation continue to be hermaphrodites, so we have an “unbalanced” species with two sexes, females and hermaphrodites, but no males. Although unusual at first, such a system can remain stable if reproduction occurs through cross-fertilization and not self-fertilization. As both females and hermaphrodites need pollen (which produces the male gametes) from other plants, they can coexist as long as the pollinator carries pollen to both sexes. The same happens if the sexes are male and hermaphrodite. As long as the pollinator carries the male’s pollen to hermaphrodite flowers, both sexes can do just fine.


The plant Geranium sylvaticum includes hermaphrodites and females, but no males. Photo by Enrico Blasutto.**

Species composed of males and hermaphrodites are called androdioecious (from Greek andro-, man, male + di-, two + oikos, home, house; therefore “male in two “houses”, i.e., in two different kinds of organisms), while those composed of females and hermaphrodites are called gynodioecious (from Greek gyno-, woman, female; therefore “female in two different kinds of organisms).

Androdioecious and gynodioecious species occur among animals as well, but in this case their existance indicates something happening in the other direction, i.e., it is a transition from a dioecious species (with males and females) to a hermaphrodite species. And this is much more complicated that the other way round. Actually, it can get really, really bad for the “single-sex sex”.

This unbalanced sexual system in animals usually happens like this. There is a happily dioecious species with male and female individuals, but one day a new mutation appears and allows one of the sexes to produce both male and female gametes, thus becoming an hermaphrodite. However, such hermaphrodites are usually unable to play the role of the new sex while mating, i.e., they have the gametes, but not the tool to mate using them. Thus, the only way to use both gametes is to fertilize themselves.

One problem that comes from doing that is inbreeding. When you fertilize yourself, you are not increasing genetic diversity. On the contrary, you have very high chances of producing offspring with two copies to the same gene, thus decreasing genetic diversity. In order to continue to have recombination, you must mate with the single-sex individuals, which means you can only play the role of your original sex and your hermaphroditism is irrelevant. You are producing useless gametes. Or are you?


A male and a hermaphrodite of the nematode Caenorhabditis elegans an androdioecious species. Credit to Worm Atlas.

The problem with inbreeding happens when an organism ends up with two copies of a deleterious gene, which is fairly common in species where cross-fertilization is the rule and such deleterious genes are maintained in the population through individuals with a single copy that is not enough to cause any trouble. That is why having kids with your parents, children of siblings is usually a bad idea. When a species evolves from a system of cross-fertilization to one of self-fertilization, inbreeding can be a serious problem at first, producing many descendants that will die soon. However, eventually this will “purge” the set of genes. If individuals only mate with themselves, the number of deleterious genes will sharply decrease after some generations and inbreeding will not be such a big problem anymore.

When this happens in a species with unbalanced sex, the single-sex individuals will be in trouble. Two androdioecious animals have been studied regarding this conflict, the nematode and model organism Caenorhabditis elegans and clam shrimps of the genus Eulimnadia, such as Eulimnadia texana. In both groups, the hermaphrodites do not seem to be very interested in mating with males. They have even lost most phenotypic clues that help males identify them as potential mates. The only thing left for the males is to insist, to look for hermaphrodites and force them to mate with them, but it is a hard battle. Even when mating does occur, the hermaphrodite usually discards the male’s sperm.


A hermaphrodite (left) and a male (right) of the clam shrimp Eulimnadia texana. Credits to arizonafairyshrimp.com

The persistence of males in the population depends basically on their ability to fertilize hermaphrodites against their will and the sex-determination system of the species. When hermaphrodites produce males by self-fertilization, they are destined to remain for at least some time even if they cannot fertilize that much. Now if self-fertilization only produce hermaphrodites, the poor males have to be really persistent or otherwise they will soon perish.

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You may also like:

Having more females makes you gayer… if you are a beetle

Endosperm: the pivot of the sexual conflict in flowering plants

Gender Conflict: Who’s the man in the relationship?

Male dragonflies are not as violent as thought

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References and further reading:

Chasnov JR 2010. The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp. Journal of Evolutionary Biology 23: 539–556.

