Category: reproductive behaviour (Page 4 of 4)

Squid male changes its mating tactic when growing larger

When becoming sexually active, male squids are little successful at first. Only later they perform better, increasing their chances to sire offspring. This development includes major changes, Lígia Apostólico and José Marian discovered.

In squid like Doryteuthis pleii, a species living off the coast of Brazil, small males are able to mate, but they have to do it at an inappropriate time and in a little successful way, as sneakers. Large males act much more effectively as real partners or consorts, as Lígia Apostólico and José Marian report.

Shooting mechanism

When squid mate, s male delivers sperm packages to a female. With a special arm, a male takes the packages, spermatophores, from the spermatophoric sac, where they are produced, and places them on the body of a female with a rapid movement. Then he is done, the sperm packages themselves will do the rest of the work. With a shooting mechanism (ejaculatory apparatus), a package turns inside out, and when evaginated, it attaches onto the female’s body and the sperm cells swim out.

A large male delivers its sperm packages neatly. He approaches a female that is about to release her eggs, places himself next to her with his head pointing in the same direction as hers, moves his special arm behind her head under the mantle that surrounds her body and places his spermatophores near the opening of the oviduct from which the eggs will be released in capsules. The sperm cells have immediate access to the eggs. The male guards the female and tries to keep rivals at bay with flickering colour patterns, because if another male also mates with her, his sperm will have to compete with that other male’s sperm.

Aggregate

A small man does not stand a chance against a large one, so he can only mate at a less exciting time, when no eggs are to be released soon. He doesn’t put his arm under the female’s mantle, but he assumes a head-to-head position and places his sperm packages under her beak, that is between the arms. When she releases the eggs, she holds the capsules for a while near the beak before depositing them on the substrate, and then a sneaker’s sperm cells have a chance – as far as the eggs are not fertilized already by a consort’s sperm.

The sperm cells of sneakers are adapted to the unfortunate site where they are placed and the wide time interval between mating and fertilization chances, and their spermatophores differ from those of consorts. Sneakers have smaller and thinner spermatophores; after evagination, they are short and club-like shaped. The sperm cells come out slowly and aggregate at the exit, having nothing to do there for the time being. The spermatophores of consorts, in contrast, are larger and after evagination, they are long and hook-like shaped. The sperm is quickly discharged in a powerful flow and sperm cells immediately diffuse, so the eggs that are released will pass through a cloud of them.

Now in Doryteuthis pleii, Apostólico and Marian found some males, intermediate in size between sneaker and consort (about 17 centimetres mantle length), that produce sneaker-like spermatophores as well as consort-like spermatophores, and often also an intermediate form. The sneaker-like packages are oldest and stay in the anterior part of the spermatophoric sac, the consort packages are youngest and reside in the posterior part, and the intermediate packages are to be found in between.

Fast switch

This indicates that a male starts as a sneaker and, if he exceeds a certain size limit, he will go on as a consort, implementing all changes that are required by the transition. Age estimates show that sneakers are indeed younger than consorts; the estimates are based on the size of small particles in the organs that enable the animals to control their position and balance; every day these particles, statoliths, increase a little in size. The switch from sneaker to consort must take place very fast, as only few males are found that are in transition.

So, during their lives, which lasts less than a year, the males go through a major development. They are small when at summer the mating season starts, but still they mature sexually, so that they can begin to reproduce – although for the time being only as little successful sneakers.

But perhaps not all males follow that path, Apostólico and Marian think. Males that were born early, in late summer or autumn, have much time before the mating season starts. They can grow to a large size before they become sexually active, and then they can be consorts from the start.

Willy van Strien

Photo: Alvaro E. Migotto (Cifonauta. Creative Commons CC BY-NC SA 3.0)

Sources:
Apostólico, L.H. & J.E.A.R. Marian, 2018. From sneaky to bully: reappraisal of male squid dimorphism indicates ontogenetic mating tactics and striking ejaculate transition. Biological Journal of the Linnean Society 123: 603-614. Doi: 10.1093/biolinnean/bly006
Apostólico, L.H. & J.E.A.R. Marian, 2018. Dimorphic male squid show differential gonadal and ejaculate expenditure. Hydrobiologia 808: 5-22. Doi: 10.1007/s1075
Apostólico, L.H. &  J.E.A.R. Marian, 2017. Dimorphic ejaculates and sperm release strategies associated with alternative mating behaviors in the squid. Journal of Morphology. 278: 1490-1505. Doi: 10.1002/jmor.20726

Disabled cactus bug produces more sperm

With their enlarged hind legs, male cactus bugs fight with each other to defend a territory or to achieve access to a female. What will become of a male that lost one of those weapons, Paul Joseph and colleagues wondered.