Ellis RE & Schärer L 2014. Rogue Sperm Indicate Sexually Antagonistic Coevolution in Nematodes. PLoS Biol 12: e1001916.

Ford RE & Weeks SC 2018. Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males? Ethology 124: 357–364.

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Cat-handedness: can cats be left- or right-handed?

by Piter Kehoma Boll

In humans, as you may know, there is usually a preference for using one side of the body to perform a task, a thing called laterality. And we have a strong tendency to be right-handed, with about 90% of humans using their right side to perform most unilateral tasks. Several studies revealed that many other animals, at least among vertebrates, display laterality as well.

A recent study investigated laterality in the domestic cat during spontaneous behaviors in contrast with the more common experiments using forced behaviors, such as making the cat try to reach food. They looked for a side preference in cats during the behaviors of lying side, stepping down and stepping over.


Photo by Juan Eduardo de Cristófaro.*

The result indicated that about one third of the cats is left-pawed, one third is right-pawed and one third is ambidextrous while moving up and down, but there is no clear preference for lying on their right or left side. Thus, we can see that, differently from humans, there is no strong bias to use one side of the body in cats, at least not when looking at cats in general.

When we consider sex, though, there was a significant difference: male cats tend to be left-pawed, while female cats are usually right-pawed. That would be very useful if cats danced the waltz.

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McDowell, L. J.; Wells, D. L.; Hepper, P. G. (2018) Lateralization of spontaneous behaviours in the domestic cat, Felis silvestris. Animal Behavior135: 37–43. https://doi.org/10.1016/j.anbehav.2017.11.002

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Badass females are unpopular among praying mantids

by Piter Kehoma Boll

One of the most iconic representations of praying mantids is that of a female eating the male after (or during) sex, an unpleasant scenario that starts with a beheading before the poor male even finishes his job.


Delicious male meal. Photo by Wikimedia user Classiccardinal.*

According to some studies, when the male is beheaded, he increases the pumping of semen into the female, thus increasing the chances of fecundation. This could make one think that being eaten is actually an advantage to the male, as it makes him have more offspring.

Several observations with different species show the opposite though. Males make everything they can to avoid being eaten by the female, as it allows them to copulate with additional females. But how can they escape from such a gruesome destiny?

It is known that hungry females are more eager to eat the partner than satiated ones. Well-fed females (fat ones) are also less likely to have a meal in bed than malnourished ones. Males can tell whether a female is hungry or malnourished and thus avoid those in such conditions. They like fat and fed females. But this is not the only thing that males take into account when choosing the appropriate mother for their children.

A study from 2015 by researchers of the University of Buenos Aires have shown that males of the species Parastagmatoptera tessellata, found in South America, also choose females based on their personality.

In a laboratory experiment, a male was put in a container where he could see two females, one aggressive and one non-aggressive. Another male was presented to both females (which were unable to see each other) and the aggressive female always attacked the male, while the non-aggressive one never did. After watching how each female behaved, the male received access to both and could choose his favorite one.

And guess what? The non-aggressive one was chosen most of the time. This means that males are not only able to tell whether they are likely to be eaten based on the female’s hunger and nutritional condition, but also by analyzing the behavior of the female towards other males.

See also:

Gender conflict: Who’s the man in the relationship?

Male dragonflies are not as violent as thought

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Lelito, J., & Brown, W. (2008). Mate attraction by females in a sexually cannibalistic praying mantis Behavioral Ecology and Sociobiology, 63 (2), 313-320 DOI: 10.1007/s00265-008-0663-8

Scardamaglia, R., Fosacheca, S., & Pompilio, L. (2015). Sexual conflict in a sexually cannibalistic praying mantid: males prefer low-risk over high-risk females Animal Behaviour, 99, 9-14 DOI: 10.1016/j.anbehav.2014.10.013

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What is behavior? Baby don’t ask me, don’t ask me, no more

ResearchBlogging.orgby Piter Kehoma Boll

One of the most difficult concepts to explain in biology is certainly life itself. But I am not here today to talk about the definition of life, but rather of another puzzling concept: behavior.

Behavior is the central subject of ethology and psychology, but it is also something more commonly understood by personal intuition, just like life, but no formal and widely accepted definition exists.

The simplest definition would be that behavior is something that is done. But in that case we fall into another difficult concept, the concept of “doing”, because what exactly is doing something?


Nobody doubts that a spider building a web is a behavior. Photo by Hedwig Storch.*

Some definitions of behavior that have been published are the following:

  • Tinbergen (1955): “The total movements made by the intact animal”. According to this definition, only animals can behave, so an unicellular alga swimming towards light, or a plant closing its leaves after being touched cannot be considered behaviors. On the other hand, the fact that an animal is orbiting the Sun because it is on Earth could be a behavior.
  • Beck et al. (1991): “External visible activity of an animal, in which a coordinated pattern of sensory, motor and associated neural activity responds to changing external or internal conditions”. Here again only animals would behave and only animals with some sort of nervous system. A behavior needs to include a response to a changing condition, i.e., a stimulus.
  • Starr & Taggart (1992): “A response to external and internal stimuli, following integration of sensory, neural, endocrine and effector components. Behavior has a genetic basis, hence is subject to natural selection, and it commonly can be modified through experience.” This definition does not use the word “animal”, but includes the need for neural components, which is almost the same thing.
  • Wallace et al. (1991): “Observable activity of an organism; anything an organism does that involves action and/or response to stimulation”. A more simple and broad explanation that encompasses many things that the previous definitions would exclude, but still includes at least the criterion that it is a response to stimuli.
  • Raven & Johnson (1898): “Behavior can be defined as the way an organism respond to stimulation”. A definition similar to the previous one.
  • Davis (1966): “What an animal does”. Very ambiguous and contradictory, as the same book includes a section on behavior of plants.
  • Grier & Burk (1992): “All observable or otherwise measurable muscular and secretory responses (or lack thereof in some cases) and related phenomena such as changes in blood flow and surface pigments in response to changes in an animal’s internal and external environment.” Another confuse, long, complex and ambiguous definition.

When a venus flycatcher closes its leaf to capture a fly, is it behaving? Photo by Stefano Zucchinali.*

Trying to find a way to create a unified definition of what is behavior, a group of researchers from the University of California, Berkeley, made a survey, published in 2009, in which they presented two lists to several biologists. The first contained a series of statements regarding behavior and the respondents should agree or disagree with the statement based on their assumption of what is behavior. The 13 statements were:

(A) ‘A developmental change is usually not a behavior.’
   (B) ‘Behavior is always a response to the external environment.’
(C)’A behavior is always an action, rather than a lack of action.’
(D) ‘All behaviors are directly observable, recordable and measurable.’
   (E) ‘People can all tell what is and isn’t behavior, just by looking at it.’
   (F) ‘Behavior is always influenced by the internal processes of the individual.’
   (G) ‘Behavior always involves movement.’
   (H) ‘Behaviors are always the actions of individuals, not groups.’
   (I) ‘Behavior is something whole individuals do, not organs or parts that make up an individual.’
   (J) ‘A behavior is always in response to a stimulus or set of stimuli, but the stimulus can be either internal or external.’
   (K) ‘Behavior is something only animals (including people) do, but not other organisms.’
   (L) ‘In humans, anything that is not under conscious control is not behavior.’
   (M) ‘Behavior is always executed through muscular activity.’


An arctic fox changing its fur color between seasons is a behavior or not? Photo by Wikimedia user Longdistancer.*

The second list included a set of 20 phenomena and the respondents should say whether they considered each phenomenon as representing a behavior or not. (In parentheses are the above statements under which the phenomenon would not be considered a behavior).