The leaf-footed cactus bug Narnia femorata can drop (autotomize) a leg when this leg is grasped by a predator, entrapped or damaged. Thanks to such self-amputation the bug survives the incident, but from now on it has only five legs left to stand on and to walk on; a leg that is lost is not regenerated. For a male, it is extra annoying if it has to sacrifice one of its two hind legs, because it uses them to fight with other males for the possession of a territory or the access to a female. However, if it loses a hind leg before it is fully grown, it can compensate for it, write Paul Joseph and colleagues.

In the southwest of the United States, Mexico and parts of Central America, the bugs live on cacti, for instance on the prickly pear cactus Opuntia mesacantha. They feed on the plants, preferably on the ripe fruits, and females lay their eggs on them, selecting parts with ripe fruits.

Fierce fight

Males try to defend a territory on a cactus. If an intruder shows up, both males position themselves rear to rear to display, kick and wrestle with their hind legs until one of them gives up. In the presence of a female – when there is a lot at stake – the fight is fiercer, and the male with the largest hind legs will be the winner. The hind legs of males are real weapons, they are enlarged and serrated.

A male that loses one of its hind legs is in problems. It cannot defeat an intact rival and the chance that it will mate a female has decreased considerably. But it may compensate for its disability, Joseph hypothesized, by growing larger testes. This would be possible if the leg is lost before the male is full-grown; bugs don’t go through a complete metamorphosis with a pupal stage, but they grow gradually.

In order to find out whether juvenile males grow larger testes after losing a hind leg, Joseph experimentally induced juvenile bugs to drop a leg by grasping the leg with a pair of forceps and tickling with a small paintbrush, mimicking what can happen in the wild. As expected, after such treatment the testes grew extra large, while everything else developed as it normally does.

More sperm

And is it useful to have enlarged testes? The researchers paired disabled and untreated males each with a female for 24 hours. Afterwards, they counted how many eggs the females laid and how many of them hatched, meaning that they had been fertilized. They noticed that most females produced about twenty eggs, independent of whether or not they had mated. Clutches of females that had been paired with an untreated male were more likely to contain eggs that hatched than clutches of females with a disabled partner. Apparently, males that dropped a hind leg less often succeeded in mating.

But if disarmed males managed to mate, they fertilized a larger proportion of the eggs. Their enlarged testes produced more sperm, and so they sired more offspring than intact males.

In conclusion, males can compensate for the loss of a weapon by investing more in testes growth – but only if they lose it when still young. Otherwise, it is just bad luck.

Willy van Strien

Photos
Large: leaf-footed cactus bug Narnia femorata; male that dropped a hind leg. ©Christine Miller
Small: leaf-footed cactus bug male on cactus fruit. Cotinis (via Flickr; Creative Commons CC BY-NC-SA 2.0)

Sources:
Joseph, P.N., Z. Emberts, D.A. Sasson & C.W. Miller, 2017. Males that drop a sexually selected weapon grow larger testes. Evolution, 20 november online. Doi: 10.1111/evo.13387
Procter, D.S., A.J. Moore & C.W. Miller, 2012. The form of sexual selection arising from male-male competition depends on the presence of females in the social environment. Journal of Evolutionary Biology 25: 803–812. Doi: 10.1111/j.1420-9101.2012.02485.x

Male spider cheats female with densely wrapped rubbish

A male nursery web spider may offer its partner a worthless package instead of a decent nuptial gift. He wraps such a fake present in many layers of silk, Paolo Ghislandi and colleagues show, so that it takes longer before the female detects the deceit and sends him away.

When you give someone a cheap gift, you’d better wrap it well. At least, that is the rule in the nursery web spider (Pisaura mirabilis), a hunting spider that occurs throughout Europe, as Paolo Ghislandi and colleagues report. A male usually carries a nuptial gift when he is looking for a female to mate with. It should contain one or more prey items that he has caught to offer her and wrapped in white silk. A female, happy to get a nice meal, will allow the male to mate her, while she often rejects a male without a present, as Maria Albo had shown.

Worthless

But instead of a meal, a female often finds the hard leftovers of an arthropod prey or some plant parts after removing the silk – an inedible gift that is worthless. Is a male giving such a gift in bad condition and unable to capture a prey and offer it? Or couldn’t he find anything better?

No, instead of inability it is pure deception, as Ghislandi concludes from field observations and behavioural experiments in the laboratory. Even a male that is well-fed and heavy – and therefore capable to catch and offer a prey – often cheats its partner with wrapped rubbish.

And he is successful, for as a female is unable to determine whether a white package contains something edible or not, she will accept a male with a fake present as readily as a male that carries an edible gift.

Punished

But ultimately, a cheating suitor will still be punished: the mating lasts briefly. A male can transfer its sperm while the female consumes her gift; it she is finished, he has to go. Consequently, when the gift is inedible, the mating will end soon, so a cheating male will transfer less sperm than a honest male. That is a disadvantage, because a female mates with several males and their sperm must compete for the eggs to be fertilized. The more sperm cells a male transfers, the more offspring he will sire.

More silk

Ghislandi also discovered that fake presents are wrapped in more layers of silk than real gifts, so cheating males invest a lot in wrapping. Probably, this is a trick to prolong mating, because the more silk is wrapped around the gift, the longer it takes a female to detect the deceit and stop the copulation.

Still, a really long mating will not ensue. And maybe that’s not so bad after all: a male cheating a female with a fake present may fertilize less eggs, but he saves time and energy to find other females, thereby increasing is lifetime reproductive success as well.

Willy van Strien

Photo: ©Paolo Ghislandi

Sources:
Ghislandi, P.G., M. Beyer, P. Velado & C. Tuni, 2017. Silk wrapping of nuptial gifts aids cheating behaviour in male spiders. Behavioral Ecology, online February 23. Doi:10.1093/beheco/arx028
Ghislandi, P.G., Albo, M.J., Tuni, C. & T. Bilde, 2014. Evolution of deceit by worthless donations in a nuptial gift-giving spider. Current Zoology 60: 43-51. Doi: 10.1093/czoolo/60.1.43
Albo, M.J., G. Winther, C. Tuni, S. Toft & T. Bilde, 2011. Worthless donations: male deception and female counter play in a nuptial gift-giving spider. BMC Evolutionary Biology 11: 329. Doi: 10.1186/1471-2148-11-329

Only old and bright males seek extra-pair mates

Red-backed fairy-wren males know how to behave to maximize their fitness, Denélle Dowling and Michael Dowling show. A male that has a bright breeding plumage invests in courting extra-pair females, whereas a man with a dull appearance invests in mate guarding.

Like most songbirds, red-backed fairy-wrens, which live in Australia, form socially monogamous pairs, and a couple may stay together for years. This doesn’t mean that the birds are faithful, however: roughly half of the young is not sired by the social father. Adultery is the rule.

In the breeding season, a male may adopt one of two alternative strategies. He can either invest in seeking extra-pair copulations to gain extra-pair offspring in addition to within-pair offspring, or he can stay on his own territory to defend it the against other couples together with his mate, to help provision the young – and to keep other males away from his mate to minimize the risk of being cuckolded.

Preference

What strategy is the best strategy? Jenélle Dowling and Michael Webster argued that the answer differs among males. It just depends on how attractive a male is to other females.

And the males differ greatly in attractiveness. Some have a bright, black and red plumage, while others have a dull, brownish plumage, much like a female. Almost all males aged more than two years are brightly coloured, among young males about half is bright. It was already known that females prefer bright males. Also, they prefer old males, as their age indicates that they are of good quality.

So, old black-red males are the most attractive ones. The best strategy for them will be to foray to neighbouring territories and court other females, Dowling and Webster assumed, as they stand a real chance to succeed. For dull males, on the contrary, it will be better to stay with their partner, as other females will be reluctant to copulate with them. Moreover, a dull male runs a high risk of being cuckolded by his social partner whenever an attractive male approaches her when she is alone. Young black-red males can try to gain extra-pair copulations, but they will be less likely to succeed than old males.

The researchers set up an investigation to find out whether red-backed fairy-wren males act in their best interests. And it turns out that they do. Old black-red males frequently leave to search for extra-pair females, while brown males mostly remain on their territory. Young black-red males adopt an intermediate strategy.

Cuckolded

DNA analyses of parents and young birds revealed that black-red males (young and old) sire more extra-pair young, as expected, but less within-pair young than dull males; apparently, bright males are cuckolded more often.

The latter result is not undisputed. In other studies, including that of Jordan Karubian, brown males were found to have less young than black-red males and to be cuckolded more frequently, even though they guarded their mate closely. But still, the best strategy for them is to stay on their territory. Otherwise, they will probably be cuckolded even more.

Willy van Strien

Photo: Red-backed fairy-wren, bright male. Jim Bendon (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Dowling, J. & M.S. Webster, 2017. Working with what you’ve got: unattractive males show greater mateguarding effort in a duetting songbird. Biology Letters 13: 20160682. Doi: 10.1098/rsbl.2016.0682
Karubian, J., 2002. Costs and benefits of variable breeding plumage in the red-backed fairy-wren. Evolution, 56: 1673-1682. Doi: 10.1111/j.0014-3820.2002.tb01479.x