  1. Ants that are physiologically capable of laying eggs do not do so because they are not queens. (C, G).
  2. A sponge pumps water to gather food (B, M).
  3. A spider builds a web.
  4. A rabbit grows  thicker fur in the winter (A, G, I, M).
  5. A plant’s stomata (respiration pores) close to conserve water (I, K, M) .
  6. A plant bends its leaves towards a light source (K, M).
  7. A person’s heart beats harder after a nightmare (B, I, L).
  8. A person sweats in response to hot air (G, I, L, M).
  9. A beetle is swept away by a strong current (F, M).
  10. A rat has a dislike for salty food (B, C, G, J, M).
  11. A person decides not to do anything tomorrow if it rains (B, C, G, J, M).
  12. A horse becomes arthritic with age (A, B, E, G, M).
  13. A mouse floats in zero gravity in outer space (E, F, G, M).
  14. A group of unicellular algae swim towards water with a higher concentration of nutrients (F, H, K, M).
  15. A frog orbits the Sun along with the rest of the Earth (F, M).
  16. Flocks of geese fly in V formations (H).
  17. A dog salivates in anticipation of feeding time (B, G, I, M).
  18. Herds of zebras break up during the breeding season and reform afterwards (H).
  19. A chameleon changes color in response to sunlight (G, M).
  20. A cat produces insulin because of excess sugar in her blood (B, G, I, M).

Only four statements (A, F, I, J) were generally agreed, while seven (B, C, E, G, H, L, M) had general disapproval and two (D, K) were neither strongly approved nor strongly disapproved.

Considering the phenomena, seven (2, 3, 11, 14, 16, 17, 18) met the criterion for approval as behaviors based on the results of the statements and seven (4, 8, 9, 12, 13, 15, 20) met the criterion for rejection. The remaining six phenomena (1, 5, 6, 7, 10, 19) had major divergences as to whether they were behaviors.

Several respondents contradicted themselves. For example, many of them agreed that only animals can behave (statement K) but also considered that algae swimming towards water with higher concentration of nutrients is a behavior (phenomenon 14).


Most people would not consider that dandelion fruits carried by the wind are behaving.

Despite the high rate of disagreement, the group decided to propose a definition of behavior. And it is:

“Behavior is the  internally coordinated responses (actions or inactions) of whole living organisms (individuals or groups) to internal and/or external stimuli, excluding responses more easily understood as developmental changes.” (Levitis et al., 2009)

The question is not settled, though, and probably never will. Later, Dr. Raymond M. Berger, discussing the same subject, tells us that under the view of Descriptive Psychology, a behavior always includes eight parameters in the following formulation:

<B> = <I, W, K, K-H, P, A, PC, S>, in which:

B = behavior (e.g., Mary playing her queen of hearts in the contexts of a game of bridge).
I = identity of the person whose behavior it is (e.g., Mary)
W = want, the thing the person is attempting to achieve (e.g, to win a trick in the bridge game).
K = know, the cognitive parameter, the knowledge of how things work (e.g., queen vs. king, hearts vs. diamonds).
K-H = know-how, the ability to do what is being done (e.g., the ability to understand the rules of bridge, or the ability to move physical objects).
P = performance, the bodily processes involved in the behavior (e.g., Mary grasping and laying down her queen).
A = achievement, the outcome of the behavior (e.g., Mary takes the trick).
PC = personal characteristics, the indivudual’s difference parameter (e.g., Mary’s deep knowledge of strategy).
S = significance, what the behavior mean (e.g., Mary is playing bridge).


Playing a cardgame is certainly a behavior. A very complex one.

Such a “definition” is, in my opinion, too complex for most circumstances of animal behavior. However, I’m not completely happy with the definition by Levitis et al. either. I think it is hard to tell the difference between a response given by the whole organism vs. one of its parts. For example, when I cough because water entered my trachea, is my whole organism responding or only part of it? Would that be a behavior?

Also, I’m not sure whether we should really consider developmental changes as something different from other responses. If I had to define behavior, I most likely would say it is:

“An activity performed by an organism that is a response to a stimulus and is dependent on the organism’s internal processes”.


Why should a pupil changing size according to the environmental light not be considered a behavior?

And you? What do you think is behavior?

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Bergner, R. (2011). What is behavior? And so what? New Ideas in Psychology, 29 (2), 147-155 DOI: 10.1016/j.newideapsych.2010.08.001

Bergner, R. (2016). What is behavior? And why is it not reducible to biological states of affairs? Journal of Theoretical and Philosophical Psychology, 36 (1), 41-55 DOI: 10.1037/teo0000026

Levitis, D., Lidicker, W., & Freund, G. (2009). Behavioural biologists do not agree on what constitutes behaviour Animal Behaviour, 78 (1), 103-110 DOI: 10.1016/j.anbehav.2009.03.018

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